ISSN 1346-7328
国総研資料 第 519 号

ISSN 0386-5878
土研資料

第 4132 号

平 成 21 年 2 月

国土技術政策総合研究所資料
土

木

研

究

所

資

料

TECHNICAL NOTE of
National Institute for Land and Infrastructure Management, No.519
andPublic Works Research Institute, No.4132

February 2009

国土交通省河川砂防技術基準同解説計画編（英訳）The Japanese Ministry of Land, Infrastructure, Transport and Tourism
Technical Criteria for River Works: Practical Guide for Planning国土交通省 国土技術政策総合研究所
独 立 行 政 法 人

土 木 研 究 所

National Institute for Land and Infrastructure Management ,Ministry of Land, Infrastructure, Transport and Tourism, Japan
Incorporated Administrative Agency Public Works Research Institute国土技術政策総合研究所資料
土木研究所資料

Technical Note of NILIM, No.519

第 519 号

and PWRI No.4132

第 4132 号

February 20092009 年 2 月

国土交通省河川砂防技術基準同解説計画編（英訳）金澤 裕勝 *・吉谷純一 **・新宅幸夫 ***The Japanese Ministry of Land, Infrastructure, Transport and Tourism
Technical Criteria for River Works: Practical Guide for Planning
Hirokatsu KANAZAWA*, Junichi YOSHITANI**, Sachio SHINTAKU***

概要
本資料は、国土交通省河川局監修、社団法人日本河川協会編による「国土交通省河川砂防技術基準同解説計画編」を英訳したものである。

キーワード： 技術基準、河川計画、砂防計画、海岸保全計画、情報の共有、流域との連携、モニタリング、施設配置計画

Synopsis
This note is the translation of "The Japanese Ministry of Land, Infrastructure, Transport andTourism’s Technical Criteria for River Works: Practical Guide for Planning." (Editorial
Supervisor: River Bureau, Ministry of Land, Infrastructure, Transport and Tourism, Editor: Japan
River Association)
Key Word :Technical Criteria for River works, River Planning,
Erosion and Sediment Management Planning, Seacoast Preservation Planning,
Information Sharing, Cooperation with Communities in River Basins,
Monitoring, Planning for Facility Installation*

国土交通省国土技術政策総合研究所河川研究部河川研究室長
独立行政法人土木研究所水災害・リスクマネジメントセンター防災チーム上席研究員***
国土交通省国土技術政策総合研究所河川研究部河川研究室主任研究員**

The Japanese Ministry of Land, Infrastructure,
Transport and Tourism
Technical Criteria for River Works: Practical Guidefor
Planning

Editorial Supervisor: River Bureau, Ministry of Land, Infrastructure, Transport and Tourism
Editor: Japan River Association
Gihodo Publishing Co., Ltd.

Introduction
With the severe economic climate coupled with a background of serious concern over rapid aging and low
birthrates, demands for administrative services are changing, and more effective and efficient development andimprovement of social infrastructure are now called for. Technological development has often contributed to thesolution of these social problems, and the mission of socially supportive technologies will never decline inimportance.
As part of the 1997 revision of the River Law, river improvement and conservation were added as objectives,
and this necessitated the creation of a basic policy for river improvement and river improvement plans. In
addition, the same year saw the enactment of the Environmental Impact Assessment Law, and in 1999 the RiverCouncil released a report on amendments to the Coast Act, construction of a healthy hydrologic cycle, andcomprehensive sediment management.
In 2005, 7 years after the initial revision, the technical issues needed to properly address these new social
demands for river administration were documented as the revised “Technical Criteria for River Works: Planning”.
To facilitate smooth application of the criteria, this practical guide has been published to provide descriptions ofrelated concepts and case examples.
The major revisions include the following:(1) Documentation of the concept of environmental improvement and conservation and issues for investigation(2) Documentation of harmonious improvement and management of disaster prevention, utilization, and theenvironment(3) Specification of policies on the "sharing of information and cooperation among river basins" and"monitoring" for the more adequate improvement and management of rivers as natural and public objects(4) Specification of the role of the basic policy for river improvement and river improvement plans(5) Separation of the basic considerations of individual plans from facility rearrangement plans to form a Basic
Planning section
These technical criteria and descriptions, if used as a matter of form, could impede the development of newtechnologies. This guide is merely a compilation of the ideas that are deemed appropriate at the present point.
Technologies should be continuously replaced with more useful ones; therefore, any efforts aimed at developing
new and original ideas and technologies while at the same time taking advantage of the policy criteria but not
resolutely sticking to the criteria should be valued.
It is expected that this guide will be kept at hand by many river engineers; that the meanings of the technicalrequirements described here will be fully understood; and that the guide will be used in a wide range of fields
and will prove to be a foundation for further technological development.

iFinally, I express my deepest appreciation to all those who were involved in the revision work.
Masato Seiji
Director General, River Bureau
Ministry of Land, Infrastructure and Transport
July 2005ii

About this Revision of the “Technical Criteria for River Works: Planning"
By River Information Office, River Planning Division, River Bureau, Ministry of Land, Infrastructure andTransport
The River Bureau of the Japanese Ministry of Land, Infrastructure and Transport has revised the Planningvolume of "Technical Criteria for River Works" (hereinafter referred to as "the Criteria"), which has been used asa guideline for the technical aspects of river administration. Notification of the revision was sent from the
Director-General, River Bureau, to Regional Development Bureaus and prefectures on 30 March 2004.
Whereas the last revision, which was carried out 7 years ago in 1997, involved mainly minor changes inconnection with the shift to the International System of Units, the revision of the Planning volume of the Criteria,
this time with major changes in its content, was the first substantial revision since 1976—an interval of 28 years.
The revised edition supports the changes in statutory plans in connection with the revision of the River Law and
the Seacoast Law, and it introduces new concepts such as "construction of a healthy hydrological cycle system"and "comprehensive sediment management", which were presented at the Council for Social Infrastructure. Also,
this edition contains full descriptions of environmental improvement and conservation and the implementation ofproper monitoring, the importance of which has been pointed out in recent years.
The order of the contents in the Planning volume has been extensively rearranged to form two major parts: a
Basic Planning part and a Facility Arrangement Planning part.

About the Criteria
The purpose of the Criteria, which were established in 1958, is to compile technical knowledge in the areas ofriver, erosion, and sediment control and the management of landslides, steep areas, avalanches, and seacoasts(hereinafter referred to as "rivers, etc."); systematize this knowledge; and clarify the current technical standards,
thereby constructing a foundation for future development. In addition to the revised Planning volume, theCriteria, which were released by Director-General of the River Bureau, include an Investigation volume and a
Design volume, as well as the Maintenance volume, which is currently being experimentally reviewed.
It should be noted that the Criteria are general standards for technical issues related to rivers, etc. In cases where
there is a more appropriate method to achieve the intended goal by the implementation of an actual measure, theexistence of the Criteria should in no way discourage the adoption of such a method.

About the revision of the Planning volume of the “Technical Criteria for River Works”(1) Background of the revision
In preparation for the revision of the Planning volume of the Criteria, a review aimed at an across-the-board
revision was started immediately after the last revision in 1997, and a revision draft was prepared mainly by theRiver Bureau of the Ministry of Land, Infrastructure and Transport. Since the Criteria cover a wide range ofareas and include highly technical elements, in the preparation of the original draft not only a review by related
sections of the River Bureau but also a detailed review from both the engineering and administrative points ofview was conducted. These reviews were also based on the opinions of staff of Regional Development Bureaus
and on knowledge supplied by the National Institute for Land and Infrastructure Management.
iii

In addition, to collect extensive opinions from people with knowledge and experience and to ensure that therevision would reflect these, the Study Committee on Technical Criteria for River and Coastal Works (see below)
was organized in 2002, and it has provided active discussion and valuable opinions.
Study Committee on Technical Criteria for River and Coastal Works: Member List
Chairperson: Shoji Fukuoka Professor, Graduate School, Hiroshima University
Masahiko Isobe, Professor, Graduate School, University of Tokyo
Takehiko Ota, Professor, Tokyo University of Agriculture
Tetsuya Kusuda, Professor, Graduate School, Kyushu University
Shinji Sato, Professor, Graduate School, University of Tokyo
Kaoru Takara, Disaster Prevention Research Institute, Kyoto University
Kazumi Tanida, Professor, Osaka Prefecture University
Tetsuro Tsujimoto, Professor, Graduate School, Nagoya University
Hiroyuki Nakamura, Professor, Graduate School, Tokyo University of Agriculture and Technology
Futoshi Nakamura, Professor, Graduate School, Hokkaido University
Takahisa Mizuyama, Professor, Graduate School, Kyoto University
Kazuo Murata, Japan Civil Engineering Consultants Association(2) Change in organization
To allow users to quickly access an item, the items were rearranged so that the Planning volume, which used to
be in one part, now has two parts: Basic Planning and Planning for Facility Arrangement, etc.
The Basic Planning part includes descriptions of statutory plans associated with projects operated by riveradministrators, such as river improvement plans and the basic doctrine necessary for systematic water andsediment management.
The Facility Arrangement part includes descriptions of river channel plans as well as plans associated with theactual arrangement of facilities. Both parts are divided into chapters based on the areas of river, erosion, andsediment control, and seacoast an independent chapter, which includes items commonly associated with theseareas, provides topics such as information sharing, cooperation with the river basin, and the importance ofmonitoring, thus contributing to the integration of doctrine throughout all areas.
Descriptions of legal stipulations, such as the Cabinet Order Concerning Structural Standards for RiverManagement Facilities, etc., have been excluded from these Criteria.
(3) Points of revision
Please refer to "Points of Revision" at the end of the book.

iv

Changes in organization
Old edition of the Volume of Planning

Related areas

Chapter 1 Comprehensive River PlanningRiver

Chapter 2 Basic Planning for Flood Prevention
Chapter 3 Basic Planning for Low Water

Sabo

Chapter 4 Basic Planning for Erosion and Sediment Control

General

Chapter 5 Basic Planning for Environmental Conservation
Chapter 6 Coastal Basic Planning

Coast

Chapter 7 Basic Planning for Landslide Prevention
Sabo

Chapter 8 Basic Planning for Steep Slope Failure Prevention
Chapter 9 Basic Planning for Avalanche Control
Chapter 10 River Channel and River Structure PlanningRiver

Chapter 11 Multipurpose Facility Planning
Chapter 12 Dam Facility Planning
Chapter 13 Erosion Control Facility Planning
Chapter 14 Landslide Prevention Facility Planning

Sabo

Chapter 15 Steep Slope Failure Prevention Facility Planning
Chapter 16 Avalanche Control Facility Planning

New edition of the Volume of Planning

Related areas[Basic Planning]
General

Chapter 1 Basic Policy
River

Chapter 2 River Planning
Chapter 3 Erosion and Sediment Control Planning

Sabo(Countermeasures for Sediment Disasters, etc.)
Coast

Chapter 4 Seacoast Preservation Planning
Chapter 5 Information Sharing and Cooperation with the River

GeneralBasin
Chapter 6 Monitoring[Planning for Facility Arrangement, etc]
Chapter 1 Improvement and Conservation of the River

GeneralEnvironment, etc., and Comprehensive SedimentManagement
Chapter 2 Planning the Arrangement of River Facilities
Chapter 3 Planning the Arrangement of Erosion and Sediment

River
Sabo

Control Facilities, etc.
Coast

Chapter 4 Planning Seacoast Preservation Facilities
Chapter 5 Planning the Arrangement of Information Facilities

v

GeneralIn response to economic and social trends and the progress and maturation of technological development, thetechnologies that need to be applied as standards are changing from day to day. Since the establishment of any
technical criteria in ways that impede the introduction of new technologies may hamper future technologicaldevelopment, the Criteria are basically descriptions of the technical items and standards that should be compliedwith. Further social changes and fast-evolving technological innovation call for new criteria, so constant review
will be indispensable to keeping them useful. In the future, we intend to follow up the utilization of the Criteria
and undertake a revision of the Research and Design volumes that will reflect the purpose of the revision of the
Planning volume.
Although the word "Proposal" was appended to the title of the Criteria in the old editions, because many years
have passed since the establishment of the original Criteria we decided not to append "Proposal" to the title fromthis revised edition onward.
Finally, we express our deepest gratitude to all parties concerned who participated in the preparation of thisedition, and to the Japan Institute of Construction Engineering for its cooperation in the coordination work.

vi

CONTENTS
GENERAL SPECIFICATIONS

1BASIC PLANNING

3

GENERAL DESCRIPTION

4CHAPTER 1

Basic policy

8

SECTION 1

General information

8

SECTION 2

Disaster prevention and mitigation

11

2.1

Overview

11

2.2Countermeasures to flood disasters

11

2.3

Countermeasures to sediment disasters

12

2.4

Countermeasures to earthquake disasters

13

SECTION 3

Proper use of rivers, maintenance of normal water flows, andimprovement and conservation of riverine environments

15

Comprehensive sediment management

17

River planning19

The basics of river planning

19

1.1

General Overview

19

1.2

Fundamental river management policy and river improvement plans

20

Basics of flood-prevention planning

21

2.1

Overview

21

2.2

Method of determining the design flood

22

2.3

Subject rainfall

23

2.4

Design reference point

23

2.5

Determining the design scale

24

2.5.1

Scale of plan

24

2.5.2

Consistency of the scale of plan within the same river system

25

SECTION 4

CHAPTER 2
SECTION 1

SECTION 2

2.6

Selection of subject rainfall

25

2.6.1

Determining the amount of subject rainfall

25

2.6.2

Review of past floods

26

2.6.3

Duration of subject rainfall

26

2.6.4

Determining the time and areal distributions of the subject
rainfall

2.6.5

2.7

26

Adjustment of the actual rainfall duration against the subject
rainfall duration

27

Determination of the design flood

29

1

2.8

2.7.1

Determining the design flood

29

2.7.2

Conversion of subject rainfalls into a flow discharge

30

2.7.3

Determining constants for the flood runoff model

30

2.7.4

Consideration of landside waters

31

Design flood discharge

31

2.8.1

Design flood discharge

31

2.8.2

Items to be examined in determining the design flood flow
discharge

2.9
SECTION 3

32

Measures against excess floods

32

Basics of appropriate utilization of rivers and maintenance of normalriver water function

33

3.1

Overview

33

3.2

Normal flow discharge

33

3.3

Determining the maintenance flow discharge

34

3.4

Determining the water utilization flow discharge

37

Basics of improvement and conservation of the river environment

38

4.1

Overview

38

4.2

Conservation and restoration of a healthy environment for plants and

SECTION 4

animals38

4.3

Maintaining and enhancing landscape quality

39

4.4

Maintenance and creation of places for activities that bring humans into

4.5
CHAPTER 3

close contact with rivers

40

Preservation of water quality

41

Erosion and sediment management planning (countermeasures for
sediment disasters, etc.)

42

SECTION 1

General information

42

SECTION 2

Erosion and sediment management master plan

43

2.1

Overview

43

2.2

Basic items associated with river system erosion and sedimentmanagement

44

2.2.1

Overview

44

2.2.2

Design scale

45

2.2.3

Design control points in the erosion and sediment management
master plan

46

2.2.4

Design sediment volume

46

2.2.5

Sediment management plan

47

2.2.6

Sediment generation control plan

47

2.2.7

Sediment transportation control plan

48

2

2.3

2.4

2.5

2.6

SECTION 3

Basic items associated with debris flow control

49

2.3.1

Overview

49

2.3.2

Design scale

49

2.3.3

Design control point, etc.

50

2.3.4

Basic items associated with countermeasures

50

Basic items associated with prevention of large woody debris

51

2.4.1

Overview

51

2.4.2

Design scale

51

2.4.3

Design control point, etc.

52

2.4.4

Basic items associated with countermeasures

52

Basic items associated with volcanic sediment and erosion control

52

2.5.1

Overview

52

2.5.2

Subject phenomena, etc.

53

2.5.3

Basic items associated with countermeasures

53

Prevention of abnormal sediment disasters

54

2.6.1

Overview

54

2.6.2

Target phenomena, etc.

55

2.6.3

Basic items associated with countermeasures

55

Landslide prevention plan

56

3.1

Overview

56

3.2

Basic items associated with landslide prevention

57

3.2.1

Target phenomena, etc.

57

3.2.2

Basic countermeasure principles

57

SECTION 4

Steep slope failure-prevention plan

58

4.1

Overview

58

4.2

Basic items associated with steep slope failure-prevention measures

58

4.2.1

Target phenomena, etc.

58

4.2.2

Basic countermeasure principles

58

SECTION 5

Avalanche prevention plan

59

5.1

Overview

59

5.2

Basic items associated with avalanche-prevention measures

59

5.2.1

Target phenomena, etc.

59

5.2.2

Basic countermeasure principles

60

Comprehensive sediment disaster-prevention plan

60

6.1

Overview

60

6.2

The elements of comprehensive sediment disaster-prevention

60

6.3

Urbanized foothills greenbelt development plan

61

6.3.1

61

SECTION 6

Overview

36.3.2

Basic countermeasure principles

62

Consideration of the natural environment

62

Seacoast preservation planning

64

SECTION 1

General information

64

SECTION 2

Basic items associated with coastal protection

69

2.1

Overview

69

2.2

Factors of the design coastal form

70

2.3

Design sea level

72

2.4

Design ocean wave

73

2.5

Design tsunami

74

2.6

Basic policy for coastal protection

75

2.6.1

Overview

75

2.6.2

Coastal erosion

76

2.6.3

Storm surge

81

2.6.4

Tsunamis

82

SECTION 7
CHAPTER 4

SECTION 3

Basic items associated with coastal environment improvement andpreservation

84

3.1

Overview

84

3.2

Preservation and restoration of hospitable habitat for animals and plants

85

3.3

Preservation and restoration of coastal landscape, etc.

87

Basic items associated with coastal utilization

88

SECTION 4
CHAPTER 5

Information sharing and cooperation with communities in river
basins90

SECTION 1

General information

90

SECTION 2

Cooperation for disaster prevention and mitigation

91

2.1

Overview

91

2.2

Measures in flood runoff areas

92

2.3

Measures in flood-prone areas

92

2.4

Flood fighting

93

2.5

Evacuation

93

SECTION 3

Cooperation for appropriate river utilization, maintenance of normal
functions of river water, and improvement and preservation of the riverenvironment

3.1

94

Cooperation for appropriate river utilization and maintenance of thenormal functions of river water

3.2

SECTION 4

94

Cooperation for improvement and preservation of the river environment,
etc.

95

River improvement in cooperation with city planning

96

4CHAPTER 6

Monitoring

97

SECTION 1

General information

97

SECTION 2

Monitoring of water, sediment, etc.

98

Monitoring associated with flood control

98

2.1.1

Overview

98

2.1.2

Water flow volume monitoring

98

2.1.3

Sediment monitoring

99

2.1.4

Inner water monitoring

100

2.1

2.2

2.3

SECTION 3

Monitoring of appropriate river utilization and maintenance of normal
functions of river water100

Monitoring of sediment

101

2.3.1

Overview

101

2.3.2

Monitoring in sediment yielding areas

102

2.3.3

Monitoring in a sediment transport area

102

Monitoring of land and associated space

103

3.1

Overview

103

3.2

Monitoring for flood control

104

3.3

Monitoring associated with utilization

104

SECTION 4

Monitoring of the environment

105

SECTION 5

Facility monitoring

106

5.1

Monitoring associated with the assessment of facility planning

106

5.2

Monitoring associated with functional maintenance107

PLANNING FOR FACILITY INSTALLATION

108

GENERAL INFORMATION

109

CHAPTER 1

Improvement and preservation of the river environment andcomprehensive sediment management

110

Improvement and preservation of the river environment

110

1.1

Overview

110

1.2

Understanding the characteristics of the river environment

111

1.3

Setting goals for improvement and preservation of the river environment

112

1.3.1

Determining the direction

112

1.3.2

Setting goals

115

SECTION 1

1.4

Measures for improvement and preservation of the river environment

116

Comprehensive sediment management

120

2.1

Overview120

2.2

Basics of facility installation planning

121

River facility installation planning

123

SECTION 2

CHAPTER 2

5

CHAPTER 2–1
SECTION 1
1.1

1.2

River channel and river structures

123

River channel planning

123

Basics of river channel planning

123

1.1.1

Basics of river channel planning

123

1.1.2

Procedure for establishing a river channel plan

124

Design high water level

125

1.2.1

Basics of determining the design high water level

125

1.2.2

Design high water levels of branches within a backwater section

1.2.3
1.3

1.4

of the main stream

126

Design high water level in estuaries

127

Planar shape and longitudinal and transverse shapes of a river channel

127

1.3.1

Basics of the planar shape of a river channel

127

1.3.2

Normal line of the levee

128

1.3.3

Shape of a tributary confluence

129

1.3.4

Longitudinal shape of a river channel

130

1.3.5

Transverse shape of a river channel

131

1.3.6

Low water channel width and high water channel height

132

1.3.7

Installation of a levee-side riparian forest zone

133

Estuary planning

133

Cutoff channel and discharge channel

134

2.1

Cutoff channel and discharge channel planning

134

2.2

River tunnel structures

136

2.2.1

Basics of planning

136

2.2.2

Cross-section and longitudinal slope

137

SECTION 2

SECTION 3
3.1

3.2

3.3

Reservoir (dam)137

Flood control planning

137

3.1.1

Design high water discharge for dams

137

3.1.2

Flood control method

138

3.1.3

Flood control capacity

139

Other plans

140

3.2.1

Planning for sediment inflow control

140

3.2.2

Planning for prevention of landslides around a reservoir

140

3.2.3

Planning to prevent leakage around a reservoir

140

3.2.4

Planning for private hydropower generation

141

Environmental considerations

141

Retarding basins etc.

143

4.1

Basics of planning

143

4.2

Site selection for retarding basins etc.

143

SECTION 4

6

4.3

Flood control planning

144

4.3.1

Flood control facility planning

144

4.3.2

Flow at the start of control

144

Weirs, water gates, and sluiceways

144

Establishment basis

144

Weir flooding level

145

Fishways in weirs

146

SECTION 6

River projects for flow regime adjustment

146

SECTION 7

Channel control facilities

146

7.1

Basis of a channel control facility design program

146

7.2

Revetment planning

147

7.3

Groin planning

148

7.4

Consolidation work planning

149

7.5

Fishways for groundsills

149

River mouth improvement

150

8.1

River mouth improvement planning

150

8.2

Selection of rivermouth improvement methods

151

Super broad levee with multiple functions

154

9.1

High-standard embankment installation section

154

9.2

Height of high-standard embankments

155

9.3

Backwater

SECTION 5

SECTION 8

SECTION 9

sectionsofbranches

flowing

into

sections

where

high-standard embankment is to be installed

156

Adjustment for projects associated with regional management

156

Drainage facilities for the inner water

160

SECTION 1

General information

160

SECTION 2

Determination of inner water drainage method

161

SECTION 3

Selection of landside waters for examination

164

SECTION 4

Determination of the probability assessment method

165

SECTION 5

Determination of the scale of inner water drainage facilities

167

Multi-purpose facilities

169

General information

169

1.1

Definition

169

1.2

Basics of multi-purpose facility planning

169

1.3

Multi-purpose facility location

169

Facility basic plan

170

2.1

Plan adjustment

170

2.2

Consideration of economic efficiency (cost-benefit analysis)

170

2.3

Multi-purpose reservoir planning

170

9.4
CHAPTER 2–2

CHAPTER 2–3
SECTION 1

SECTION 2

7

CHAPTER 2–4

2.3.1

Evaluation of necessary capacity and its allocation

170

2.3.2

Flood control capacity

171

2.3.3

Capacity for maintenance of normal functions of river water(unspecified capacity)

171

2.3.4

Capacity for irrigation

172

2.3.5

Capacity for municipal water

173

2.3.6

Capacity for power generation

173

2.3.7

Storage capacity for sedimentation

174

Water quality preservation facilities

175

SECTION 1

General information

175

SECTION 2

River water quality preservation measures

176

SECTION 3

Water quality preservation measures for reservoirs, lakes, etc.

180

CHAPTER 3

Installation plan for erosion and sediment control facilities

183

CHAPTER 3–1

General information

183

CHAPTER 3–2

Installation plan for erosion and sediment control facilities

183

SECTION 1

General information

183

SECTION 2

Installation plan for sediment yield control facilities

184

2.1

Overview

184

2.2

Hillside preservation works

185

2.2.1

Overview

185

2.2.2

Hillside works

185

2.2.3

Hillside conservation works

186

2.3

Erosion control dikes

187

2.4

Consolidation works

188

2.5

Riverbed girdles

189

2.6

Anti-erosion works

189

2.7

Torrent-preservation works

189

Planning for sediment transport control facilities

190

3.1

Overview

190

3.2

Erosion control dikes

190

3.3

Consolidation works

191

3.4

Riverbed girdles

191

3.5

Groins191

3.6

Anti-erosion works

192

3.7

Sand-retarding basin works

192

3.8

Torrent-preservation works

192

3.9

Training dikes

192

Planning the installation of woody debris control facilities

193

SECTION 3

SECTION 4

8

4.1

Overview

193

4.2

Woodendebris control facilities

193

4.2.1

Facilities for the control of wooden debris

193

4.2.2

Wooden debris trapping facilities

193

SECTION 5

Planning volcanic sediment and erosion control facilities

194

5.1

Overview

194

5.2

Planning for volcanic mudflow control facilities

195

5.3

Planning for lava flow control facilities

195

Planning landslide control facilities

196

SECTION 1

General information

196

SECTION 2

Planning landslide control facilities

196

2.1

Basis of planning landslide control facilities

196

2.2

Selection of construction methods

196

SECTION 3

Control works

197

SECTION 4

Prevention works

198

Planning for steep slope failure prevention facilities

200

SECTION 1

General information

200

SECTION 2

Planning steep slope failure prevention facilities

201

2.1

Basis of planning steep slope failure prevention facilities

201

2.2

Selection of construction methods

201

Planning for avalanche control facilities

203

SECTION 1

General information

203

SECTION 2

Planning for avalanche control facilities

203

2.1

Basis of the plan for avalanche control facilities

203

2.2

Selection of construction methods

203

2.3

Prevention works

204

2.4

Protection works

204

Planning comprehensive sediment disaster control facilities

205

SECTION 1

Basis of planning for comprehensive sediment disaster control facilities

205

SECTION 2

Basis of planning for urbanized foothill greenbelt facilities

205

Planning coastal protection facilities

206

SECTION 1

General description

206

SECTION 2

Erosion control facilities, etc.

206

2.1

Overview

206

2.2

Allocation between sediment transport facilities and beach nourishment

207

2.3

Sediment transport control facilities

209

2.3.1

Selection of facilities

209

2.3.2

Detached breakwaters

209

CHAPTER 3–3

CHAPTER 3–4

CHAPTER 3–5

CHAPTER 3–6

CHAPTER 4

9

2.3.3

Artificial reefs

210

2.3.4

Headlands

210

Beach nourishment212

Storm surge protection facilities

215

3.1

Overview

215

3.2

Allocation of dikes or revetments and wave-dissipation facilities

216

3.3

Dikes or revetments

216

3.3.1

Types of dikes or revetments

216

3.3.2

Normal line of dikes or revetments

218

3.3.3

Gradient of Foreshore

218

3.3.4

Design runup height and design wave-overtopping rate

219

3.3.5

Freeboard

219

3.3.6

Height of the design dike or revetment

219

2.4
SECTION 3

3.4

SECTION 4

Wave-dissipation facilities

220

3.4.1

Types of wave-dissipation facilities

220

3.4.2

Detached breakwaters

221

3.4.3

Artificial reefs

221

3.4.4

Wave-dissipation works

222

Tsunami control facilities

223

4.1

Overview

223

4.2

Allocation among dikes or revetments and wave-dissipation works

223

4.3

Design tsunami runup height

224

4.4

Tsunami breakwaters

224

Planning for the installation of information facilities

225

SECTION 1

General information

225

SECTION 2

Collecting information and ordering, provisioning, and commoditizing

CHAPTER 5

the system

225

SECTION 3

System for gathering information, data, etc.

226

SECTION 4

Development of a quality-assured database

227

SECTION 5

Construction of a network for sharing information and data

229

10

General Specifications

1

General Specifications
1. Purpose of the Criteria
The “Technical Criteria for River Works” (hereinafter referred to as "the Criteria") stipulate the technical items
necessary for the implementation of research, planning, design, and maintenance of rivers, erosion and sedimentcontrol, and the management of landslides, steep areas, avalanches, and seacoasts (hereinafter referred to as"rivers, etc."). The Criteria are aimed at the proper management of land and water, which are importantcomponents of the national estate, with a particular focus on river basins, and they aim to systematize thetechnologies associated with rivers, etc., and to thereby contribute to the maintenance and improvement oftechnological standards.

2. Content of the Criteria
If technical standards have been legally stipulated, then the implementation of research, planning, designing, andmaintenance needs to comply with these legal standards. The Criteria describe the standards for technical items
associated with rivers, etc. as supplementary to these standards. Therefore, if a more appropriate method exists
for the achievement of the original purpose when a certain measure is being implemented, then the Criteria
should in no way prevent the adoption of such a method.
The Criteria consist of four volumes: Research, Planning, Design, and Maintenance, and the content of theCriteria will be revised in response to the future improvement of technical standards, etc.

3. Application of the Criteria
The Criteria, as a rule, apply to all rivers, etc., but may not apply to rivers for which application of the Criteria is
not rational. An example would be a section of a river where disaster-restoration works, which are urgent and/or
need to take into consideration consistency with the upper and lower streams, are to be carried out.

2Basic Planning

3

General Description
With the nation’s peculiar climate condition, severe geomorphologic features, and social condition under whichnarrow alluvial plains are densely inhabited and the land there is densely used, proper land management is
indispensable to support safe and comfortable daily lives of the citizens as well as advanced economic andsocial activities while conserving a rich natural environment. Also with arising many water-related problems
caused by global warming and those concerning the safety of water including dioxin pollution, it is anticipatedthat land management associated water will grow to be increasingly important and complicated.
In order to properly implement land management, it is important to comprehensively manage land and water,which are important components of the national land, from the view points of disaster prevention, proper use ofresources, and environmental conservation. For smooth, efficient, and comprehensive management of land andwater, it is necessary to share with the citizens issues and goals of management of water, sediment, etc. and to
implement hardware-based measures such as the development and improvement of erosion and flood control
and seacoast preservation facilities which are organically linked with software-based measures such as land
utilization guide and damage mitigation countermeasures.
Moreover, these measures need to be implemented within a system in which research of the present condition,planning of measures, implementation of measures, monitoring and assessment, and feedback are conducted inthis order.

Explanation
Due to its natural conditions such as its geographical, morphological, geological, and climatic features, Japan has
a land which is vulnerable to disasters and draughts caused by typhoons, rain and snow storms, earthquakes, andvolcanic eruptions, and it has suffered a series of major disasters caused by floods, landslides, and volcanic
eruptions in recent years. Also, because the nation’s population and assets are densely concentrated in certain
areas, a major disaster in such areas may result in extensive, catastrophic damages. These natural and social
conditions also make it difficult to implement the use of resources such as land and water and the development
and improvement of social infrastructures such as transport networks. Moreover, in years to come during the 21st
century, occurrences of abnormal rain storms and unexpected disasters associated with drastic climate changes
caused by global warming, etc., further progression of desertification, occurrences of acid rain, reduction ofwater usable as resources, frequent occurrences of droughts, and other problems caused by expected changes in
the water environment which will give adverse effects to the desired establishment of a healthy hydrologicalcycle system are anticipated. These problems have been added to by the problem of the contamination of waterby substances such as dioxin being diffused through water. The following is an overview on the nation’s major
natural and social conditions.

1. Natural conditions
With steep backbone mountain ranges running through the central part of the long and narrow archipelago ofJapan, most of its rivers are also steep. Also, since it is located in the Asian monsoon region, Japan ischaracterized by the concentration of rainfalls during the rainy season and the typhoon season. This causes water
to flow downstream for short periods of time when a flood occurs, and the peak flow volume becomes extremely4large compared to the normal flow volume while a long duration of no rainfall causes the amount of water todrastically decrease easily resulting in a water shortage. With a wide extension of the vulnerable geology which
has been produced as a result of crustal deformation caused by many tectonic lines such as the Median Tectonic
Line combined with the rainfall and geographical characteristics, landslide disasters tend to occur easily. Thus, inJapan floods and landslide disasters happen very easily, and its geological features make it difficult to stably
utilize rainfalls as a source of water.
At the time of the Jomon Transgression approximately 6,000 years ago, the majority of Japan’s plains where
many big cities such as Tokyo, Osaka, and Nagoya are now located presumably used to have their coastlines
much toward inland from the present ones. Most of these plains lie on a young weak foundation called alluvium
which cumulated within the past 10,000 years. Therefore, compared with the urban areas of European countries,
which developed on the strong diluvium formed approximately 2 million years ago, solid safety measures forbuildings and other structures are very important in Japan’s major cities. Moreover, in the urban areas of ourcountry, there are many cases that the water levels of the river at the time of a flood are higher than thesurrounding elevations. In addition, excessive pumping of groundwater has invited land subsidence, and as aresult, the downstream areas near the river mouths spread at sea level, and they are endangered by probabledisasters caused by not only floods but also high tides.
In the 21st century, due to global warming and resulting changes in climatic conditions, the global water
environment is anticipated to change resulting in a number of issues to arise from the viewpoint of establishmentof a healthy hydrological cycle system.
Furthermore, despite its land area counting only about 0.3% of the global land area, Japan is one of the mostearthquake-ridden countries, experiencing approximately 20% of the earthquakes larger than magnitude 8.0
which occurred in the world between 1900 and 1994 in or around its land area. Also, Japan is one of the premiervolcanic countries where about 10% of all the active volcanoes of the planet exist.

2. Social conditions
Along with the progression of urbanization, the population and assets of our nation have been highly densely
concentrated in certain areas. For example, approximately 90% of the entire population dwell in the city
planning areas which count approximately 25% of the national land area, and approximately 50% of the entire
population and approximately 75% of the nation’s total assets are concentrated within the flood susceptible areasof the alluvial plains which occupy about 10% of the national land area. For this reason, once a disaster occurs, it
may result in enormous damages. While its water resources have been used from old ages as agricultural water
as well as for power generation, industrial use, and water supply, wastewater is returned to rivers after processingin the sewerage system, etc. and efficiently reutilized for various purposes further down the rivers. Recent years
have also seen the rise of water safety problems such as diffusion of dioxin and other toxic substances throughwater.

3. Natural environment
The major factor which allows diversified organism species to live and grow in our country is its multiphasenatural environment featured by the particular climate zones ranging from the subtropical to subarctic zones as5well as many uphills and downhills of widely different elevations. In addition, rivers not only have a flood
control function and is used for various beneficial purposes, but they also function as places where an affluent
and healthy living and regional culture that take advantage of the rich natural environment and waterfront spaces
are fostered.
In order to achieve optimized conservation and proper use of the national land with such natural and social
conditions, it is necessary to appropriately manage its components including the land, water, sea areas,
atmosphere, transport, and cities, etc. while also considering the effects that are given between them. Amongthese components, it is particularly important to comprehensively manage the land and water from theviewpoints of disaster prevention, proper use of resources, and environmental conservation. To facilitate its
achievement, comprehensive water and sediment management measures that combine hardware-based measuresthat require the development and improvement of erosion and flood control and coastal preservation facilities,
and software-based measures that require efficient utilization of existing facilities and land utilization guide, as
well as natural environment conservation. In addition, it is also important to provide information to and sharetasks with the citizens, thereby supporting them start thinking about water-related issues and act on them on theirown initiative.
National land management should be implemented within the framework of a system which ranges fromplanning of measures based on the present condition, implementation of measures, monitoring and assessment,
and feedback to be reflected on future planning in this order. This is because these processes are closely relatedwith each other, and the natural and social conditions are constantly changing.
Chapters 1 through 6 of the Basic Planning part are based on the following policies of description.
1.

The basic policy of water and sediment management on which plans in Chapter 1 are based is described
before descriptions of individual plans.

2.

Chapters 2 through 4 describe basic concepts on which various statutory plans are based.

3.

Chapters 5 and 6 describe those items included in the policy description in Chapter 1 which could not be
stipulated because of existing statutory plans, the necessity for cooperation with the river basin, and thenecessity and important points of the system which encompasses monitoring and assessment, as well as
feedback to them.

6

Importance of national land management- Disaster prevention- Proper uses of resources- Environmental conservation

Sea areas

Land and water management

Transport

Water and sediment management taking into consideration
the viewpoint of the river basin is important.

Hardware-based measures

Software-based measures

7

Urban

Chapter 1 Basic Policy
Section 1

General Information

Water and sediment management for the purposes of disaster prevention, appropriate resource use, andenvironmental conservation involves the comprehensive management of land and water resources, importantcomponents of the national estate.
Water and sediment management needs to be implemented systematically and from a long-term point of view.
It needs to aim at securing a domestic balance as well as overall balanced river systems for disasterprevention and mitigation, appropriate use of rivers, maintenance of normal water flows, improvement andconservation of riverine environments, maintenance of a healthy hydrological cycle, and comprehensive
sediment management. At the same time, regional characteristics, project costs, including life cycle costs, andthe effects and influences of all individual management projects, have to be considered.
To comprehensively implement such management programs, various plans must be established. They include
basic policies for river improvement and river improvement plans, erosion and sediment control master plans,
landslide prevention plans, and plans for seacoast preservation. Such plans are aimed at proper water andsediment management. They are established at the river basin level and should be consistent with each other.
Policies based on these plans should be organically linked and implemented.
Management of land including water and sediment can achieve optimal effect if the people who live andwork there recognize its tasks and importance, and positively tackle the tasks. Therefore, it is important toshare information and collaborate with related administrations, residents, enterprises, or organizations.

Explanations
River systems are an essential part of the hydrological cycle. They carry precipitation to the sea, transport erodedsoil and sand, and provide habitat for flora and fauna. The relationship between rivers as natural objects andhumanity originated in the use of rivers by people and in the natural processes presented by rivers to people invarious forms. Since rivers faithfully reflect the status of their basins, human activities in the rivers and theirbasins change the rivers and the relationship between the rivers and the people.
For instance, urbanization reduces the water retention and flow retardation functions increasing flood runoff andthe occurrence of urban flood disasters. In turn, these events lead to loss of the alimentation function[replenishment] of ground water, depletion of spring water, and reduced river flow. To address these problems,comprehensive flood-control projects have been introduced in some river basins to promote the proper use ofland through such measures as: the installation of rainwater storages at the time of new development, enhancing
water retention in the river basins, through forest and paddy fields; maintenance and securing of flow-retardation
functions; publication of inundation records, and recommendations for dwelling styles that take the possibility offlood damage into consideration[development control].
As lifestyles and industry have developed, pollutants have begun to be discharged into rivers at levels beyond thepurification capacity of river systems, resulting in water quality deterioration. The original features of rivers havebeen lost, and riverine environments have deteriorated as a result of reductions in river volume caused by therationalization of agricultural water intake and the formation of river bypasses for channel-type power generation,
8and occurrence of recession areas. In some regions, changes to the hydrological cycle have created serious
negative impacts, such as ground subsidence from overextraction of ground water and the lowering of watertables in urban areas. In addition, as energy-intensive urban activities have led to the increased heat-island
phenomena, attention has turned on the thermal amelioration effects of rivers and riparian areas in urban areas.
Consequently, because proper regulation of the effects of rivers on people and efficient use of rivers by people
are interdependent, it is vital to optimize the status and expected functions of rivers and their basins from ageneral viewpoint that is based on natural and social conditions.
Here, the ideal situation of a river and its basin determines their desirable status and that of the relationshipbetween, as compared with their current natural and social states. The expected functions of the river and its
basin determine what can be expected from them when their status is optimized.
In light of this point, in the implementation of water management it is important to promote the necessary
policies and projects as part of integral and comprehensive management of the volume and water quality of theriver. The aim should be to establish a healthy water cycle system that takes full advantage of the various
functions that the river system originally had. To this end, plans that aim to establish a healthy water cycle
system and take into consideration not only the river volume but also the water intake and discharge system,
including the networks of channels in the river basin and the ground water, are called for.
In contrast, in mountainous areas, on plains, in coastal areas, and in estuaries, various problems associated withsoil and sand movement are apparent, including degradation, estuary closure, and shoreline retreat. To solvethese problems, improve the stability of channels, secure flood control, manage rivers, improve the functionalityof coastal facilities, and conserve the environment, it is necessary to promote comprehensive sedimentmanagement by identifying not only the water system but also coastal areas as part of the entire sedimenttransport system in terms of quantity, quality, and time.
Projects for the proper management of land and water need to be implemented systematically, taking into
consideration the project cost, including the life cycle cost and the effects and influences of the project.
Therefore, assessments of the effects of a project need to take into consideration its life cycle, the benefit–costratio, and any peripheral influences (external economy, etc.).
In order to properly manage water and sediments, in river planning, including planning for flood control, water
utilization, and environmental improvement and conservation (Chapter 2); erosion control planning, including
countermeasures for sediment disasters and planning for environmental improvement and conservation (Chapter3); and seacoast preservation planning, including coastal protection and environmental improvement,
conservation, and utilization (Chapter 4), it is necessary to take a whole river basin perspective and adjust theseplans to secure a balance in the entire river system. This shift helps in disaster prevention and mitigation
management, proper use of rivers, maintenance of normal water flows, improvement and conservation ofriverine environments, and comprehensive sediment management to realize the above goals and construct a
healthy water cycle. In focusing on sediment movement it is necessary to consider not only the river system, but
the entire sediment system, including seacoast areas.
When we examine these issues, we need to examine plans involving boundary areas between erosion control andriver, and those between rivers and seacoasts, but since the phenomena occurring around these areas arecomplicated and there is much that is still unexplained, studies need to continue in future.
9

Also, we need to conduct studies to develop a comprehensive plan for basic policies, including that for river
basins.
In developing the plan it is important to achieve accountability, and it is necessary to consider consistency withother plans and the ensuring of transparency. Also, the people need to take an interest in water-related matters,develop solutions, and then act to support these issues. We therefore need to gather and improve all water-related
information and provide this information to the people so that they can share and utilize it.

Water and sediment management should be performed frome a whole of river
basin perspectiveComprehensive Planning
Disaster prevention andmitigation

River planning(Chapter 2)
Erosion control planning(Chapter 3)
Seacoast protection planning(Chapter 4)

Appropriate use of rivers
and maintenance of
normal water flows

Improvement andconservation of the riverenvironment

Construction of a healthy water cycle system Comprehensive sediment management

Fig. 1–1 Comprehensive planning

10Section 2 Disaster Prevention and Mitigation
2.1 Overview
Disaster countermeasures need to be implemented systematically and from a long-term viewpoint to ensure
safety and security in a fragile land, to develop a sustainable society, and to realize the efficient use of landand environmental conservation.
Planning of disaster countermeasures has to not just attempt to prevent disasters from external forces of acertain magnitude, but also to minimize damage if such forces are exceeded.

Explanations
It takes a long time for disaster countermeasures to actually start functioning, and although in some cases theybring instantaneous effects when they are implemented, in other cases they do not have effects until a certain
degree of improvement occurs. Therefore, the systematic implementation of disaster countermeasures whileconstantly adopting a long-term perspective makes the countermeasures more efficient and effective.
In planning countermeasures to possible disasters, consideration of damage prevention against a certain
magnitude of external force in terms of the capacity of future and present scales of planning and the present
facilities is not enough; it is also important to minimize the damage by implementing both hard (structural) andsoft (non-structural) measures jointly with the river basin if an excessive external force should occur.
This section describes countermeasures to flood, sediment, and earthquake disasters.

2.2 Countermeasures to flood disasters
A flood disaster can be defined as damage to human lives, properties, and social and economic activities
through floods or tidal waves.
Planning of countermeasures to possible flood disasters should be primarily based on the prevention or
mitigation of flood disasters against a certain magnitude of external force such as rainfall, and it should take
into consideration the minimization of damage if a phenomenon that exceeds a certain magnitude should occur.
Also, it is important for countermeasures to flood disasters to take into consideration the characteristics of theriver and flood, the form of the flood disaster, and the situation on the floodplain, and to aim to achieve a properbalance in the entire river system, including a balance between the upper and lower reaches and the main stream
and branches.

Explanations
Since countermeasures to flood disasters need to address problems caused by natural phenomena, and there aresocial, financial, and technical constraints, it is impossible to totally prevent flood disasters caused by a flood or
tidal wave of any magnitude. Therefore, countermeasures to flood disasters should be planned on the basis ofprotection against a flood or tidal wave of a certain magnitude under various constraints.
Although flood disasters caused by phenomena exceeding a certain magnitude occur rarely, if they do occur the
resulting damage can be anticipated to be enormous. Consequently, the implementation of countermeasures to11flood disasters when a phenomenon greater than a certain magnitude occurs should include software-based
measures aimed to mitigate the flood disaster as much as possible.
In addition, it is necessary to plan countermeasures to flood disaster that are properly balanced as part of theentire river system. Because, in general, a flood in the lower reaches of a river or in the main stream brings agreater scale of disaster than one in the upper reaches of the river or in a branch, it is important to maintain abalance between the upper and lower reaches and between the main stream and branches in the planning ofcountermeasures and the improvement of facilities. Planning of countermeasures to flood disasters needs to take
into consideration various factors, including the past flood disaster record, the characteristics of the flood, thescale of damage potential on the floodplain, the form of land use, and the relationship between the floodwaterlevel and the ground level in landslide-prone areas. Also, in cases where inland flooding occurs frequently but
there is a high level of security against foreshore water flooding, or the level of security against inland is high but
there is a low level of security against foreshore flooding, there is not a proper balance between protection
against floods and countermeasures to landside water. In cities and their surrounding areas, changes in runoff
into rivers may be caused by urbanization and residential land development, and high concentrations of
population and assets within the flood area may exacerbate flood disasters. In such cases, it is necessary to makeefforts over the entire basin toward not only drainage and river improvement but also runoff control bycontrolling land use and by building water storage and infiltration facilities. In the broad sense of the term, thisalso constitutes the securing of a balance between the river basin and the river.

2.3 Countermeasures to sediment disasters
Sediment disasters can be defined as damage to human lives, properties, and public facilities throughsediment movements such as landslides and erosion of mountainsides or slope surfaces, sediment flows,
earthflows, or landslides.
Countermeasures to sediment disasters should be implemented efficiently and effectively and based on
knowledge of the mechanisms of sediment movement and disaster occurrence. They should appropriately
combine structural measures such as the building of facilities and soft measures such as the establishment of
warning and evacuation systems. These should also take into consideration as much as possible, and as
required, the balance of sediment movement over the entire sediment transport system, including in seacoast
areas.
Furthermore, the implementation of countermeasures to sediment disasters needs to be aimed at not only
preventing disasters on a projected scale but also minimizing the damage if a disaster of excessive scale
should occur.

Explanations
Sediment movement takes place in the following three major forms:- Failure, erosion, and landslide on a slope such as a mountainside or cliff due to the action of rain and groundwater (slip erosion and landslide)- Flood caused by the accumulation of aggregates of water and sediment, which are transported downstream
in a steep stream (mass transport)
12- Downstream transport of sediments created by riverbed and riverside erosion (individual transport).
Sediment disasters occur as a result of one of these three forms of sediment transport.
Slope failure and earthfalls usually occur on steep mountainsides or slopes and directly damage houses andarable land. Since it is not clear in many cases how landslide occurrence is related to rainfall, we need to develop
a proper understanding of the geography and geology of areas where landslides may occur.
Other sediment disasters include those caused by snowslides in the presence of snow cover and drift wood, andlava or pyroclastic flows caused by volcanic eruptions.
Because of the complicated nature of the occurrence mechanisms of sediment disasters, the realization ofcountermeasures takes a long time, and in implementing such plans, it is necessary to ensure that early effects
can be obtained, not only against phenomena on a projected scale but also against more frequently occurring
sediment disasters on a smaller scale.
In addition, countermeasures to sediment disasters need to be implemented effectively, not just withhardware-based measures such as construction works, but also with a combination of software-based measures
such as the establishment of warning and evacuation systems and the restriction of land use.

2.4 Countermeasures to earthquake disasters
The purpose of a countermeasure to an earthquake disaster is to protect river administration facilities, erosion
control facilities, and coastal protection facilities from disasters caused by seismic motion; to prevent andmitigate resulting secondary disasters such as flood and sediment disasters; and to prevent and mitigate
disasters caused by tsunami.
For protection against seismic motion the necessary seismic performance should be determined on the basisof the characteristics of the river and the erosion control and coastal facilities. For protection against tsunami,countermeasures that include coastal protection should be implemented.

Explanations
Countermeasures to earthquake disasters include the securing of seismic performance against seismic motion anddisaster prevention measures against tsunami. The following should be taken into consideration in the planningand implementation of countermeasures to seismic motion disasters.
1.

On the basis of the seismic characteristics of the region and river of interest, erosion control and coastal
facilities and their ability to withstand earthquakes should be secured. Design methods are described in“Technical Criteria for River Works (Draft) Design”.

2.

For important facilities, it is important to adopt a design that takes into consideration the long recurrence
interval of disastrous earthquakes and to implement soft measures such as a regional disaster prevention
plan.

The following should be taken into consideration in the planning and implementation of countermeasures totsunami disasters:
1.

In coastal areas where the possibility of earthquake occurrence has been pointed out but no major
earthquakes have occurred for a long time, it is necessary to consider countermeasures to an earthquake and13

tsunami of the maximum scale predictable from existing knowledge.
2.

Countermeasures to tsunami disasters need to be comprehensive, combining the improvement ofdisaster-prevention facilities, city and town planning from the viewpoint of tsunami disaster prevention, andthe establishment and improvement of disaster-prevention systems. Countermeasures for coastal areas
should take into consideration the fact that disasters can take diverse forms, depending on the geography,
use of coastal areas, and land uses.

14Section 3 Proper Use of Rivers, Maintenance of Normal Water Flows, andImprovement and Conservation of Riverine Environments.
The purpose of proper use of rivers, maintenance of normal water flows, and improvement and conservationof riverine environments is to ensure a safe and secure living environment, sustainable development ofsociety, efficient use of national land, and environmental conservation. To this end, we need to achieve thefollowing for not only rivers but also their basins:
1.

Proper use of rivers and maintenance of normal water flows

2.

Conservation and restoration of a good living and growing environment for plants and animals

3.

Maintenance and formation of an aesthetically pleasing landscape4.

Maintenance and formation of places for activities that bring humans into contact with rivers

5.

Conservation of good water quality.

Explanations
The purpose of proper use of rivers, maintenance of normal water flows, and improvement and conservation ofriverine environments is to ensure a safe and secure living environment, sustainable development of society,efficient use of national lands, and environmental conservation. These goals are positioned as important elementsof national land management, as described in the Section 1 General Information.
Here, ‘river environment etc.’ includes riverine, coastal, and mountain stream environments.
To achieve proper use of rivers, maintenance of normal water flows, and improvement and conservation of theriver environment, the following goals need to be realized at the same time as efforts are made within the river
basins:1.

Proper use of rivers and maintenance of normal water flows

Proper use of rivers and maintenance of normal water flows are aimed at maintaining the original functions of
the running water in rivers. These functions comprehensively include the occupation of running water;
navigation; fisheries; tourism; conservation of clear running water; prevention of salt damage; prevention ofestuary closure; protection of river administration facilities; maintenance of ground water levels, landscapes, andlives and the growing environments of plants and animals; and procurement of places for activities that bringhumans into contact with rivers.
2.

Conservation and restoration of a good living and growing environment for plants and animals[The aim of …]Conservation and restoration of a good living and growing environment for plants and animals
aims to conserve and restore biological communities and living and growing environments that are in harmony
with the river, and to maintain these over many years to come.
3.

Maintenance and formation of an aesthetically pleasing landscape [Preservation of Landscape values]

Maintenance and formation of an aesthetically pleasing landscape means the maintenance and formation of awater-based landscape in which rivers are naturally present; this landscape should be based on the natural riverlandscape and the historical and cultural background of the region.
4.

Maintenance and formation of places for activities that bring humans into contact with rivers [Enhancing
interaction between humans and rivers]
15

Maintenance and formation of places for activities that bring humans into contact with rivers enables humans tocontact nature in a symbiotic relationship without having a major influence on nature.
5.

Conservation of[Protecting] water quality

Conservation of good water quality encompasses the securing of a level of water quality necessary for the properuse of rivers, the maintenance of normal water flows, and the conservation of river environments.

16Section 4 Comprehensive Sediment Management
To achieve disaster prevention and mitigation, proper use of rivers, maintenance of normal water flows, andimprovement and conservation of the river environment it is important to promote comprehensive sedimentmanagement.
Comprehensive sediment management is implemented to address the problems associated with sediment
movement that are occurring in fields in mountainous and piedmont areas, in alluvial fans, on plains, and inestuarine and coastal areas. If individual countermeasures against the problems faced by erosion control,
dams, rivers, and seacoasts cannot solve these problems, then comprehensive sediment management should
not be limited to individual solutions for these fields. Instead, it should identify sediment movement as part ofthe concept of sediment transport and should provide the necessary countermeasures, such as
sediment-generation control and adjustment of sediment runoff, as part of the entire sediment transportsystem in order to solve these problems.

Explanations
Since Japan is located in the circum-Pacific orogenic zone, has a fragile geology, and is covered with precipitous
terrain, the environment is marked by the high rates of erosion associated with steep and mountainous terrain.
Sediments produced in the mountains and plateaus are carried downstream by rivers, to form alluvial fans andfloodplains, and/or are transported to the coast where they enter the coastal sand drift system to form coastal
sedimentary topographies. In Japan, space used as a primary source of people’s livelihoods comes from the land
formed by sediments transported by rivers.
Some forms of sediment transportation impair the security of our lives; degrade the functionality of flood-control
and water-utilization facilities; and reduce the functionality of rivers, mountain streams, and seacoasts as naturalenvironments for plants and animals to live and grow in and as places for activities that bring humans into
contact with nature.
In particular, problems associated with sediment transport that are occurring in different areas of river basins,
including mountains and piedmonts, alluvial fans, plains, estuaries, and coasts, may not be solved simply byimplementing countermeasures to the problem in individual areas such as erosion control facilities, dams, rivers,and coasts.
In such cases, we must not limit the measures to each individual area; instead, we must identify the entire
process of sediment transport from the mountain and piedmont to the estuary and coast through the concept of asediment transport system. We need to investigate and examine the entire system, comprehensively understandsediment transportation, and plan and implement countermeasures.
On the basis of the characteristics of each river and seacoast, comprehensive sediment control as part of national
land management should aim to properly control sediment production and adjust sediment runoff in the entire
sedimentary system, which has both temporal and spatial expansion. Thereby, it will be able to prevent disasters
caused by sediment transportation, conserve the ecological system and landscapes of rivers and seacoasts, utilize
open spaces along the rivers and seacoasts, and realize a rich and vital society.
Since it is important in the implementation of comprehensive sediment control to understand the characteristics
17

of sediment transportation in the entire sedimentary system and improve the accuracy of research and prediction
methods, sediment transportation over the entire sedimentary system should be monitored.

18

Chapter 2 River Planning
Section 1 The Basics of River Planning
1.1 General Overview
The aim of river planning should be to achieve a harmonious balance between flood control, water utilization,and environmental function and should also consider comprehensive sediment management.
River planning should also consider the quality of the various data used, such as hydrological data, including
rainfall and flow rate, and environmental data.

Explanation
This chapter covers the basic elements of planning for flood prevention, appropriate use of rivers, maintenanceof the designated functions of river water, and improvement and conservation of the river environment. Theseissues should be considered from the viewpoints of flood control, water utilization, and the environment as partof the establishment of a basic policy for river improvement and river improvement plans, as stipulated in theRiver Law.
In this chapter, the descriptions of river planning have been divided into "Basic Flood Prevention Planning","Basic Appropriate Use of Rivers and Maintenance of Normal River Water Function", and "Basic Improvement
and Conservation of the River Environment" for purposes of convenience. However, flood control, water
utilization, and environmental functions are in reality closely interrelated, and it is not possible to completely
separate them in practice. Comprehensive sediment management is also very important along with rivermanagement, including maintenance, flood prediction, and facility operation. In practice, river planning needs tobe undertaken so that these various functions are comprehensively reinforced.
The following are some of the more important considerations when undertaking river planning:- The present state and historical changes to the natural environment of the basin- The present state, historical changes, and future outlook for land use and the social environment of the basin- History of disasters and measures taken- Achieving the required level of flood control- Measures for mitigation of damage when the design flood level is exceeded- Comprehensive sediment management- The present status and future prospects for water utilization- Securing of a healthy water cycle system- Maintenance and management of rivers- Conservation and restoration of the natural environment- Maintenance and formation of an aesthetically pleasing river landscape- Maintenance and formation of places for human interactions with rivers- Linkages between river management and regional development- Economic rationality..
River planning requires various data. In flood prevention planning, for example, requires data on rainfall, flow19

rates, water levels, floodmarks, sea levels, cross-section surveys, to name just a few. Before these data are used,their accuracy should be thoroughly confirmed by a method such as comparison with the results of observations
in nearby areas. If necessary, the data should be corrected.

1.2 Fundamental river management policy and river improvement plans
The fundamental river management policy for a broad geographic area (country, province region), prescribe
the long-term policy for river improvement and the basic types of improvement planned for each river system
while taking into consideration the entire balance across the region and the characteristics of the individual
rivers and their basins.
A river improvement plan, on the other hand, should prescribe the specific improvements to be carried out
over a period of about 20 to 30 years in accordance with the stipulations of the fundamental rivermanagement policy while taking the needs of the inhabitants of the river basin into consideration.

Explanation
The implementation of a river improvement project involves first setting the goals to be achieved and thenimplementing the measures aimed at achieving these goals. Since river improvement requires enormous cost andtime, the general practice is to set medium-term goals with staged implementation while maintaining good of thelevel between different river systems, rivers, etc. It is not just the rivers themselves that change in response tonatural processes; the social situation within the river basin also continually changes over time, leading to direct
influences on the river and changes in the river-related needs of the local residents. Therefore, it is impossible tocomprehensively determine at any given time the specific improvements that will reflect the long-term goals or
the specific procedures and timing needed to achieve these goals. Consequently, a target planning period should
be set that is appropriate for achieving the immediate needs and providing local residents of a sense that tangible
results will occur in a realistic timeframe. After choosing the planning period, a river improvement plan thatdefines the medium-term improvement goals should be set in accordance with the fundamental rivermanagement policy and its long-term goals.
The following items are to be described in the fundamental river management policy:
1.

The basic policy on comprehensive conservation and utilization of rivers in the given river system

2.

Basic river improvementa.

Design flood and flood diversion to river channels and flood control facilitiesb.

Design flood discharge at major locationsc.

Design high water levels at major locations and associated river widthd.

The flow discharge needed to maintain normal functions of river water at major locations.

The following items are to be described in the river improvement plan:
1.

The goals of the river improvement plan

2.

Information on the implementation of river improvements, including:a.

The purpose and types of work, locations, and an outline of the function of any river management20

facilities to be builtb.

The location, purpose, and type of any required river maintenance.

As described above, a river improvement plan identifies the specific components of a river’s improvement. In
executing the actual improvement work within cost and time constraints, it is essential to determine the order ofthe works and coordinate the initiation and progress of all projects. To this end, the following should beconsidered:
1. The basic unit of a river improvement plan is a section of river in which a series of river improvement
works is to be implemented.
2. Generally, the planning period is set at around 20 to 30 years, which is regarded as the time required tocomplete a set of river improvement works for a particular section. However, in cases where there areuncertainties about specific improvements (e.g. if an extended period is needed for research and study),
then these improvements should be omitted from the plan to enable a shorter planning period to be
adopted. Alternatively, the issues of uncertainty should be specified for further investigation and the plan
reviewed at a later time after they have been clarified.
3. The plan should be periodically reviewed and revised as necessary with respect to changes in thecircumstances surrounding the subject river, including regional planning objectives.
4. Reviewing the river improvement measures should consider the allocation of funds to allow the plan to berealized within the planning period and to be compared with alternatives.
5. The river improvement measures should be described so that the need for them and effects they will bringare clearly presented.
6. In deciding the composition of each item of river improvement works, the need to minimize future
follow-up work should be considered as much as possible. However, because a particular improvement
work may be urgent, and because there is also a need for the facility to be durable, future follow-up work
may be unavoidable.
7. The maintenance plan identifies the maintenance tasks to be systematically undertaken, not just one-off
and urgent maintenance works. Items that are needed for river monitoring, such as observations andsurveys, should also be included.
8. The river improvement plan should also describe any tasks that are needed as preconditions for river
improvement.

Section 2 Basics of Flood-prevention Planning
2.1 Overview
A flood prevention plan first needs to adopt a flood hydrograph (hereinafter referred to as design flood). This
design flood forms the basis of the plan at design control points and is adopted with the aim of ensuring thedesired flood behavior modification to prevent or mitigate river flooding disasters.
For this reason, the flood prevention plan should be prepared in such a way that the facilities to be built fromthe design flood are technically and economically well balanced throughout the river system and achieve the21desired level of functionality.
In the preparation of a flood prevention plan, it is necessary to comprehensively examine the functions of theriver system, including flood control, water utilization, and environmental functions. It should be noted thatthe plan does not aim to control the maximum possible flood in the subject river. The plan aims to not justcontrol the design flood but also to be able to deal with the occurrence of a flood that exceeds the design
flood (hereinafter referred to as excess flood), wherever necessary.
The fundamental river management policy should specify the peak flow of the design flood at design control
points, deployment of river channels and flood control facilities, and the design flood discharge at major
locations. The river improvement plan should specify target years for staged goals to be achieved and should
aim to prevent flooding within a certain magnitude and, where necessary, mitigate damage from floods thatexceed the design flood. Also, the plan should emphasize the efficient use of existing facilities and non
structural measures and consider countermeasures to be implemented by communities in the river basin.

Explanation
In the preparation of a flood prevention plan, coordination between the plan and other river plans and facility
plans for the subject river and its basin is necessary. In general, for example, this means coordination between
the river course plan and the erosion and sediment management plan, between the flood control plan and thewater utilization plan, and between the river course plan and the environmental conservation plan, etc.
A flood prevention plan should be technically and economically appropriate and ensure that sufficient effects areproduced.
Technical and economic consideration should be given not to concentrate the damage caused by excess floods to
the greatest extent possible. Consideration should also be given to ensuring the availability of appropriate
countermeasures in the event of an excess flood by elucidating the damage that such a flood would cause toaffected regional communities.
If it is sufficient for the purpose of the plan, the design flood could be defined only by peak discharge instead ofdefining design hydrographs.

2.2 Method of determining the design flood
Although several different methods exist for determining the design flood, the most common, and the one thatshould be used as standard, is based on rainfall analysis.
The design flood should be determined for each design control point.

Explanation
The design flood is estimated from the probability of occurrence of a flood of the scale represented by thehydrograph, i.e., the occurrence probability. The occurrence probability indicates the degree of security adopted
for this flood prevention plan, or the safe level of flood control: the smaller the occurrence probability, thegreater the degree of security.
However, a flood hydrograph itself is not necessarily useful for calculating the occurrence probability. In many22

cases, a flood hydrograph, as used for statistical analysis from peak flow or total volume, presents difficulties
such as a lack of precision because of data shortages, and the need for complex calculations.
Therefore, the method that focuses on the rainfall that causes the flood involves selecting a subject rainfall levelwith a probability of excess corresponding to the established safe level of flood control. A hydrograph is thenplotted from this subject rainfall. This method should be used as the standard approach, though in some cases, amethod more appropriate to the subject river may need to be adopted.
While a number of flood hydrographs may be calculated using a flood runoff model based on the subject rainfall,
the hydrograph that is used as the basis of the flood prevention plan represents the “design flood". The selectionof a design flood requires comprehensive review for setting an appropriate peak flow that corresponds to thescale of the design. Since the design flood serves as the basis for the flood prevention plan, the flood hydrographmust be one that has not been affected by any artificial manipulation such as flood control. The design flood is
not necessarily the one with the maximum peak flow or maximum total flow among the calculated floodhydrographs.
Incidentally, the phrase "subject rainfall" is used here rather than the more conventional phrase "design rainfall".
This avoids the interpretation of "design rainfall" as meaning only one type of rainfall event that is used todetermine the design flood. As described in Section 2.7 of this chapter, a design flood can be produced by aseries of rainfall events, and such a group of rainfall events that are used to determine the design flood arereferred to as the "subject rainfall".

2.3 Subject rainfall
The subject rainfall is selected for each design control point. Subject rainfall is represented by three elements:
the amount of rainfall, the temporal distribution of the rainfall, and the regional distribution of the rainfall.

Explanation
This section shows the elements of the subject rainfall as used in the standard method for determining a design
flood.
The concept of selecting subject rainfall will be described in Section 2.6 of this chapter.

2.4 Design reference point
The design control points should be chosen to have sufficient hydrological data available, should serve as
locations for hydrological and hydraulic analyses, and should be closely related to the overall plan. The design
control points should be set wherever necessary for the plan.

Explanation
Design control points are used as points at which to evaluate the target safety level. Suitable locations are rivergauging stations or dams or other major flood control facilities that can serve as a basis for hydrological analysis.
Fig. 2-1 shows candidate reference points: a gauging station at A on the main river course, gauging stations on
tributaries B and C, and dams D and E.

23

The scale of the subject rainfall can differ from point to point, and if there are different facilities subject to theplan differ at the same location, then the subject rainfall may differ for each set of facilities.

Fig. 2–1

2.5 Determining the design scale
2.5.1 Scale of plan
In determining the planning scale, the importance of the subject river should be emphasized, and the actual
damage caused by past floods, economic effects, and other factors should all be taken into consideration.

Explanation
The planning scale refers to the level of safety against flood disasters to be provided in the area affected, and it isdesirable that a good balance be maintained between the upstream and downstream areas, as well as between themain stream and the tributaries, depending on the importance of each river. This balance also needs to be viewed
from a national perspective.
The importance of the river should be defined from the purpose of the flood prevention plan and in considerationof various elements, such as the extent of the river basin, the social and economic significance of the subject
region, the quantity and quality of the assumed damage, and the history of past disasters. Approximate references
for determining the scale of the subject rainfall when rivers are categorized into importance classes A, B, C, D,
or E are shown in Table 2–1.
For estimated damage, refer to Economic Research in the Research volume.
Generally, a river that has major sections of class I river is designated in importance as class A or B; one that has
minor sections of class I urban river and class II urban river is designated as class C. Classes D and E are
generally designated class ‘general’ rivers, depending on their importance.
For regions that have suffered enormous damage in the past, it is generally undesirable to determine the planning
scale without considering the prior catastrophic floods. In such cases, a plan is usually set up to stabilize people’s
livelihoods such that future floods of the same scale will not cause disasters.
However, in this case, there is also a need to consider the balance between upstream and downstream areas andthe balance between the main stream and tributaries.
Once the design scale has been determined, the subject rainfall, which is necessary for engineering design, canbe selected. The subject rainfall is characterized by the amount of rainfall, temporal distribution, and regional
distribution; the scale of the subject rainfall, however, is generally evaluated by its return period.
24Therefore the return period of such subject rainfalls does not necessarily correspond to the return period of peakdischarge.
However, since the return period of the peak discharge of the design flood has an important meaning in a flood
prevention plan, if there is a significant difference in return period may arise between subject rainfall and peakdischarge, it will be necessary to clarify the relationship and consider the adoption of some other method todetermine the planning scale.

Table 2–1 Importance of river and planning scale
Importance of river

Planning scale (Return period in year of subject rainfall) *Class A

Over 200

Class B

100–200

Class C

50–100Class D

10–50

Class E

Below 10* Inverse of the annual probability of excess

2.5.2 Consistency of the scale of plan within the same river system
When setting flood prevention plans within the same river system, consideration should be given to
maintaining sufficient consistency between the upstream and downstream areas with regards to the planning
scale, as well as between the main stream and tributaries.

Explanation
The purpose of a flood prevention plan is to prevent floods of the design scale.
Maintenance of consistency does not necessarily mean that the design scale will continue between the upstream
and downstream areas or between the main stream and tributaries. Especially, when two or more design control
points have been determined within the same river system, there will usually be no correlation between thesubject rainfalls in terms of the amount and duration of rainfall, etc. Simply using the same planning scale forboth the upstream and downstream areas or for the main stream and tributaries will, in many cases, endanger thedownstream or mainstream regions if an excess flood occurs. Therefore, the appropriate scale for the particularsection of river should be taken into consideration to achieve consistency in the level of protection.
In addition, consideration should be given, wherever necessary, to ensuring that the damage caused by an excess
flood will be distributed as much as possible and will not be excessively concentrated on specific areas.

2.6 Selection of subject rainfall
2.6.1 Determining the amount of subject rainfall
The amount of subject rainfall should be determined by setting the planning scale in accordance with thestipulation in Section 2.5.1 of this chapter, and also by setting the duration of rainfall.

25

2.6.2 Review of past floods
The review of past floods should include the characteristics, duration, and spatial distributions of the amountsof rainfall that caused them; their water levels and hydraulic and hydrological data, such as rates of discharge;
and the flood situation and the reality of damage, etc.

Explanation
Data on the rainfall, water level, and flow discharge associated with past floods are critical for hydraulic andhydrological analysis. To clarify the temporal and spatial distributions of rainfall, it is necessary to collect as
many hourly rainfall data as possible for major locations within the river basin.
Actual measurements of flow discharge at major locations is desirable. If unavailable, estimates should beobtained using an appropriate method, such as by inverse calculation from floodmarks. The highest possible
accuracy is required in estimating the impacts of flooding and water retardation from particular flow discharges.
The characteristics of past flooding and damage it caused should be thoroughly investigated because this will beof prime importance in judging the effects and significance of the project.

2.6.3 Duration of subject rainfall
In determining the duration of the subject rainfall it is necessary to consider the size of the river basin, theproperties of the rainfall, the pattern of flood runoff, the type of project facility, and any difficulties inaccessing past data etc.

Explanation
The duration of the subject rainfall should be based on the size of the river basin, the duration of flooding, andthe cause of rainfall (typhoon or front). The type of counter measures that would be studied should also be
considered.
Since the data necessary to determine rainfall duration are not always available, the duration is often set at 1 to 3
days for the purposes of statistical analysis and other reasons.
Also, in the case of a river where a flood can travel from the most distant point of the basin to the river withinseveral hours, it is necessary to separately examine those rainfall events whose durations dominate the size of thepeak discharge of the flood.

2.6.4 Determining the time and areal distributions of the subject rainfall
The temporal and spatial distributions of the subject rainfall should be determined for a considerable number
so that each subject rainfall would have equal amount of rainfall to that of planning scale determined in
Section 2.5.1 of this chapter.
It should be corrected if significant inconsistency arises from simply extending the distributions.

Explanation
Once the total rainfall for the subject rainfall is given, the remaining two elements—that is, the temporal and26

spatial distributions—should be determined to define the subject rainfalls.
After having total volume of the subject rainfall, you should determine temporal and spatial distribution of thesubject rainfalls.
In general, the following two methods are available.
One method is to clarify the statistical or meteorological relationships between these three elements (i.e. theamount of rainfall, temporal distribution, and spatial distribution) and determine the temporal and spatial
distributions for given rainfalls from these relationships.
Another method is to determine the amount of rainfall and then create the temporal distribution and spatial
distribution by simply expanding or contracting some past rainfall patterns. Unless they are regarded as being
unlikely to occur in consideration of the statistical relationships between these elements, they will be adopted.
Since it is usually simple and easy to understand, the latter method is used here. In selecting past rainfall events,
care must be taken that rainfall events that have caused severe floods or have high recurrence patterns in thebasin are not excluded. The number of rainfall events to be selected varies depending on the length of time forwhich the data have existed; the maximum extension rate is set to about 200% in many cases.
The rainfall patterns that have extensive differences in spatial distribution or have high intensity during a part in
temporal distribution may arise remarkable discrepancies because rainfall intensity during the hours that
dominant the peak discharge tends to be extremely high in such rainfall patterns.
The following examples are considered as specific processing methods:
1.

If extension of rainfall that has extensive differences in spatial distribution causes rainfall in someparts of the basin to be significantly large, and the return period of that rainfall is significantly
different from the return period of the design scale, then the expanded rainfall of that rainfall patternshould be excluded from the subject rainfalls, since its inclusion is deemed inappropriate.

2.

If extension of a pattern where short duration, high intensity rainfall causes the return period of
rainfall intensity within the duration that is predominant over the peak discharge of a flood to beremarkably different from the return period of the design scale, then the extended rainfall of thatrainfall pattern should be excluded from the subject rainfalls since its inclusion is deemed
inappropriate.

3.

Subject rainfalls for the rainfall patterns described in 1 and 2 above should be adopted after
correction of the spatial and temporal distributions as well as any remarkable differences in return
period.

2.6.5 Adjustment of the actual rainfall duration against the subject rainfall duration
If the actual rainfall duration selected in Section 2.6.4 differs from the subject rainfall duration, then thefollowing adjustment is to be made, depending on the duration:
1. When the actual rainfall duration is shorter than the subject rainfall duration
Leave the actual rainfall duration as it is, and extend only the amount of rainfall to that of the subject rainfall.
In this case, however, if any inconsistency as described in Section 2.6.4 arises, correction should be madewithin that range.
2. When the actual rainfall duration is longer than the subject rainfall duration27

As a rule, the solution described in 1 above should be adopted. However, if the amounts of rainfall after
extension have become fairly large compared with the amount of subject rainfall, then, as a rule, extend theamount of rainfall within a time equivalent to the duration of the subject rainfall only, and use the actual
rainfall for any rainfall before that time.

Explanation
It is extremely rare for the actual rainfall duration selected in Section 2.6.4 of this chapter to agree with thesubject rainfall duration. However, even the data used for determining the design scale of the subject rainfall in
Section 2.5.1 normally do not have rainfall durations that match the subject rainfall durations; therefore,
normally, it is not necessary to make any adjustment at all. On the other hand, if the actual rainfall duration ismuch longer than the subject rainfall duration, and the amount of rainfall after extension is much greater thanthat of the subject rainfall, then not making any adjustment will result in inconsistency.
In this case, as a rule, with the part that is identified as the main constituent of the series of rainfall events
positioned at the center, the amount of rainfall within the time corresponding to the subject rainfall duration
should be extended so that it equals the amount of the subject rainfall, and the actual rainfall should be used as it
is for preceding rainfall that continues to the subject rainfall (Fig. 2–2).
Various other adjustment procedures may be possible. However, because in river planning there are many cases
in which comparison with other rivers is required, we have decided to propose this procedure as a standard.
When the construction of a detention reservoir is anticipated, however, it is necessary to take into consideration
the series of rainfall events, including rainfall before and after.

Fig. 2–2 Adjustment of rainfall durations
28

Importance of the region, past floods, beneficial
and adverse effects
Importance of the river
Determination of the design
return period

Actual rainfall(s)Subject rainfall(s)Hydrograph(s)Verification based on probability ofdischarge and specific discharge
Determination of the design flood

Fig. 2–3 Determination of the design flood

2.7 Determination of the design flood
2.7.1 Determining the design flood
The design flood should be determined from flood hydrographs plotted for the subject rainfalls selected in
Section 2.6 of this chapter. An appropriate flood runoff model will be used, and there will be comprehensive
consideration of the properties of past floods, project facilities, etc.

Explanation
Since the subject rainfalls have already been selected, it is easy to calculate the hydrograph of a flood using anappropriate runoff model, but a careful examination is needed to select the hydrograph that will be used as thebasis for determining the design flood.
The process of determining the design flood should be as shown in Figure 2–3.
To select (a group of) subject rainfalls, the spatial and temporal distribution should be examined as described in
Section 2.6.4 of this chapter. The rate of extension should be about 200% in most cases.
Flood control facilities such as dams and flood control basins should be ignored in the hydrograph calculations,and for water utilization dams such as hydroelectric storages, operating rules for flood conditions should betaken into consideration.
In most cases, since inappropriate rainfalls have already been rejected from the examination of spatial andtemporal distributions, etc., the hydrograph that shows the maximum discharge among the calculatedhydrographs in the group should be selected to give the peak discharge of the design flood.
When there is a sufficient record of discharge data, a discharge probability should be used. For small and29

medium-sized rivers, the peak discharge of the design flood should be verified by a method such as the rational
formula. It is also necessary to use the unit discharge to check the relative balance between the main stream andtributaries, between the upstream and downstream, climate characteristics, and other rivers of similar design
scale.
Another method of determining the design flood includes determining the peak discharge for the design scale byevaluating the probabilities of rainfall of different magnitudes along with their spatial and temporal distributions
based on extensive amounts of accumulated data.

2.7.2 Conversion of subject rainfalls into a flow discharge
A runoff calculation method that is best suited to the characteristics of the subject river should be used toconvert the subject rainfalls into a flow discharge. The rational method can be used for rivers in which flood
storage does not have to be taken into consideration.

Explanation
One of the various calculation methods for converting rainfalls into flow discharge that can adequately reflect
the runoff characteristics of the subject basin should be selected. Runoff calculation methods include, among
others, the unit hydrograph method and the storage function method. The rational method is widely used wherestorage phenomena are barely observed in the river channel or basin, where there is no need to take storage
phenomena into consideration, and where there are no construction plans for dams, flood control basins, or thelike.
For details of these runoff calculation methods, please refer to the Technical Criteria for River Works (Draft)
Research.

2.7.3 Determining constants for the flood runoff model
When determining constants for the flood runoff model being used to convert the subject rainfalls into a flow
discharge, the following factors need to be considered:
1.

Possible difference between the actual and design flood scales2.

2. Possible changes in the basin conditions since the flood through land development, afforestation,
deforestation, etc..

Explanation
Since parameters for the flood runoff model are usually obtained from actual data from smaller scale floods, caremust be taken to avoid irrational results when determining them.
In determining the constants, sufficient consideration must be given to possible changes in basin conditions thatthat may have occurred since the time of the actual flood, such as those caused by land development. In
particular, the runoff ratio needs careful examination since it can change considerably in response to the state of
the basin and can greatly influence the flood runoff volume as well as the flood peak discharge.
The following default values may be used for the runoff coefficient when using the rational method, and inlet

30

time may be used to calculate the time of concentration (time for the flood to travel from the most distant pointof the basin to the point in the river channel under consideration). For details about formulas for determining thetime of concentration, please refer to the Research volume of Technical Criteria for River Works (Proposal).
<Runoff coefficient>
Densely built-up area

<Inlet time>
0.9

Mountainous river basin

2 km2:

30 min2

General built-up area

0.8River basin with very steep slopes:

2 km

20 minField and plain

0.6

Area with sewerage system

2 km2:

30 minPaddy field

0.7

Mountain

0.7

2.7.4 Consideration of landside waters
When inner waters are deemed to have a great influence, that influence should be taken into consideration.

Explanation
In the downstream part of a river or in a basin, inner waters may have a strong influence over the peak dischargeof the design flood.
In such a case, the existing inner water drainage plan should be taken into consideration. In addition, even if
there is no specific drainage plan currently available, an appropriate amount of drainage discharge should be
added to the calculated design flood, as required, taking into consideration the future development situation of
the subject area and referencing drainage plans, etc. for other similar areas. Methods for this purpose include onethat calculates the drainage discharge from an inner water analysis model, and one that uses a unit discharge ofapproximately 5 m3 s–1 km–2 for an urban area and approximately 2 m3 s–1 km–2 for a general area.

2.8 Design flood discharge
2.8.1 Design flood discharge
In flood-prevention planning, the high water discharge to be used as the basis for river channel and damplanning for major locations should be determined by rationally routing the design flood through river channels,
dams, etc. This is called the design flood discharge.

Explanation
For river channel and dam planning, the high water discharge at design control point is of primary importance.
However, the flood control capacity of a dam is influenced by the shape of the hydrograph, the capacity of thedam that could be built, the rules of operation of the dam, etc. in addition to the high water discharge. Therefore
these elements must be thoroughly examined when determining design flood discharge. The runoff discharge atthe design control point and other major locations is calculated from one or more design floods and varies
depending on the channel characteristics and hydraulic conditions such as the capacity of the dam, etc. Rational
distribution of the design flood to river channels and dams means the distribution of these flow volumes to31channels, dams, etc. while considering consistency between upstream and downstream and between the mainstream and tributaries and examining the items described in Section 2.8.2 below.

2.8.2 Items to be examined in determining the design flood flow discharge
When setting the design flood discharge for a river channel, dam, flood control basin, etc., it is important tofully consider each of the following:1.

Study on installation of flood control facilities such as dams, regulating reservoirs, and flood control
basins from technical, economic, social, and environmental conservation points of view.

2.

Study on improvement of current river channels and distribution to cutoff channels, discharge channels,
and tributaries, etc. from technical, economic, social, and environmental conservation points of view.

3.

Issues concerning the coordination of current and future regional development plans along the river andother projects related to the river.

4.

Perspectives and policies on strategies to counteract future increases in flood discharge in areas subject
to increasing urbanization.

5.

Technical, economic, and social measures for dealing with excess floods.

6.

Evaluation at each phase of project implementation.

1.

7. The difficulty of operation and maintenance of the subject facilities.

Explanation
The items listed above need to be thoroughly examined before determination of the design flood discharge to be
used as the basis for each river facility plan.

2.9 Measures against excess floods
For rivers that are anticipated to cause serious damage in floods exceeding the design level, measures tomitigate the effects of such excess floods should be planned.

Explanation
River embankments are constructed or improved to withstand the normal action of flowing water below thedesign flood level (or design storm surge level if applicable). However, since floods are normally caused bynatural rainfall, there is always some probability that a flood exceeding the design flood discharge may occur. If
such an excess flood occurs, river embankments may be destroyed, resulting in enormous damage. In particular,
the failure of river embankments that protect areas possessing high population density, extensive infrastructure,,or central administrative functions will not only take a heavy toll in the area but will also cause great damage tothe entire country, both socially and economically.
For this reason, the consequences of floods exceeding the design scale should be considered when establishing afundamental river management policy and river improvement plans. Construction of embankments with higher
standards of protection than for the design flood should be planned for particular river sections if the predicteddamages from excess floods are deemed unacceptable.

32Section 3 Basics of Appropriate Utilization of Rivers and Maintenance of Normal River
Water Function
3.1 Overview
The basic items associated with the proper utilization of rivers and the maintenance of normal river water
function should be aimed at proper management of rivers. They should serve as a basis for determining theflow discharge necessary to maintain the normal function of river water and for establishing a policy forsecuring this flow discharge while ensuring consistency with flood control functions.

Explanation
For appropriate utilization of rivers and maintenance of normal river functions, the basic aim is to manage rivers
to ensure that they are kept in their normal condition as part of a comprehensive river management strategy.
The flow discharge necessary to maintain the normal function of river water should be determined from thecurrent uses to which the river water is put, the present condition of the river environment and the natural andsocial environments of the river basin and their historical contexts.
The formulation of policies for securing the discharge levels needed to maintain normal river functions should
fully consider structural measures such as dams and non structural measures such as rationalization of diversionof flow and more effective use of existing facilities. The policy should ensure that the flow discharge needed tomaintain normal river functions can be secured for at least up to a 1 in 10 year drought. It is also desirable todevelop measures for droughts that may exceed this design scale.

3.2 Normal flow discharge
There are two types of flow discharge to be determined. The first is the flow discharge determined in total
consideration of water functions such as navigation; fisheries; tourism; maintenance of clean river water;
prevention of salt damage; prevention of river-mouth clogging; protection of river management facilities;
maintenance of groundwater levels, landscape values, and ecosystems; and the securing of opportunities forhuman interactions with the river. This discharge is hereinafter referred to as "maintenance flow discharge".
The second is the flow discharge necessary for water utilization downstream of the point for which themaintenance flow discharge has been determined (hereinafter referred to as "water utilization flow
discharge"); this discharge is determined at a point that serves as the reference point for appropriate rivermanagement.
The normal flow discharge, if necessary, should be determined for each of the river reach represented byannual fluctuations in the maintenance flow discharge and water utilization flow discharge.

Explanation
The flow discharge required to maintain the normal functions of the river water should be secured throughout theyear, and it should be determined in consideration of fluctuations in river discharge, in addition to other factors.

33

For the main stream and major tributaries, one or more reference points for monitoring normal flow discharge toensure appropriate low water management. It is desirable to select reference points at which sufficienthydrological data have been obtained and which have good flow discharge correlations with other flow discharge
observation points under normal river conditions, or at points for which the relationship between upstream anddownstream flows is clearly understood and which is close to the where the river water is utilized.
The normal flow discharge should be determined by investigating the water balance between these reference
points from data on the inflow and quantity of water intake of the subject river; the value chosen for normal flow
discharge should satisfy both the maintenance flow discharge and the water utilization flow discharge.
The normal flow discharge should be determined for each of the river reach determined in consideration of theannual fluctuation patterns of both the maintenance flow discharge and the water utilization flow discharge.

3.3 Determining the maintenance flow discharge
The maintenance flow discharge should be determined for each of the different sections of the river, divided
from the similarity of their properties.
The maintenance flow discharge, if necessary, should be determined for each of the river reaches,.

Explanation
The maintenance flow discharge should be determined in total consideration of river functions such as
navigation; fisheries; tourism; maintenance of clean river water; prevention of salt damage; prevention ofriver-mouth clogging; protection of river management facilities; maintenance of groundwater level, landscapevalues, and ecosystems; and the securing of opportunities for human interactions with the river. Among these
conditions, the needs for tourism are considered to be largely satisfied if conditions such as landscape values andriver and navigation are maintained. The securing of opportunities for human interactions with the river are
considered to be basically satisfied if the flow discharge, determined from such conditions as river andnavigation, fisheries, and maintenance of clean river water, landscape values, and ecosystems, is maintained.
Consequently, the maintenance flow discharge can often be determined by comprehensively taking into
consideration the above-mentioned nine conditions, with the exception of tourism and the securing ofopportunities for human interactions with the river.
In determining the maintenance flow discharge, it is desirable to also consider fluctuations in flow discharge.
1. River and navigation
River and navigation was originally carried out within the context of natural fluctuations in river water level thatwould occasionally cause suspension of service in times of drought. Even if river and navigation services aretemporarily suspended when a drought occurs, they can be substituted with alternative services such as landtransport and can return to normal operations as soon as adequate water levels are restored.
However, depending on the amount of water traffic and availability of alternative means, river and navigation
can play an important role in physical distribution and public transport. Also, the public need for river andnavigation is high where it is associated with tourism or has historical significance. In such cases, it may be
34

necessary to specify a target and secure a certain flow discharge (width of water surface and depth of water)
based on it.
2. Fisheries
The flow discharge necessary for fishery operations can usually be satisfied by the flow discharge necessary for
maintaining ecosystems. However, further examination will be necessary in the case of rivers for which special
conditions, such as for laver (nori seaweed) cultivation, are needed.
3. Maintenance of clean river water
There is a need to control the deterioration in water quality that occurs as a consequence of reductions in water
volume. Essentially, the quality of the river water should be maintained by implementing pollutant
source-control measures in the river basin. Therefore, reduction of the pollutant load in the basin should be given
first priority in determining the necessary flow discharge. However, since it may be difficult to secure high water
quality through pollutant control alone, it is necessary to also consider the possibility of a measure for increasing
the flow discharge.
4. Prevention of salt damage
The flow discharge necessary to prevent salt damage should be determined such that the effects of saltwater
intrusion on river fisheries and ecological systems are minimized, as are the effects of an increase in the salinityof water and groundwater taken from the river for domestic water supply and agriculture.
5. Prevention of estuary clogging
A certain flow rate should be ensured for rivers where the estuary is prone to clogging or is expected to be
subjected to clogging in the future.
6. Protection of river management facilities
It is necessary to secure a flow discharge sufficient to prevent the degradation of wooden facilities (e.g.
revetment foundations and pile hurdles) because of water level reductions caused by decreases in flow discharge.
7. Maintenance of groundwater level
It is necessary to secure an adequate level of flow discharge if a reduction in river flow discharge causes aremarkable reduction in groundwater level and directly and extensively influences groundwater recharge.
8. LandscapeThe flow discharge necessary for maintenance of landscape values is to be determined such that the landscape isvisually satisfying. Drought-affected river landscapes are a natural phenomenon, but a permanently degradedriver landscape due to low flows caused by large-scale water intake is undesirable. In particular, it is necessary tosecure adequate flow discharge to maintain widely popular river landscapes that are valuable for tourism, scenic
values, or holding special events.
35

9. Status of ecosystems
The flow discharge should allow maintenance of river ecosystems. Diverse ecosystems are formed in rivers withfluctuating discharge. Although the occurrence of natural droughts is an element of this fluctuation, a reductionin flow discharge through large-scale artificial intakes can cause a drastic deterioration in the habitat of animals
and plants.
It is particularly important to secure a level of flow discharge that will maintain habitat in riffles, inlets, etc.,
which are vulnerable to drastic reductions in flow discharge.
In addition to the above items, if there are other items that need to be taken into consideration as measures affectadversely the present condition of the subject river, then those items should also be examined.
Since the elements described above change along the river course because of changes in natural and social
factors, it is necessary to divide the subject river into multiple sections of relatively uniform conditions anddetermine the maintenance flow discharge for each section based on the prevailing conditions.
Classification of river sections should be conducted by comprehensively examining various factors, such as theriverbed configuration, tributary inflow, condition of the river channel, ecosystem conditions, river water quality,
and river utilization.
The items needed to maintain the flow discharge may differ for each river reach of each particular river. For thisreason, in determining the maintenance flow discharge, the necessity for river reach division should be examined.
If necessary, river reach division taking into consideration the reach-specific pattern of necessary flow discharge
by item should be performed, and the maintenance flow discharge should be determined for each river reach.
Facility installation or improvement, etc. may be a possible measure in the river section and ensuring a particularlevel of flow discharge may then not be the best river management strategy. Therefore, the possibility of such
facility installation or improvement should be taken into consideration when determining an appropriate
maintenance flow discharge.

36

3.4 Determining the water utilization flow discharge
The water utilization flow discharge should be determined for each of the locations selected as appropriate
for the actual situation of river water utilization. This discharge should be determined for each of the sections
determined by river reach division in consideration of the annual water utilization pattern, as required.

Explanation
The water utilization flow discharge is the level of river flow discharge necessary for occupancy of river water. It
should be determined from the water levels and volumes necessary at each intake location. To maintain waterlevels high enough for the intake may require the installation of weirs or improvement of intake facilities, etc.
The flow discharge to be ensured for the subject river should consider licensed water rights and customary waterrights. Both licensed water uses and customary water uses should be thoroughly investigated to clarify the
purpose of use, water volume used, period of use, etc.
The water utilization regulation needs to be reviewed at appropriate intervals in response to changes in actual
water utilization.
In determining the water utilization flow rate, the intake volumes and locations for various water uses should belongitudinally pigeonholed to select appropriate locations and determine the water utilization flow rate for eachlocation. If the need arises, river reach division should be conducted in consideration of the annual water
utilization pattern, and the water utilization flow rate should be determined for each of the periods.

37Section 4 Basics of Improvement and Conservation of the River Environment
4.1 Overview
The fundamentals of improving and conserving the river environment involves: the conservation andrestoration of a sound habitat for animals and plants; the maintenance and enhancement of a good landscape;the creation and maintenance of places for enabling human interactions with the river; and the preservation ofwater quality.

Explanation
With the modern demand for affluent, comfortable, lives in a high-quality environment, rivers are increasingly
expected to provide pleasant riparian leisure space and healthy habitat for plants and animals. In addition toproviding flood control and supplying water, rivers are also seen as important elements of regional landscapes,
nature, and culture. For this reason, it is necessary to define the fundamental elements of "improving andconserving the river environment".
The basic items associated with the improvement and conservation of the riverine environment need to be
examined, together with the basic items associated with flood-prevention plans (Section 2) and the basic items
associated with appropriate river use and the maintenance of normal river water functions (Section 3).

4.2 Conservation and restoration of a healthy environment for plants and animals
In river improvement and management, efforts should be made to maintain the biotic communities andhabitat that will benefit the river in the future, taking into consideration the present status and past changes inbiotic communities and in the habitat of the river.

Explanation
Conservation and restoration of habitat is important for maintaining diverse ecosystems, sustainable utilizationof biotic resources, and opportunities for human interactions with nature. A river changes its physical form as it
flows downstream and eventually reaches the estuary; its waters also change under the influence of various
elements in the basin. From their source to the estuary, rivers change in character because of differences in
topography, soil, water, exposure to sunlight, etc. in different parts of the river and the different river habitats
such as the acquatic environment, the water’s edge, sand bars, and so on. Diverse communities of plants andanimals live and grow in these diverse environments. It is necessary to fully understand the vital role that rivers
play in conserving the diversity of biotic communities, providing regionally significant ecosystems in a healthy
condition.
To this end, it is important to determine which biotic communities and habitats are suitable for the river. There isa need to focus on the objects and lives that are scientifically significant or rare—those that are typically seen in
the river, highly dependent on the river, and maintained by the dynamism of the river, and those that are
indicators of continuity between upstream and downstream. There is also a need to consider the environmentsthat are atypical of the river, and to take into consideration the present situation and the historical trajectory and38

background.

Fig.2-4

Stochastic illustration of the diversity and continuity of inhabitation and growth of the animals
and plants

In focusing on these points of view, it is important to maintain, improve, and manage rivers so that thecommunities and habitats suitable for the river can be conserved, restored, and maintained in the future.

4.3 Maintaining and enhancing landscape quality
In the maintenance and management of rivers, efforts should be made to maintain and enhance landscapes
formed around river water, taking into consideration the natural attributes of the river landscape and thehistorical and cultural background of the region.

Explanation
A riverfront landscape basically consists of a dynamic microtopography formed by the interaction between water39

flow, sediment movement, and the plants and animals (especially plants) living there. The present appearance ofthe landscape is the result of interactions between these natural processes and human interventions. The riparianlandscape also reflects the living and growing environment for animals and plants. Therefore, changes in riparianlandscapes occur in tandem with changes in the living and growing environment for animals and plants. The
fundamental components of the natural riverside landscape are the microtopography, vegetation, and water flow,
and these need to be emphasized in the maintenance and building of a good landscape.
From ancient times people have interacted with rivers in various ways such as flood prevention, waterborne
traffic, utilization of river water for daily life and productive activities, expression of religious faith, etc., and thepresent river landscape has been formed through these historical and cultural backgrounds. For example, somecities that have developed as pivotal points of waterborne traffic still have boatslips, old clay structures, etc.
along the riverside, and there are cases where these are valuable heritage items that represent the uniqueness of
the region. Where the river is used in people’s daily lives, there may be stairways that lead to washing places or
the river itself. Structures for controlling the river flow and water level also constitute elements of the landscape.
Revetments, embankments, flood damage-protection forests, etc. are also important elements of the landscape.
The elements behind these landscapes, which have been formed through the interactions of human activity andthe natural events associated with rivers, need to be well understood in terms of the historical trajectory and theregional uniqueness.
With its horizontal surface, water provides visual relief to people. It harmonizes with the treescape and helps toform unique, integrated scenery. In an urban city forested with buildings, a wide horizontal landscape provides apleasant open space. A rippling water surface reflecting light presents various elements of expression and beauty,
depending on how the water is illuminated and on the different angles of the waves—an effect only possible withwater. Thus, efforts should be made to maintain and build water-based river landscapes by identifying water
itself as an important element of these landscapes.
The values of river landscapes include their value as natural landscapes created through the interactions among
the microtopography, water, and the creatures and plants of the river; their value as a human and social landscapefostered through the long relationship between people and the river; and their value as water-based landscapes.
Although all of these values are important in the maintenance and building of a good landscape, it is important tojudge, from the properties of individual rivers, which of these should be more important in the conservation andimprovement of riverside landscapes.
To maintain and build a beautiful landscape centered around water as the key element, harmony with thesurrounding landscape is important. It is also necessary to induce and control the landscape through coordination
with relevant bodies, etc.

4.4 Maintenance and creation of places for activities that bring humans into close contact with rivers
In the maintenance, improvement, and management of a river, efforts should be made to conserve the riverine
environment and maintain and improve riparian places so that people can gain close contact with the riverand coexist with nature.

40

Explanation
Activities that bring humans into contact with nature are not possible in all parts of a region. Therefore, it isimportant to plan activities that take advantage of the scenery and freshness of riverside areas, as well as thecharacteristics of the river or the region, such as historical and cultural resources, festivals and other special
events. It is also important that people deepen their understanding of the river environment itself, as well as ofthe various natural and social factors peripheral to the river that affect them, and that the human interactions withthe river will themselves not have any adverse impacts on the river environment.
In maintaining and creating places for human interactions with a river, it is important not only to build, improve,and maintain facilities and places but also to conserve the entire riverine environment itself, which includes thenatural environment and landscapes as a background for these activities. It is also important to install, improve,and maintain facilities and places that integrate educational and welfare functions. In addition, although the useof a river is, as a rule, based on individual responsibility, it is important to have sufficient provision ofsafety-related information, education of river users, cooperation between relevant agencies and bodies in theriver basin, and preparation for emergencies.
In the improvement and maintenance of facilities or places, efforts are to be made to ensure that the environment
will be maintained in the future, taking into consideration the river shape and natural processes of the river (e.g.
erosion and accumulation of sediment, movement of sandbanks).

4.5 Preservation of water quality
In river improvement and management, efforts should be made to preserve the quality of river water so thatthe river is properly utilized, the normal function of the river water is maintained, and the river environment
is conserved.

Explanation
The water quality appropriate to maintain the normal function of river water and conserve the river environmentis that which does not impair the various river-associated functions, including water utilization (e.g. water fordomestic supply, agricultural water, industrial water, fishery water); conservation of habitat; securing of places
for human interactions with the river; landscape management; conservation of peripheral living environments;
tourism; protection of river management facilities; and maintenance of groundwater quality.
The term “water quality” includes the concept of riverbed condition.
In preserving water quality, it is important to grasp the relationship between the river basin and the water area,
and it is also necessary to reflect the natural and social environments of the basin and their historical trajectories.
This includes organisms that use to live and grow there in the past, as well as the relationship between changes inhuman interactions with the river and water quality. It is also important to address future changes in the conditionof the river basin and changes in the residents’ attitudes in relation to sustainable production activities in thebasin and requests made by riverside residents in regard to the river.
In addition, it is also necessary to examine possible countermeasures against unexpected water quality hazards
and cases of abnormal water quality.

41

Chapter 3 Erosion and Sediment management Planning (Countermeasuresfor Sediment Disasters, etc.)
Section 1 General Information
Erosion and sediment management planning (countermeasures for sediment disasters, etc.) is the process of
preparing master plans aimed at preventing or mitigating sediment-related disasters. Such plans include:
landslide prevention plans; steep slope failure prevention plans; avalanche control and impact mitigationplans; and comprehensive sediment disaster prevention and impact mitigation plans aimed at combinations ofdebris flows, landslides, and steep slope failures.

Explanation
The term “river basins” also includes volcanic areas and areas at the base of volcanoes, steep slopes, etc.
“Sediment yield” means the production of unstable sediment through phenomena such as failure and erosion of amountainside or slope as a result of heavy rainfall, snowmelt, earthquake, etc.. Mechanisms of sediment
movement include debris flow, landslides, river bed or bank erosion, etc. The aim of prevention and mitigationof sediment disasters caused by sediment yield and discharge is to protect the lives of the general public,
property, and the living and natural environments from direct disasters caused by the failure or erosion of amountainside or slope, direct impact by debris flow, etc. They also aim to provide protection from indirect
disasters caused by the burial of flood control reservoirs with sediment, flooding induced by riverbed accretion
etc.
The following is a list of issues to be taken into consideration in erosion and sediment management planning.
- Social environment, including land use in the river basin, etc.
- History of past disasters and projects- The level of safety against sediment disasters to be secured- Comprehensive sediment management of the entire sedimentary system- Conservation and restoration of a good natural environment- Maintenance and building of a good landscape- Utilization of river basins, etc.

42Section 2 Erosion and Sediment management Master Plan
2.1 Overview
Erosion and sediment management master plans should be designed in such a way that hazardous sedimentcan be rationally and effectively managed within the design area to prevent and mitigate sediment yield in thesubject river basin and sediment disasters caused by sediment discharge.
Erosion and sediment management master plans include river system erosion and sediment management
plans, debris flow control plans, large woody debris prevention plans, volcanic sediment and erosion control
plans, and disaster prevention plans for abnormal sediment such as from landslide dams. These plans are to
be developed and implemented in accordance with the disaster phenomena and the reasons for taking
countermeasures.

Explanation“Hazardous sediment” means yielded or discharged sediment likely to cause a sediment disaster.
Depending on the disaster phenomena and the purpose of the countermeasure, erosion and sediment management
master plans can be divided into five categories:
1)

river system erosion and sediment management plans, which aim to prevent or mitigate
sediment disasters by controlling the movement of hazardous sediment into the river systemfrom mountainsides and slopes, whereby sediment is yielded to torrents, the river, andeventually the seacoast;2)

debris flow control plans, which aim to prevent or mitigate disasters caused by debris flows;3)

large woody debris prevention plans to prevent or mitigate disasters caused by floating logs
discharged with sediment;

4)

volcanic sediment and erosion control plans, which aim to prevent or mitigate disasters causedby rainfall, volcanic activity, etc. in volcanic sediment and erosion control areas; and5)abnormal sediment disaster prevention plans, which aim to prevent or mitigate disasters
caused by abnormal sediment movement induced by landslides, dam collapse, etc.

Multiple plans from these five categories can be prepared for the same region simultaneously, depending on thedisaster phenomena and the purpose of the countermeasure. In such cases, the plans should be developed
separately for different disaster phenomena, and it is necessary to adjust each plan to maintain consistency
between plans.

432.2 Basic items associated with river system erosion and sediment management
2.2.1 Overview
A river system erosion and sediment management plan should be established for the purpose of securing theflood control and water utilization functions of the river. It should also aim to conserve the environment
through the prevention and mitigation of sediment disasters by controlling the movement of hazardous
sediment into the river system from the mountainside, whereby sediment is originally yielded to torrents andeventually to the river.
As part of the river system erosion and sediment management plan, a sediment management plan forrationally and effectively managing hazardous sediment should be designed based on the design sediment
discharge, etc.
In river systems where problems related to sediment movement are evident, the plan should be established bytaking into consideration the promotion of comprehensive sediment management.

Explanation
In designing a river system erosion and sediment management plan, there are three elements that need to beconsidered: 1) sediment discharge, 2) sediment quality (grain size), and 3) the time scale of sediment movement.
A conceptual diagram of sediment movement in a river system erosion and sediment management plan formed
around these elements is shown in Figure 3–1. Since a river system erosion and sediment management plan must
to be based on an understanding of the processes occurring within the system, and thus accumulated data, it isnecessary to conduct sediment monitoring.
The subject time scales for sediment movement should be categorized into short-term, medium-term, andlong-term, and sediment movement phenomena should be determined for each of these periods.
As a guide, the short-term period should be determined from the duration of a series of rainfall events that cause
the design-scale phenomenon.
The medium-term period should be set to somewhere between a few years and several decades, which isconsidered to be the period of time necessary for the sediment yielded by short-term rainfall events to move andstart having effects on the surrounding environment.
The long-term period should be set to between a few and several decades or longer that covers the short-term andthe medium-term periods.

44One toseveraldays
Time scale

A few years to several
decades(Design duration of
rainfall)(Period that the sediment
discharge and the fluctuationof the riverbed at thereference point for the saboplan turn to steady state within the tolerance range)

Short term

Medium term

Extent of centenarian(Return period of the design rainfall)

Time spanLong term

Limit rainfall of collapse
Rainfall

Unstablesediment is
not yielded
newly
significantly

Sediment yield of
unstable sediment

Sediment
amountby theclasses ofgrain size

Sediment
Discharge

Riverbed
fluctuation

Sediment
yield frowlarge scalecollapse

Sediment
discharge
to becontrolled

Fluctuation
to becontrolled

Conutermeasures
for sediment yield
field

Sediment discharge tobe controlled

Sediment discharge to be olelayed and leveledRiverbed fluctuation ofnatural river channel
Fluctuation to be regulated and leveled

Sediment yield anddischarge

Sediment yield and discharge to be
regulated to the ordinary scaleFig.3-1 conceptual diagram of Sabo plan for the whole river system

2.2.2 Design scale
The design scale of a river system erosion and sediment management plan should generally be determinedfrom an evaluation of the probability of the subject rainfall exceeding the annual precipitation, while also
taking into consideration various factors such as past disasters in each river system, the importance andbeneficial and adverse effects of the planning area, etc.

Explanation
The subject rainfall is determined by three elements: rainfall precipitation, temporal distribution, and spatial
distribution. Refer to Section 2.6 of Chapter 2 for how to determine the subject rainfall.
The design scale should be appropriately determined from the subject rainfall, taking into consideration the45

occurrences of sediment movement phenomena in past disasters. The design scale of sediment discharge by bed
load transport should be determined in accordance with Section 2.5 of Chapter 2.

2.2.3 Design control points in the erosion and sediment management master plan
Design control points in are points at which targets are set for sediment volume.
Design control points are to be set in the lowest reach of the target design area, at locations associated withriver planning, in upper reaches that are targets for conservation, and in the lowest reach of areas wheresediment is likely to be yielded, among others.
Auxiliary reference points are to be set in case it is necessary to determine sediment movement at a point
where the sediment movement pattern changes, for example, in the upper reach of a tributary that is the target
for conservation or at a confluence of the main stream and a tributary.

Explanation
Design control points are to be positioned where the regional characteristics are well reflected, in order to clarify
the subject of the river system erosion and sediment management plan and ensure consistency with the sedimentmanagement plan within the entire planning area of the river system erosion and sediment management plan.
Auxiliary reference points should be set at more than one location if necessary.

2.2.4 Design sediment volume
The design sediment yield, the design sediment discharge, and the design tolerable sediment discharge should
be determined as the design sediment volumes necessary to develop a sediment management plan as part of
the river system erosion and sediment management plan.

Explanation
The design sediment yield includes newly eroded sediment from mountainside or riverside collapse, sedimentfrom the expected expansion of old collapses, and residual sediment from old collapses that is likely to bedischarged at the time of collapse occurrence or is accumulated on the river bed and becomes the subject of
secondary erosion. The design sediment yield is determined from data from surveys of the present status, surveys
of past disasters, surveys in similar areas, etc.
The design sediment discharge is the part of the design sediment yield that is transported by the tractive force ofa debris flow or runoff from design scale rainfall and which reaches the design control points. It is to be
determined in consideration of past sediment discharges, the topography of the basin, the adjustment capacity of
the river channel, etc.
When computing the tractive force, it is desirable to use the flow rate of the runoff calculated in consideration of
the characteristics of mountain streams.
The design tolerable sediment discharge is a sediment volume that presents no hazard at the design control point,
the downstream area, or the sea coast. It should be transported as necessary sediment and is determined inconsideration of the tractive force of the water flow, the grain size of the discharged sediment, the present status46

of the river channel, river channel planning, etc. In river systems where problems related to sediment movement
are evident, the design tolerable sediment discharge should be determined by considering overall sedimentmanagement.
It is desirable that the design sediment yield, the design sediment discharge, and the design tolerable sediment
discharge are represented by the sediment quantity and quality (grain size), depending on the temporal variabilityof sediment movement. Efforts should be made to determine the design sediment yield, taking into consideration
not only the sediment quantity and quality (grain size) but also the sediment yield pattern, place of yield, andoccurrence timing.

2.2.5 Sediment management plan
The sediment management plan aims to rationally and effectively manage the sediment volume, which is thedifference between the design tolerable sediment and the design sediment discharge; this is subject tosediment management at the design control points. The sediment management plan consists of a sedimentproduction control plan and a sediment transportation control plan, which are interrelated.

Explanation
In developing a sediment management plan, the design sediment yield control volume, which is necessary for the
sediment yield control plan, together with the design sediment discharge control volume and the design sediment
discharge adjustment volume, which are necessary for the sediment transportation control plan, needs to be
determined for the relevant design control points (auxiliary reference points). They should satisfy the following
formula:E = (Q + A – B) (1 – α) – C – D
E: Design tolerance sediment discharge volume
Q: Design sediment discharge volume at the auxiliary reference point immediately upstream of the relevant
design control point (or auxiliary reference point)
A: Design sediment yield volume
B: Design sediment yield control volume
α: Ratio of the sediment volume, which is adjusted in the river channel and does not flow downstream below thedesign control point (or auxiliary reference point), against (Q + A – B)
C: Design sediment discharge control volume
D: Design sediment discharge adjustment volume
α is determined in consideration of the conditions of the river basin. Refer to Section 7 of this chapter forconsiderations regarding the natural environment and landscapes.

2.2.6 Sediment generation control plan47

A sediment generation control plan aims to rehabilitate sediment producing areas, prevent the occurrence of
new sediment sources, and suppress hazardous sediment generation by controlling hillside failure, landslides,
riverbed, and riverbank erosion.
In designing the plan, the design generated sediment needs to be rationally partitioned between hillside
works, erosion control dikes, etc., taking into consideration the conditions in the sediment production area,
the sediment production pattern, the sediment discharge pattern, the features requiring protection, etc.

Explanation
The targets of a sediment generation control plan are mountainous hillslopes, the primary source of sediment,
and river channels, secondary sources of sediment.
The design amount of sediment to be managed by erosion and sediment management facilities is determinedaccording to the capacity of existing erosion and sediment management facilities, and the topography, geology,
vegetation, stability of the ground, and other relevant factors.

2.2.7 Sediment transportation control plan
Sediment transportation control plans are aimed at controlling hazardous sediment discharge with facilities
that have sediment capture and adjustment functions, etc., and to allow non-hazardous sediment needed in thelower reaches to travel safely downstream.
In designing the plan, the design sediment discharge volume and design sediment discharge adjustment
volume need to be rationally distributed to erosion control dikes, etc., taking into consideration the sedimentyielding pattern, sediment volume and grain size, subjects for protection, geography, bed slope, present statusof the river channel, etc.

Explanation
Sediment transportation control plans are concerned only with the river channel.
The design sediment discharge control volume should include that portion of the mobile sediment that could be
stored in fixed amounts by facilities such as erosion control dikes. When debris-exclusion works are planned, themobile sediment produced from such works can be included.
Generally, it is necessary to set the design amount of sediment discharge to be controlled to the volume between
the stable slope of the sediment stored in fixed amounts in facilities such as erosion control dikes and theaccumulating gradient of sediment that is anticipated to be produced by a flood. Since the sedimentation area ofan erosion control dike is often designed to provide a river course adjustment function, the design sediment
discharge adjustment volume from an erosion control dike needs to be determined by newly increased volume.
If the sediment capture and adjustment functions are enhanced by the installation of a silt dam, etc., then theeffective volume needs to be evaluated appropriately.

48

2.3 Basic items associated with debris flow control
2.3.1 Overview
The purpose of a debris flow control plan is to protect people’s lives, property, and public facilities fromdebris flows.

Explanation
Debris flows are a natural phenomenon involving the fluidized flow of earth, rocks, vegetation, etc. produced
from hillside failure, usually along the sides of mountain streams, with the flow tending to be concentrated
within the stream course where additional material may be recruited to the flowing mass from the stream bed orsides.
Disasters caused by debris flows are divided into two categories: 1) disasters caused by a direct hit from a debris
flow; and 2) disasters caused by flooding following the debris flow, etc.
Disasters associated with direct hits from debris flows are caused by boulders, which travel at the tip of the flow,
directly colliding with man-made structures, including houses.
Disasters caused by flooding from the flows succeeding debris flows occur from the accumulation of floodwaters
behind the tip of the debris flow.
An example of a typical discharge pattern of a large-scale debris flow is shown in Fig. 3–2.

Boulders (Disaster caused by a direct hit
from the debris flow)
Mixed with boulders (Disaster caused bya direct hit from the debris flow)

Mixed with cobbles (Disaster caused byflooding from successive flow)

Muddy water (Disaster caused byflooding from successive flow)

Fig. 3–2 Conceptual diagram of debris flow flooding

2.3.2 Design scale

49

The design scale of a debris flow control plan should be determined by taking into account various factors, suchas the characteristics of the basin, the benefits and potential adverse impacts of the project, etc. It is generally
determined by evaluating the probable sediment discharge volume from debris flows and the return period of
the subject precipitation.

Explanation
The design scale should be determined in consideration of the characteristics of the basin, previous sediment
movement phenomena during past disasters, etc. In evaluations that use the probability of the subject rainfall
exceeding the annual precipitation, it is necessary to determine the scale of sediment movement that could be
anticipated from a debris flow caused by that rainfall.
In mountain streams where debris flows occur frequently, the design scale may be determined from data on
previous debris flows.
Since the behavior of debris flows caused by large-scale failure and landslides are not necessarily correlated with
rainfall, it is necessary to set the design scale with reference to actual values associated with past debris flows insimilar geography and geology in neighboring areas and other data.

2.3.3 Design control point, etc.
The design control point is the point at which the sediment volume for the debris flow control plan is to be
determined. Generally the design control points should be established upstream, above the subjects to beprotected. When it is necessary to grasp the state of sediment movement at points where the sediment
movement pattern changes, auxiliary reference points should be established.

Explanation
Generally, when a facility is installed at the outlet of a valley upstream that is the subject for protection, or at thestart point of debris flow accumulation, or downstream of the start point of debris flow accumulation, the design
control point should be set downstream of the facility.

2.3.4 Basic items associated with countermeasures
A debris flow control plan aims to prevent or mitigate disasters caused by debris flows. It should consist ofcomprehensive measures properly combining structural measures for suppressing debris flow occurrence andcontrolling discharge (such as construction and improvement of erosion and sediment management facilities,etc.) with non-structural measures (such as establishment of a warning and evacuation system, land use
control, etc.).

Explanation
Structural measures such as construction and improvement of erosion and sediment management facilities are
aimed at preventing or mitigating disasters caused by debris flows. They should be designed such that they can
50rationally and effectively prevent debris flow occurrence and control discharge.
Debris flows can cause serious damage, including loss of life. Therefore, in parallel with structural measures,
efforts should be made to predict debris flow occurrences as an example non-structural measures. It is necessary
to plan comprehensive measures, including non-structural measures for minimizing the damage caused by debris
flows, by establishing warning and evacuation systems and information systems, controlling land use, etc.
In the course of developing a debris flow control measure, the design sediment volume should be determined inaccordance with Section 2.2.4 of this chapter, and a sediment management plan should be designed inaccordance with Section 2.2.5 of this chapter. If marked changes take place in the basin owing to natural factors
such as a new slope failure, debris flow occurrence, destabilization of slopes as a result of earthquake, etc., orowing to anthropogenic factors such as land development, then the design sediment volume, etc. need to bereviewed and the debris flow control plan needs to be revised, as required.

2.4 Basic items associated with prevention of large woody debris
2.4.1 Overview
Large woody debris prevention plans for river basins where floating logs and large tributaries are a feature aim
to protect the lives and property of the general public, as well as public facilities, etc. from disaster associatedwith woody debris discharged with sediment.

Explanation
Generally, when a failure occurs in steep river basins in forested mountains, logs coupled with sediment runoff
may clog narrow parts of the river, bridge openings, box culverts, etc. as they travel downstream, resulting inflooding of sediment, etc. and runoff from bridges, etc. and posing a serious hazard to lives, housing, and public
facilities such as roads.
A large woody debris prevention plan should be developed to control damage from the flow of large woody
material due to slope failure, debris flows, and riverbed and side erosion, as well as from re-mobilization of past
deposits of fallen trees and hewn logs.

2.4.2 Design scale
The design scale of a large woody debris prevention plan should be determined in consideration of various
factors such as the characteristics of the basin, the volume of woody material discharged to the design control
points, etc.

Explanation
The volumes of woody material in the plan should be represented as an actual volume of wood and calculated on
the assumption that there are no facilities for preventing or reducing the occurrence and runoff of sediment andlarge woody debris in the basin.
Where it is predicted that the species, age, and timber volume of trees from the source area will not change, thevolumes of timber runoff should be calculated from the areas of new hillside failure. This is determined from site51

investigations and in the erosion and sediment management master plan.
The volumes of large woody debris (trees, logs, tributaries) that are deposited on the riverbed should bedetermined from their lengths and diameters through site investigations. Artificially produced floating logs, suchas runoff of hewn logs and converted timbers, should be excluded from the plan.

2.4.3 Design control point, etc.
Generally, the design control points, etc. should be established upstream of the area where the subjects forprotection are located. They should be identical to the design control points determined in the river systemerosion and sediment plans, debris flow control plans, etc.

Explanation
Since large woody debris prevention plan are established in conjunction with river system erosion and sedimentmanagement plans, debris flow prevention plans, etc., and not by alone, the design control points must be
identical to those used for the other kind of control plans.

2.4.4 Basic items associated with countermeasures
From the design sediment volumes determined in the river system erosion and sediment plans, debris flow
prevention plans, etc., a large woody debris prevention plan should be established in such a way that it isconsistent with the sediment management plan, erosion and sediment management facilities are properly
located, and sediment is rationally and effectively controlled.

Explanation
Large woody debris prevention generally involves measures for preventing the generation of floating logs andlarge tributaries and measures for capturing such material in the river channel to prevent them being transported
downstream. A large woody debris prevention plan must be consistent with the facilities in the river systemerosion and sediment management plans, debris flow prevention plans, etc., taking into consideration eachsediment occurrence and the behavior of floating logs, depending on the sediment runoff pattern.
If the situation in the basin, such as the forest status, drastically changes, the volumes of floating logs in the planshould be reviewed and the large woody debris prevention plan revised.

2.5 Basic items associated with volcanic sediment and erosion control
2.5.1 Overview
A volcanic sediment and erosion control plan aims to protect lives and property, public facilities, etc. fromsediment disasters caused by rainfall events and volcanic activities in a volcanic sediment and erosion control
area.

52

Explanation
A volcanic sediment and erosion control area means an area subject to damaging sediment disasters as a result of
volcanic phenomena or movement of volcanic ejecta.
Disasters associated with volcanic eruptions can be highly destructive and extensive and their social impacts canbe devastating and last for a long period of time after the eruption. Therefore, a volcanic erosion and sedimentmanagement plan needs to be consistent with regional plans, and, in parallel with the plan, construction andimprovement of erosion and sediment management facilities and establishment of warning and evacuation
systems need to be implemented in order to promote city-planning centering around safety and disasterprevention.

2.5.2 Subject phenomena, etc.
The phenomena subject to a volcanic erosion and sediment management plan include debris flows and volcanic
mudflows caused by rainfall and volcanic events including lava flows, etc in area of the volcanic erosion andsediment management area.
The design scale of the target sediment movement phenomena should be determined in consideration of various
factors, such as the natural and social characteristics of the area to be controlled, past volcanic activities anddisasters, beneficial and adverse effects of the project, etc.
The design control points, etc. should be determined in accordance with Section 2.2.3 of this chapter.

Explanation
The target sediment movement phenomena in a volcanic erosion and sediment management plan should bebasically debris flows and volcanic mudflows. Lava flows, etc. are included as necessary.
Other phenomena, such as pyroclastic flows, which are obvious when they occur and whose area of influence ispredictable, need to be included in the plan if there is a likelihood of their occurrence. In such cases, ash falls,
cinders, etc. should be evaluated as factors that affect the sediment-yielding region. Since sediment movement
phenomena caused by rainfall events, etc. in volcanic erosion and sediment management areas are characterized
by extremely different sediment-yield and runoff conditions (such as extraordinary erosion) compared to normal
mountainous river basins, these should be taken into consideration in planning.

2.5.3 Basic items associated with countermeasures
A volcanic erosion and sediment management plan should be established as a comprehensive measure thatappropriately combines structural measures (such as improvement of erosion and sediment management
facilities) with non-structural measures (such as establishment of warning and evacuation systems, land use
control, etc.). In this case, it should be established in accordance with Section 2.3.4 of this chapter.
A plan that targets lava flows, etc. at the time of eruptions should be established if this is deemed necessary
from the consideration of past volcanic activities.

53

Explanation
In designing a volcanic erosion and sediment management plan, the design sediment volume, etc. should bedetermined for target phenomena in the volcanic erosion and sediment management area in accordance with
Section 2.2.4 of this chapter. The design sediment volume, etc. should be properly determined from past disasterand eruption records, etc., taking into consideration the characteristics of sediment movement phenomena in thesubject volcanic erosion and sediment management area.
Debris flows caused by rainfall events, etc. should be determined for each sediment yielding area and basin incompliance with the basic items associated with river system erosion and sediment management in Section 2.2,
the basic items associated with debris flow control in Section 2.3, and the basic items associated with large
woody debris prevention in Section 2.4 of this chapter.
A volcanic erosion and sediment management plan targeted at eruptions should be established in consideration ofvarious structural measures (such as improvement of erosion and sediment management facilities) andnon-structural measures (such as establishment of warning and evacuation systems). It is desirable to design
non-structural measures that assume phenomena exceeding the design target phenomena. For appropriate
provision of information by municipal governments about warning and evacuation, non-structural measures
should include preparation of volcanic hazard maps, establishment of warning and evacuation criteria, andmonitoring of volcanic activities and sediment movement.
After the preparation of volcanic hazard maps, efforts should be made to publicize them so that local publicbodies can guide land use based on them. It is necessary to keep in mind the fact that land use control can also be
one of the non-structural measures used to facilitate prompt action at the time of disasters, such as those causedby eruptions.
Equipment for monitoring sediment movement, etc. should be selected in consideration of the characteristics ofthe subject volcanic erosion and sediment management area, and it is desirable to establish a system that allows
appropriate information provision through the combined use of multiple equipment, etc.
In order to quickly and properly deal with disasters such as a sudden volcanic eruption, structural emergency
plans such as construction of guide banks and flow channels, as well as non-structural measures such as
monitoring of volcanic activity, should be planned..
In planning facility locations, it is necessary to thoroughly consider the local conditions, such as the naturalenvironment (refer to Section 7 of this chapter).
When a remarkable change occurs, such as the status of volcanic activity or a new occurrence of sediment
movement, the target phenomena in the plan should be reviewed and the volcanic erosion and sedimentmanagement plan revised if required.

2.6 Prevention of abnormal sediment disasters
2.6.1 Overview
Plans for preventing abnormal sediment disasters should be aimed at protecting lives and property and public
facilities from sediment disasters such as the formation and collapse of landslide dams or from other types ofrare events.

54

Explanation
Although disasters caused by the collapse of landslide dams, etc. do not occur frequently, their occurrence will
generate large amounts of sediment resulting in tremendous damage. Therefore, a plan for preventing abnormal
sediment disasters such as those associated with landslide dams should be designed when a landslide dam forms,
depending on the phenomena that occur, etc. When a sediment disaster can be predicted in advance, it isnecessary to take preventative measures.

2.6.2 Target phenomena, etc.
The target phenomena in a plan for preventing abnormal sediment disasters include: the potential submersion of
items requiring protection upstream of a landslide dam formed in a river channel due to heavy rainfall or anearthquake; large-scale debris flows caused by the collapse of a landslide dam; debris flows caused bylarge-scale failure due to earthquakes.
The target scale of sediment movement phenomena in the plan should be determined in consideration of various
factors, such as the phenomena of sediment runoff downstream caused by the collapse of landslide dams.
The design control points, etc. should be determined in accordance with Section 2.2.3 of this chapter.

Explanation
The design control points, etc. should be determined in accordance with Sections 2.2.3 and 2.3.3 of this chapter,depending on the target phenomena.

2.6.3 Basic items associated with countermeasures
A plan for preventing abnormal sediment disasters should aim to prevent or mitigate the damage likely to becaused. It should be a comprehensive plan combining structural measures (such as the construction andimprovement of erosion and sediment management facilities, including drainage channels for draining theflooding caused by a landslide dam) with non-structural measures (such as determination of the hazard
expansion area, monitoring of landslide dams, etc.).

Explanation
A plan for preventing abnormal sediment disasters such as those caused by landslide dams will combine
structural measures (such as emergency measures for preventing or mitigating abnormal sediment disasters) with
non-structural measures (such as: assessments of slope stability and the risk of landslide dam collapse;
preparation of hazard maps of areas at risk due to flooding in the event of landslide dam formation and collapse;
monitoring of landslide dams).
Emergency measures should include measures both for the landslide dam site itself and for the area downstreamof the landslide dam, depending on the constraints of construction and the stability of the landslide dam.
Emergency measures at the landslide dam site include excavation of sediment forming the dam body (channel
installation), dam body sediment removal, and installation of cutoff walls made of steel sheet piles. Downstream
measures include the exclusion of debris from the area of sedimentation of existing erosion and sediment55

management facilities, installation of new erosion and sediment management facilities, etc. as measures
downstream.
Proactive measures include the installation of erosion control dikes, etc.
An erosion and sediment management facility installation plan needs to take the occurrence of disasters
exceeding the design scale into consideration.
Risk assessment to support warning and evacuation systems implemented by municipal governments and include
the examination of stability with respect to slope failure and landslides. Stability can be determined from thegeneral risk assessment of long slope failure and landslides. In assessing the risk of landslide dam collapse, it isnecessary to consider overflow from the dam as well as piping and slip failure of the downstream face of thedam due to infiltrated water.
The areas subject to hazard include upstream inundation areas due to the filling of the dam and downstream
flooding areas caused by collapse of the dam. Upstream inundation area should be predicted from geologicalmaps on the assumption that the highwater mark will be equal to the height of the landslide dam. In the case of alandslide dam collapse, the area of hazard should be predicted by comparing the maximum depth of flood
occurring downstream of the landslide dam with the elevation at that location.

Section 3 Landslide Prevention Plan
3.1 Overview
The aim of a landslide prevention plan aims is to protect lives, property and public facilities from landslides.

Explanation
Disasters associated with landslides occur under conditions particular to this nation, such as its characteristic
topography, geology, climate, and land uses. These disasters are roughly divided into two types: 1) direct
disasters, where the subject requiring protection is located on a landslide slope or within reach of a moving soil
mass generated by a landslide; and 2) indirect disasters, which affect areas upstream and downstream through theblocking of rivers by sediments or the secondary collapse of river-clogging sediments.
The aim of a landslide-prevention plan is to prevent or mitigate these direct and indirect landslide-related
disasters, and it should be developed from preliminary landslide surveys and the analyses thereof. The various
conditions of the landslide-prevention area, such as topography, geology, climate, land use, the status of thesubjects requiring protection, and the urgency of protection all need to be taken into account.
Since landslides generally occur on a large scale and consist of two or more movement blocks, it can take many
years are to complete construction works. On the other hand, because there are often many houses and public
facilities on the landslide slope and within reach of the moving soil mass generated by the landslide,
non-structural measures such as an appropriate warning and evacuation system are essential to cope with thethreat of the landslide being activated. For this reason, a landslide-prevention plan needs to take into
consideration not only structural measures such as construction and the improvement of landslide-prevention
facilities but also non-structural measures to ensure the establishment of a warning and evacuation system.
The plan must be kept consistent with the surrounding environment, relevant laws and regulations and regional
plans.
56

Once a landslide-prevention plan is established, it needs to be properly reflected in the landslide-prevention
works master plan stipulated in Article 9 of the Landslide Prevention Act.

3.2 Basic items associated with landslide prevention
3.2.1 Target phenomena, etc.
The target phenomenon in a landslide-prevention plan is the sliding of specific portions of land as a result ofgroundwater flows, or the resulting movement of the land.
The target planning scale should be determined in consideration of various factors, such as the landslide
phenomenon itself, the importance of the subjects requiring protection, the urgency of the project, and thebeneficial and adverse effects of the project, etc.

Explanation
In a landslide-prevention plan, measures should be planned to deal with the movement phenomenon resulting
from groundwater flows, etc., and the phenomena that follow this movement. Lands currently continuing to
move, or those that show a landslide configuration and may move, should be targeted. Since landslide
phenomena often show different movement patterns depending on natural conditions, such as topography andgeology, in establishing the plan it is necessary first to fully understand the target landslide phenomenon andthen to properly determine the target planning scale.

3.2.2 Basic countermeasure principles
A landslide-prevention plan needs to be a comprehensive measure combining structural measures with
non-structural measures such as the establishment of a warning and evacuation system, as well as land-use
control, etc.

Explanation
Structural measures for landslide prevention need to take into consideration the land uses in thelandslide-prevention area and surrounding areas, etc. These measures should be designed so that thelandslide-prevention facilities are arranged rationally and effectively to ensure the desired level of safety.
Non-structural measures mainly refer to the establishment of a warning and evacuation system and land-use
control, among others. To contribute to appropriate warning and evacuation, monitoring equipment such as
ground extensometers and ground tiltmeters need to be installed for speedy data collection, and efforts should bemade to establish an appropriate communication system that connects with relevant organizations and bodies.
In the case of a landslide occurring around a dam reservoir, it is necessary to examine the effects of filling level
fluctuation, etc.
In planning the construction or improvement of landslide-prevention facilities, it is necessary to take into fullconsideration the natural environment of the target area and surrounds, including the landscape and ecosystem,
as well as the living environment, etc. For more details, refer to Section 7 of this chapter.

57

Section 4 Steep Slope Failure-prevention Plan
4.1 Overview
A steep slope failure-prevention plan should be established for the purpose of protecting lives and property
from disasters caused by steep slope failures.

Explanation
The occurrence of steep slope failures is intricately connected with topographical, geological, vegetation, andrainfall characteristics, and it is extremely difficult to predict the location and timing of such occurrences. In
addition, with the accompanying rapid large sediment movement, once a steep slope failure occurs it often leads
to a life-threatening disaster.
Steep slope failure-prevention plans should include measures to construct and improve steep slope
failure-prevention facilities and to regulate the occurrence of new risks by establishing a warning and evacuation
system, constraining activity within a steep slope area in danger of failure, and limiting the construction ofresidential buildings. It should also ensure safety by encouraging landowners and those who may be affected byany disaster to take disaster-prevention measures and advising them on building relocation, etc.
A steep slope area in danger of failure means an area of land for which control of activity is required so thatsteep slope failure is not accelerated or triggered on the steep slopes or on neighboring land where a considerable
number of residents or developments may be subjected to damage due to the failure.

4.2 Basic items associated with steep slope failure-prevention measures
4.2.1 Target phenomena, etc.
The target phenomena in a steep slope failure-prevention plan are the failures of steep slopes as a result of
natural phenomena such as rainfall events and earthquakes.
The design planning scale should be determined in consideration of various factors, including the presumed
phenomena of steep slope failure, the importance of the subjects requiring protection, the urgency of the
project, and the beneficial and adverse effects of the project, etc.

4.2.2 Basic countermeasure principles
A steep slope failure-prevention plan needs to be a comprehensive countermeasure that combines structural
measures such as construction and improvement of steep slope failure-prevention facilities with non-structural
measures such as the establishment of a warning and evacuation system, land-use controls, etc.

Explanation
The construction of a steep slope failure-prevention facility should be planned within a steep slope area in dangerof failure in consideration of the importance of the subjects to be protected and the beneficial and adverse effectsof the facility.
A warning and evacuation system should be established to predict disaster occurrence in advance and support
early evacuation of community members from the area that is anticipated to receive damage.

58

To support smooth warning and evacuation processes by municipal governments, it is vitally important toquickly and accurately grasp the climatic conditions, the predicted rainfall, the standard rainfall level at which
warning and evacuation are required, and the presence of precursory phenomena. To quickly and accurately
gather such information and to share disaster-related information among relevant organizations and bodies andcommunity members, efforts are needed to establish an appropriate information and cooperation system.
In establishing a steep slope failure-prevention plan, it is also necessary to adequately coordinate the plan withother projects that are planned from other laws and regulations and institutions, and to maintain consistency
regarding the positioning of the subject slope in regional plans, etc.
For considerations associated with the natural environment, refer to Section 7 of this chapter.

Section 5 Avalanche Prevention Plan
5.1 Overview
An avalanche-prevention plan should be established for the purpose of protecting the lives of the general publicfrom disasters associated with avalanches.

Explanation
An avalanche-prevention measure aims to protect people’s lives from disasters caused by avalanches, and it
should be appropriately implemented through the construction and improvement of avalanche-prevention
facilities, the spread of information on places where avalanche hazards are likely to occur, and the establishmentof a warning and evacuation system.
An avalanche-prevention measure should target mainly hillside slopes and their surrounding areas.
In cases where there are items to be considered regarding relevant laws, regulations, and institutions, as well as
aspects of the surrounding environment other than those stipulated in this chapter, the plan also needs to takethese into consideration.

5.2 Basic items associated with avalanche-prevention measures
5.2.1 Target phenomena, etc.
The target phenomena in an avalanche-prevention plan are surface and total layer avalanches as well as snow
cover movement (creep or glide) on slopes. The design planning scale should be determined in consideration ofvarious factors such as predictable avalanche phenomena, the importance of the subjects requiring protection,
the urgency of the project, and beneficial and adverse effects of the project, etc.

Explanation
An avalanche-prevention plan centers around areas in danger of avalanche. The design scale should bedetermined by selecting areas with a high avalanche hazard and determining the extent of the likely avalanche.
This depends on the type and scale of predictable avalanches, as determined from the findings of topographic
surveys, vegetation surveys, and other avalanche-associated surveys, while taking into consideration thecharacteristics of the area requiring protection.

59

The area in which a measure or measures are required in accordance with the design scale is the design targetarea.
An avalanche hazard area should cover an area with a slope of 18 degrees or more when the point at which thesurface gradient becomes less than 15 degrees in the upper part of a hillslope with a surface slope of 15 degrees
or more and a height of 10 meters or more is viewed.

5.2.2 Basic countermeasure principles
An avalanche-prevention plan should be a comprehensive measure that properly combines structural measures
to constructing or improving avalanche-prevention facilities with non-structural measures such as theestablishment of a warning and evacuation system.

Explanation
Avalanche-prevention facilities should be planned in consideration of the importance of the subjects forprotection and the beneficial and adverse effects of avalanche-prevention facilities, etc.
The establishment of a warning and evacuation system aims to predict the occurrence of avalanche disasters andsupport early evacuation from areas that are predicted to receive damage. This should be done at the prefecturaland municipal levels and for relevant organizations, bodies, and community members. To this end, the planneeds to take into consideration the provision to community members of various details such as the predictedavalanche occurrence area, the avalanche path, the sedimentary province, and the movement pattern of theavalanche. It should also encompass the establishment of an appropriate communication and cooperation system
for information sharing among the relevant organizations and bodies and community members.
For considerations related to the natural environment, etc., refer to Section 7 of this chapter.

Section 6 Comprehensive Sediment Disaster-prevention Plan
6.1 Overview
The aim of a comprehensive sediment disaster-prevention plan is to prevent or mitigate complex sedimentdisasters caused by multiple disasters, such those caused by sediment yield and runoff in the basin, landslides,
steep slope failures, etc., and it should be established combining structural and non-structural measures.

6.2 The elements of comprehensive sediment disaster-prevention
Comprehensive sediment disaster-prevention should be planned by determining the target phenomena, scale,and extent from the regional characteristics and land-use status. It should combine structural measures with
non-structural measures such as the establishment of a warning and evacuation system and land-use controls,etc.

Explanation
To support the development of the plan, detailed surveys of the regional characteristics should be conducted,
including the natural conditions (topography and geology), regional plans of the municipal government, and the60status of land-use controls based on relevant laws and regulations, etc.
A comprehensive plan must be established after identifying the area at risk and special damage potential zone inaccordance with the Act for the Promotion of Measures for Sediment-related Disaster Prevention in Damage
Potential Zones, etc.. It should incorporate the establishment of a warning and evacuation system; theimplementation of land-use controls; the regulation of building structures; and the relocation of residences.
In designing warning and evacuation systems, the plan should be designed to take into consideration thecomprehensiveness and efficiency of observations of the phenomena that may cause sediment movements
associated with debris flow, landslides, and steep slope failure; the establishment of an information-gatheringand communication system that facilitates fast and accurate forecasting; and the establishment of an information
system that allows interactive information exchange with community members, etc.

6.3 Urbanized foothills greenbelt development plan
6.3.1 Overview
An urbanized foothill greenbelt development plan should be planned as a comprehensive measure based on theurbanized foothill greenbelt master plan. It should aim to develop a region that is resistant to sediment disaster,
maintaining consistency with regional plans, etc., and making use of the various functions and effects of trees.
It should also properly combine structural measures such as the construction or improvement of erosion- andsediment-control facilities, landslide-prevention facilities, steep slope failure-prevention facilities, etc. with
non-structural measures such as land-use controls.

Explanation
An urbanized foothill greenbelt is a slope greenbelt that consists of a series of forests on hillsides, in mountainstream areas, and on the foothill slopes neighboring urbanized areas for the purposes of sediment disasterprevention and mitigation. These greenbelts are also intended to help form good urban environments andpleasant scenery and landscape; they should conserve and foster ecosystem conservation in hilly urban fringes
that may be susceptible to sediment disaster.
In light of their purpose, urbanized foothill greenbelt master plans are established by municipal governments ascomprehensive plans that stipulate the basic functions and measures for future urbanized foothill greenbelt
development in cooperation with relevant organizations and bodies and by coordinating land-use controls, etc.
based on relevant laws and regulations. These master plans utilize greenbelts for provision of extensive
disaster-prevention zones.
An urbanized foothill greenbelt development plan should be based on a master plan and include a comprehensive
sediment disaster-prevention plan with specific measures for erosion and sediment management to be
implemented by the responsible agencies.
In an urbanized foothill greenbelt development, it is important to implement the necessary measures such as theconversion of tree species to make full use of the various functions and effects of trees, in addition to thecomprehensive development of erosion- and sediment-control facilities, landslide-prevention facilities, steep
slope failure-prevention facilities.
61

The plan needs to be consistent with regional plans and with land-use control plans based on relevant laws andregulations.

6.3.2 Basic countermeasure principles
An urbanized foothill greenbelt development plan should be established in such a way that forest conservationand cultivation and forest structure improvement are implemented to maintain and improve the forest’s functionof controlling sediment yield and discharge due to surface erosion, etc. The plan should also be designed tocreate and conserve areal disaster-prevention space by the implementation of measures such as construction or
improvement of erosion- and sediment-control facilities, etc. It is particularly important for urbanized foothillgreenbelts to be coordinated with other projects and land-use controls based on relevant laws and regulations so
as to prevent uncontrolled urbanization and ensure the security of the subject region.
In developing a plan, sufficient consideration needs to be given to the building of a good urban environment,
the creation of scenery and landscapes, ecosystem conservation, and the provision of places for healthy
recreation, etc.

Explanation
A comprehensive measure should be planned to form a series of areal woody corridors neighboring an urban
district and to construct or improve erosion-control dykes and erosion- and sediment-control facilities throughhillside works, landslide-prevention facilities, and steep slope failure-prevention facilities, etc. The aim is toprevent and mitigate disasters due to sediment discharge, such as debris flows, landslides, steep slope failures,etc., on hillsides, in mountain stream areas, and in foothills on the outskirts of cities within urbanized foothills. It
is particularly desirable that the plan is designed not only in cooperation with other projects, such as park
projects, through the designation of damage-potential zones, etc., but also in appropriate cooperation withland-use controls etc. based on laws and regulations such as the City Planning Law for the purpose of preventing
uncontrolled urbanization.
Since it is expected that appropriate conservation and growth of forests and improvement of forest structure willbring about beneficial effects such as the formation of a good urban environment, the preservation of sceneryand landscapes, the conservation of habitat for animals, and the provision of places for healthy recreation, thedesign of the plan needs to take into consideration the actual status of the region for which these effects areplanned.

Section 7 Consideration of the Natural Environment
In establishing an erosion- and sediment-control master plan, landslide-prevention plan, steep slope
failure-prevention plan, avalanche-prevention plan, or a comprehensive sediment disaster-prevention plan,sufficient consideration needs to be given to the natural environment and landscape in and around the design
area.

62

Explanation
Mountainous slopes, mountain streams, river channels and areas adjoining such natural spaces are associatedwith distinct ecosystems and landscapes.
In establishing an erosion- and sediment-control master plan, landslide-prevention plan, etc. for a particular river
basin, or other geographic unit, it is necessary to fully consider existing conditions, including habitat for animals
and plants, the landscape, water quality, etc., as well as the location, shape, and structure of proposed facilities tosecure the continuity of the living and growing environment for plants and animals and a good quality landscape.
For details of the promotion of comprehensive sediment management, including the securing of continuity of
sediment movement in sediment transport systems, refer to Section 4 of Chapter 1.
Since one of the purposes of an erosion- and sediment-control master plan is to restore degraded naturalenvironments to their original state, efforts need to be made to use native species when introducing vegetation toa degraded area. It is desirable to select species that are in harmony with the natural environment of the local anddownstream areas. In the process of re-vegetation, goals need to be set, and it is desirable to perform
maintenance as required.
Existing vegetated slopes in an urban district are important components of the urban landscape and are valuable
spaces for conservation of plant and animal habitat. Therefore, measures to construct or improve erosion- andsediment-control facilities, landslide-prevention facilities, steep slope failure-prevention facilities, andavalanche-prevention facilities need to consider the surrounding living environment, etc., the conservation ofexisting trees, and the introduction of new vegetation by using native species.

63

Chapter 4 Seacoast Preservation Planning
Section 1 General Information
A seacoast preservation plan aims to preserve, restore, and improve the functions of a seacoast and thereby
improve and preserve coastal protection and the coastal environment, ensure appropriate use of the publicseacoast, and create a harmonized coastal space.
This chapter describes basic items associated with coastal protection, improvement and conservation of thecoastal environment, and seacoast utilization. These are basic items that need to be examined in theestablishment of a seacoast preservation plan.
A seacoast preservation plan must be a comprehensive plan based on all these items.

Explanation
Seacoasts are valuable assets for humankind, and they need to be properly evaluated as land, places for marine
recreation, parks, places for agriculture and fisheries, scenic sites, ecological systems, historical and cultural
spots, livelihood spaces, etc., and passed down to future generations. On the other hand, severe natural
conditions and changes in social and economic environments have caused disasters due to storm surges,
tsunamis, and coastal erosion.
Historically, there have been cases in which a coastal environments have been damaged and access to theseacoast prevented due to coastal protection works. There have also been cases in which gently sloping coastal
dikes and revetments constructed to secure access to a beach have reduced the natural beach area, prevented useof the beach, eliminated the plant and animal habitat, and increased wave overtopping rates.
In public administration, the term “seacoast preservation” has been used mostly to mean coastal protection as in
the term “coastal protection facility”, but a seacoast preservation plan involves “preservation” as acomprehensive term, the definition of which includes preservation of the coastal environment and beach use (Fig.
4–1).

Fig.4-1 Direction of seacoast preservation
Figure 4–2 shows the organization and flow of plans. A plan consists of a seacoast preservation plan, which is amaster plan, and a seacoast preservation facility arrangement plan, which defines the type, arrangement, and64

scale of facility, etc. A seacoast preservation plan is composed of basic items associated with seacoastpreservation, improvement and conservation of the coastal environment, and seacoast use, all of which are thepurposes of seacoast preservation. A seacoast preservation plan should not aim to provide the best outcome forthe individual areas of coastal preservation, environmental conservation, or use of the seacoast, but must aim tocomprehensively optimize these three features. To this end, there are cases in which plans may need to bereexamined.
In a seacoast preservation facility arrangement plan, the organization of seacoast preservation facilities, etc. thatsatisfy the basic items defined in a seacoast preservation plan, as well as the arrangement and scale needed toacquire the necessary performance, should be determined. Since erosion, storm surge, and tsunami facilities areplanned as integral parts of the plan, there are cases that call for reexamination. Also, in the process of examining
a facility arrangement plan, it is necessary to confirm that the coastal preservation, environmental conservation,and use goals determined in the seacoast preservation plan are satisfied. If these goals are not satisfied, then it isnecessary to reexamine the seacoast preservation plan. The seacoast preservation plan and the seacoastpreservation facility arrangement plan need to be established in such a way that they are integrated with eachother.

65

Seacoastpreservation plan
Coastal environment
Living and growing
environment
Coastal landscape

Coastal protection
Design beach profile
Design sea
level,design ocean
wave and design
tsunamiSea coast utilization
Comfort
SafetyProtection measure
Coastal erosion
Storm surge
TsunamiSeacoast
preservation facility
arrangement plan
Storm surge facility
Layout planning ofembankment,revetm
ent facilities andwave barriers
Embankment andrevetment
wave-energy
dissipating facilities

Erosion control
facilities
Layout planning oferosion control
facilities and beachnourishment
Erosion control
facilties
Beach nourishment

Tsunami control
facilities
Layout planning ofembankment,
revetment andtsunami breakwaters
Embankment andrevetment
Tsunamibreakwaters

Fig.4-2 Organization and flows of plans
The aim of “Coastal protection” is to protect the lives and property of the general public from storm surges,
tsunamis, and ocean waves, and to protect seacoasts, which are public assets, from erosion. The aim of“improvement and conservation of the seacoast environment” is to: conserve the diverse spaces themselves,
including mud beaches, sandy beaches, shingle beaches, and cliffed coastlines, which form the substrate of
ecosystems, history, and culture; to preserve coastal landscapes, which are historically and culturally important;and to improve degraded coastal environments wherever necessary. The aim of “Seacoast utilization” is tocoordinate diverse overlapping or conflicting use purposes such as recreation, distribution, and fisheries, and topromote safe and comfortable seacoast utilization.
In a seacoast preservation plan, the terrestrial and marine areas that influence the seacoast are treated as one
integral area. The plan should include measures to be undertaken in the seacoast preservation area by seacoast66

administrators as well as measures to be implemented through coordination with the people concerned fromsurrounding areas.
In many cases the protection line is set on the basis of the current shoreline when examining coastal protection.
From the viewpoint of overall national land management, however, although the protection line seems to be
regarded as a matter-of-course condition, its appropriateness needs to be reexamined in view of proper land use,etc.
A seacoast is an area with diverse natural conditions, including its meteorology, oceanographic phenomena,
topography, and geology, etc. Social and economic conditions such as population, assets, and land use aredifferent from one seacoast to another. Consequently, care must be taken not to design a seacoast preservation
plan in a standardized manner, but to take into consideration the particular characteristics of each subject region.
Along a seacoast, natural processes such as ocean waves, tides, winds, and the activities of organisms areconstantly taking place, and give rise to a particular coastal geomorphology and coastal environment. By valuing
and making use of these natural processes, it is possible to preserve the natural habitat in good condition and toconserve and develop a safe and beautiful seacoast. For this purpose, in designing a seacoast preservation plan itis necessary to fully understand natural forces.
Seacoasts are generally divided into two types: 1) beaches composed of mud, sand, and shingle; and 2)
cliff-lined coasts, with exposed base rocks and many reefs. Beaches are further classified into three types
depending on their bed materials: 1) mud beach; 2) sandy beach; and 3) shingle beach. An extensive flat area
composed of sand and mud with an intertidal zone is called a tidal flat. Sandy beaches, being the primary targetof coastal protection, are where seacoast preservation facilities are often constructed or improved by seacoast
administrators and where conflicts between coastal protection, environmental conservation, and use can easily
occur. Figure 4–1 shows the functions of the sandy beach. In building or improving a seacoast preservation
facility at a sandy beach, it is essential to understand that sandy beaches have important functions related tocoastal protection, environmental conservation, and utilization, as they absorb the energy of waves, purify
seawater, provide habitat for flora and fauna, provide spaces for recreation and sports, and work space for
agriculture and fisheries.

67

Table.4-1 Functions and capabilities of the sandy beach
Function

Capability

・Wave over topping prevention

Reduction efficiency of run-up height
and wave overtopping rate, and stabilityof coast line

・Water puritication
・Living spase for plants and animals
・Landscape

Efficiency of purification, living
conditions for plants and animals(grain size of bed material and waterguality, etc)and landscapeobstrucation(crest height, etc)

Utilization

・Swimming beach・Fishing
・Place for festive events

Convenience(access time, etc),comfort(rrain size and color of bed material,water guality, etc)

Other

Foundation of structures
such as embanlcments

Support strength of rock bed,scour rateand stability

PurposeProfection

Environment

The following items need special consideration in designing a seacoast preservation plan.
1. Consistency with related plans
Consistency with other related plans such as national land utilization or development plans and regional plansneeds to be assured.
2. Cooperation and coordination with the administrative authorities concerned
Sufficient cooperation and close coordination with seacoast-related administrative authorities are necessary.
3. Community participation and information exchange
Seacoast-related information needs to be distributed widely during plan preparation to improve the transparencyof the project and convey what the situation concerning coastal protection, environmental conservation, andutilization would be like as a result of plan implementation. Community members should be invited toparticipate in public hearings during both the plan development stage and the plan implementation stage as
required so that the plan can be implemented effectively and efficiently.
4. Review of the plan as required
The achievements of the plan and construction or improvement of planned seacoast preservation facilities need
to be reviewed as required. Any changes will depend on the results of the monitoring and will be in accordancewith changes in the physical, social, and economic conditions of the subject region.

68Section 2 Basic Items Associated with Coastal Protection
2.1 Overview
The aim of coastal protection is to mitigate damage to human lives and assets and impacts on various activities
taking place on land owing to storm surges, tsunamis, ocean waves, and sediment regimes

while maintaining

harmony with the coastal environment and utilization.
A defined section of seacoast should be determined as the target area from the target external forces; thesituation of the area prone to flooding due to wave overtopping or overflowing; and the continuity of beachdrift.
This section describes the elements of a coastal protection plan as well as the basic policies of coastal protection
against erosion, storm surges, and tsunamis. A coastal protection plan should provide comprehensive treatmentof three aspects: erosion, storm surges, and tsunamis. In particular, it is necessary to examine the presence oferosion in order to confirm the adequacy of the design beach geometry elements on which storm surge andtsunami control measures are based.

Explanation
Under their severe natural conditions, including such phenomena as earthquakes, typhoons, low pressure systems,
and winter ocean waves, the coastal areas of Japan are prone to disasters caused by tsunamis, storm surges,
ocean waves, coastal erosion, etc., and many parts of Japan have actually received major damage in the past.
Figure 4–3 shows the place of coastal protection in a seacoast preservation plan. In a seacoast preservation plan,
the goals associated with coastal protection, environmental conservation, and utilization should be set inaccordance with the current situation of the subject seacoast. From these goals, the design ocean wave, design
tsunami, and design sea level, which form the basic parameters of the seacoast preservation plan, should bedetermined, and the target coastal features should be determined as the elements of the design coastal form. Next,
protection measures against coastal erosion, storm surges, and tsunami should be examined by using the plan’sbasic parameters. In this examination process, measures for dealing with coastal erosion, storm surges, andtsunamis need to be planned as a single integrated project, and there may be cases that call for reexamination aspart of this process. If it turns out that no effective measure can be planned as a result of the examination, then areview of the goals of coastal protection, environmental conservation, and utilization may be necessary.
The development of countermeasures against coastal erosion, storm surges, and tsunamis requires acomprehensive examination, since control measures for storm surge and tsunami control are likely to differ
depending on the target coastal features of the plan. For example, because the wave overtopping rate increases as
the beach profile becomes smaller due to erosion, the elements of the design coastal form are a prerequisite forstorm surge and tsunami control measures. Therefore, the long-term stability of the coastal features subject toprotection (i.e. the necessity for a coastal erosion control measure) needs to be confirmed before developing astorm surge or tsunami control measure.
A coastal protection plan is part of a seacoast preservation plan, and the seacoast preservation plan must be acomprehensive, well-integrated plan in which measures for coastal protection, environmental conservation, andutilization are in harmony.
The design scale should be determined in consideration of past storm surges and ocean waves, the current status69

of the subject seacoast, the beneficial and adverse effects of the project, and the importance of the project area.
Seacoast
preservation plan
Coastal environment
Living and growing
environment
Coastal landscape

Coastal protection
Design beach profile
Design sea
level,design ocean
wave and design
tsunamiSea coast utilization
Comfort
SafetyProtection measure
Coastal erosion
Storm surge
TsunamiSeacoast
preservation facility
arrangement plan

Fig.4-3 Positioning of coastal protection in a seacoast preservation plan

2.2 Factors of the design coastal form
The elements of the design coastal form should be determined as coastal features that are deemed necessary forcoastal protection, environment, and utilization, based on an analysis of natural conditions such as meteorology,
oceanographic phenomena, and topography, while taking into consideration the population, asset concentration,utilization, economy, etc. of the hinterland.

Explanation
The key elements of coastal form include backshore width, backshore height, foreshore width, foreshore slope,nearshore width, nearshore slope, and shoreline shape (Fig. 4–4). When determining the elements of the design
coastal form, coastal features that are necessary for maintaining and improving the functions (protection,
environment, and utilization) of the target seacoast should be determined.

70

Backshore
Backshorewidth

Nearshore

Foreshore
Foreshore
width

Revetment

Offshore

Nearshorewidth

Backshore height

H.W.L
L.W.L

Foreshore slope
Nearshore slope

Fig.4-4 Elements of coastal form
In view of coastal protection, a shore sectional area, rather than individual elements of the coastal form, should
be determined as the design shore elements in a storm surge or tsunami control measure in such a way that the
run-up height does not exceed the design height (see Fig. 4–5). For coastal erosion control, the national land baseline, the backshore width used as the foundation of the seacoast preservation facility construction, and theshoreline shape for control of the long-shore sediment transport rate should be determined. For coastal
environment conservation, the necessary elements should be determined on grounds of the backshore width
necessary for coastal vegetation and egg deposition, etc., Plant and animal habitat, and the coastal landscape.
Elements necessary for a seacoast utilization measure should be determined based on the regional culture andlivelihood as well as the number of seacoast users (see Table 4–1).
The sectional form of a shore constantly changes owing to short-term variabilities caused by ocean waves andtidal currents coupled with long-term variabilities such as erosion caused by imbalanced beach drift. In
determining the design coastal features, a shore design that permits long-term stabilization should be assumed.
The necessary elements of the design shore form must be ensured despite the predictable short-term variabilities.
If the current shore elements are not effective in securing the elements of the design shore form, static beachnourishment may need to be considered to secure them.

71Run-up height < Crest height of the embankment

Seashore profile when ocean waves are highCurrent seashore profile = Design seashore profile(a) Current profile is according to the conditions of the design profile
Run-up height > Crest height of the embankment

Design seashore profile

Seashore profile when ocean waves are highCurrent seashore profile(b) Current seashore profile is not according to the conditions of the design profile
Embankment collapse

Design seashore profile

Seashore profile when ocean waves are highCurrent seashore profile(c) Current seashore profile is unmatched to the conditions of the design profile remarkably
Fig. 4-5 Concept of the design seashore profile
If the design shore form dimensions cannot be met by the application of a coastal erosion control measure, or ifthey are insufficient as storm surge or tsunami control measures, then the design shore form elements or thegoals for coastal protection, environmental conservation, and utilization should be revised.

2.3 Design sea level
The design sea level should be determined based on tides, storm surges, seiches, secondary oscillations, etc.
The frequency, period, duration, etc. of waves must also be taken into consideration.
As a general rule, the design sea level should be one of the following:
1. Maximum high water in the past
2. High water + Largest sea level departure from normal in the past
3. High water + Estimated largest sea level departure from normal
In cases 2 and 3, corrections can be made based on the frequency with which high water and largest sea level
departure from normal occur simultaneously.

Explanation
Among the design sea level determination methods listed above, in cases 2 and 3, high water during the typhoonseason instead of high water can be used, as in the bays of Tokyo, Ise, and Osaka. When the estimated largest sea
level departure from normal obtained from a prediction formula or simulation using past meteorological data is72

employed, the Ise Bay Typhoon is often used as the design meteorological condition. For areas that arepresumably prone to receiving damage at extraordinary levels, such as Tokyo, Ise, or Osaka Bays, for example,
the Ise Bay Typhoon and other past typhoons are used to create models, and the design meteorological condition
is determined through storm surge simulations that assume several different courses. The provisory clause isstipulated to allow revision of the design sea level if, from past data, an examination of the frequency with whichhigh water and largest sea level departure from normal occurred simultaneously necessitates it. This would be
likely in such cases as when the design sea level drastically fails to meet the content of the actual plan.

2.4 Design ocean wave
The design ocean wave height and period should be determined from an estimate obtained from long-term
measured values, wave prediction using long-term meteorological data, or measured values for a neighboring
coast. In either case, significant waves should be used in principle.
The design wave direction should be the direction that is expected to most significantly affect the run-up height,
wave overtopping rate, and the scale and stability of structures, etc.

Explanation
The design ocean wave should be a deep water wave and should be determined, as a general rule, from measured
values. Generally, one of the following should be used as the design ocean wave for use in a storm surge control
measure or structural design:1.

Largest measured ocean wave

2.

Ocean wave predicted from an abnormal weather model3.

Probable ocean wave obtained through probability prediction based on statistical data estimated from
actual measurements or past meteorological data

As a general rule, significant waves should be used for ocean wave extremal statistics. Actual measurements
used for ocean wave statistics should preferably be those recorded over a considerable period of time (more than
10 years). However, since there are cases in which abnormal ocean wave occurrences are concentrated over
several years, careful analysis is essential when deciding to choose an observation period from the available data.
In cases where there is a lack of actual measurements, the following data or material may be used as
supplements:
1. Values obtained by using measured data to correct predictions computed from meteorological data recorded
over 30 recent years
2. Long-term estimates obtained through wave prediction from meteorological materials such as weather charts
When using measured data to correct estimates obtained from meteorological data, the measured data should befrom at least 3 years of records and should include a considerable number of abnormal ocean wave events.
However, if a wave is observed during an abnormal weather event that may or may not occur once in several
decades, and if that wave exceeds the estimated value, then the observed value may be used to determine thedesign ocean wave. Any records of abnormal waves from outside the period used for the estimations based on73meteorological data (e.g. before the period) need to be considered. Furthermore, if measured data and estimates
are available for a neighboring coast that has similar submarine topography, meteorological conditions,oceanographic phenomena, and other natural conditions, then the wave for the subject coast may be estimated byusing these materials as reference.
Depending on the submarine topography, tide level, or the way in which the coastal structures have been
installed, the wave height reaching coastal structures may not be the peak for the design ocean wave.
Consequently, it is necessary to consider waves that are lower than the design wave height.
When waves hit a coastal structure, those approaching perpendicular to the length of the structure are usually themost dangerous. However, if this is not the case—for example, when the properties of entering waves are largely
dependent on the angle of entry, or if there is an interception, or if the wind area is limited in a bay—then thedesign wave direction must be one that most affects the stability of structures, etc.
The ocean wave data used as part of a coastal erosion control measure are the wave height and prevailing wave
direction that yield the mean ocean wave energy based on actual measurements when the shoreline variability is
within the normal range. If there are large seasonal variations, ocean waves may be determined for each season.
If the variation is temporary, the maximum annual wave or waves hitting several times per year, as determinedfrom actual measurements, may be used.
When examining coastal utilization, ocean waves should be determined depending on the use.

2.5 Design tsunami
The design tsunami should be determined through examination of past tsunamis in consideration of thebeneficial and adverse effects of the project, the importance of the project area, etc.

Explanation
Because of the low frequency of occurrence of tsunamis, in many cases (except in some areas such as the
Sanriku coastal zone) sufficient data and experience from past tsunami disasters are not available for use indesigning a tsunami disaster prevention plan. In addition, the length of the tsunami recurrence interval to beconsidered in determining the design tsunami has not been clearly specified, and the frequency of occurrence andscales differ considerably from region to region. For this reason, the wave height of the design tsunami should
generally be determined from examinations of past tsunamis and in consideration of the beneficial and adverse
effects of the project. In some cases, the maximum run-up height of a past tsunami is used as the design run-up
height.
Since tsunamis cause tremendous damage very quickly, when the project area is an area of high importance, anassumption should be made about the magnitude and hypocenter of the earthquake. From this assumption, and inconsideration of the run-up height estimated through numerical simulation, the design tsunami should bedetermined. In cases where there are no reliable data available or where the coastal geometry or facility
arrangement has been changed because of the installation of a new coastal structure, etc., then the sameprocedure should be followed. For a region that may be hit by a tsunami occurring in an inter-plate seismic gap,a tsunami assumed with a virtual fault model may be used, since the details of past tsunami events are not known
in many cases.
74

Because tsunamis have extremely long wavelengths and their propagation even affects the submarine
topography—including abyssal floors—non-ria coasts may also receive serious damage. Although tsunami
disaster-prevention measures are usually designed from past tsunami marks, or heights, when a measure isdeveloped from an assumed tsunami, tsunami deformation in shallow waters needs to be considered.
Because tsunamis are a low frequency phenomenon, there is no need to include the external forces targeted inmeasures against storm surges.

2.6 Basic policy for coastal protection
2.6.1 Overview
Coastal erosion, storm surges, and tsunamis should be considered in a comprehensive manner in designing acoastal protection measure. Coastal protection needs to be in harmony with the target coastal environment andutilization. The development of coastal erosion control measures must precede storm surge and tsunami control
measure considerations, since the design shore form elements, which are preconditions of storm surges andtsunamis as well as of the coastal environment and its utilization, must be determined in the process ofdeveloping a coastal erosion control plan.

Explanation
A basic policy for coastal protection should define the goals and means of protection against coastal erosion,
storm surges, and tsunamis. These should not be developed separately, but need to be designed as integral partsof a comprehensive measure (Fig. 4–6). As this flow diagram indicates, in developing a coastal protection
measure, the design shore form and the current shore form should be compared, and the need for a coastal
erosion control measure examined, to ensure that the design shore form elements, which are preconditions forstorm surge and tsunami control plan development, are obtained. For a seacoast for which it has been judged thata measure is necessary, first, a coastal erosion control measure and then a measure for securing the design shore
form should be examined. Next, it should be determined whether a storm surge or tsunami disaster prevention
measure is necessary against the design tide level, the design wind wave, and design tsunami; if a measure isrequired, it should then be examined. At this point, if the measure is expected to adversely affect the maintenanceof the design shore form, then the coastal erosion control measure needs to be reexamined. There may be cases
in which the design shore form elements constrain the content of the measure against storm surges. In such cases
there is a need to revise the design shore form elements and establish a more appropriate plan. ,The need may
also arise to modify the design shore form elements in the process of developing a coastal protection-related planin view of coastal environment conservation and utilization factors.

75

Design seashore profile

Evaluation of the current and the
Measuresfuture conditions through theare requiredcomparison with the design seashore
profile.
Coastal erosion
control measures- If the condisions of the design
seashore profile are required tobe rivised.
- If the design seashore profile isaffected by the countermeasures.

Measuresare required

Evaluation of the current seashore
profile with the design sealevel andthe design ocean wave.

Measuresagainst storm
surge
Measuresare required

Evaluation of the current seashore
profile with the design tsunami.

Measuresagainst tsunami
Assessment of the influence ofmeasures on the environmental
conservation and utilization of
seashore.

- Review of the measures.
- Review of the planned target ofprotection, environmental
conservationi and utilization of
seashore.

Establishment of the seashore
protection measures

Fig. 4-6 Flow chart of the seashore protection planning
Through this process, a seacoast preservation plan should be optimized during its development with adjustments
and coordination between measures against coastal erosion, storm surges, and tsunamis. The same optimization
process through periodic examination is indispensable for coastal protection, environmental conservation, andutilization measures.
In particular, examination of coastal erosion to confirm the design shore form elements needed for coastal
protection, environmental conservation, and utilization must be included as a precondition for developing any
seacoast preservation plan.

2.6.2 Coastal erosion
A coastal erosion control plan aims to attain the design shore form elements, where appropriate, through (static)
beach nourishment; improvement of the sediment regime; and preservation of the design shore form elements
by ensuring well-balanced littoral sand drift through (dynamic) beach nourishment.
The project area, in principle, should be a littoral transport system, but it may include coastal land areas asappropriate.

Explanation
A coastal erosion control measure aims not only to prevent loss of national land and disasters caused by
overflowing or wave overtopping, but also to pass down precious coastal environments to future generations andsecure utilizable coastal spaces. To this end, the design shore form elements should be achieved through (static)
beach nourishment. Equilibrium of sediment transport rate should be restored through sediment regime
76improvement, construction of a littoral transport control facility, or (dynamic) beach nourishment so that thedesign shore form elements can be preserved.
Figure 4–7 shows the typical flow of coastal erosion control planning. In designing a coastal erosion control plan,
it is necessary to first specify a littoral transport system with a closed sediment budget, except in cases where thesediment is discharged to a submarine canyon or deep sea. Figure 4–8 shows the pattern of the sediment budget
in a littoral transport system. The primary contents of the sediment budget in a littoral transport system are: 1)
sediment supplied from the river and cliffs; 2) sediment discharged out of the zone as blown sand; 3) sedimentflowing in or out the zone as littoral drift sand; 4) sediment flowing in or out the zone as cross-shore drift sand;5) sediment discharged to a submarine canyon or the deep sea; and 6) sediment artificially moved out of the
zone.

77

Fig.4-7 Flow chart of erosion control planning

78

Loss to submarine valley
Sediment supply from sea cliff

Retrogression of sea cliff

Sediment supply from riverLong shore drift

Long shore drift

Long shore drift

Sandy shore
Loss as blown sandFig.4-8 Schematic diagram of sediment balance of seashore
Next, from the status of past and present sediment budgets, the erosion mechanism should be elucidated, efforts
to improve the sediment regime should be made through coordination with relevant organizations and bodies,and a future target sediment regime should be set. To this end, it is important to grasp the sediment regime
evolution as a longshore sediment transport rate distribution or a sediment budget. The sediment regime should
be investigated at points above and below the point where sediment change its features as a results of flows into
the target littoral sediment transport system, the point where sediment is discharged out of the system, and thepoint where littoral sand drift is changed by a coastal structure, etc.
Because changes in coastal topography due to impacts such as environmental changes (e.g. changes in wind
waves and sediment supplies) may be delayed, occurring over an extended period of time, an erosion mechanism
is often associated with multiple factors from different generations. When investigating an erosion mechanism,
the history of the sediment environment of the river basin and coastal zone must be examined and analyzed in acomprehensive manner, and the sediment volume supplied to the coast and the littoral sediment movement mustbe elucidated.
In cases where improvement of the sediment regime only is not effective for maintaining the design shore form
elements from social and economic points of view, these elements must be maintained through the constructionof a littoral sediment management facility or (dynamic) beach nourishment. The feasibility of installing aplanned sediment management facility and using (dynamic) beach nourishment measures should be examinedfrom economic and environmental viewpoints. If they are not feasible, then these measures should be
reexamined and the design shore form elements and the goals for coastal protection, environmental conservation,and utilization should be modified as appropriate.
In examining a series of coastal erosion prevention measures, it is necessary to harmonize the measures withfactors related to the coastal environment and utilization and to take full advantage of the various properties of
the target seacoast.

79

Sediment supply(small amount)Long shore drift

Erosion control measures
Sea cliff(Erosion)

Sediment
Discharge(small amount)

Decreasing of sediment supply

Wave

Break waterLong shore drift

Sedimentation

Long shore drift

Expansion of eroded area

Disruption of sediment transport

Incident wave

Break water

ErosionSedimentation

Formation of a shielded area

Fig.4-9 Patterns of coastal erosion
Coastal erosion is caused mainly by land subsidence or imbalances in longshore sediment transport. Erosion
caused by the latter may arise from the following mechanisms:
1. Reduction of sediment supply from rivers and coastal cliffs (Fig. 4–9 (a))
Sediment discharged from river mouths decreases because of various factors, such as the presence of erosion
80control dikes located in the upper reaches of rivers that discharge into the subject seacoast, sedimentation indams, reduced sediment discharge associated with reduced flood frequency, increased sedimentation in the riverowing to the presence of groundsills, etc., and decreased river sediment loads due to gravel quarrying. In aseacoast that has been maintained by sediment supplies from coastal cliffs, coastal cliff erosion control measures
will cause a sediment budget imbalance, resulting in erosion.
2. Disruption of littoral sand drift continuity (Fig. 4–9 (b))
If structures such as a breakwater, training dike, landfill revetment, jetty, or offshore dike are installed in aseacoast where littoral sand drift is prominent, then part or all of the coastal sand drift will be blocked anderosion will occur down current of the structure.
3. Dredging and gravel quarrying
If excavation is undertaken offshore by dredging of navigation channels or estuaries, or by gravel quarrying in an
estuarine or sea area, drift sand may be trapped in the excavation marks, or beach profile changes that fill the
excavation marks may occur, resulting in erosion.
4. Shielded area formation (Fig. 4–9 (c))
Even on seacoasts with little littoral sand drift, where the wave approach angle is almost perpendicular to theshoreline, a shielded area is formed behind a large-scale breakwater or pitched island installed in the sea area.
Sediment moves from the outside to the inside of the shielded area near the shore, resulting in erosion. On theother hand, on seacoasts with little apparent sediment transport due to seasonal variation in the wave approachangle, the sand drift direction is not reversed even when the wave direction is reversed within the shielded area.
Therefore sediment deposited as a result of littoral drift sand being blocked by the anthropogenic structure
cannot move out of the shielded area. This causes an imbalance in the sediment transport rate, facilitating the
rapid development of erosion.
Since the short-term changes on a beach dominated by cross-shore sediment transport are, in most cases,
reversible processes, and can be treated as probabilistic, the variability range should be reflected in the elementsof the design shore form. In addition, since the occurrence of non-reversible cross-sectional changes has been
confirmed, care must be taken when interpreting the results of actual section measurements.

2.6.3 Storm surge
A storm surge control plan aims to prevent wave overtopping and inland seawater flow by using embankments
and wave dissipation structures for the design tide level, design wind wave, and design shore form elements,
thereby protecting human lives, assets, and activities in the area subject to protection.
The mitigation of disasters caused by external forces exceeding the design scale should also be counted.

Explanation
Storm surge countermeasures are aimed at preventing disasters caused by storm surges and high wind waves.
81

The “storm surge” phenomenon is an abnormal rise of the tide level in marine coastal areas because of
barometric depression and winds caused by typhoons, low pressure systems, etc. However, “measures against
storm surges” target both abnormal tide levels and high wind waves caused by strong winds. Patterns of damage
caused by storm surges and high wind waves are classified into the following two categories: 1) damage fromflooding in the sheltered area due to wave overtopping and inland seawater flow, and 2) damage caused by theenergy from the overtopping sea water and by inflowing objects. The objects to be protected can be divided into
three categories: 1) human lives, 2) assets, and 3) activities such as business activities and transportation.
In most cases, the milder a coastal slope is, the greater the wind setup becomes. Seacoasts with mild slopes, suchas those of the bays of Tokyo, Ise, and Osaka and the Sea of Ariake, have historically received serious,
large-scale storm surge disasters. On the other hand, on seacoasts such as that of Suruga Bay, which ischaracterized by a steep submarine topography, although the wind setup is comparatively small, because storm
surges can approach without being attenuated by the submarine topography, the embankments needed are higher
than for the aforementioned enclosed bays.
The preliminary basic research that must be performed before a storm surge countermeasure is developed should
be conducted as described in the Research volume on Technical Criteria for River Works (Proposal). Not only
natural conditions, such as climate and oceanographic phenomena, but also social conditions such as the
population and industries in the project area should be studied. In particular, disaster research needs to be aimed
at grasping not just the hydraulic data but also detailed information on the actual situation so that it can be usedeffectively as the basis of determining the design scale.
It has been confirmed that wind waves attacking a seacoast at the time of a typhoon, etc. include long-period
waves whose period is as long as several minutes, and these are largely assumed to be the cause for high run-up
heights and wave overtopping. However, because there have not been sufficient data available to justify thequantitative treatment of long period waves, here, a storm surge countermeasure should be developed from thedesign tide level plus the effect of the design wind wave, as has been conventionally done. Efforts are needed togather data to be used to grasp the characteristics of long period waves in accordance with the relevant
descriptions in the Research volume.
Before a storm surge countermeasure is developed, coastal erosion should be examined to ensure that the design
shore form elements will be secured.
In establishing a storm surge countermeasure, in view of the damage mitigation against external forces
exceeding the design scale, not only structural measures of facility construction or improvement but also
non-structural measures such as rapid information transfer for effective evacuation should be examined.
The content of a storm surge countermeasure needs to be harmonized with coastal environment conservation andutilization factors so that various characteristics of the subject seacoast can be fully utilized.

2.6.4 Tsunamis
A tsunami control measure should aim to mitigate damage to human lives and assets as well as influences on
activities in sheltered areas by the construction of a coastal dike designed in accordance with the design
tsunami.

82

Mitigation of the damage from external forces exceeding the design scale should also be taken into
consideration in the design of a tsunami control measure.

Explanation
A tsunami control measure should aim to prevent tsunami disasters. Patterns of damage from tsunamis can beclassified into the following two categories: 1) damage from flooding in the sheltered area due to wave
overtopping and seawater inflow; and 2) damage caused by the energy of overtopping seawater and frominflowing objects.
Surrounded by quake-prone marine areas, Japan has often received damage from tsunamis since ancient times.
Tsunamis are caused by a sudden deformation of the sea floor and seawater surface (e.g. fault movement,
topography failure, meteorite fall, etc.) and are often caused by submarine earthquakes. Tsunamis can be divided
into two categories: 1) far field tsunamis that reach the coast in several hours after the occurrence, and 2) nearfield tsunamis that reach the coast within several tens of minutes after the occurrence. The frequency oflarge-scale tsunamis hitting a particular region is extremely low, although once they do hit, they cause
destructive damage to the region, resulting in serious disasters. Although studies are being conducted on thecharacteristics of tsunami-causing earthquakes, so far there has not been enough progress to make it possible topredict the occurrence of tsunamis. On the other hand, seismic gaps at plate boundaries have been recognized asorigins of the earthquakes likely to cause tsunamis.
To develop a tsunami control plan, research is needed into natural conditions such as the topography and social
conditions such as the population and industries of the project area. When considering the establishment of anevacuation system, etc., it is necessary to investigate the land uses in the subject coastal area, including thesuitability of coastal buildings to the coastal environment. Research on past disasters, in particular, needs to be
aimed not just at gathering hydraulic data, but also at investigating the actual circumstances under which the data
were collected. Since human memories and experiences of past disasters can easily fade with time, special
attention should be paid to the collection and storage of data and materials as sustainable records.
In view of the need to mitigate the damage caused by excess external forces and establish risk management,
there is a need to construct or improve coastal protection facilities such as embankments and breakwaters. Thereis also a need for comprehensive measures that combine these structural measures with non-structural measures
such as the establishment of a disaster prevention system in cooperation with communities in the subject region;
the securing of areas for evacuation and escape routes; evacuation guidance for users; and adjustment of land
uses. Furthermore, the occurrence of earthquakes with relatively small tremors that cause major tsunamis(tsunamigenic earthquakes) needs to be taken into consideration in the design of prevention measures.
Since the occurrence of tsunamis is not associated with other oceanographic phenomena, measures for fast
recovery after coastal protection facilities have been destroyed by tsunamis need to be established for seacoasts
where storm surges or high wind waves often occur.
The content of a tsunami disaster prevention measure needs to be harmonized with coastal environment
conservation and utilization factors so that the various characteristics of the subject seacoast can be fully utilized.

83Section 3 Basic Items Associated with Coastal Environment Improvement andPreservation
3.1 Overview
The aim of coastal environment improvement and preservation is to protect or restore the ecosystem andlandscape of the subject coast in harmony with coastal protection and utilization.
Goals for coastal environment improvement and preservation should be based on the current and past status of
the coast as well as the situation on neighboring coasts. The goals should aim to preserve and restore the diverse
elements of the coastal environment, including beaches and cliff-lined coasts, while making efforts at consensus
building among the parties concerned.

Explanation
Seacoasts are an interface between the land and the sea. They provide diverse habitat for animals and plants,
such as beaches (including tidal flats) and coastal cliffs, and there are many indigenous animals and plants thatare dependent on these peculiar natural conditions. There are also seacoasts where beautiful natural landscapes,
such as those with white sand and green pines, are seen, or where natural parks are located. The primaryenvironmental functions of seacoasts that need to be improved and preserved include: 1) a space for animals to
live, breed and raise young; 2) biological and physical water purification; 3) physical and mental healthimprovement; 4) a space for environmental education; and 5) carbon dioxide sequestration function.
A coastal environment improvement and preservation plan should cover not only the subject seacoastpreservation area but also the neighboring coast, hinterland, and offshore sea areas, whenever this is necessary.
In coastal environment improvement and preservation, behaviors that have adverse effects on the coastal
environment should be avoided if at all possible. Efforts should be made to protect the coastal environment, andlost natural conditions and landscapes should be restored as required. To this end, construction or improvementof seacoast preservation facilities should be implemented in accordance with the natural characteristics of theindividual seacoast. Also, in view of the need to procure living and growing spaces for animals and plants, it isnecessary to examine the existing facilities with a view to rebuilding them to be friendly to the coastal
environment.
As shown in Fig. 4–10, trade-offs may occur between coastal protection, environmental conservation, andutilization factors. In addition, because a coastal environment is composed of diverse elements, the value
judgments of those elements are diverse. For this reason, if the target of the plan is limited to particular elements,
the plan may adversely influence other elements and invite conflicts of value. To minimize these trade-offs andconflicts and set coastal environment goals sensitive to the natural and social characteristics of the subject area, it
is important to build consensus between the concerned parties (seacoast administrator, local government, experts,
community members) and implement preservation and restoration measures through their collaboration (Fig.
4–11).

84

[The case of protection conscious][The case of environment conscious]Protection

Protection
Constraints on construction
or improvement ofprotection facilities

Protection facilities
affect environment

Environment

Constraints on useof seacoast or
construction of
convenient facilities

Protection facilities
inhibit use of seacoast

Environment

Use

Use

Fig.4-10 Trade-offs between coastal protection, environmental conservation and utilization
Data and information on the subject seacoast environment should be collected, documentated, and analyzed. By
providing and publicizing the results, such data and information should be shared between the concerned parties
to bring about a common understanding of the coastal environment to be preserved. Cooperation with otherprojects, establishment of a support network with experts and NGOs, and application of adaptive management,
etc. are also important. In the process of developing the plan, the "Guidelines for Promoting Coastal
Development in Symbiosis with Nature" would be a useful reference.

Fig.4-11 Seacoast development well-balanced in protection, conservation and utilization

3.2 Preservation and restoration of hospitable habitat for animals and plants
Plans or measures for the preservation and restoration of sustainable habitat for animals and plants should bedesigned from research conducted in the past or in the course of plan development. The plans or measures
should aim to preserve a highly stable living and growing environment to sustain diverse fauna and flora.
Efforts should also be made to restore the environment if this is necessary.

85Explanation
Preservation and restoration of sustainable habitat for animals and plants should be evaluated from the following
three points of view: 1) stabilization of sustainable habitat for animals and plants; 2) smooth matter cycle; and 3)
biodiversity.
1. Stabilization of sustainable habitat for animals and plants
Such habitat should be regarded as having been stabilized if utilized by indigenous species with no significant
decrease or increase in the number of individuals for several years or longer, or if the numbers of individuals of
those species match the constituent ratios appropriate for the habitat. Because changes in the situation of ahabitat greatly influence the species living in that habitat, the degree of balance between the factors forming thehabitat, the bed material and water quality, and other significant factors should be determined.
2. Smoothness of material cycle
The material cycle mechanism should be comprehensively elucidated from an examination of various
components, including inflow from rivers, seawater exchange volume, biological production volume, internal
production rate, settling volume, solution volume from the sea floor, solution rate, etc.
3. Biodiversity
The degree of biodiversity should be determined from the numbers of indigenous species higher and lower in thefood chain in the habitat. In this process, not only the variety of the entire biocenosis but also the varieties ofindigenous species and their abundance as a proportion of the entire community should be considered, and it isalso necessary to take heed of rare species, the coastal environment status, and the conditions enabling
preservation of region-specific species.
Depending on the substrate material, seacoasts are categorized as mud beach, sandy beach, shingle beach, coral
reef, or cliffed, and each habitat has a peculiar ecosystem with different characteristics. For example, sandy
beaches, provide egg-laying and nesting habitats for animals such as sea turtles and little terns. Mud beaches
provide habitat for a diverse range of organisms and feeding sites for wild birds. Seaweed beds and forests
support the basic production of the marine food chain and also provide spawning sites and shelter for fry.
Furthermore, seacoasts are composed of zones parallel to the shore: offshore, nearshore, foreshore, andbackshore. The functions of these zones, whether for breeding, growing, or living, depend on the space, anddifferent spaces are closely interrelated. An ecosystem is formed thanks to the presence of these diverse spaces,
and therefore it is important to understand their ecological functions and how they are related to each other as a
network.
Ecosystems contain an extremely wide variety of species, and it is not possible to grasp them all. An effective
approach is to select a target species or community that has an important function or can act as an indicator for
preservation of the entire ecosystem and then examine the preservation of the community or species.
It is impossible to grasp any ecosystem thoroughly, and because of the uncertainty it is also impossible to predict
all influences. For this reason, it would be effective to adopt adaptive management for correction as needed,while using monitoring to identify effects and influences.
86

In the case of the use of cars on a beach or spoilage of the beach by the trampling of habitat, which can destroy
habitat for animals and plants, it is necessary to consider restrictions on when and how the beach should beavailable to users.
It is not possible to quantify the effects of coastal protection facilities on an ecosystem. However, the extinctionof coastal vegetation and destruction of nesting sites for sea turtles have been reported from the construction ofsea embankments and revetments. Detached breakwaters have been reported to obstruct turtle access to thebeach for nesting. On the other hand, the presence of reef organisms on or around detached breakwaters,
seaweed propagation on the crowns of man-made reefs, and the presence of fish around new types of detachedbreakwaters have also been reported. In designing a coastal protection facility construction or improvement plan,
it is necessary to examine records of past cases to determine how the plan or facility would influence the subject
coastal environment.

3.3 Preservation and restoration of coastal landscape, etc.
Preservation and restoration of the coastal landscape should be viewed in combination with consideration forprotecting forests, streetscapes, etc. in the hinterland. Efforts to preserve the landscape and its characteristic
components, such as the sea surface, horizon, shoreline, and land area, should be made. Landscape restoration
should be implemented if necessary.
Elements of coastal flavor, such as the sound of the waves and the tang of sea air, also need to be taken into
consideration.

Explanation
Coastal landscapes, as typified by white sand and green pines, are deeply and extensively related to theformation of the culture and identity of Japan as a maritime country. At the same time, Japan is prone to natural
disasters caused by such phenomena as storm surges, erosion, and tsunamis, and with extensive utilization ofcoastal land, disaster-prevention measures are indispensable for the stabilization of people’s livelihoods. For thisreason, consideration of the coastal landscape was likely to have been lacking in the scope of disaster-prevention
measures that were hastily established and implemented within limited budgets. A seacoast preservation plan
must identify diverse values of the subject seacoast and comprehensively evaluate the coast’s values. It should
also aim to preserve or restore the coast from these values.
The landscape of a natural seacoast is composed of the sea surface, islands in the sea, shoreline, and thehinterland as integral elements. Some coastal protection facilities may affect the water surface, horizon, coastal
shape, and other elements of the natural seacoast landscape, reducing the coast’s value. For this reason, in
selecting the type of coastal protection facility, it is necessary to fully understand the natural agents that form thecoast and its value, and efforts should be made to preserve and restore the coastal landscape wherever necessary.
Adjustable measures should be also necessary for preservation or restoration of the hinterland landscape.
There are also cases that necessitate the artificial creation of a landscape through proactive utilization of seacoastpreservation facilities, etc. Since simultaneous landscaping of the hinterland is often required in many cases,
special consideration is needed for integration with the hinterland. Furthermore, artificial formation of alandscape should also be based on an understanding of the natural environment and implemented within an87adaptive and controllable scale.
A seacoast influences people in various ways through their five senses, thus creating coastal values. The primaryelements are the landscape, which is perceived visually; the soundscape, which is perceived aurally; and the tangof the sea air, which is perceived by the sense of smell. All of these contribute to the creation of an atmospherethat is unique to each individual seacoast. In principle, these elements should preferably be preserved as one unit.

Section 4 Basic Items Associated with Coastal Utilization
Coastal utilization should aim to secure safety, comfort, and convenience, depending on the characteristics anduse pattern of the subject seacoast. The utilization function should be preserved and improved in harmony withcoastal protection and environmental conservation.
Coastal utilization zones should be based on the coastal utilization pattern, and regulations and guidance forappropriate use by the general public should also be taken into consideration. For facilities used by the generalpublic, user safety needs to be considered.

Explanation
Since ancient times seacoasts have been used for festive and other cultural events by regional communities.
Seacoasts have played important roles in the formation and succession of regional cultures. With the recent
advancement and diversification of people’s needs, seacoasts have been used for various types of sport andrecreation, for experiential and learning activities, and for seaside healing and relaxation. They are also usedconventionally as bathing spots.
Since seacoasts are prone to natural influences, their use has been limited. For this reason, installation or
improvement of facilities that increase the convenience of use may be necessary to enhance the utilization
function of seacoasts. Coastal convenience facilities include access to the seacoast, parking lots, public lavatories,
rest areas, and information facilities, etc. In the installation of convenience facilities, the question of who will be
the constructing or improving body and managing body needs to be considered.
In cases where access to the seacoast is hampered by the presence of a coastal embankment, access to the coast
should be ensured by installing a stepped structure or a gently sloped embankment, as appropriate. In developing
a plan for securing coastal access, barrier-free measures such as installation of a ramp should be considered sothat the elderly and disabled people will have access to the seacoast. This will enable them to easily andcomfortably visit the coast and come into contact with coastal nature in the course of their daily lives. Rubbishthat has drifted ashore ranges widely from natural objects such as driftwood to anthropogenic objects such as
plastics and resin pellets; enormous quantities of rubbish drift ashore. There is also illegal waste dumping andlittering by coast users (e.g. empty cans, cigarette butts, fishing tackle, etc.). Since these interfere with
comfortable use of the coast, the division of collecting responsibilities needs to be clarified among the parties
concerned, and measures to enhance a sense of public responsibility among coast users and residents in river
basins need to be implemented to control the generation of rubbish.
There are many cases in which the construction or improvement of coastal protection facilities and utilization
conflict with each other, as exemplified by the construction of coastal protection embankments preventing accessto the coast, or of detached breakwaters or artificial reefs preventing fishing or surfing. There are also problems
88

associated with harmonization between coastal protection, environmental conservation, and utilization, such as
the destruction of coastal vegetation or nesting sites by vehicles driven on beaches. Moreover, friction arises
from the simultaneous different use of beaches and coasts, for example, for sea bathing, surfing, jet skiing,fishing, etc.
In planning a coastal use adjustment measure, the functions necessary for the relevant community should bedetermined from the current uses of the subject coast and the needs of users. In consideration of the need forcoastal protection and preservation of the coastal environment, conflicting uses of the coast should becoordinated with each other, safe use of the coast should be guided, and the coastal utilization patterns that meetthe characteristics of the subject coast should be determined. It may also be necessary to restrict utilization forthe purpose of coastal environment preservation.
When introducing a new coastal usage pattern, it is necessary to examine whether safe and comfortable use ispossible in view of the natural properties of the coast; whether it will conflict with the existing usage pattern; andwhether it will adversely influence the coastal environment. When a coast is used in multiple ways, the space
and time for each usage should be specified to prevent accidents and friction among users.
In determining the coastal usage pattern and areas for use, opinions should be gathered from seacoast users,
regional community members, fishery operators, and ecosystem experts, etc. whenever this is necessary.
Although a seacoast can be used by anyone freely at their own risk and responsibility, the natural conditions canlead to people being involved in accidents due to the occurrence of nearshore currents, sudden tidal waves, etc.
When coastal protection facilities are made available for public use despite the risks, high safety levels should beensured during the use of these facilities. This necessitates the incorporation of safety measures into the designof the coastal protection facility whenever such measures are deemed necessary in consideration of the external
forces that are at work, the performance conditions, costs, and durability.
Some seacoasts may not be suitable for use because of their ocean currents, topography, and other natural
conditions, or because coastal structures may have been built on them. Also, there are cases in which a seacoast
may become dangerous because of the occurrence of tsunamis. For these reasons, the provision of sufficient
safety information, education of seacoast users, etc., and cooperation with coast-related organizations and bodies
are important for safe coastal utilization.
Reference
1) Guidelines for Promoting Coastal Development in Symbiosis with Nature. Written and edited by the ResearchInstitute for Promoting Coastal Development in Symbiosis with Nature. Supervised by the Seacoast Office, Land
Conservation Division, River Bureau, Ministry of Land, Infrastructure and Transport (MLIT) of Japan. Issued by
the National Seacoast Association, p. 73, 2003

89

Chapter 5 Information Sharing and Cooperation with Communities inRiver Basins
Section 1 General Information
Water and sediment management needs to be implemented in close cooperation with organizations and bodies
and concerned parties in the subject river basin. Such management exceeds the sole capacities of administratorsand corporations associated with the river. With changes in the riparian environment, the increasing interest of
the general public in administrative information, and the advancement and promotion of information
technologies, we are facing a new challenge for the realization of a society where shared water-related
information is readily available. In view of this arising social demand, it is vitally important to promote theestablishment of a safe country with diverse cultures through the sharing and utilization among members of thegeneral public of all types of water- and sediment-related information that are collected and organized.

Explanation
The goals of the basic policy described in Chapter 1 cannot be attained through the sole efforts of theadministrators of rivers, sediment and erosion control measures, and seacoasts, or of related corporations. It istherefore important to establish cooperative and collaborative relationships between the organizations and bodies
in the river basin as well as with concerned parties. It is also important to monitor changes in the natural andsocial conditions in the basin and to implement various projects in concert with all related parties.
Water is indispensable for all life and is a vital resource for securing comfortable and cultural lifestyles for thegeneral public, for promoting agricultural and industrial development, and for maintaining cleanliness in ourdaily lives.
For this reason, the numbers of people seeking a sound water environment and the level of public concern forwater-related information have been increasing in recent years.
In the 21st Century, further changes in the global environment relating to water and resulting problems from theviewpoint of securing a healthy water cycle are anticipated. These changes include the occurrence of heavy,
intensive rainfall events that exceed the design scale because of the wider variability of the meteorologicalenvironment (attributable to global warming, etc.); unexpected disasters; exacerbation of desertification; and theoccurrence of acid rain. They also include a reduction in the amount of water usable as a resource, and water
shortages. In addition, water safety is at risk because of the diffusion of toxic substances such as dioxin via
water.
With its steep mountain ranges and frequent typhoons, Japan is in a geographic and meteorological situation
whereby it must be consistently prepared for flood disasters, droughts, and sediment disasters, etc.
With the adoption of the recommendations of comprehensive studies on the revision of official school
curriculum guidelines, contributions from the disclosure of river-related data on hydrology, water quality, andecological systems are expected.
Also, from the viewpoint of preserving a healthy water cycle, river basin administrators are seeking the
integrated management of water-related information, such as on water intake and discharge and groundwater.
Thanks to the recent improvement of information infrastructure (e.g. by the construction and expansion of90

fiber-optic networks) and the advancement of new technologies such as GIS and GPS, the provision anddisclosure of information to the public have been accelerated, and an environment for providing enormous
amounts of visual and other data to the general public in an easy-to-understand manner is now taking shape.
The general public is now demanding a higher standard of river administration services, including streamlined
administrative structures, more efficient river management through proper implementation of information
gathering and management, and the realization of accelerated and transparent tendering and contracting
procedures.
In view of this situation surrounding river administration, we need to develop more advanced and efficient riveradministration services that will enable the sharing of water-related information by the general public.

Section 2 Cooperation for Disaster Prevention and Mitigation
2.1 Overview
The current status of flood control facilities in Japan has not reached a satisfactory level. A huge amount ofexpenditure over a prolonged period is required before sufficient flood control at the design scale can beachieved. Since water and sediment disasters are caused by natural phenomena, there is always a risk ofdisasters occurring that exceed the design scale.
Cooperation with communities in the river basin can bring more economical and effective results than whenflood control facilities only are used. For these reasons, to mitigate damages to human lives and assets by waterand sediment disasters as much as possible, and to promote river improvement and sediment disaster prevention
and coastal protection works, flood control measures that aim to reduce flood runoff volumes, minimize flood
damage and the damage caused by storm surges, tsunamis, wind waves, and sediment disasters, and establish
evacuation systems must be developed and implemented in cooperation with communities in the subject river
basin.
Development and implementation of flood control measures in cooperation with communities in the river basincannot be realized through the efforts of river administrators alone. It is indispensable for these administrators
to collaborate closely with other administrative organizations and bodies involved in land use, city planning,
sewerage, housing, agriculture, and forestry as well as communities in the basin and the mass media, etc.

Explanation
In planning measures for disaster prevention and mitigation, it is always necessary to take into account thepossibility that external forces leading to events exceeding the design scale of the flood control facility may
occur. This is because the collection period of hydraulic and hydrological data on which planning is based is alittle over 100 years at most, and therefore not long enough to enable prediction of the most extreme events withlong return intervals. Another reason is that using flood control facilities alone against external forces ofextraordinary scale with low probability of occurrence requires more land and facilities than using flood control
facilities in combination with measures implemented by communities in the river basin. The former approach is
not necessarily economic or efficient.
For this reason, it is necessary to actively exchange sufficient information on flood disaster countermeasures91with relevant organizations and bodies, community members, mass media, etc. during normal times, and, at thesame time, it is important to promote due consideration of disaster prevention in land use and city planning. It isalso necessary to establish information sharing and cooperative systems to smoothly carry out flood prevention
and evacuation activities in times of disaster.
For example, in future development and land use planning it is important to consider preserving the water
retention and retarding functions of the river basin. In addition, it is necessary to establish appropriate role
sharing among communities in the river basin and to actively plan and implement measures over the entire river
basin, including the securing of storage and infiltration functions coupled with city planning.
For a region with a concentration of population and assets, it is desirable to also consider the promotion of city
planning in combination with the installation of flood- and earthquake-resistant embankments of a high standard
that allow easy access to the river and maintain high scenic value.

2.2 Measures in flood runoff areas
To control increases in runoff due to development in the river basin, measures to optimize storage, infiltration,and retention functions inherent to the flood runoff area should be actively promoted in cooperation with
communities in the basin.

Explanation
Measures in flood runoff areas include, among others:
・ the preservation of natural land uses (such as forestry and agriculture),
・ the maintenance of retarding areas (such as paddy fields),
・ the introduction of facilities that promote rainfall infiltration (such as permeable pavements andpermeation-type sewerage systems),
・ the storage and utilization of rainwater, and・ the installation of reservoirs for flood disaster prevention.
To reduce flood runoff volumes, these measures in the flood runoff areas must be actively promoted incooperation with communities in the river basin.

2.3 Measures in flood-prone areas
To mitigate disasters caused by flooding, measures for controlling flood flows and maintaining flood-retarding
functions should be actively promoted in cooperation with communities in the river basin.
Especially in regions and facilities that make use of underground areas such as underground shopping areas,
underground railroads, and basements (since these areas are at high risk of serious damage and fatalities),
subsurface construction and underground utilization and improvement that take inundation into consideration
should be promoted, together with the construction and improvement of inundation prevention and emergency
drainage facilities and the establishment of evacuation systems.

92

Explanation
Measures in flood-prone areas include measures enabling residents to protect themselves, non-structural
measures, and measures for subduing flood flows. Flood-flow control measures include the construction of ring
levees, secondary levees, open levees, overflow-resistant levees, roads with raised heights, tree belts, and flood
prevention forests. Direct self-protective measures include residential land raising, construction of houses withelevated floors, installation of watertight doors, and possession of emergency boats. Non-structural measures
include the introduction of inundation-tolerant land uses, establishment of warning and evacuation systems, andobtaining water damage insurance. To mitigate damage due to flooding, these measures should be actively
promoted in the flood-prone area in cooperation with the communities in the river basin.

2.4 Flood fighting
A flood fighting measure allows the maximum utilization of flood control facility functions and is extremely
important for flood disaster mitigation. The primary responsibility for flood fighting rests with municipal
governments (or district flood fighting authorities or flood-fighting associations). However, since effective andappropriate flood-fighting activities are impossible without the cooperation of river and seacoast administrators,
close collaboration between municipal governments, district flood fighting authorities, flood-fighting
associations, and river and seacoast administrators is necessary in all phases of flood fighting. At the same time,
river and seacoast administrators need to proactively participate and be cooperative in the establishment offlood fighting plans from a broad-based point of view so that flood disasters extending beyond the territories ofmunicipalities and prefectures can be properly dealt with.
River and seacoast administrators should promote, in an appropriate manner, the securing of resources needed
for flood-fighting activities conducted in times of flood disasters, as well as the improvement of emergency
response centers. In addition, they need to develop and install flood and storm surge forecasting systems, and(due to the Flood Fighting Act) issue timely and effective flood warnings.

Explanation
River and seacoast administrators must proactively participate and cooperate in designing the contents of floodfighting plans. To properly deal with flood disasters that extend beyond the territories of municipalities andprefectures, these administrators must determine important flood fighting locations and foster support andcooperation so that the flood fighting plans based on the Flood Fighting Act developed by related municipalities
meet the requirements for prevention and mitigation of flood disasters. If the need arises to review the floodfighting plan because of river improvement, changes in the river channel status, etc., then administrators need toinduce municipalities to revise their flood fighting plans as appropriate.
Flood and storm surge forecasting systems must be developed and installed, and efforts should be made toconstantly enhance the precision and functions of the forecasting systems.

2.5 Evacuation
To protect people from being killed or injured in flood and sediment disasters, rapid and adequate evacuation

93

must be conducted if the public is endangered. To this end, municipal governments, river and seacoast
administrators, and corporations that operate sediment managements—i.e. the bodies primarily responsible forregional disaster prevention measures—should work in close cooperation with each other.
For rapid and adequate evacuations, municipal governments, river and seacoast administrators, andcorporations that operate sediment managements need to collaborate closely with each other to inform
community members of flood-prone areas, areas in danger of sediment disaster, refuge routes, refuge shelters,
etc., and establish evacuation systems in advance. Also, they should publicize the use of river terraces or otherriver spaces as evacuation routes and shelters in times of earthquake and should improve such spaces for these
purposes.
In times of disaster, river administrators and corporations that operate sediment managements should prepare
and announce, with appropriate timing, disaster forecasts such as the predicted arrival times of flood flows,
inundation depths, inundation times, and predictions of damage from volcanic eruptions, etc.

Explanation
Information about changes in conditions in the river channel and basin such as facility construction or
improvement and advances in technology always needs to be kept up to date.
River and seacoast administrators and corporations that operate sediment managements are expected to provide
information on flood and sediment disasters and the like. Municipal heads are responsible for issuing evacuation
orders and directives to their community members. Because they need to base their judgments on thisinformation, it must be reflected in the disaster prevention plans developed by the municipalities. Operators of
sediment managements also include corporations and public administrators of areas designated for erosion
control works, landslide-threatened areas, and steep slope areas in danger of failure.
For the provision of disaster information, systems for directly providing the necessary information to community
members should be established with the understanding and cooperation of the mass media andtelecommunication companies, and by use of the Internet.
Since rapid and adequate evacuation is not possible if all community members lack an awareness of the risks,river and seacoast administrators and corporations operating sediment managements must provide community
members with disaster prevention education during disasters and non-disaster periods alike.

Section 3 Cooperation for Appropriate River Utilization, Maintenance of Normal
Functions of River Water, and Improvement and Preservation of the River Environment
3.1Cooperation for appropriate river utilization and maintenance of the normal functions of river water
Cooperation between local public bodies, relevant private organizations and bodies, and community members,
etc. should be established to enable the entire river basin to promote measures for appropriate river utilizationand maintenance of the normal functions of river water.
Water utilization measures in the river basin include measures against rainwater infiltration, storage measures,
facility-based measures such as rainwater utilization, reuse of treated waste water, elimination and94

consolidation of withdrawal and drainage systems, and measures for guidance on appropriate water utilization.
In order for these measure to be efficiently and effectively implemented in the entire river basin, it is necessary
to accurately grasp the local needs, ensure close coordination with relevant organizations and bodies, andconsider appropriate role sharing.

Explanation
Appropriate river utilization and maintenance of the normal functions of river water require the participation of
all groups within the river basin. It is also important that plans for future development and land use maintain theretention and retarding functions of the basin. At the same time, it is necessary to determine appropriate sharingof roles among the communities in the river basin and to actively promote measures involving the entire basin,such as measures for securing the retention and retarding functions, which are coupled with city or town
planning.
To secure appropriate utilization of the river and maintenance of the normal function of the river water incooperation with local public bodies, relevant private organizations, and local community members, etc., riverand seacoast administrators should release and routinely exchange information related to the river, etc., with suchgroups as well as with citizens interested in the river, people with experience, academics, and other concernedbodies.
It is also important to establish a mechanism that allows smooth coordination through close cooperation andcollaboration between participants to promote efforts by communities and entities in the river basin as a whole.
Examples of major structural- and non-structural measures are as shown below.
Structural measures- Infiltration measure: Stormwater infiltration inlet, infiltration trench, etc.
- Storage measure: Facilities for rainwater storage, etc.
- Reuse: Utilization of rainwater, reuse of treated waste water (for miscellaneous, agricultural, or industrial use),etc.
- Elimination and consolidation of withdrawal and drainage systems- Sewerage system improvement, pollution source control, preservation of water conservation forests
Non-structural measures- Appropriate water utilization: Water conservation (by water consumers and water suppliers), rationalization ofwater utilization- Guidance: Regulation, guidance, awareness raising, education, etc.

3.2 Cooperation for improvement and preservation of the river environment, etc.
To promote basin management for the preservation of natural environments such as river environments, and forthe improvement and preservation of river environments as accessible environmental spaces, local needs shouldbe accurately grasped and cooperation between local municipalities, relevant private organizations and bodies,and local community members, etc., should be established.

95

Explanation
In promoting the preservation of a natural environment and the improvement and preservation of riverenvironments as accessible environmental spaces, cross-cutting measures for improvement and preservation of
the environment of peripheral areas should be coupled with their utilization and preservation. This should occur
through measures to improve and preserve the longitudinal river environment, as characterized by thecontinuation of water and vegetation in an upstream–downstream direction, as well as through cooperation withsurrounding areas. To this end, it is important to implement river improvement and management by gaining theunderstanding and cooperation of the citizens, academics, and the bodies concerned. In an effort to determine
what the ideal river environment should be like and to establish plans for river improvement and management, it
is desirable to continuously release information on the subject river and to routinely exchange information andopinions with citizens interested in rivers, persons of experience or academic standing, and the bodies concerned.

Section 4 River Improvement in Cooperation with City Planning
In consideration of the land use, history, landscape, nature, climate, and culture of the basin, it is necessary toestablish cooperation between local municipalities, relevant organizations and bodies, and regional community
members to promote city planning that takes advantage of the characteristics of the river and provides places
for regional interactions that make use of the river.
Especially in the case of urban rivers, consideration needs to be given to their multiple roles in addition to theirflood fighting function, such as in urban disaster mitigation, providing an accessible environment, andsupporting various urban activities.

Explanation
City and town planning for riparian areas requires autonomous local governments that play the major role in city
planning to cooperate with river administrators.
In city planning, continuity as an integrated space consisting of the river and riparian area (e.g. topography,function, landscape, etc.) should be secured by taking advantage of the natural topography.
In river improvement, on the other hand, it is important to improve the functions of the environment andappropriate river utilization in addition to securing flood fighting functions.
Riverfront improvement should take into consideration the regional history, landscape, nature, climate, andculture so as to enliven the townscape in harmony with the riparian area and create a new attractiveness.
It is desirable to implement construction, improvement, and utilization measures by relating broad-based
river-related plans to policies established by prefectures for the improvement, development, and preservation ofcity planning areas (City Planning Area Master Plans). Similarly, relationships with individual cities and towns
should be associated with the basic policies established by municipal governments for city planning (Municipal
Master Plans).
The following points need to be taken into account for plans and measures concerning urban rivers:
1. Securing of disaster prevention function
Efforts should be made to enhance the urban disaster prevention functions of rivers, including their roles as96

sources of water for firefighting in times of disaster and domestic water for post-disaster use and as fire breaks,
refuge shelters, evacuation routes, and emergency mass transport routes.
2. Space supporting urban activities
In rivers and riverside areas, appropriate utilization of river spaces to enable diverse urban activities to be
conducted while at the same time maintaining an appropriate balance with preservation of the naturalenvironment should be promoted.
In areas with concentrated population and assets, the promotion of city planning in combination with theconstruction of flood- and earthquake-resistant embankments of a high standard that facilitate access to the riverand the securing of high scenic value should be considered.
Utilization of river transport should be promoted to mitigate urban land traffic and reduce environmental burdens
such as greenhouse gas emissions. Also, construction and improvement of nodal points connecting to land traffic
should be implemented.

Chapter 6 Monitoring
Section 1 General Information
For appropriate management of water and sediment, etc., investigation, planning, construction, andmaintenance processes need to be perceived as a series of processes in an integrated system, and it is important
to monitor and assess each process as it is implemented so that the results can be reflected in each process forreexamination and improvement.
Since natural and social conditions keep changing in the basin, including in the river itself, it is important thatmonitoring, assessment, and feedback are always properly implemented. This chapter describes the basic
concept of monitoring.

Explanation
Bed configurations, flow volume, and the environment of a river are closely related to and change with thenatural and social conditions in the basin. For this reason, a mechanism that allows the communities in the basin,
including in the communities in the river itself, to be kept in optimum condition should be established throughthe monitoring of water, sediment, land and space, and facilities for the purpose of appropriate assessment of thecurrent situation. From these results, the mechanism should enable feedback to planning, construction, andmaintenance whenever necessary.
Moreover, since this goal cannot be attained through the sole efforts of river, sediment management, andseacoast administrators and corporations, it is important to enhance cooperation with the organizations, bodies,parties, and persons concerned with the basin to grasp changes in the natural and social conditions in the basin
and to collaborate with these parties to implement various projects.
This chapter is based on the above-mentioned concepts and is divided into sections on monitoring of water andsediment, land and space, the river environment, and facilities, each describing the basic concept of monitoringnecessary for the target element. Monitoring of these elements needs to take into account the relationship97

between the entire water cycle system and the entire sedimentary system, and it is necessary to allow
coordination between these systems.

Section 2 Monitoring of Water, Sediment, etc.
2.1 Monitoring associated with flood control
2.2.1 Overview
Monitoring associated with flood control means comprehensive monitoring of precipitation, water flow, andsediment volumes of the river and is conducted to enable appropriate planning, construction, and maintenanceprocesses as a series of processes in an integrated system. The prevailing situation of flood control should beassessed from monitoring results and feedback should be given to planning, construction, and maintenanceprocesses as appropriate.

Explanation
To grasp the current status of the measures developed and implemented from “River Planning” in Chapter 2,“Erosion and Sediment management Planning (Countermeasures for Sediment Disasters, etc.)” in Chapter 3, and“Seacoast Preservation Planning” in Chapter 4, it is important to appropriately monitor hydrologic and hydraulicinformation such as precipitation, flow volumes, wind, waves, and sediment-related information such changes in
sediment load, the river bed, and the shoreline, and to assess their current status on the based of information
obtained through monitoring.
The results of the monitoring should be used to judge whether the above measures based on flood control plans
are taking effect as expected or not; if they are not, then the monitoring results are used to determine whether theconditions in the basin or river have changed or whether appropriate maintenance has been implemented. Fromthese judgments, necessary feedback should be given to the planning, construction, or maintenance processes.
In monitoring related to flood-control, water and sediment phenomena are interrelated with plans for flood
prevention, sediment disaster prevention, and coastal protection. Therefore, appropriate coordination between
monitoring and these plans is necessary.

2.2.2 Water flow volume monitoring
Water flow volume monitoring means the monitoring of precipitation at predetermined locations and of waterlevels, tide levels, and flow velocities (flow rate), etc. at the time of flooding; this information contributes to theprediction of flooding, the provision of flood information, and the operation of facilities.
From the results of monitoring, the runoff rate, channel roughness, etc. should be assessed, and feedback should
be given to the basic items associated with flood prevention and those associated with coastal protection, as
required.

Explanation
The target phenomena here are mainly floods and storm surges. The monitoring methods to be employed should98

be those described in the Manual for Hydrological Observation and in the Research Volume of the Technical
Criteria for River Works (Draft).
The items to be monitored first when a flood has occurred or is likely to occur include the rainfall in the river
basin, the water surface elevation and flow velocity (flow rate), and the tide level at predetermined locations of
the river. The results of monitoring should be used for the operation of river management facilities such as dams
and weirs; the preparation of flood forecasts and flood fighting warnings; and the provision of flood forecasts tothe communities in the basin.
Locations for monitoring should be determined in advance based on the natural and social conditions in thebasin; usually, the locations determined in the flood prevention plan or flood fighting plan should be used.
Depending on the predicted scale of the flood and the status of the flood control facilities, flood prediction may
be necessary; therefore, it is necessary to note that, in some cases, it will be appropriate to set additional target
locations.
The results of monitoring are used to evaluate the relationships between rainfall amount and runoff volume andwater level and discharge rate. If a flood water level is found to be significantly high or low compared with past
floods of comparable size, then other factors, such as the relationship between water level and channel roughness,should be investigated. If necessary, the essential elements of flood control (design scale, facility arrangement,etc.) should be reexamined and the results reflected in the maintenance of trees in the river channel and river,
improvement and preservation of the river environment, erosion and sediment management planning(countermeasures for sediment disasters, etc.), and basic items associated with coastal protection.
It is also important to assess the attributes being monitored and locations from the results of monitoring.
Monitoring methods such as the use of rainfall radar should be upgraded at appropriate times in response toadvances in technological development.

2.2.3 Sediment monitoring
Sediment monitoring means measurement of the longitudinal profile and cross sections of the river, shape of theshoreline of sea coasts, and, if necessary, river bed materials and the sediment transport rate, etc. Measurements
are made at pre-determined locations periodically and after the occurrence of abnormal phenomena such as
floods and storm surges. This monitoring helps to provide flood information and in flood prediction.
The results of monitoring should be used to assess sediment movement and riverbed deformation and then fed
back to review the major elements of flood prevention, and coastal protection, or erosion and sedimentmanagement planning (countermeasures for sediment disasters, etc.) as required.

Explanation
The items that need to be monitored during a flood or when a flood is likely include sediment movement andwater volume. Monitoring should cover the entire sediment system, including the sediment yield area, sediment
movement in torrents, riverbed deformation, suspended sediments, and bed load sediments.
Sediment monitoring is important not only for erosion and sediment management planning (countermeasures for
sediment disasters, etc.) but also for providing feedback to river planning and seacoast preservation planning.
Since the water surface elevation at the time of an actual flood cannot be predicted without knowledge of99

sediment movement during the flood, it is necessary to track riverbed deformation to determine the adequacy of
the river planning and, especially, the water level at the design flood discharge.

2.2.4 Inner water monitoring
Inner water monitoring means monitoring away from the main river channel at pre-determined locations. Items
monitored include rainfall, the water levels of minor rivers and their receiving waters, and the working ofdrainage pumps, etc. Such monitoring contributes to the prediction of landside flooding, the provision of inner
water information, and the appropriate operation of drainage facilities. From the results of monitoring, inner
water treatment should be assessed, and feedback should be given to the inner water treatment plan and facility
operation.

Explanation
Many inner water disasters are caused by the overtopping of runoff from medium-sized and small rivers,
waterways or sewerage systems because their drainage capacity is exceeded or from rainfall exceeding thecapacity or the design scale of the subject facilities. In a region with landside areas that are becoming
progressively urbanized, in particular, reduced permeability and other factors are likely to increase the run-off
coefficient, potentially leading to design drainage capacity being exceeded.
Sometimes the high water level of the receiving river makes it difficult for the feeder river to discharge excesswater, even if the flow volume is within the normal drainage capacity of the feeder river. Therefore, it isimportant to assess drainage by monitoring the rainfall in the landside basin, the water levels of the landside riverand the receiving river, and the working of drainage pumps.
The monitoring methods to be employed should be those described in the Manual for Hydrological Observationand in the Research volume of the Technical Criteria for River Works (Draft).

2.2 Monitoring of appropriate river utilization and maintenance of normal functions of river water
Monitoring of appropriate river utilization and maintenance of normal functions of river water means themonitoring of water volume, water quality, etc., to determine the degree to which normal flow volume issecured and the effectiveness judged. In accordance with the results of monitoring, the water quality, etc. should
be assessed and feedback should be given to measures for appropriate river utilization and maintenance of thenormal functions of river water, as appropriate.
In selecting locations for monitoring, the locations that were examined in the basic items associated with themaintenance of normal functions of river water should be included in the examination, and locations
appropriate for monitoring should be selected.

Explanation
Monitoring of the water volume and quality should be conducted for the following purposes:
1. To grasp the degree to which normal flow volume is secured
Monitoring should be conducted to check whether the predicted normal flow volume occurs as planned.

1002. To judge the effects of securing normal flow volume
Monitoring should be conducted to check whether the achievement of normal flow volume causes any problems.
3. To judge the effects of measures taken to secure normal flow volume
Monitoring should be conducted to determine whether the implementation of individual measures has been
progressing as planned, the expected effects have been achieved, and the entire plan has produced the expected
effects at representative locations.
From the results of the above monitoring, the basic items associated with the maintenance of normal functions ofriver water should be reviewed wherever necessary.
Important data for determining the normal flow volume can be obtained especially at the time of extraordinary
drought; therefore, substantial monitoring of water volume and quality has to be conducted to determine:
1. How well the environmental standards in the subject public water body are met
2. The extent to which the water quality permits comfortable recreation in the subject river
3. The appropriateness of the water quality for water utilization
4. The appropriateness of the water depth, current velocity, and water quality for preserving healthy habitat forplants and animals
5. The extent to which the surface width and water quality (including freedom from litter) necessary for creatinga good landscape are obtained.
In monitoring the water volume, locations representing the flow regime of the target water area should beselected from longitudinal characteristics such as the water balance (merging, branching, etc.) and water
utilization, and the flow volumes of the main stream and tributaries should be determined.
The water balance of the river can be affected by changes in drainage discharge due to the construction or
improvement of sewerage systems, changes in rivers as receivers of discharge due to the construction of
bypasses, and changes in groundwater utilization, etc. Therefore, data and general information on the amount ofdrainage discharge, the amount of water intake, the groundwater level, the amount of groundwater extraction, etc.
should be gathered as required.
In principle, the observation of water volume is based on continuous observations, such as conversion (discharge
curves) of the values obtained from automatic observations of water level.

2.3 Monitoring of sediment2.3.1 Overview
Monitoring of sediment needs to be conducted, because to implement comprehensive sediment managements
and prevent disasters caused by the movement of sediment, etc., it is important to determine the volume and101quality (grain size) of sediment from the viewpoint of spatial and temporal continuity and to predict their futurechanges.

Explanation
To prevent disasters associated with sediment movement, to preserve river and coastal habitats and landscapes,
and to properly utilize the river or sea coast it is necessary to comprehensively and properly manage sediment
movement and the resulting topographical changes over the entire sediment system. To this end, it is necessary to
conduct sediment monitoring, since it is important to survey not only the amount of sediment movement in theentire sedimentary system but also the channel morphology and quality (grain size) as well as the current statusof their temporal change. From these results, it is necessary to predict short-term and medium- to long-term
future changes.
In this section, monitoring of sediment is divided into two categories: 1) monitoring of sediment yield anddischarge in sediment yielding areas, and 2) monitoring of riverbed deformation and sediment transport in thesediment transport area. However, these divisions are inseparably interrelated and have to be in harmony witheach other in space and time.

2.3.2 Monitoring in sediment yielding areas
The aim of monitoring in the sediment yielding areas is to determine the sediment yield and sediment runoff,
etc. by measuring the erosion rates and amounts of deposition in torrents on hillside slopes, mountain-ringed
areas, and alluvial cones, etc., and the sediment discharge in torrents, etc. In conducting monitoring, efforts
should be made to identify the grain size distribution of the sediment.

Explanation
Sediment yield on a hillside slope occurs as a result of slope failures, landslides, erosion, and expansion of
already collapsed areas. In torrents in mountain-ringed areas and alluvial cones, etc., sediment is produced as aresult of riverbed and side erosion. Sediment yield should be determined from the erosion and deposition
volumes obtained by measuring the amounts of topographical change of hillside slopes, torrents, etc.
The sediment runoff volume from a sediment yielding area can be determined by measuring the sedimentdeposits in erosion control dikes or directly measuring sediment concentration in runoff at the same time as riverdischarge is measured.
Topographic or sediment deposition surveys and observations should be conducted periodically or before andafter a flood. Measurements of sediment runoff discharge should be conducted at the time of flood, with secured
safety to determine the variation. The temporal variation in sediment yield and discharge in the sedimentyielding area should be determined by conducting surveys and observations continuously from upstream todownstream.

2.3.3 Monitoring in a sediment transport area
The aim of monitoring in a sediment transport area is to determine the sediment movement rate by measuring102

topographical changes in the middle and lower reaches of the river, the estuary, and the shoreline, as well as thesediment load of the river and the longshore sediment transport rate along coasts.
In the process of monitoring, efforts should be made to identify the grain size distribution (quality) of thematerial comprising riverbeds, beaches, etc.

Explanation
River channel erosion, sediment deposition, and changes in the shape of the shoreline occur as sediment moves
in the sediment transport area. Topographic changes in the channel and the shape of the shoreline should bemeasured, and the sediment movement itself should also be measured by investigating the sediment transport
rate both in the river and in the longshore sediment transport area.
Sediment transport monitoring should be conducted not only before and after a flood or storm surge but also
periodically, so that the sediment transport during normal periods can be grasped. Temporal changes in sedimenttransport should be observed by continuous measurement.
Since riverbed variation itself is important when considering the safety of the river channel against floods within
the design high water level or the effects of sediment movement on the river environment, not just the sediment
movement, but also the riverbed variation itself, needs to be monitored.

Section 3 Monitoring of Land and Associated Space
3.1 Overview
Monitoring of land and space is aimed at comprehensively monitoring the landform, usage pattern, vegetation,etc. in river basins, seacoast preservation areas, erosion and sediment management project areas, etc. to preventand mitigate disasters, achieve appropriate use of space, and improve and preserve the environment.
Assessments should be made from the results of monitoring, and each plan and maintenance program should be
reviewed as required.

Explanation
For the measures implemented in accordance with “River Planning”, “Erosion and Sediment management
Planning (Countermeasures for Sediment Disasters, etc.)”, and “Seacoast Preservation Planning” (described in
Chapters 2 through 4) to remain effective, it is essential to properly manage the land and space of rivers, erosion
and sediment management areas, and seacoasts.
“Land and space” here includes ground surface, underground areas, water surfaces, and airspace in an area
designated by law. The longitudinal and transverse forms of the river, including its low flow channels, water
surface, use of high water channels, vegetation in high water channels, soil cover condition, etc. should bemonitored through measurements such as surveys. In addition, the approval of permits for area occupancy needs
to be included in these considerations.
The results of monitoring, together with the results of water and sediment monitoring, should be evaluated fromvarious viewpoints, such as disaster prevention and mitigation, appropriate use of space, and improvement and103

preservation of the environment. If it is determined that the expected effects have not been obtained, thenfeedback should be given to each plan, construction, and maintenance program.

3.2 Monitoring for flood control
Land and space monitoring for flood control means the comprehensive monitoring of the river channel andbeach profile, etc. through longitudinal and transverse surveys and vegetation distribution investigations inorder to manage land and space for the prevention and mitigation of disasters caused by floods and storm
surges. The results of monitoring should be used to assess flood containment capacity etc. and, if necessary, toreview the basic elements of flood prevention, coastal protection, and erosion and sediment management plans(countermeasures for sediment disasters, etc.), and the maintenance of these.

Explanation
Land and space monitoring for the prevention and mitigation of flood disasters should be conducted for
longitudinal and transverse river channel management to secure an appropriate level of flood containment
capacity. It especially aims to manage the bed slope, plain figure, flow section, and roughness. In monitoring,attention should be paid to changes in temporal and spatial conditions.
Flood and sediment runoff characteristics vary depending on the land use and vegetation in the river basin, theinstallation of structures, and the vegetation distribution in the river channel, etc. For this reason, to allow a flood
within the design high water to flow safely, it is important to assess the flood-carrying capacity by means of
longitudinal and transverse surveys of the river channel, vegetation surveys, discharge observations during an
actual flood, etc. The trend in temporal variation and riverbed deformation during a flood also needs to be taken
into account. If it is determined that the expected discharge capacity is not satisfied, then section excavation,
dredging, cutting of vegetation, and installation of river structures such as spur dikes should be executed. If
necessary, the essential elements of flood prevention and erosion and sediment management plans(countermeasures for sediment disasters, etc.) should be reviwed.
On the other hand, the targets of land and space monitoring for erosion and sediment management planning(countermeasures for sediment disasters, etc.) include hillsides and torrents, which are sediment yielding areas,and sediment transport areas, etc. The longitudinal and transverse forms of the river, slope conditions, etc. should
be monitored and assessed. The concept of monitoring here is the same as for land and space management,
aiming to prevent and mitigate flood disasters as described above.
Land and space monitoring for the prevention and mitigation of storm surge disasters targets seacoasts andestuaries, and it should be conducted in the same way as for the aforementioned land and space management toprevent and mitigate flood disasters. For coastal monitoring, the main targets of monitoring are the sandy beachand beach profile; permit approval should be obtained for occupied areas.

3.3 Monitoring associated with utilization
Land and space monitoring for utilization aims to monitor the statuses of land occupancy and utilization inorder to optimize river and seacoast utilization and secure healthy habitat for animals and plants. Assessment

104should be carried out from the results of such monitoring, and if necessary, plans related to the fundamentals ofmaintenance, improvement and preservation of river environments should be reviewed.

Explanation
River areas and spaces consisting of water surfaces, high water channels, etc. and coastal land and spaces
consisting of seawater surfaces, sandy beaches, etc. are valuable spaces for fostering diverse ecosystems andcreating therapeutic landscapes. Combined with the surrounding nature and streets, they also form valuable
landscapes. In addition, they are public spaces for utilization by community members living in the basin andcoastal areas and are spaces that can be used as emergency evacuation sites and recovery bases in times ofearthquake disaster.
In the utilization of land and spaces along rivers and seacoasts, in principle, utilization for public purposes
should be given priority while considering the fact that they serve as habitat for animals and plants. And, sincefairness is important in the utilization of the limited land and spaces to avoid conflict in use, comprehensive
monitoring should be conducted for permit granting for land and space occupancy. Investigations are also
necessary for the use of water surfaces, high water channels, and sandy beaches, etc.
The results of such monitoring should be evaluated from the viewpoints of transparency, fairness, and orderly
utilization as well for the effects on environmental improvement and preservation. If it is determined that theland and spaces are, in general, not properly being used, then use zoning should be reexamined and permissions
for land and space occupancy and their maintenance should be reviewed.
Moreover, it is desirable to simultaneously implement projects for improvement of access to the river or seacoastby installing gently sloping dikes, etc. and by tree planting and installation of public lavatories, as well as byimplementing other improvement measures that will promote utilization, such as the installation of signs.

Section 4 Monitoring of the Environment
Monitoring of the environment means monitoring of river and seacoast shapes, ecosystems, landscapes, water
quality, etc. The results of monitoring should be evaluated and used to review the items associated with riverenvironment improvement and preservation, coastal environment improvement and preservation, and erosion
and sediment management planning (countermeasures for sediment disasters, etc.).

Explanation
Monitoring of the environment should be conducted to monitor the river morphology (including riffles andpools), habitat for animals and plants, landscapes, locations used for river-based human activity, (such as
walkable embankments), and water quality. Such monitoring should aim to:
1. Determine the level of goal achievement
Monitoring should be conducted to check whether the goals for improvement and preservation of theenvironment that were set at the time of plan establishment have been achieved as expected.
2. Determine the beneficial and adverse effects and impacts of goal achievement
Even if the goals set for improvement and preservation of the environment have been achieved, the uncertainties

105

that surround river and coastal environments necessitate that monitoring should be conducted to check whether
the expected effects have been obtained, whether unpredicted impacts are noticeable, and whether issues
identified at goal setting have been resolved. If the results of monitoring show any deviation from expectations,
it is important to analyze the cause.
3. Determine the effectiveness of measures
Monitoring should be carried out to judge whether the measures for improvement and preservation of theenvironment were most appropriate for achieving the goals that were set.
4. Feed back the results to goal setting and measure establishment
The results of environmental monitoring should be fed back into the goals for improvement and preservation of
the environment and the establishment of measures.
Preliminary surveys are necessary to determine and evaluate the effects and impacts. During this process, it isimportant to grasp the status of not only the river, but also its basin.
Substantial monitoring should be conducted during abnormal drought or periods of abnormal water quality orafter floods. Such monitoring provides important data and information for use in considerations for improvement
and preservation of the river environment, etc.
It is important that the results obtained from such monitoring be used effectively for maintenance.

Section 5 Facility Monitoring
5.1 Monitoring associated with the assessment of facility planning
Monitoring associated with the assessment of facility planning means comprehensive monitoring to check
whether the expected functions of the facilities and measures implemented as a result of river planning, erosion
and sediment management planning (countermeasures for sediment disasters, etc.), and seacoast preservation
planning are being achieved despite changes in the natural and social conditions that have taken place after theirinstallation. The results of monitoring should be assessed and used to review facility plans and revise them as
required.

Explanation
Facilities for river management, erosion control, and seacoast preservation and related structures are planned andconstructed in accordance with River Planning (Chapter 2), Erosion and Sediment management Planning(Countermeasures for Sediment Disasters, etc.) (Chapter 3), and Seacoast Preservation Planning (Chapter 4).
The question of whether the expected functions of these facilities and structures have actually been fulfilled ismore important than their existence. There will be cases in which the arranged facilities are not fulfilling their
expected functions due to changes that have taken place in the natural and social conditions of the river basin or
in the river and coastal characteristics after construction of the facilities.
For this reason, monitoring and assessment should be carried out through the implementation of monitoring as
described in Sections 1 through 4 of this chapter to check whether the arranged facilities, etc. are fulfilling their
expected functions, and facility planning should be reviewed and revised as required. Relevant examples include106re-examination of the intake rate due to reduction in the amount of agricultural land, reconstruction of intake
facilities due to degradation of the riverbed, and beach nourishment in response to coastal erosion. Functional
degradation of facilities due to deterioration, etc. will be referred to in Section 5.2 below.

5.2 Monitoring associated with functional maintenanceMonitoring associated with functional maintenance means comprehensive monitoring to check whether thefacilities, etc. arranged in accordance with river planning, erosion and sediment management planning(countermeasures for sediment disasters, etc.), and seacoast preservation planning are fulfilling their expectedfunctions. The results of monitoring should be assessed and fed back into facility plans as required.

Explanation
River management facilities such as river dikes and groundsills and seacoast preservation facilities such as
coastal dikes and beach nourishment facilities must possess the functions that are stipulated in the Cabinet Order
Concerning Structural Standards for River Management Facilities, etc. and the Criteria for Seacoast Preservation
Facility Construction, etc., and it is important for the management of rivers, etc. that these functions arecontinuously maintained without deterioration.
The planning and design of plans for the management of water, sediment, etc. and arrangement of facilities
should be carried out in such a way that the functions expected of the facilities can be fulfilled in considerationof the level of maintenance. It is important to conduct thorough monitoring through surveys and measuring
instruments and, from the monitoring results, to assess the functionality of the facilities to check whether thefacilities are fulfilling their expected functions.
The items, timing, and frequency of monitoring should be dependent on the purpose and expected functions ofeach facility. In general, though, monitoring should be conducted through periodic inspections, inspections at thetimes of floods and storm surges, and inspections after such events. Measurements that require specializedmeasuring equipment should be conducted more efficiently if such considerations are included in the design andconstruction of the facilities.
It is important that the results of monitoring be organized using predetermined formats for the assessment offacility functionality. If it is determined that the functions of the facility have deteriorated and are not satisfying
the expected standard, the results should be fed back into the facility plan (which may lead to reconstruction) or
maintenance so that proper functions of the facility should be achieved.
Reference
1) Hydrological Observation, 2002 Edition, Authored and edited by the Public Works Research Institute,
Supervised by the River Bureau of the Japanese Ministry of Land, Infrastructure and Transport. Published by theJapan Construction Engineers’ Association

107

Planning for Facility Installation

108General Information
General Information
A facility installation plan shall be designed in such a way that prevention or mitigation of disasters caused byfloods and storm surges, etc., improvement and preservation of the river environment, etc., and comprehensive
sediment management are properly implemented.
The project facilities must be constructed, improved, maintained, and managed in consideration of therelationship between project costs (including appropriate life-cycle costs) and the beneficial and adverse effects
and impacts of the project.
When planning each individual facility installation plan, consistency with other facility installation plans andcooperation with non-structural measures need to be sufficiently taken into consideration.
When designing a facility installation plan, thorough consideration must be given to the natural as well as social
characteristics of the river basin and community.

Explanation
Since prevention or mitigation of disasters due to floods and storm surges, etc., improvement and preservation ofa healthy river environment, and comprehensive sediment management are closely interrelated with each other,
consistency among these must be secured in the development of a facility installation plan. Also, it is necessary
to take into consideration consistency with related facility plans and other plans, as well as with excessive
external forces. In this part, Chapter 1 describes the improvement and preservation of river environments, etc.
and comprehensive sediment management; these are themes common to all sections in Chapter 2 and thereafter,
where individual arrangement plans will be discussed.

109

Chapter 1 Improvement and Preservation of the River Environment andComprehensive Sediment Management
Section 1 Improvement and Preservation of the River Environment
1.1 Overview
The goals for developing a plan to improve and preserve a river environment, etc. shall be established on thebasis of an understanding of the characteristics of the subject river environment, the natural and social
environments of the basin and seacoast, and their historical changes. They should also take into considerationconsistency with the flood control and water utilization functions, and the measures taken to achieve these goals
shall be designed accordingly. In this process, preservation or restoration of a healthy living and growing
environment for plants and animals, formation and maintenance of good landscapes, creation and maintenance of
places for activities that bring humans into contact with the river, etc., and preservation of good water quality
shall be comprehensively considered.

Explanation
When developing a plan for a river environment, bibliographic surveys, field surveys, and discussions with thepersons concerned shall be conducted, and the following step-by-step process needs to be followed: 1) based on
an analysis of the aforementioned surveys, develop an understanding of the characteristics of the subject riverand its basin, as well as their historical transition; 2) determine the direction for improvement and preservation of
the river environment.; 3) establish goals for improvement and preservation of the river environment, through acomprehensive examination of this direction and of flood control and water utilization factors; and 4) establish
measures for improvement and preservation of the river environment on the basis of these goals.
Establishment of a plan shall follow the process shown below:

Understandings of the characteristics of the river environment, the basin or the seacoast, etc. , as well as
those historical changes

Setting the objective and direction for improvement and preservation of the river environment , etc.

Establishment of the policy of the improvement and conservation of the river environment , etc.

The following points need special attention in the development of measures for improvement and preservation of
the river environment.:
1. A thorough examination of the nature of the individual features of the river shall be conducted, giving due
110consideration to the characteristics of the river and the basin, and their historical transition.
2. Surveys of existing documents, field surveys, and discussions with experts, the local entities concerned, civicorganizations, shall be adequately conducted.
3. The information obtained from such surveys shall be compiled into a river environment information map (adiagram showing in an easy-to-understand manner the riverbed configurations, vegetation status, living andgrowing environments for plants and animals, characteristics of the river environment, that is aimed at providing
appropriate information about the river environment to define it’s characteristics.
4. Environmental considerations, together with considerations about flood control and water utilization, shall be
examined from the early stages of plan development onward, not after the flood control and water utilization
factors have been examined.

1.2 Understanding the characteristics of the river environment
The characteristics of the subject river environment shall be understood through surveys of the characteristics ofthe river, seacoast, or mountain stream; the habitats of plants and animals; the utilization of the river, seacoast, or
mountain stream; the natural and social environments of the basin and the seacoast; and their historicaltransition.
The results of these surveys shall be systematically organized for the entire river, and for each area.

Explanation
1. Method of understanding the river environment, etc.
The characteristics of a river environment shall be discovered through investigation of not only the current
situation but also the historical transition of the river. In this process, changes in the statuses of gutters, riffles
and pools, river forms, river vegetation, rivers and surrounding land uses, as well as changes in the basin status,
shall be determined by arranging aerial photos, topographical maps, plane figures, longitudinal profiles, andcross-sections, in chronological order.
On the basis of an analysis of the gathered information, the subject river shall be zoned according tocharacteristic similarities (e.g. river course, natural environment, social environment characteristics, etc.). A riverenvironment information map shall be prepared for each zone to give an understanding of the characteristics ofthe river, the typical features of the river, and the problems to be solved.
An effective method to this end is to organize materials about the river course transition, river landscapetransition, river zoning, and the relationship between environmental zoning and organisms, and to prepare riverenvironment information maps (overall area, wide area, and section). When the specific contents are being
examined, some existing documents 1) should be useful as references.

2. Method of determining water quality
As part of the information on the river environment, information on water quality in particular shall be organizedin terms of its current situation as well as its historical transition, depending on the characteristics of the river. In
this process, it is necessary to take into account the local characteristics.
111

On the basis of the organized information on the current status of water quality, the relationship between thebasin and river flows and water quality, and the characteristics of river water quality change, shall be determined.
The mechanism of water quality change in the river, as well as the causes for this mechanism, shall be
determined through analysis of the current water quality on the basis of the aforementioned information.
Using information such as that on the current water quality, an analysis shall be conducted to forecast how thewater quality will change in the future. This analysis shall be conducted by using an appropriate method thatshould be selected in accordance with the characteristics of the target water body, the required level of accuracyof the prediction, and the data available, etc. Since there are periods when the water quality is important for the
lives of organisms and for water utilization, forecasts for not only drought and low-flow periods but also for all
year around are necessary.

1.3 Setting goals for improvement and preservation of the river environment
1.3.1 Determining the direction
The direction of improvement and preservation of the river environment shall be determined based on thecharacteristics of the river environment, basin, or seacoast from the viewpoints of preservation and restoration ofhealthy habitats for plants and animals; formation and maintenance of good landscapes; creation andmaintenance of places for activities that bring humans into contact with the river; and preservation of good water
quality.

Explanation
The “direction” means the direction for improvement and preservation of the river environment, etc., determinedon the basis of the discovered characteristics of the river environment, etc. and the attributes of the basin. It is
important to determine a direction for each zone.
In determining the direction, the following points need to be observed:- Local opinions obtained through hearings, etc. should be taken into due consideration.
- Historical transition of the river environment, etc. should be taken into due consideration.
- From the viewpoint of the natural environment, the discovery of the “original state” (i.e. original landscape)of the river should be helpful, although it is difficult to determine the original state of the river. The majorityof rivers in Japan have been artificially tampered with since ancient times, and, as a result, they have been
gradually changed and the riverside land has been utilized accordingly. Therefore, it is impossible todiscover what the river and its course used to look like before any artificial tampering. For this reason,
discovery of the “original state of the river” before large-scale river improvement projects in recent years
and when there was relatively little artificial intervention (e.g. the river shape and environment before
large-scale training during the high-growth period of the economy) would be a secondary option. In the caseof an urban river, a direction that aims to create a riverine environment that meets the conditions of thecurrent and future social environment should be determined.
- In order to discover the “original state of the river”, old topographical maps, longitudinal profiles, andcross-sections, as well as aerial photos, municipal histories, local histories, and interviews with local elders
should be helpful as references. Also, the state of a river that is close to the target area and maintains a good112

river environment without much artificial intervention, or of a river with similar natural conditions (such as
topography, geology, and river shape) that maintains a good river environment without much artificial
intervention can be informative.
1. Determining the direction for improvement and preservation of a healthy habitat for plants and animals
The direction for improvement and preservation of a healthy habitat for plants and animals shall be determinedbased on the following points:
1) Preservation and restoration of the river environment typical of the subject river
Efforts shall be made to preserve and restore the habitat for the animals and plants that the subject river
originally had. To this end, the river’s original shape shall be preserved and restored by taking full advantage ofthe river’s natural restoring force. It is also important to respect the dynamism of the river.
2) Securing a continuous environment
Some of the creatures living in a river travel between upstream and downstream, between the main stream andbranches, canals, or ponds, between the river and its peripheral areas, or even between saltwater and freshwater.
Some creatures need both water and land areas, and there are still some other creatures that move around inplant-growing areas. In order not to prevent such creatures from traveling from one place to another, acontinuous environment in longitudinal (upstream-downstream) and lateral directions as well as continuity withthe surrounding environment shall be secured. Because, in a tidal zone, wave run-up from the sea upstream into
the river occurs as the tidal sea level changes, the characteristic ecosystems formed in such an environment needconsideration.
3) Preservation and restoration of habitats for plants and animals typical of the subject river
Efforts shall be made to preserve and restore species that are important academically or in view of scarcity,
important communities, and notable habitats, and, at the same time, it is important to consider the preservation
and restoration of habitats for plants and animals typical of the river by focusing on the plants and animals living
and growing in the characteristic environments of the river, including riffles and pools, river beaches, riverside
forests, inlets, swamps, etc.
Countermeasures against alien species shall be considered if necessary.
4) Securing the water cycle
Stream flow is closely related to peripheral ground water, and the level of ground water affects the growth ofriver vegetation. Where there is spring water (which is limpid and often stable in temperature) in the river, anenvironment different from those in the main stream is formed, substantially adding to the diversity of the riverenvironment. Consequently, it is necessary to secure the water cycle by securing riparian permeability in such a
way that the natural water flow is not intercepted.
2. Determining the direction for formation and maintenance of a good landscapeThe direction for formation and maintenance of a good landscape shall be determined based on the following
113

points:
1) Respecting the typical dynamic features of the river, such as the river shape, composed of riffles, pools, andriver beaches, which are typical characteristics of the subject river; variability in the water volume;
continuity, etc.
2) Adequacy of a structure that combines functionality and appropriate structural beauty
3) Preservation of water quality
4) Measures to suppress factors that disturb the river landscape5) Building and maintenance of a good landscape that produces the distinctive features of the subject locality,
coupled with city or town planning based on the area and river as well as the streetscape, etc.
Here “landscapes” include not only places of scenic beauty and major appearances formed by exceptional naturalelements, but also neighboring landscapes such as other views and sceneries.
Measures for the formation and maintenance of major appearances shall take into account the typical features ofthe locality’s landscape, major viewpoints, and landscape resources (natural elements).
3. Determining the direction for creation and maintenance of places for activities that bring people into contact
with rivers
The direction for creation and maintenance of places for activities that bring people into contact with rivers shallbe determined based on the following points:
1) The river shape, water volume, and water quality, etc. in addition to the living and growing environment forplants and animals.
2) The status of utilization and needs.
3) Comfort and safety for users.
4) Places for experiential activities and environmental learning that take advantage of the natural environmentof the river.
5) Local history and culture.

4. Determining the direction of preservation of good water quality
The direction for preservation of good water quality shall be determined based on water utilization, preservationof habitats for plants and animals, securement of places for activities that bring humans into contact with theriver, landscape, preservation of living environment, tourism, maintenance of groundwater quality, etc., andindices that represent them shall be set.
Although BOD (biochemical oxygen demand) or COD (chemical oxygen demand) has been widely used as theindex for rivers, reservoirs, lakes, or swamps, the following indices shall be selected as appropriate, dependingon the river characteristics.
1) In cases where water utilization (use for a water supply system) is the primary element, ammonia, THMFP
114

(trihalomethane formation potential), 2-MIB (2-methylisoborneol), odor, or chlorine ions, etc. can be used.
2) In cases where preservation of habitats for animals and plants is the primary element, SS (suspended solids),
turbidity, DO (dissolved oxygen), chlorine ion content, water temperature, etc. can be used.
3) In cases where the securing of places for activities that bring humans into contact with the river is the primaryelement, SS, turbidity, color, transparency, coliform count, odor, etc. can be used.
4) In cases where landscape is the primary element, SS, turbidity, color, transparency, etc. can be used.
5) In cases where suppression of internal production is the primary element, chlorophyll a, nutrients, DO, etc.
can be used.
6) It is also effective to first set qualitative goals such as the suppression of algal blooms or maintenance of alevel of water transparency that enables visualization of the riverbed, and to then set quantitative goals by using
indices such as chlorophyll a or transparency to realize the qualitative goals.

1.3.2 Setting goals
Goals for improvement and preservation of a river environment, etc. shall be established for each area zoned on
the basis of the characteristics of the river environment, etc. They shall be based on the pursued direction inconsideration of a good balance between flood control, water utilization, and environmental factors.

Explanation
A direction to be pursued shall be determined for each of flood control, water utilization, andenvironment-related endeavors. Then, specific goals shall be established by optimizing trade-offs and othercorrelations among them.
Goals for improvement and preservation of the river environment, etc. shall be set for each area zoned on thebasis of the different characteristics of the river environment, such as those for the upper and lower reaches, etc.
In this process, the goals need to be examined through an adequate exchange of information and opinions on thesubject region.

115

Conservation of river environment

Direction to
pursueFloodcontrolWaterutilization

Preservation
and restorationof good
habitat forplants andanimals

Formation andmaintenanceof goodlandscapeCreation andmaintenanceof places for
activities thatbring humans
into contact
with river

Preservationof good water
quality

Overall arrangement

Goals

Flood
controlWaterutilization

Conservation of river environment

Fig.1-1 Goal-setting Flowchart for improvement and preservation of the river environment
For water quality, target values shall be determined for each index representing the determined direction.
Since the water quality of the river shows longitudinally different properties because of natural as well as
anthropogenic factors, in establishing the goals for water quality preservation the river shall be zoned into
multiple areas based on the longitudinal properties, and then goals for each area shall be examined.
For reservoirs, lakes, and swamps, too, water area zoning may be required before examination is performed to
set the goals.
Target values shall be set in consideration of drought and water utilization periods as well as important periods
for the life and growth of animals and plants, as required. In cases where the river flows into a closed water areasuch as an enclosed bay, goals that take wide-area water quality preservation into consideration shall be
investigated, if this is necessary.
In establishing goals for water quality preservation, coordination with relevant organizations and bodies in thebasin and consideration of environmental standards are necessary.

1.4 Measures for improvement and preservation of the river environment
Measures that are necessary for achieving the goals for improvement and preservation of the river environment,
etc. shall be established.
In examining the measures, cooperation and role-sharing with community members and relevant organizations
and bodies in the basin shall be considered.

Explanation
Multiple examination cases shall be set in consideration of flood control, water utilization, and environmental
116

factors as well as the planar, longitudinal, and transverse shapes, and the vegetation, etc., of the river, etc. The
channel shape and coefficient of roughness shall be determined for each case examined, in consideration of the
future maintenance level and tree vegetation status.
Future channel status (riverbed evolution, etc.) and river environment, etc. shall be predicted for each case, and it
is important to conduct a comprehensive assessment of the effects on flood control and water utilization as wellas the impacts on the environment. For this reason, it is important to predict how river improvement will affectthe river environment by studying the content of the proposed river improvement against river environment
information maps.
If significant effects and impacts are predicted, the plan shall be revised, as appropriate, so that the entire plan
will be a well balanced one.
In the process of developing a plan, it is desirable that frequent exchange of opinions and information takes placewith the local public, as well as with other persons concerned and with civic organizations that are associatedwith the river.
1. Measures for preservation and restoration of areas that are good habitats for plants and animals
Measures for the preservation and restoration of areas that are good habitats for plants and animals shall beestablished based on the following:- Changes in areas that are good habitats for animals and plants should be avoided as much as possible.
- The diversity of the river shape shall be preserved and restored.
- Continuity of habitats in the river shall be secured.
- Expansion of nature in the basin shall be considered.
- Threatened organism species, etc. shall be protected.
- A healthy water cycle shall be secured.
- The dynamism of the river shall be secured.
- Understanding and cooperation shall be obtained from citizens, academics, and experts, and from interested
organizations, etc.
The following are specific examples of viewpoints for examination:- Low-flow channels should have a naturally shaped bed, not an artificially flattened bed.
- The width of a low-flow channel or a gutter shall be determined on the basis of the width of the original
river.
- The transverse shape of a low-flow channel shall be determined on the basis of the transverse shape of thelow-flow channel of the original river.
- The normal line shape of a low-flow channel shall be a gentle meander, determined on the basis of thenormal line shape of the low-flow channel of the original river.
- The longitudinal shape of a riverbed shall be determined on the basis of the longitudinal shape of theoriginal riverbed.
- If the original river had riffles and pools, these shall be formed.
- Riffles in portions of the river where a cliff, a forest, etc. adjoins the river should be preserved if at all117possible.
- Intricately indented portions and curves of the riverbank shall be preserved so that slack water areas aremaintained.
- Continuity between branches and canals shall be secured.
- If at all possible, waterside areas should not be covered with concrete.
- The height of a high (higher) water channel shall be determined on the basis of the future vegetation andsediment deposition statuses.
- Riverbed slope gradients shall preferably be as gentle as possible, but there are cases in which a steep
gradient will allow the formation of more diverse environments if there are highly restricting conditions
related to land use, etc.
- Riverside forests should be preserved and restored to as great an extent as possible.
2. Measures for formation and maintenance of good landscapes
Measures for the formation and maintenance of good landscapes shall be established based on the following:- Harmony with the characteristics of the subject river landscape shall be maintained.
- Harmony with the subject locality’s natural environment shall be maintained.
- Harmony with the history, tradition, and culture of the subject locality shall be maintained.
- The appearance of river structures and permitted structures shall be considered.
- Preservation of a good river landscape shall be induced.
- Understanding and cooperation shall be obtained from citizens, academics, experts, interested organizations,etc.
The following are specific examples of viewpoints for examination:- There is growth of plants on the riverbed and revetment and has the appearance of a natural environment.
- The river course meanders gently.
- The top of the levee has a rounded surface and a gentle line.
- The high-water channel has trees that arrest the view.
- The river edges are delicately indented where plant growth is luxuriant.
- The flow has planar variation caused by the presence of riffles, pools, and shoals, etc.
- The water is limpid.
- The width of the water surface is large.
- The dry riverbed provides a long walkway and easy access to the waterfront.
- Distant views are secured.
- A continuous mountain ridgeline is visible.
- Anthropogenic structures and objects are colored to blend in with their natural surroundings.
- Objects blocking the landscape view, such as signs, utility poles, and steel towers, are not conspicuous.
- There is a row of trees, or trees or a forest that serve to break the view around the river.
- There is a riverside park.
- There are no elevated roads running along the river.
118

3. Measures for creation and maintenance of places for activities that bring humans into contact with the river
Measures for the creation and maintenance of places for activities that bring humans into contact with the river,etc. shall be established based on the following:- Utilization and the natural environment shall be harmonized.
- Good places for activities shall be maintained.
- Access to the waterfront shall be improved.
- Places for activities shall be provided according to the actual utilization and the need for utilization.
- Cooperation and collaboration with local community members and civic organizations, etc. shall bepromoted.
The following are specific examples of viewpoints for examination:- Healthy habitats for plants and animals shall be taken into consideration.
- Consideration of the structure and its materials and finish is necessary so that users (especially the elderly,
children, and disabled people) can comfortably use the place without extra physical effort.
- Safety shall be considered.
- Utilization as a place for environmental learning shall be considered.
- Use of natural materials, and their sizes and shapes, shall be considered; they should suit the historical andcultural backgrounds of the locality and the river.
- Economic efficiency and ease of management shall be considered.
- Acts that hamper the activities of the general public should be constrained.
4. Measures for the preservation of good water quality
Since water quality is affected to a large extent by the natural and social environments of the basin, itspreservation needs to be dealt with by the entire basin. To this end, close cooperation and collaboration withmeasures by relevant organizations and bodies in the basin and appropriate role-sharing among them shall bepromoted.
Relevant organizations and bodies in the basin include local public entities, local interested organizations, etc. It
would be effective to set up a council for each basin through collaboration between these and riveradministrators.
Since the measures implemented by river administrators and relevant organizations and bodies in the basin have
limitations in terms of scope of implementation, coordination among these bodies may be necessary. It is also
necessary to give feedback on these measures (role-sharing).
Even for water areas where water quality-related problems are not apparent, plans for the preservation of goodwater quality need to be established in view of future changes in the basin.
Measures for the preservation of river water quality shall be developed to achieve water quality goals in
consideration of the pollution loading distribution in the basin, the characteristics of water quality change, andthe effectiveness of basin management, etc.
In the process of developing measures for the preservation of river water quality, measures that are most119effective for achieving the water quality preservation goals shall be selected, their efficacy for water purification
shall be estimated, and the estimated results will need to be incorporated into considerations concerning
role-sharing.
Selection of measures needs to take into account the characteristics of the water to be purified; the impacts onother water quality items and the ecosystem; the maintenance requirements; and the necessary length of theoperation in years, etc.
Water quality preservation measures for rivers, reservoirs, and lakes and swamps can be divided into thefollowing functional categories, and measures for preservation of water quality shall be developed byappropriately combining these:- Load reduction- Load separation- Securement of flow volume- Reinforcement of the purification function of the river- Water temperature and flow control- OtherIf early achievement of water quality preservation goals is impossible owing to a wide discrepancy between thecurrent water quality and the target quality, or to difficulties with the technical and economic feasibility of
implementing the measures, etc., a staged program or alternative plan shall be considered if this is necessary.
Measures to counter the occurrence of water quality hazards caused by abnormal water quality, etc. shall be
examined through cooperation and collaboration with relevant organizations and bodies, information gathering,and the preparation of resources and equipment, etc.
Measures for water quality preservation in the basin shall be planned and implemented primarily by relevantorganizations and bodies in the basin. It is important to make these measures effective by combining functions
and in consideration of the effects on the environment in the lower reaches.
Refer to Chapters 2 through 4 for water quality preservation measures that can be implemented in the basin by
river administrators.

Section 2 Comprehensive Sediment Management
2.1 Overview
In the process of developing plans for comprehensive sediment management, the actual state of sediment
movement shall be adequately determined on the basis of the characteristics of the river or seacoast, and thenecessary surveys shall be conducted in consideration of the relationships between the spatial as well as
temporal continuity associated with sediment movement, the quantities and quality (grain size) of the sediment,
and the river flow, etc.

Explanation
In a sediment transport system where problems related with sediment movement are apparent, efforts shall be120

made to understand the characteristics of sediment movement, etc. through determining the characteristics of theriver or seacoast and performing monitoring surveys of sediment movement.
Comprehensive sediment management needs to take into consideration spatial continuity such as longitudinalcontinuity from upstream to downstream of the river or continuity in the longshore direction of the seacoast.
Since riverbed configurations are changed by sediment movement not only caused by flooding but also occurringin normal times, sediment movement at normal times should be included in considerations of temporalcontinuity.
In addition, since a mixture of diverse grain sizes exists in areas from mountain lands and hillside lands to theestuary, and since ecosystems and natural environments that are suitable for grain size distribution and river flow
as well as different patterns of utilization of river spaces exist, not just the sediment quantity but the river
channel morphology and the quality of sediment (grain size, etc.) need to be considered.
Although sediment travels in a river discontinuously, its primary moving force is flowing water. For this reason,
it is important to examine the quantitative characteristics of the river flow regime in the sediment transportsystem for adequate implementation of sediment management.

2.2 Basics of facility installation planning
If the basin, etc. has apparent problems related to sediment movement, a facility installation plan shall be
designed by appropriately combining the multiple measures needed for comprehensive sediment management in
the subject sediment transport system.

Explanation
A facility installation plan shall be designed by combining the necessary measures selected from the following:
1. Erosion control by flowing sediment
Measures for the prevention of disasters caused by sediment movement, such as hillside failures, landslides, anddebris flows, as well as for the development and preservation of fluvial forest zones as part of the disasterprevention measures, shall be implemented. In addition, the status of the basin, such as its topography andgeology, the functions expected of erosion control dikes, and the effects on subjects requiring preservation in thelower reaches shall be comprehensively examined. Thereby, an erosion control dike shall be installed that is
capable of capturing hazardous sediment and safely allowing the necessary sediment (in terms of quantity andquality, i.e. grain size) to flow downstream.
2. Reuse of sediment within the sediment transport system
Sediment deposited at river administration facilities such as dams and groundsills, as well as in erosion andsediment control facilities such as erosion control dikes and sediment-retarding basins, shall be removed andcarried out within the extent that its purpose and function are not damaged. It shall be reused for a seacoast thatneeds beach nourishment or for a remarkably degraded river, etc.
3. Sand removal plan for a dam
Measures for discharging sediment from a dam and appropriately flowing sediment downstream without
undermining the expected purpose of the dam shall be developed.
4. Appropriate maintenance of river structures
121

For cross-river structures, etc. whose safety is deteriorating due to riverbed degradation, reinforcement measures
such as revetment attachment or bed protection work shall be implemented as appropriate.
Direct supply of the sediment downstream shall also be examined.
5. Appropriate gravel quarrying
For a river that has a sufficient cross-sectional area, if future sediment supply in great amounts is not expected,
gravel quarrying must not be carried out because it causes excessive riverbed degradation and has adverse effects
on the environment. For a river where sediment deposition is apparent, if no problems are expected to arise interms of flood control, water utilization, and environment preservation, well-planned gravel quarrying shall beappropriately carried out in consideration of the balance between river channel or seacoast maintenance andsediment supply.
Reference
National Census on River Environment (River Edition) Guide for Surveys for Creation of General Material
Concerning River Environment (Draft), 2001 Edition: Supervised by River Environment Division, River Bureau,
Ministry of Land, Infrastructure and Transport (MLIT). Issued by Foundation for Riverfront Improvement andRestoration, 2001

122

Chapter 2 River Facility Installation Planning
Chapter 2–1 River Channel and River Structures
Section 1 River Channel Planning
1.1 Basics of river channel planning
1.1.1 Basics of river channel planning
A river channel plan shall be established for the purpose of safe downstream flow of flow volumes below thedesign high water discharge and in consideration of the improvement and preservation of the river environment.
In the process of plan development, not only the status of land use in riverside areas, but also comprehensive
sediment management, if necessary, shall be considered.

Explanation
A “river channel” means the land space in which the water travels, and it usually means the portion surroundedby the embankment or the riverbank and riverbed. Since many changes take place in a river channel, andsediment and other things flow in the water, it is important to thoroughly examine whether the functions related
to the planned target can be maintained after the project and what the necessary maintenance will be. For rivers
that show obvious sediment movement, this should especially be taken into due consideration. Also, in the caseof rivers that are in basins below mountains that actively yield sediment, or that flow to beaches whose
sediments are supplied from the river, it is desirable to consider the relationship between these areas andcomprehensive sediment management.
River channel planning shall be based on nature-oriented river reconstruction that takes into consideration thediverse channel topography formed by the characteristics of the subject river course, as well as the naturalenvironment and landscapes formed there, and it is necessary to take into consideration river utilization and thediverse aspects of the river, such as its history and culture and the day-to-day relationship between the river andits people.
In the process of developing a river channel plan, it is important to determine the optimal longitudinal andtransverse shapes and the planar shape, etc., through a comprehensive examination of quantitative stability(securing of discharge capacity), securing of qualitative stability (safety of river management facilities, safety of
infiltration surfaces of embankments, etc. against erosion caused by sediment movement), minimization of total
cost, and improvement and preservation of the river environment (preservation and restoration of the naturalenvironment, harmonization with river utilization).
In conventional expressions such as "design bed slope" and "design bed height", use of the word "design", may
lead to a false understanding that uniform longitudinal and transverse heights must be secured; these expressions
have therefore been corrected.

123

1.1.2 Procedure for establishing a river channel plan
In establishing a river channel plan, the following procedure shall be followed to carry out a detailed
examination while considering the current issues associated with the channel, the status of the peripheral area,
the natural environment of the locality, and the social environment, as well as their historical transition.
1.

Determine the design high water level.

2.

Determine project sections, depending on the reasons for improvement.

3.

Establish multiple examination case studies in regard to the planned line and the longitudinal and transverse
shapes of the river channel.

4.

Establish plans for river structure construction, etc.

5.

Comprehensively assess the possible effects and impacts on flood control, water utilization, and theenvironment.

On the basis of the comprehensive assessment, the necessary corrections shall be made to the plan until it
becomes well-balanced.

Explanation
A river channel plan shall be established in such a way that it meets well the purposes stipulated in "Chapter 1:
Basic Policy" of the Basic Planning part. First, the characteristics of the river, including the natural conditions
such as topography, soil nature, etc., and the natural and social environments of the locality and their historicaltransition (based on Chapter 2 in the Basic Planning section) shall be determined. On the basis of these and inconsideration of the goals for flood control, water utilization, and environmental preservation, etc. (based on
Chapter 2 in the Basic Planning section), a detailed examination of a possible river channel plan shall be carriedout. After determining the design high water level on the basis of the concept described in the next section, thequestion of whether the design high water discharge can flow shall be examined. On the basis of the results ofthis examination, too, any lack of discharge capacity, the existence of crossings that hinder flood discharge,
inadequate traits of the river channel, and causes of past major disasters, etc. shall be investigated, thereby
identifying the reasons for improvement and determining the sections of interest. On the basis of these
examinations, multiple examination case studies of the planar and longitudinal and transverse shapes of the river
channel shall be established.
In each examination case, a plan for the installation of structures to prevent and control erosion, corrosion, andsediment deposition in the channel and thus enable prolonged channel stability through the use of the planar andlongitudinal and transverse shapes, etc. that have been determined shall be examined. If necessary, newconstruction, reconstruction, or elimination and consolidation of weirs, sluices, water gates, and other facilities(including structures whose main purpose is other than flood control) shall be examined. In establishing ameasure for river facility installation, the river water behavior at normal times and in times of flood, as well aschanges in the riverbed and riverbank shape characteristics, the soil and geological properties, and the sedimenttransport characteristics, should be taken into due consideration. Its positioning and role in river channel
planning, and the criteria used to order the priority and timing for installation shall be clarified. A measure that124

draws the expected effects from the minimum necessary construction of new facilities or reconstruction ofexisting facilities without overly depending on structures, while taking into due consideration the improvement
and preservation of a healthy river environment, shall be developed.
The effects and impacts on flood control, water utilization, and the environment shall be comprehensively
estimated for each examination case.

1.2 Design high water level
1.2.1 Basics of determining the design high water level
When reviewing a river channel plan for a river for which a design high water level has already been
determined, as a general rule, the revised level shall not exceed the existing design high water level. In cases
where partial raising of the design high water level is unavoidable, the area for its application shall be kept to aminimum; it is desirable for the design high water level to be kept under the highest high water level of past
floods, if at all possible.
When determining a new design high water level for a river for which no such level has been determined (forexample, in the case of construction of a new stream such as a cutoff channel or discharge channel, or for a riverfor which overall river improvement is to be implemented), the design high water level shall be determined insuch a way that the height exceeding the riverside ground level is minimized, in consideration of the design highwater level of connecting rivers and the regional characteristics, etc. Especially, for a river whose design scale issmall and for which a sufficient water surface gradient can be secured in consideration of the conditions of thedownstream river channel, the design high water level shall be set approximately to the ground level.

Explanation
The design high water level is the water level below which the design high water discharge can flow, and it is abasic determinant used for various purposes such as determining an embankment height or bridge beam height,
or for planning a branch channel and drainage of the inner water. It is also the most important planning factor
associated with river management and may even determine the scale of disaster when a flood levee failure occurs.
Therefore, in the process of examining a river channel plan, the question of how the design high water levelshould be determined is the key issue to be examined. The majority of rivers have already had design high waterlevels set, and river improvement and management have been implemented on the basis of these. Consequently,
in this section, the basics of determining the design high water level will be described in the following two cases:
1) when the design high water level has already been determined; and 2) when the design high water level has
not been determined, or when it has been determined but full-scale improvement, such as that for a new river(often a medium-sized or small river) is to be implemented.
In the case where a river channel plan is reviewed for a river section for which a design high water level was
determined in the past, raising the design high water level would be tantamount to full-scale river improvement.
This would be unrealistic, except in cases where the design high water level is raised for limited portions only. It
is also contrary to the broad principle of flood control, which is to discharge flood water at a water level as low
as possible; therefore, in most cases the existing design high water level is adhered to.
When determining a new design high water level for a river for which a level has not been determined in the past125

(for example, in the case of new river improvement, or a river for which the existing design high water level
does not have to be adhered to because a full-scale river improvement project is being conducted), the design
high water level shall be determined in such a way that the height exceeding the riverside ground level will beminimized, in consideration of the regional characteristics such as topography and land-use status, etc. For ariver with a small design scale, especially, since the possibility of flood occurrence exceeding the design highwater level is high, positive consideration should be given to planning excavated river channel. In this case, dueconsideration needs to be given to securing the groundwater level at times of low water discharge, securing theintake level of various service waters, and measures for maintaining the normal functions of river water andpreserving the river environment. If the design high water level for an upstream river channel is madeexcessively low compared with the landside ground level on the basis of considerations of the safety of the entire
river system, then the discharge capacity of that section will be substantially enhanced and a flow volume
exceeding the design level can be discharged. Therefore, any flow volume exceeding the design level will bedischarged to a downstream river channel with embankments, causing a serious problem in terms of downstream
river safety. Consequently, it is very desirable to determine a design high water level that is approximately equal
to the ground level.

1.2.2 Design high water levels of branches within a backwater section of the main stream
The design high water levels of branches within a backwater section of the main stream shall be determined on
the basis of the water level described in the following 1 or 2, whichever is higher:
1.

The water level obtained by backwater calculation when a volume of the design high water level is flowedin the main stream and a combined volume corresponding to the peak discharge of the main stream isflowed in the branches.

2.

The water level obtained by backwater calculation using, as the starting water level, the water level of themain stream corresponding to the flow volume of the main stream when the design high water discharges
from the branches join the main stream.

In cases where the ratio of the design high water levels of the branches to that of the main stream are relatively
small, the water level obtained by uniform flow calculation for the design high water discharges of the branches
may be used instead of the water level obtained by 2.

Explanation
Ideally, the design high water level of branches within the backwater section of the main stream shall be
determined by including the high water levels of the branches based on the flood discharges of the branches
corresponding to each water level of the main stream, although, in general, such an extensive examination is notnecessary. It is safe to regard the following two cases as criteria: 1) when the main stream flow reaches thedesign high water level; and 2) when the branch flows reach the design high water discharge.
If the situations in the main stream basin and the branch basin are radically different and seem to have little to dowith the peak occurrence, then the backwater of the main stream is presumably almost horizontal. If this is thecase, the water level in 1 above shall be that which is horizontal to the water level of the main stream at theconfluence.
1261.2.3 Design high water level in estuaries
The design high water level in estuaries shall be determined from the hydraulic and meteorologicalcharacteristics of the river and sea areas around the estuary and in consideration of the river channel
characteristics at, and around, the estuary, as well as of future measures for estuary improvement.

Explanation
With waves, flows, and tides, and the coexistence of freshwater and seawater, the hydraulic phenomena
occurring in and around a river estuary are physically very complicated. Therefore, the design high water level
for an estuary area needs to be determined in consideration of phenomena such as riverbed deformation in timesof flood, the effects of the chloride content of seawater, and the hydraulic phenomena around the estuary, such assea level departure from normal—whichever are regarded as factors necessary for the plan. If the estuary may bereclaimed or dredged in the future, the resulting effects should be taken into due consideration.
1. Riverbed deformation at times of flood
There are rivers for which the effects on the water level of the presence of a sand spit at the river mouth,
topographical deformation of the sand spit at the river mouth, riverbed raising, etc. in times of flood should not
be ignored. For such rivers, topographic deformation of the estuary in times of flood needs to be taken into
consideration.
2. Effects of chloride content of seawater
In some cases, the density difference between river water and seawater in the estuary causes a saltwater wedge,
making it impossible to regard the area between the water surface and the riverbed as an effective cross-sectional
area. In such cases, the amount of rise in water level due to the effects of the chloride content of seawater needs
to be taken into consideration.
3. Co-occurrence of a flood and storm surge
For rivers where the possibility of simultaneous occurrence of a flood and a storm surge due to a typhoon is high,the relationship between flood peaks and sea level departures from normal in past flood instances shall beanalyzed. The design high water level for the estuary shall be determined as appropriate on the basis of thehighest high tide level or watermarks in times of flood and the water level that takes into consideration the sameprobability deviation as the scale of the flood prevention plan. If the hinterland around the river mouth isespecially important, it is necessary to also examine water levels that take into consideration the probability
deviation corresponding to the scale of the flood prevention plan.

1.3 Planar shape and longitudinal and transverse shapes of a river channel
1.3.1 Basics of the planar shape of a river channel
The planar shape of the project section requiring improvement shall be determined primarily on the basis of the127planar shape of the current channel and in accordance with the directions in terms of flood control, water
utilization, and environmental preservation. Discharge channels and cutoff channels shall be designed to haveappropriate planar shapes in consideration of their effects on the surrounding environment.

Explanation
In general, river improvement works are carried out along the current river channel, taking into consideration the
topography, land-use pattern, and difficulty of land acquisition along the current channel. Accordingly, the design
planar shape shall be determined primarily from the planar shape of the current channel and in accordance withthe directions in terms of flood control, water utilization, and environmental preservation.
In many cases it is more advantageous to determine a planar shape similar to that of the current channel in viewof the need to preserve the natural environment. In cases where the area where the river is broadened has avaluable natural environment, however, a different planar shape needs to be examined, depending on thenecessity. If the current river course is extremely tortuous, or has a crowded, large-scale residential area along it,it may be necessary to examine a planar shape that includes the construction of a new channel, such as adischarge channel or cutoff channel, etc. In such a case, several planar shapes shall be examined for
combinations of the current channel and new channel. From among these, the optimal planar shape shall beselected through examination of topography, geology, current and future land use (note that area separation needs
to be considered), administrative divisions or districts, irrigation and drainage canal systems, effects on
groundwater, landside water drainage, effects on areas upstream and downstream of the project section, naturalenvironment, landscape, economic efficiency, and maintenance after improvement, etc.

1.3.2 Normal line of the levee
The normal line of the levee (including sections of excavated river channels) shall be determined in a way thatthe necessary river width is secured in view of various factors, including the design high water discharge,
riverside land utilization, natural environment, flow regime in times of flood, current river channel, future
maintenance of the river channel, and economic efficiency, etc.

Explanation
The normal line of the levee defines the river width on the basis of which the planar shape of a river necessary
for flowing the design high water discharge is determined. It is the most important condition that constrains theuse of the landside area. Appropriate river widths vary depending on the water depth, gradient, and riverbed
roughness, even if rivers have the same design high water discharge. They also differ depending on the status of
the existing levees, the crowdedness of riverside residential houses, the natural environment and river utilization,and the land acquisition status. Therefore, the river width needs to be defined as part of a comprehensive
examination of river channel planning. In the process of determining the normal line of the levee, theabove-mentioned and the following points must be taken into due consideration:
1. The relationship with the natural environment typical of the subject river and the status of river utilization

128

shall be taken into due consideration, so that improvement and preservation of the river environment will befacilitated.
2. Even if the current river width is large enough in terms of discharge capacity, it is desirable, in general, tosecure a width deemed sufficient to take advantage of the river’s storage effect. Although it is the general rulethat the flow volume reduction effect due to channel storage is not included in the consideration of the expected
effects (because it differs depending on individual flood), this does not mean underestimation of channel storage
effect.
3. Various factors such as the securing of levee safety, maintenance of the river channel flow against erosion andsediment deposition, etc. shall be examined in a comprehensive manner on the basis of the flow regime in timesof flood. For flash streams, the normal line of the levee is usually defined as a nearly straight line. For slow
streams, it does not have to be a straight line, but an extremely tortuous line should be avoided. In some cases, anappropriately meandering line may be used to limit the scope of the need for levee and river bank erosion
control.
4. When choosing to use a meandering shape, the current river channel, hinterland topography and geology, andland utilization, etc. shall be taken into consideration, and attention should be given to avoiding locating areas
with densely built houses and closure points of the former river at water-collision fronts.

1.3.3 Shape of a tributary confluence
The shape of a tributary confluence, as a general rule, shall be one that allows the tributary to smoothly join themain stream, and it shall be determined in consideration of the flow regime as well as the corrosion anddeposition status of the confluence. This is not the case, however, if the design high water discharge of thetributary is much smaller than that of the main stream, thus having only a negligible effect on the confluence. In
determining the longitudinal shape of the confluence, consideration should be given to the free migration of
aquatic biota.

Explanation
At a tributary confluence, because (more than) two streams that are different in flow direction and velocity join,
the flow regime is more complicated than that in a normal channel section. Consequently, this may cause a highflow velocity or the formation of dead water zones along levees, as well as large-scale corrosion and sedimentdeposition and upstream water level rises due to flow resistance. Therefore, as a general rule, a tributary must be
made to join the main stream smoothly. This should stabilize the flood flow before and after the confluence andprevent riverbed corrosion and sediment deposition. This is not the case, however, if the design high water
discharge of the tributary is much smaller than that of the main stream, thus having only a negligible effect on
the confluence. In addition, since any difference in the level of the beds of the main stream and tributary at theconfluence will interfere with free migration of aquatic biota, the longitudinal shape of the confluence shall be
determined by taking the freedom of migration of aquatic life into consideration.
1291.3.4 Longitudinal shape of a river channel
The longitudinal shape of a river channel shall be determined in relation to the normal line of the levee and thetransverse shape of the channel and in consideration of the landside ground level, river environment, riverbed
stability, and economic efficiency, etc. In general, the longitudinal shape of the current river channel shall be
used as the primary determinant. For ordinary rivers, the riverbed slope is made steep to gentle from upstream todownstream, and consideration shall also be given to the groundwater level, the intake level of service water,
and the foundation heights of existing important structures, etc.

Explanation
While the transverse shapes of a riverbed are not of even height owing to the action of flowing water, thelongitudinal shape is, in general, determined from the average riverbed height of low water channels (riverbeds
not including riverbanks when using single cross-sections), and its gradient is determined as the bed slope. The
longitudinal shape of an ordinary river channel shall be determined from the longitudinal shape of the averageriverbed height of the current low water channels in accordance with " Research on River Channel
Characteristics" (Chapter 19 of the Research volume of the Technical Criteria for River Works [Proposed]). Thisis because using the current bed slope as a base, in general, is advantageous for future river channel maintenanceunless local changes are in progress in the current river channel. It is important that due consideration is given to
the foundation heights of important structures, the intake level of service water, the riverbed height of the mainstream at the confluence (if the subject river is a tributary), the riverbed heights at points where bedrock is
exposed, and the peripheral groundwater levels, etc. Since the longitudinal shape of a river channel is closely
related to the freedom of migration for aquatic biota, as well as to the quality of habitat for plants and animals,
such as riffles and pools, and to river utilization, etc., river environment factors such as the securing of
longitudinal continuity need to be given sufficient consideration.
If it is necessary to partly change the bed slope, as in the case of a cutoff channel, the longitudinal shape shall be
determined in consideration of the upstream and downstream bed slopes. When drastically changing the river
regime, the longitudinal shape needs to be determined by taking into account the transverse shapes and inconsideration of future river channel stability. For an ordinary river, it is the general practice to make the bed
slope steep to gentle from upstream to downstream.
If there is a need for riverbed stability, groundsills etc. can be installed, depending on the riverbed status. In thiscase, the position and direction of installation shall be determined in consideration of the planar shape of thechannel as well as the freedom of migration of aquatic life.
Because of riverbed deformation and partial deep corrosion etc. caused by flooding, the actual riverbed height
may be lower than the average bed height of the low water channel assumed at the time of planning. For thisreason, in deciding the longitudinal shape of the river channel it is important to pay adequate attention to
long-term, local riverbed deformation and to determine the bed height needed for structure planning anddesigning. Especially, for the upstream and downstream areas of groundsills, deepening of embedment andfoundation works for peripheral structures shall be examined on the assumption of possible bed deformation
occurrence. A groundsill installation plan shall be established in accordance with Section 7 of this chapter. For
130the examination of bed deformation, the description in "Sediment Load Research" (Chapter 14 of the Researchvolume of Technical Criteria for River Works [Draft]) should be useful.
In channels in mountainous land and upstream channels in alluvial fans where sediment yield is active, theriverbed may suddenly rise owing to sediment discharge caused by flooding. Although the occurrence of such aphenomenon is technically hard to predict, comprehensive countermeasures coupled with sediment discharge
control by sediment and erosion control facilities needs to be examined on the basis of past experiences. The
longitudinal shape of an estuary shall be determined in accordance with Section 8 of this chapter.

1.3.5 Transverse shape of a river channel
The transverse shape of a river channel shall be determined by taking into due consideration various factors suchas the longitudinal shape of the channel and the topography, geology, river environment (including habitat forplants and animals), and riverside land utilization, as well as long-term local bed deformation. If there is a low
water channel, the design position of the low-water channel bank shall be determined on the basis of the low
water channel bank location needed for levee protection (levee protection lines), and, if necessary, the low water
channel bank location needed for high-water channel utilization (low-water channel bank management lines).

Explanation
Since the difference between the minimum and maximum flows of rivers in Japan is generally large, compoundcross-sections are usually employed for channel stability. In the long run, the transverse shape of a river channel
naturally changes because of floods, etc., and local deep corrosion associated with the normal line andlongitudinal shape may also occur. The transverse shape shall be determined and the river channel control
facilities shall be planned and designed in due consideration of these points.
When a wide, flashy stream has several watercourses that fluctuate, in many cases it is impossible to clearly
differentiate between the low water channel and high water channel from the viewpoint of channel maintenance.
For a river with a small design high water discharge, the transverse shape is usually a single cross-section.
The "design transverse shape" does not mean that all elements of a transverse shape should be determined asplanned items, but it identifies minimum necessary essentials such as the levee height and width, high-water
channel height and width, and low water channel bank position, all of which are necessary for river management.
It should be noted that the design transverse shape needs to take into consideration various factors such as
riverbed deformation caused by natural force.
The transverse shape of a river channel is equally important for preservation of the natural environment, river
utilization, etc., and the flow regime at normal times and at times of flood should be determined so that thetransverse shape of the river channel that is appropriate in consideration of the effects of natural forces such as
flowing water can be determined. If the ratio of the flow section to the cross-section within the set normal line of
the levee needs to be relatively large, excavation of a low water channel bed, widening of a low water channel,
or excavation of a high water channel shall be considered. Whichever is the case, an appropriate method needs tobe employed by comprehensively examining various factors such as river channel stability, effects on rivermanagement facilities, river space utilization, and impacts on the river environment, etc. When low water
channel excavation is necessary, the transverse shape of the low water channel to be formed needs to be designed
131to take a riverbed or riverbank shape that is appropriate for preservation and restoration of riffles and pools aswell as healthy habitat for plants and animals, as based on the original state of the river.
The setting of a low water channel is especially important when determining the transverse shape. In therelationship with the planar shape of the river channel, some sections of the channel are more vulnerable to bankerosion due to flooding than other sections, depending on the normal lines or whether they have sand banks. For
wide, large rivers, lines for the purpose of levee protection (levee protection lines) and lines for the stabilizationof the low water channel bank (low water channel bank management lines) shall be determined, and it isimportant to designate bank erosion prevention points in consideration of those lines as well as the current
profile.
The purpose of levee protection lines is to protect a levee from destruction due to bank erosion caused by floods.
Therefore, it is necessary to set these lines for all sections of the subject river that have levees. As a general rule,
the positions of levee protection lines shall be determined in a way that the levee will not be endangered by asingle flood event. In cases where a necessary high water channel width cannot be secured, bank protection (by,for example, the use of revetments) shall be applied.
Low water channel bank management lines indicate a section of a river where bank erosion needs to beprevented in order to secure the expected functions of the river channel in view of flood control, water utilization,and environment preservation, etc. They shall be set as appropriate for the purpose of low water channel bankstabilization. The location of a planned low water channel bank, as determined from the positions of levee
protection lines and low water channel bank management lines, shall be set as the design transverse shape.

1.3.6 Low water channel width and high water channel height
The low water channel width and the high water channel height shall be determined in consideration of river
channel maintenance, flood frequency of the high water channel, utilization, and preservation of habitats forplants and animals, etc.

Explanation
The low water channel width shall be generally determined with the emphasis on the current channel shape, andthe high water channel height shall be usually determined through estimation that assumes a discharge capacity
allowing one flood occurrence per several years. However, since high water channel utilization patterns arediverse and vary from area to area, it is necessary to take into consideration various factors such as importance ofpreservation and restoration of healthy habitats for animals and plants in the channel and surrounding areas,
predicted future transition of vegetation, installation of structures, etc. in determining the low water channel
width and high water channel height. While the high water channel height shall be determined based on thehistorical transition of the longitudinal as well as transverse shapes of the channel and flood frequency and inconsideration of the low water channel width, it is not desirable, from the viewpoint of high water channel
maintenance, to let the current velocity of the high water channel become excessively high in order to secure its
stabilization at times of flood.

1321.3.7 Installation of a levee-side riparian forest zone
A levee-side riparian forest zone shall be installed, if necessary, for a section in possible danger of severe
damage caused by levee break or flooding.

Explanation
A levee-side riparian forest zone shall be installed to prevent levee break due to prevention of corrosion at timesof overflow and to prevent widening of levee break caused by flooding flow at a time of levee break when thesubject area is prone to serious damage due to levee break and flooding.
When installing a riparian forest zone, harmonization with the surrounding natural environment including
vegetation and utilization of landside area shall be taken into consideration.

1.4 Estuary planning
In developing plans for an estuary, the longitudinal and transverse shapes shall be deliberately determined taking
into due consideration both the river and sea conditions so that they satisfy the following conditions.
1.

They are well capable of handling the design high water discharge, as well as storm surges and tsunami incase such events should occur.

2.

Future maintenance is easy.

3.

They do not interfere with water utilization around the estuary at times of low water.

4.

Due consideration is given for the natural environment around the estuary.

Explanation
Plans for an estuary are roughly divided into river channel planning and planning for measures against storm
surge around the estuary. A channel plan requires examination of the design high water level, longitudinal andtransverse shapes, and measures against estuary closure (such as the installation of a guide levee, etc.) or estuary
improvement measures. In cases where it is deemed necessary to consider measures against storm surges ortsunamis, such measures need to be consistent with storm surge measures etc. for the seacoast. Measures against
storm surges include the installation of a tide gate at the estuary and installation of a dike for the designated hightide section. If a measure against storm surges is required, the most appropriate method shall be chosen throughexamination of the tide level, flood and wind wave characteristics, economic efficiency, method for estuary
improvement, etc.
An estuary receives forces acting from both the river and the sea, and therefore overall consideration of both theriver and sea conditions is demanded. In establishing a plan for an estuary, examination based on an appropriate
survey shall be conducted in accordance with "Estuary Research" (Chapter 9 of the Research volume ofTechnical Criteria for River Works [Proposed]). With waves, flows, tides, and the presence of both freshwaterand seawater, hydraulic phenomena and sediment movement around an estuary are extremely complicatedphysically, making it technically difficult to assess the effects and impacts of improvement works, etc.
Consequently, river channel planning for an estuary requires examination based on adequate field surveys and inreference to the case of a similar river, and, if necessary, the conducting of a hydraulic model experiment and use

133

of a numerical simulation.
Although the river width at an estuary is considered to depend on the river flow, bed slope, bed material, andsediment load, etc., there is no definite rule on this point. The present conditions should be maintained to as greatan extent as possible, unless river basin adjustment or some other significant changes (such as those in normal
flow or low flow) take place. If there is a sand spit at the river mouth and maintenance of the river channel
without the sand spit is presumably impossible; or if removal of the sand spit is expected to damage theprotection function against wind wave and salt water invasion and bring serious adverse effects; or if the sandspit is playing an important role in the estuary ecosystem, then the sand spit should be maintained in its current
condition, if at all possible, and longitudinal and transverse shapes that take the presence of the sand spit into
consideration should be determined. Even if there is no sand spit at the river mouth, widening or bed excavation
at the estuary must be avoided as much as possible, since this may adversely affect the maintenance andmanagement of the estuary because of its function as a sediment deposition space. If widening or excavation isabsolutely necessary, sufficient examination should be conducted to establish adequate measures.
The transverse shape of an estuary is usually determined from the transverse shape of a section slightly upstream,in an area that has river course characteristics identical to those in the estuary. When widening the current rivermouth, however, the same considerations are required, because the widening is related to estuary maintenance.
Since an estuary has a river environment with the natural characteristics of a brackish-water region and a tidal
flat, estuary planning needs to be harmonized with the local natural environment. However, because we haveinsufficient physicochemical and ecological knowledge of the environmental characteristics of estuaries,
deliberate research and examination based on information on similar rivers, and past experience, are important.

Section 2 Cutoff channel and discharge channel
2.1 Cutoff channel and discharge channel planning
A cutoff channel (or short-cut) is a waterway constructed to short-circuit an extremely tortuous river course.
A discharge channel (or diversion channel) is a waterway that branches part or all of a flood midway along ariver and discharges it to another river, back to the main stream, or directly to the sea.
In planning the construction of a new waterway such as a cutoff channel or discharge channel, an appropriate
plan shall be established that ensures safe flow of the flood, taking into consideration various factors such as theenvironment of and around the new channel; the present and future social environment; the groundwater levelsof the surrounding area; the quality of the groundwater; any irrigation and drainage canal systems; the landsidewater drainage, and post-construction river course maintenance, as well as impacts on the environments of thedestination water body and the source river.

Explanation
1. Cutoff channel and discharge channel
Many large-scale cutoff channels were constructed in the past for large rivers that had numerous meandering andtortuous sections. Today, however, because the planar shapes of the major sections of large rivers have mostly
been fixed as a result of river improvement endeavors, there are few cases of large-scale cutoff channel
134construction. On the other hand, many cutoff channels were built until recently as part of river improvement
works for extremely tortuous sections of medium-sized and small rivers.
Also, especially in large cities, the unavoidable use of tunnel structures to construct discharge channels becauseof topographic- and land-use–related constraints is not rare.
There are other cases of new channel construction, such as when moving a confluence to another location to
reduce the effects of back water from the main stream, or to short-circuit the flood flow from the mountainous
watershed of a tributary.
2. New channel construction planning(1) The bed slope of a cutoff channel is steeper than that of the main stream owing to shortening of the channel
extension. For a meandering river with a stabilized bed, bed stability will be lost. For example, increased
current velocities in the upper reaches may cause corrosion, endangering upstream structures, as well as
sediment deposition in the lower reaches, resulting in rises in water level.
For this reason, in the process of cutoff channel construction planning, the bed slope and trait of the river
channel shall be determined and a plan for improvement of the transverse shape shall be established for not
only the cutoff channel section but also the upstream and downstream sections that will be affected.
Therefore, if the need arises, investigations shall be conducted on the channel shapes of the short-cut section
and the upstream and downstream sections to be affected, as well as on the bed slope, river structures, bed
material, and bed fluctuation. The bed fluctuation based on the projected channel shall be predicted, and an
optimal plan shall be developed through predicted corrections to the new channel and repeated prediction
calculations.
(2) A discharge channel construction plan shall be designed in such a way that the flood discharge can be safely
flowed and in consideration of the surrounding environment, social environment, groundwater level,
groundwater quality, irrigation and drainage canal systems, landside water drainage, and maintenance of the
new channel, etc. A thorough examination shall also be conducted into the possible impacts on theenvironments of the destination water body and the source river, and of the economic efficiency.
a) It shall be determined whether natural diversion or artificial diversion by the use of a structure such as afixed crest weir or a water gate is to be applied. When a structure is to be installed, it shall be determined
whether the structure is necessary on the main stream side, the discharge channel side, or both sides.
b) In terms of the longitudinal shape, as in the case of cutoff channels the discharge channel section is steeper
than in the upstream and downstream sections; in many cases it has a larger bed fluctuation owing todifferences in bed materials. Accordingly, a thorough examination is necessary when determining amethod for reducing flood energy, and structural safety measures through embedment deepening etc. shallbe examined with deliberation.
c) Although low water diversion is not commonly carried out in the case of a discharge channel used for highwater diversion (in order not to limit the utilization of water in the main stream), river functions at normal
times shall also be examined (for example, in the case of the need to convey water for purification in times
135

of high water).
(3) When cutting a new channel such as a cutoff channel or discharge channel, efforts should be made toprevent new landside water problems by taking landside water drainage into due consideration. To this end,
it is important to gain sufficient knowledge and understanding of the existing discharge canal systems inriverside areas.
Measures for landside water drainage shall be developed for the leveed section such that the current
drainability is not damaged. Also, if the condition of the upstream and downstream channels permits,
excavated river channel should be planned, if this is at all possible. In this case, inflow from riverside areas
along the new channel shall be included in the design high water discharge.
Furthermore, to avoid significant trouble, adequate surveys shall be conducted to predict any potential
effects on groundwater level and quality.

2.2 River tunnel structures
2.2.1 Basics of planning
A river tunnel structure should not be installed unless it is unavoidable in light of topographic features or forother special reasons. The route shall be determined from surveys of topographic and geological conditions,overground land use, and underground installations and utilities, etc. The alignment of the river tunnel structure
should not being extremely tortuous, if at all possible.
The current river course shall be maintained, except in cases where this is impossible for unavoidable reasons.

Explanation
Compared with ordinary rivers, rivers with tunnel structures present many more problems in terms of channel
maintenance, such as cross-section occlusion caused by falling objects in times of flood. There are also
management-related problems caused by human error, etc. For these reasons, when planning cutoff channel ordischarge channel construction as part of a river course plan, it is desirable to avoid a plan that calls for theconstruction of tunnels. However, if the lower reaches of the river are urbanized, making adequate channel
widening impossible, or if a diversion channel has to be installed through a built-up area, or if the topography
does not permit the choice of an open channel, an examination, including a comparison with alternative plans,must be carried out. The examination must take into due consideration various factors, such as the possible
effects of channel widening on urban functions etc., the urgency of the flood control project, the improvement of
the river environment, constraints on construction, and economic efficiency, etc.
After the introduction of the tunnel structure channel, unless it is absolutely necessary to demolish the structure
the current channel should be maintained so that the situation will not become worse than at present, even if the
new channel becomes unusable for some reason. Securing of the current channel is also important in terms ofpreservation of valuable waterfront space in urbanized areas. Compared with other urban land uses, rivers inurbanized areas form spaces that are far superior in scale and continuity and richer in diverse environmental
functions. Therefore, it is not at all desirable to demolish or reduce the size of the current channel simply for
intensive land use after the construction of the tunnel structure channel.
1362.2.2 Cross-section and longitudinal slope
The tunnel cross-section needs to be large enough to include, as a rule, a sufficient vacant area in addition to asectional area necessary for the design flow.
The tunnel longitudinal slope shall be the most appropriate from the viewpoints of securing the flood control
function, hydraulic stability, and maintenance.

Explanation
1. Cross-section
In the case of free-flow channel-type tunnels, driftwood, suspended debris, etc. can cause discharge failures, andhigh-speed currents cause reductions in air pressure. To compensate for this, a vacant area based on a value of
not less than approximately 15% of the cross-sectional area necessary for the design flow shall be secured so that
there is sufficient air supply to permit airflow.
In the case of penstock-type tunnels, the cross-sectional shape shall be determined in consideration of discharge
capacity, aeration rate, negative pressure occurrence, cutoff performance, surging, and lining plans, etc., and an
inverted arch shall be introduced, depending on the maintenance plans. In planning for the construction of apenstock-type tunnel, the details of its aeration conditions shall be determined through model experiments. If
necessary, an appropriately expanded cross-sectional area shall be planned on the basis of the characteristics ofthe locality, including the presence of sediments, driftwood, and suspended debris. Furthermore, the intake or
inflow should be shaped appropriately to minimize aeration, and infrastructure for air removal from inside thetube shall be implemented.
2. Longitudinal slope
An inappropriate longitudinal slope in the tunnel may cause sediment deposition in low-gradient sections.
Therefore, the longitudinal slope needs to take into consideration the balance of scouring forces over the entire
section.
Because the flow velocity inside a penstock-type tunnel is related to the hydraulic gradient and is not dependent
on the waterway gradient, the longitudinal slope is normally determined from the viewpoint of tunnel
maintenance.
If the residual water in the penstock-type tunnel after a flood is to be drained from a downstream pumping
station, it is desirable to form the slope in such a way that the downstream side is down.

Section 3 Reservoir (Dam)
3.1 Flood control planning
3.1.1 Design high water discharge for dams
If the hydrograph for the dam location corresponding to the design flood determined in 2.7 of Section 2 in
Chapter 2 of the Basic Planning part, and the dam location itself, are the design control points, the design highwater discharge for the dam shall be determined rationally through examination of the peak discharge in the137hydrograph and the flood control capacity of the dam location.

Explanation
The design high water discharge for a dam is the basic quantity of a flood control plan for the dam, and it shallbe determined for each individual dam. Expected effects of the dam on the design flood of downstream design
control points, those on the channel directly beneath the dam location, and the harmony with flood control
facility planning for the entire river system shall be examined in a comprehensive manner.
The design high water discharge for a dam site, as a general rule, shall be determined on the basis of thehydrograph with the largest peak discharge selected from among the following:
1. The hydrograph with the largest peak discharge and the one with the highest flood control capacity for thedam location; these hydrographs are selected from among the hydrographs corresponding to the group used todetermine the design flood.
2. The hydrograph with the largest peak discharge and the one with the highest flood control capacity; these are
selected from among the group of hydrographs for the dam location obtained from the subject rainfall of the dam
area.

3.1.2 Flood control method
A dam-based flood control method shall be selected from among methods that can ensure the expected flood
control effectiveness of the downstream design control points in consideration of flood runoff characteristics,
control efficiency, sureness of operation, and ease of maintenance, etc.

Explanation
The most certain and effective method of dam-based flood control must be selected in consideration of the rivercondition, hydrologic characteristics of flood runoff, storage capacity, outlets, purpose of control, control
efficiency, sureness of operation, ease of maintenance, and channel improvement downstream of the dam, etc.
Dam-based flood control methods include the following:
1. Constant rate discharge method
When the inflow exceeds the flood control start-flow volume, this control method discharges a fixed rate of the
inflow until the peak flow is reached, and a fixed volume after the peak flow is reached. A great control effectcan also be expected for medium-sized and small floods. Generally, this method is suitable for rivers where theflood discharge from downstream of the dam constitutes a substantial portion of the flood discharge at thedownstream design control point, or for rivers whose channels leave much to be improved.
2. Constant discharge method
Because this is a peak-cut method that discharges a fixed quantity of flood volume regardless of the floodhydrograph, etc., high control effects can be expected from this method. This is especially the case for rivers
whose channels have been improved to a considerable level, although the control effect of this method isrelatively weak for medium-sized and small floods.
3. Natural regulation method138This method does not usually use a flood control gate, or, even when it does use one, the opening of the gate is
kept fixed and not operated for discharge control. Although a large flood control capacity is needed, because thismethod does not require human operation it is easy to manage and is used primarily for small-scale dams with
fast release.
4. Adaptive release-rate method
This method is adopted when there is a time lag of discharge between the dam location and the downstream
sector and control is needed, especially for the first or latter part of a flood, or when there is a need for storing
maximum flow.
For small-scale dams (approximately 20 km2 or smaller) or dams with a small flood control capacity (equivalent
rainfall* of approximately 50 mm or less), it is desirable to adopt the adaptive release-rate method in order toavoid cumbersome gate operation. For dams with an equivalent rainfall of approximately 100 mm or less,adoption of the natural regulation method should be considered.
Although the adaptive release-rate method is superior in efficiency, it demands accurate prediction of floodhydrographs.
All existing dams were constructed in response to the flood control or water utilization needs that arose at thetime of their construction. When multiple dams are constructed and operated within a single river system,
operation of individual dams does not always result in optimal operation of a group of dams in the area. If thegroup of dams includes dams that are advantageous in terms of flood control and ones that are advantageous interms of water utilization, depending on such conditions as rainfall or catchment area, better effects may beobtained by not just reconstructing them but also reallocating the flood control and water utilization capacities ofexisting dams and revising the administration rules, instead of operating the dams individually. The choice offlood control method for existing as well as new dams needs to be examined from this point of view.
*Flood control capacity / catchment area: represented in mm.

3.1.3 Flood control capacity
The storage capacity for flood control (flood control capacity) shall be determined on the basis of thehydrographs and the control methods applicable to flood control planning. An approximately 20% margin
capacity shall be included.

Explanation
For optimized utilization of the limited storage capacity, if a preliminary release system (which maintains theflood control capacity by preliminarily releasing water to lower the reservoir level at the time flood occurrence isforecast) is to be adopted to secure the flood control capacity, a thorough examination of the sureness of theoperation and the effects of the operation on the downstream sector should be conducted.
The flood control capacity of a reservoir, as a general rule, shall be determined by conducting flood control
calculations for the hydrograph for the dam location corresponding to the group of hydrographs used todetermine the design flood and the group of hydrographs for the dam location obtained from the subject rainfall139

of the dam area, then selecting the one with the largest control capacity needed

3.2 Other plans
3.2.1 Planning for sediment inflow control
A plan for sediment inflow control shall be established for the maintenance of a reservoir functions,comprehensive sediment management, and improvement and preservation of the river environment, etc., asappropriate.

Explanation
Measures for removal of sediment from a reservoir and the proper flow of sediment downstream without
detriment to the original purpose of the dam are important for comprehensive sediment management.
To this end, if the need arises, a sediment inflow control plan shall be established. It shall take into due
consideration impacts on the river environment as well as the life cycle cost by combining various measures suchas the control of sediment inflow into the reservoir (installation of a sediment trap dam immediately upstream of
the reservoir; stabilization of slopes around the reservoir, etc.); dredging and digging of sediment in thereservoir; and the installation of a sand-discharge pipe, a sand-trap scour gate, or a sand-discharge bypass

3.2.2 Planning for prevention of landslides around a reservoir
If necessary, appropriate landslide prevention works for land in or adjacent to the reservoir shall be planned inorder to prevent landslides caused by the storage of flowing water.

Explanation
To maintain the functions of the reservoir and secure safety around the reservoir it is necessary to prevent
landslides caused by the storage of flowing water.
For this purpose, landslide surveys shall be conducted around the reservoir in the investigation stage, as
appropriate.

3.2.3 Planning to prevent leakage around a reservoir
If necessary, leakage prevention works for land adjacent to the reservoir shall be planned to protect the reservoir
functions against leakage from the reservoir as a result of flowing water storage.

Explanation
Leakage from the reservoir to its neighboring land as a result of the storage of flowing water shall be prevented
in order to maintain the functions of the reservoir.
For this purpose, geological surveys shall be conducted around the reservoir in the investigation stage, as
appropriate.

1403.2.4 Planning for private hydropower generation
Installation of a private hydropower generation facility shall be examined for the rationalization of dam
management and the proper utilization of hydraulic energy resources from the dam.

Explanation
Electric power generated by private hydraulic facilities is used to run the dam maintenance facility, to power
aeration drainage to preserve the reservoir water quality, and to power fountains in the reservoir. Also, the cost of
dam management can be reduced by the sale of surplus power.

3.3 Environmental considerations
In dam construction planning, due consideration shall be given to environmental factors such as the water andsoil regimes, healthy habitats for plants and animals, activities that bring humans into contact with the river, andburdens on the environment.
Especially, it is necessary to thoroughly examine how changes in the flow regime after dam construction may
affect the lower reaches of the river.
In addition, on the basis of the natural and social environments of the basin, consideration should be given topreventing or reducing impacts on the environment and the creation of a new environment.

Explanation
1. The following items need to be considered in dam construction planning.
1) Preservation of the environment, including water and soil regimes
In dam construction planning, the following items need to be taken into consideration to prevent or reduce thepossible impacts of the dam on the water and soil regimes:- Changes in water quality, as indicated by indices of the water environment (e.g. turbid water by sediment,water temperature, eutrophication, amount of dissolved oxygen, hydrogen ion concentration)- Modifications to the topography, geology, ground, and soil, and changes in groundwater
2) Preservation of healthy habitats for plants and animals
In dam construction planning, the following items need to be taken into consideration in order to preserve
healthy habitats for plants and animals:(1) Animals- Modifications to major habitats such as breeding places of important species- Destruction of migratory pathways of important species- Modifications to noteworthy habitats- Environmental changes in the habitats of important species and noteworthy habitats (environmental changes
around the modified area, and changes in the bed material, flow regime, and water quality of the reaches
downstream of the dam)
(2) Plants- Modifications to the habitats of important species and communities

141

- Changes in the habitats of important species and communities (environmental changes around the changed
area, and changes in the bed material, flow regime, and water quality of the reaches downstream of the dam)
(3) Ecosystemsa) Modifications and changes to remarkable species of high rank aspect- Modifications to habitats such as breeding places- Destruction of migratory pathways- Changes in habitats (changes in the bed material, flow regime, and water quality of the reaches downstreamof the dam)b) Modifications and changes to remarkable species of typical aspect- Modifications to habitats- Changes in habitats (construction of a reservoir, changes in habitats due to sediment deposition in thereservoir, changes in the bed material, flow regime, and water quality of the reaches downstream of thedam)c) Modifications and changes to remarkable species of special aspect- Modifications to habitats- Changes in habitats (change in groundwater level, environmental changes around the modified area, changes
in the bed material, flow regime, and water quality of the reaches downstream of the dam)d) Modifications and changes to remarkable migratory species- Destruction of migratory pathways
3) Maintenance of places for activities that bring humans into contact with the river(1) Maintenance of a good landscape
In dam construction planning, the following items need to be taken into consideration in order to maintain goodlandscapes:- Degree of modification to major viewing points and landscape resources- Major changes in appearance- Considerations in regard to the topography, geology, and cultural assets(2) Maintenance of places for activities that bring humans into contact with the river
In dam construction planning, the following items need to be taken into consideration in order to secure places
for activities that bring humans into contact with the river.
- Degree of modification (annihilation, reduction) to places for activities that bring humans into contact withthe river and natural resources- Changes in the usability of places for activities that bring humans into contact with the river- Changes in the amenity of places for activities that bring humans into contact with the river4) Amount of environmental burden
A dam construction plan shall be established on the basis of the degree of environmental burden and inconsideration of the following items:- Amounts of construction byproducts (waste, recycling resources)
2. Securing of a flow volume necessary for the lower reaches downstream of the dam and changes in the flow
regime are the basis for dam construction planning and need to be thoroughly considered in the planning stage.
142

- Changes in the bed material due to changes in the flow regime (armoring)- Changes in flood frequency- Sections with easy access to the water- Migratory fish upstream- Changes in underground water due to changes in river level
3. Not only the viewpoint of preservation of the environment but also the viewpoint of creation of a newenvironment needs to be considered in the planning stage.
- Improvement of the water source area in harmony with the surrounding environment- Landscape design of the dam site
- Formation of places for activities that bring humans into contact with the environment and that have beenchanged by the appearance of the dam site, quarry site, and muck-disposal yard.

Section 4 Retarding Basins etc.
4.1 Basics of planning
Planning for retarding basins etc. should consider the target landform, land-use situation, groundwater level,
river situation, natural environment, river flow regulation condition, overflow frequency, economic efficiency,
maintenance, etc.

Explanation
Retarding basins etc. are mostly established not only to decrease the downstream peak runoff by storing part of
the flood waters but also, in flat areas, to perform some degree of water or effluent treatment prior to discharge
into the receiving channel.
Types of retarding basins include those that surround the reservoir with a levee, a detention reservoir partition
levee, or an overflow levee by taking advantage of natural landforms; those that ensure storage function by
excavation of the land; and those that ensure the storage function of underground space.
It is necessary to consider using the space occupied by a retarding basin for other aims in addition to the aim offlood control. Especially, in planning a multipurpose retarding basin that has a reservoir function, Chapter 2–3
should be referred to.

4.2 Site selection for retarding basins etc.
Retarding basins etc. should be established in places where flood control at the target site is certain and thestorage capacity is easily ensured.

Explanation
Although in many cases it is difficult to make an arbitrary selection of a retarding basin because of differences in
geographic characteristics, it is desirable to establish the basin as close to the target site as possible inconsideration of its flood regulation effect. However, the hydrograph of the target flood becomes progressively
flatter with distance downstream, so a retarding basin in a downstream area will need to be more capacious than143

the amount that can be cut. Thus, its area will need to be increased because it cannot have a water depth as great
as that of a reservoir in a mountainous area. Also, peripheral areas are conspicuously urbanized in downstreamsites, and site acquisition becomes more difficult, so these factors must be comprehensively considered.

4.3 Flood control planning
4.3.1 Flood control facility planning
Control facilities such as retarding basins should be planned to have sufficient control function to have different
effects for different purposes.

Explanation
Generally, unlike dams etc., in many cases these facilities have side overflows from river channels.
The items to consider in the control facility planning are given below:
1.

Height and length of the overflow levee

Although the height of the overflow levee is directly related to the flow at the start of control, the height andlength exert an influence on its control effect and control the figure of hydrograph after control, so theestablishment of the overflow levee needs to be carefully examined in consideration of its control purpose.
2.

River channel

River planning near an overflow levee needs to carefully consider the flow conditions in times of riverbed
fluctuation or flood.

4.3.2 Flow at the start of control
The flow at the start of control should be determined so as to ensure the desired effect in consideration of thecontrol effect and the characteristics of the flood runoff.

Explanation
The flow at the start of control is the one at the time inflow starts from overflow levees etc. into retarding basinsetc. Considering this flow as large, or making the overflow levee high, can produce a large effect against thedesign-scale flood but is less likely to have a sufficient effect against a small or medium-sized flood. Conversely,
making the flow small or the overflow levee lower can have a sufficient control effect against a small or
medium-sized flood, but the control effect against the design-scale flood will in many cases tend to decrease.
Thus, the flow at the start of control must be determined so as to ensure the desired effect in consideration of thecontrol purpose, flood runoff characteristics, retarding basin capacity, etc.

Section 5 Weirs, Water Gates, and Sluiceways
5.1 Establishment basis
The establishment sites of weirs, water gates, and sluiceways (hereinafter called weirs etc.) should be selected144

according to their own purposes. River bends, places with a small bed cross-section, and places with unstableriver regimes should be avoided if at all possible, in comprehensive consideration of the aspects of flood control,
beneficial water use, and environment. Also, these structures should be unified, when possible, to decrease thenumber of establishment sites needed.

Explanation
Weirs are classified into diversion weirs, final baffling weirs, and intake weirs according to their own
establishment purposes, or into fixed weirs and movable weirs according to their structures.
Water gates and sluiceways are facilities with embankment functions that are established across a river.
In selecting establishment sites for weirs etc., besides the fact that these structures should be selected so as tosufficiently accomplish their aims, it is desirable to select a site with little velocity variation because of its
straight stream axis and with little bed variation because of its stable stream axis. This is because the installationof, in particular, a weir, tends to cause disorder in the regime; the weir may act as an obstacle to water discharge
in times of flood. Although some people think that the installation of a weir in a narrow stream may be profitablein terms of savings in construction costs, installation in such places must be avoided as far as possible because it
requires special consideration of safe flow during flood or because the weir itself will be a condition constraining
the riverbed in the future.
Planning for the installation of weirs, etc., should comprehensively consider the direction to be taken in terms offlood control, beneficial water use, and environment on the basis of the natural and social environments of thesite of concern and their historical development. It should also examine the advantages, effects, and influence on
the water and soil environments, the habitats and growing environments of living things, the landscape, andplaces for activities that bring humans into contact with the rivers, so that the most suitable project is chosen.
Structural objects such as weirs etc. may be weaknesses of embankments; it is also desirable to decrease their
numbers in consideration of their maintenance, so their unification needs to be attempted as far as possible.
Additionally, in the case of planning for the installation of multipurpose weirs for their reservoir function, see
Chapters 2 and 3.

5.2 Weir flooding level
The weir flooding level should not only be at least 50 cm lower than the height of the flood plain; it should also
be the same amount lower than the inland ground level.
However, this is not the case when proper measures such as fitting etc. are taken.

Explanation
When the site and design flood level of a weir are specified, it is necessary to sufficiently consider changes in the
groundwater level and such problems as ill-drained inland. In some cases, owing to the topographic conditionsetc., it will be necessary to take appropriate measures such as embankment in the inland ground or flood plain.

145

5.3 Fishways in weirs
When there is concern that the establishment of a weir could hinder the movements of fish up and down thestream, a fishway should be installed.

Explanation
To minimize the effect of the establishment of a weir on the habitats of fish etc., it is desirable to install a
fishway for those fish that run up and down the river. Target fish not only include hat are considered importantresources.
The fishway plan should specify the target flow, water level, placement and so on so that the target fish can run
up and down the river and may not erratically enter the weir. It should consider the flow condition of the target
site, the extent of water level fluctuation on the downstream side of the weir, the time at which and the course
over which each target fish runs up and down, the manipulation of intake and discharge from the weir, etc.

Section 6 River Projects for Flow Regime Adjustment
Rivers requiring flow regime adjustment could be connected to two or more other rivers. Adjustments may beneeded for flood treatment; for conveying water to maintain flows by using the rivers’ mutual residual
discharges; for the development of new water uses; and for conveying water developed by other facilities.
To designing proper projects, the planning should examine the effects on the environment of the sites to andfrom which water is conveyed. It should also consider the environment within a new river and its surrounding
environment, its present and future social environments, the level and quality of the surrounding groundwater,
the irrigation and drainage system, and the landside water drainage control.

Explanation
When a river requiring flow regime adjustment is related to a reservoir facility such as a dam, it is necessary toshow their mutual supply relationship during the actual preparation of the project so that the amount ofdeveloped water does not overlap.
Also, in specifying the amount of raw water conveyance sufficient research must be done to ensure that thenormal discharge or water use of the river from which the water is conveyed is not disturbed.

Section 7 Channel Control Facilities
7.1 Basis of a channel control facility design program
In planning facilities to control embankment or riverside erosion and riverbed scour or sedimentation, thefacility type, the normal line of the embankment to be established, and the establishment site and its extension
should be decided on, giving sufficient consideration to long-term or local riverbed variations. These decisions
should be based on channel longitudinal and cross-sectional profiles, channel characteristics, flow conditions
such as flooding, and soil properties, river environments, etc.

146

Explanation
Facilities that control embankment or riverside erosion and riverbed scour or sedimentation need to take into
account variations in river characteristics such as erosion, scour, and sedimentation on the basis of thelongitudinal and cross-sectional profiles, channel properties, and flow conditions (e.g. during flood) in the target
river section. At the same time, for a full understanding of the characteristics of each potential facility, each one
needs to be examined, as do the combination of facilities to be established, the establishment site and its
extension, etc.
It is also necessary to attempt to maintain a desirable channel for the river environment, by fully examining theadvantages of the channel or its influence on the river environment and considering the fact that variations in theriver’s characteristics have a close relationship with the river’s natural environment.
Channel control facilities include revetments, bed groins, consolidation work (bed girdles and drops), etc. In
selecting the facilities it is necessary to understand the characteristics of these facilities sufficiently and to selectone facility or some facilities in combination that is/are the most economical and suitable for the channel control
aim. Especially because river channels change locally and secularly, it is necessary to gain a sufficientunderstanding of their characteristics so as to plan each facility. Also, the establishment of a facility should befully catered for from the perspective of security in times of flood or of river environmental conservation,
because it is likely to promote the erosion of the surrounding riverbed and sides. The functions of such structuralobjects in river channel planning, or their effects on the river environment (especially in small and medium-sized
rivers) are large, so it is necessary to plan proper structural objects in reference to past experience, the conditions
in similar rivers, the literature, and so on.

7.2 Revetment planning
Revetments are established for the purpose of protecting dikes etc. (landside areas in the cases of excavatedchannels) from erosion by flowing water by the use of flood planes and structural objects. In the installation ofrevetments, their necessity (and the sites for their establishment) and their normal lines and extensions should bespecified with a good grasp of the channel cross-sectional profile, including the flood plane width, flow
conditions in times of flood, water route variation, etc. Also, since bank protection is strongly associated withriver environment improvement, revetment installation should consider the conservation and restoration of agood habitat and growing environment for living things and the maintenance and formation of a good landscapeand places that bring humans into contact with the river.

Explanation
Revetments, commonly constructed by sediment, are established according to need to prevent dikes from being
eroded by flowing water. However, since revetments that are often established in places such as river edges
important to the natural environment of the channel have a strong relationship with the river environment, theneed for their construction should be sufficiently considered. Additionally, the plan should be suitable for theconservation and restoration of a good habitat and growing environment for living things and the maintenanceand formation of a good landscape and places that bring humans into contact with the river by letting the facility
work together with the flood plane and groin. It is important to seek economical bank protection works that fully147consider the river environment by taking an inventive approach in accordance with the river’s characteristics.
Revetments are classified into high water revetments to protect the dike’s front slope and low water revetments
to protect the river side of the flood plane or the landside ground of an excavated channel. An integrated
combination of high water revetment and low water revetment is called a levee revetment. The revetment
planning should consider the flood velocity near a levee or riverside, flood plane width, water route secularchange, etc., and should specify the work type, the establishment site, and any extension. Also, note that theestablishment of a revetment moves the water-collision front downstream in many cases.
Meanwhile, it is desirable to make the normal line as smooth as possible; this is determined by the trait profile of
the revetment, because a high water channel revetment is established along the trait of the revetment. Whereas
the normal line of a high water channel revetment should be the one that is suitable for the flow direction of highand low water, it should be determined with sufficient consideration of the width of the flood plane and of theriver environment. (See “1.3.5 Channel cross-sectional profile.”)Additionally, the necessary revetments should be established above and below a structural object such as abridge, sluiceway, water gate, weir, consolidation work, etc., in accordance with the Cabinet Order concerning
Structural Standards for River Management Facility, etc. and the associated Ministry of Construction Ordinance.
It is desirable to make the configuration of these approach revetments as smooth as possible so that they cannot
cause the formation of vortices and dead water zones.

7.3 Groin planning
Groins are constructed along with flood planes and structural objects to protect embankments (landslide areas, inthe case of excavated channels) from erosion by flowing water. Also, they are sometimes established to ensure anavigable route or to improve and conserve river environments. Groin planning should sufficiently consider theeffects of the groin on the habitat and growing environment of animals and plants; the landscape; the flow
capacity; and the areas upstream, downstream, and on opposite sides of each groin, on the basis of the plane andlongitudinal and cross-sectional profiles of the river, the channel characteristics, and the river environment.

Explanation
Since groin planning is closely related to river channel or revetment planning, it is necessary to prepare different
plans for different channel characteristics and purposes, taking note of the mutual relationship, while considering
their effects on the upstream, downstream, and opposite sides of each facility and the improvement andconservation of the river environment.
The differences between revetment and groin are described below:
1.

A revetment, which directly covers the riverside to prevent erosion, facilitates the accomplishment of its
aim.

2.

The groin aims to indirectly protect the riverside by its water-rejecting and velocity-decreasing effects, so if
its placement and construction type are not properly decided on, then in some cases it cannot achieve a
sufficient effect.
148

3.

Proper groin installation can narrow the river and thus increase the river depth when the water is at normal
flow levels.

4.

Since the groin approach to a revetment and its downstream side are likely to be eroded by complex flow intimes of flood, these areas will need attention.

5.

Note that the space between groins, on which sediment is deposited, can be a good habitat, breeding site, or
shelter for water creatures such as fish, as well as a good growing environment for plants.

As described above, since a groin has a riverside protection function and landform and river water control
function that differ from those of a revetment, it is necessary to plan the installation etc., sufficiently inconsideration of the required flood control effect and the advantages of, and influence on, river environment
improvement and conservation.

7.4 Consolidation work planning
Consolidation works, which are conducted when unavoidable for riverbed stability, should be planned in
sufficient consideration of their effects on the surrounding riversides and river control facilities, as well as on
maintenance, fish runs, etc.

Explanation
When a riverbed is eroded and lowered by flowing water, the revetment foundation floats up, causing dangerous
conditions for flood control. In addition, the lowered riverbed makes it difficult to take all sorts of service water.
In this case, so as to maintain the riverbed as high as is required for river control, and to stabilize it, a groundsill
is installed across the river. The groundsill has a significant effect on the river environment, and the stability ofthe surrounding riverbed should also be sufficiently considered, so it is desirable to avoid installing the
groundsill if at all possible. The groundsill interrupts the continuity of water flowing upstream and downstream
and thus blocks fish from moving up and down. Therefore, in the establishment of a groundsill, consideration
should be given not only to meeting the need for fish to move up and down by careful consideration of its
structure and the installation of a fishway, but also to its future maintenance.

7.5 Fishways for groundsills
When there is concern that the construction of a groundsill will have a significant effect on fish movements, afishway should be established.

Explanation
To decrease the effect of the establishment of a groundsill on the habitat of fish etc. as far as possible, it isdesirable to establish a fishway for fish that need to move up- and downstream. Target fish should not only
include those that are considered important resources.
The fishway plan should determine the target flow, water level, installation, etc., in sufficiently consideration of
the stream flow conditions at the target site, the range of water level variation above and below the groundsill,

149and the time at which, and the course whereby, each target fish species moves up and down the river.

Section 8 River mouth Improvement
8.1 River mouth improvement planningRiver mouth improvement planning should seek to maintain a balance with nature, sufficiently considering both
the river and sea conditions, allowing floods to flow in safety against river-mouth closures etc., preventing
disasters caused by flood tides, and increasing river utilization as the occasion demands.
To specify an estuary improvement plan, the following items should be considered:1.

Functional and economic balance over the whole channel plan

2.

Avoidance of disturbance of navigation etc.

3.

Future maintenance kept as easy as possible

4.

Avoidance of second damage through loss of the natural balance between estuary and sea (due to sedimentsupply from the river to the seashore)

5.

Avoidance of damage to good habitats or growing environments for the animals and plans of the estuary or
its adjacent sea, good landscapes, and good places that bring humans into contact with the river.

Explanation
River mouth improvement is roughly classified into channel planning at the river-mouth and storm surge control.
The channel plan should examine the methods that are used for estuary improvement, specifying the design highwater level in the river-mouth, giving details of any excavation and dredging of the river-mouth part, andincluding measures, such as the use of jetties, to deal with rivermouth closure.
Because the river mouth, as the boundary between sea and river, is influenced by both, its management becomescomplicated, unlike that of the upper reaches of the river.
In some estuaries, a sand spit develops at the river mouth and hinders the free flow of water. This phenomenon iscalled river-mouth closure, and it causes different problems depending on the type of closure. These problems
are roughly classified as follows:1.

Since the depth and watercourse cannot be stable at the river mouth, navigation becomes difficult.

2.

At the river mouth, silting of the harbor can cause problems of unreliable arrival of vessels because of
insufficient depth.

3.

Flood discharge is blocked.

4.

Poor drainage occurs in the hinterland of the river mouth.

Methods of removing the problems caused by river-mouth closure are:
1.

Jetty

2.

Water gate, blind ditch, and detached breakwater3.

Artificial canalization.

Consideration should be given to the maintenance of the rivermouth improvement plan under complicatedexternal forces such as seasonal changes in river flow or tide.
150

In the actual estuary improvement, it is necessary to give sufficient consideration to important factors such as
saline wetlands, tidal flats, and brackish-water regions, and to the effects of coastal drift sands and saltwater
intrusion, so as not to cause heavy damage to the natural balance at the river mouth.
Also, in the examination of the river-mouth improvement planning, it is important to understand the behavior ofany sand spit at the river mouth under normal water levels or in times of flood. Additionally, the phenomenon
should be examined by hydraulic model testing and numerical simulation, as the occasion demands, to check theplanning.

8.2 Selection of rivermouth improvement methods
The methods used for rivermouth improvement should be determined in consideration of river characteristics
such as flow changes, adjacent sea characteristics such as sand drift tide flows, the natural environment at theriver mouth, economic efficiency, future maintenance, etc.

Explanation
At the river mouth, where various phenomena that happen in both river and seacoast simultaneously or
separately interact with each other to make it a very complicated place, it is not easy to simply select and planmeasures and construction methods for rivermouth improvement. Thus, in designing facilities, it is necessary to
fully consider their effects on the river mouth.
The construction methods used for rivermouth improvement include jetty, water gate, blind ditch, detachedbreakwater, artificial canalization at a shore bar, etc.; these are described below.
1.

Jetty

Jetties should be planned so that their different functions for different purposes can be fulfilled or so that theireffects on the adjacent seaside can be reduced as much as possible.
(1) Functions of jetties
Although the functions of jetties, i.e.
a.

Fixation of the site of the river mouthb.

Stabilization of the water’s coursec.

Maintenance of rivermouth depthd.

Lowering of the water level, both at times of normal water level and in times of flood

are referred to here, in planning a jetty it is necessary to design it to serve its purpose.
(2) Length of the jetty
Although the length of a jetty differs depending on the purpose of the rivermouth improvement and the scale of
the river mouth, it is generally possible to state the following.
When the rivermouth position has not drifted and the opening position is fixed within the channel width, it isnecessary to install two jetties from the shoreline at the time of normal tide into the sea, extending across thewidth of the channel, in consideration of the seasonal change in the shoreline. When only one jetty on one side is151installed, it is extended to some extent into the sea so that the water’s course can be fixed, even if the jetty is
installed either up- or downstream of the prevailing direction of longshore sediment transport.
When the river mouth has been deepened considerably and the width of the opening maintained, it is necessary
to project two jetties into points in the water that are at considerable depth. The results of examinations of thedepth and bed fluctuation of jetty apical points in all class A rivers across the country show that if the apicalpoint depth of the jetty is 4 to 5 m in rivers facing the open sea, the depth can be almost ensured; if it is 2 to 3 m,then the mouth will become shallow; and if it is less than 1 m a bar will develop.
The foundations of a jetty should be attached to the bank or, in principle, to the revetment, if one exists. In thecase of natural seacoasts and a middle jetty, it is necessary to extend the foundations in the direction of thechannel to a position where waves cannot reach the back of the foundations in times of flood.
(3) Interval between two jetties
When rivermouth closure is prevented, the narrower the interval between the two jetties, the greater the tractive
force and its resultant flash effect. However, from the viewpoint of flood discharge, a narrow width raises thewater level at the river’s mouth and sometimes causes problems with channel planning, so it is necessary to
specify the rivermouth width on the basis of changes in the water level at the river mouth and the results of an
examination of the relationship between jetty interval and the bed height in the jetty interval.
(4) Direction of the jetty
In principle, the jetty should be made perpendicular to the shoreline. However, it does not matter if it is curved
gently or is angled offshore. If it is curved sharply, the main stream will bump into it in times of flood, causing
problems such as deep excavation. Also, if the direction of the jetty is largely bent toward the shoreline, then the
waves reflected by the jetty will cause the shoreline to back away in some cases.
River with large sediment discharges supply the discharged sediment in only one direction, changing thesediment balance at the beach near the river mouth, so sufficient countermeasures are required.
(5) Height of the jetty
The jetty needs to be at least 1 m higher than the highest bar height so that waves cannot get over the jetty’s dam
crest and take sand into the channel. Where there is large amount of sand drift by wind, it is also necessary to
prepare countermeasures against the blown sand. In terms of the height of the tip of the jetty, even if wave
overtopping is permissible, it is generally specified to be at least 2 m than high water in many cases.
(6) Width and construction of the jetty
The width of a jetty is determined by its construction. It should have such a strong construction that it cannot be
broken in times of flood, and it should be equipped with foot protection and wave dissipation works, as theoccasion demands. Impermeable construction is effective because it is more likely to facilitate fixation of theriver mouth’s position and can prevent sediment translation. However, a permeable jetty can have the same effect152if its width is enlarged and void is lessened.
2.

Water gates

When a water gate is planned, care should be taken that the discharge capacity is enough for flowing the design
flood.
A water gate is established to maintain the river mouth by the effect of flashing and to minimize corrosion of any
construction facade by waves. Additionally, in many cases the gates are likely to have a combined purpose of
preventing saltwater, waves, and high water from entering and maintaining water levels.
Whether the sand spit at river mouth is flashed by the addition of a gate needs to be judged by using the degreeof flashing of the bar in times of flood without a gate and by referring to an example of a river of the same scaleand grade scale of beach material as the target river. Model experiments are useful for examining this. Also, at abeach where the shoreline changes sharply it will be necessary to give sufficient consideration to where the gate
is to be established.
In the case of many small rivers, a water gate is established with a pump slightly up from the river mouth, which
doubles as pump drainage.
3.

Blind ditch and detached breakwater

A blind ditch built to pass through a sand spit should be planned to fulfill its function of discharging river water
into the sea almost without relation to the fluctuation of the sand spit at the river’s mouth.
This construction method is mainly applicable to inner water discharges in small rivers. In rivermouth
improvement, a single blind ditch is more effective if is combined with other constructions, such as a water gate
at the upstream end to boost the flash effect against sedimentation within the blind ditch, or a detachedbreakwater to reduce the height of the waves reaching the periphery of the river mouth and make the height ofthe sand spit, behind which the blind ditch is constructed, lower, etc.
(1) Direction of establishment of the blind ditch
In principle, the blind ditch should be placed perpendicular to the shoreline of the beach, with the exception ofcases with special conditions in which the length of the ditch itself can be the shortest possible and thus its
construction cost the least. The opening of the blind ditch is curved so that waves cannot directly invade it. At
beaches where longshore sediment transport dominates, the opening of the blind ditch should be directed toward
the downstream side of the sediment transport. When the longshore sediment transport changes in accordancewith the fluctuation in wave entry angle, it is desirable to place two or four trains of blind ditches, with their
openings at both ends.
(2) Length of the blind ditch
The upstream end of the blind ditch should be placed at the upstream side of the beach, which is not influenced
by wave run-up. The higher the blind ditch foundations, the better for sedimentation prevention within the ditch.
The tip of the blind ditch should be placed in a position where the opening cannot be buried, in accordance with153

the shoreline fluctuation. When the upstream channel bed of spit sand is too low to make the blind ditch higher,
the tip will need to be extended far into the water, where it can be higher than the sea bed. Then, in the absenceof a large difference between the river water level and tidal water level in times of flood, it will be necessary totake care that the blind ditch is not flashed by the sediment within it. Also, the tip of the blind ditch will bescoured conspicuously by waves, so sufficient countermeasures need to be taken. In extending the blind ditch
into, especially, a sandy beach with a steep slope, the downstream side of the sediment transport will be eroded,
so it will be necessary to give sufficient consideration to this point in the planning stage.
4.

Artificial bar open-cut methods

Artificial bar open-cut methods should be planned with sufficient consideration of the need for long-term
maintenance and the effects of the bar on the natural environment of the river. They have the following purposes:(1) To ensure the depth and width of a river by a large-scale excavation at the river mouth(2) To prevent the water level at the river mouth from rising by open-cutting part of the sand spit at the rivermouth, thus facilitating flashing of the bar in times of flood.
The method of ensuring the cross-sectional area of a river by large-scale excavation at the river mouth is suitablefor case such as inland seas, in which there is little sedimentation at the river mouth because of the relatively
small wave action. At river mouths where wave action is strong and sand drift causes the riverbed to rise, it isnecessary to use it and a jetty at the same time. Because, with this method, sand drift is discharged from the riverby wave and sediment action, it is necessary to consider the degree to which the excavated channel can be
maintained. Also, since such a large-scale excavation will lead to erosion of the surrounding beach, it will benecessary to take countermeasures, such as supplying the excavated sediment to the eroded beach.
The artificial open-cut is designed to facilitate flashing of the rivermouth bar in the early stages of a flood bymaking the bar crest height of the whole, or part, of the bar lower than that under natural conditions. However, inthe case of flood due to typhoons, as on the Pacific coast, the bar open-cut part is soon filled in by the swellbefore the flood, so it is quite difficult to execute this method in large rivers.
When bar open-cut is included in the plan for rivermouth improvement, the open-cut part needs constantsupervision. If a large change is caused by wave action, then it will be necessary to promptly maintain it so thatits height and cross-sectional profile can remain the same as the design height and cross-sectional profile.

Section 9 Super broad levee with multiple functions
9.1 High-standard embankment installation section
In the selection of a section requiring the installation of a high-standard embankment, it is necessary to
determine a series of sections, comprehensively considering past major floods and high waters, the disaster154

states that resulted, and the meteorological, topographic, geographic, and developmental conditions of the basin
and region in which the disaster needs to be prevented. It is also necessary to take into consideration thebalances between upstream and downstream and between the right and left sides of the river in order to prevent
catastrophic damage due to the breaching of dikes.

Explanation
Although an embankment should be improved so that its structure is safe against the normal action of a flow
whose water level is less than the design high water level, flood is caused by natural phenomena such as rainfall,etc., so there is usually a possibility that a flood that exceeds the design high water level will occur. When suchan excess flood occurs, there is an increased risk of the occurrence of a very severe disaster due to dikebreaching as a result of overflow, etc. A breach in a dike that protects, especially, a large city with a very dense
aggregation of population and property, and with central administrative functions, etc., will not only doenormous damage to the area but will also greatly shock the society and economy of the whole country.
For this, high-standard embankments should be developed such that they cannot be breached by excess floodingthat may occur beyond the design, even if channel development has been completed. The river channel planshould determine the section in which such excess flood countermeasures need to be taken.
The high-standard embankment planning should determine the sections where these embankments need to beinstalled and then develop them. To determine these sections, the following items should be sufficiently
considered:1.

The states of accumulation of population, property, etc. within the flood prone area

2.

The level of social and economical effects that dike breach would have on the whole country3.

The damage pattern, as estimated by the relationship between the design high water level and thelandform of the design flood area

4.

The trend in land use along the river.

In an embankment section that may be damaged by a flood occurring in the same area if the dikes are breachedfrom one cliff-and-wood–surrounded section to another, or from a cliff-and-wood–surrounded section to theriver mouth, it will be necessary to decide on the high-standard embankment section as a series of sections.
Also, conventionally, river embankments etc. should be constructed by balancing the upstream and downstream
areas or the right and left sides of the river. The high-standard embankment section should be determined suchthat the opposite side or downstream side of the section, along which an embankment against excess flood action
is established, has an embankment structure that is similar in principle.

9.2 Height of high-standard embankments
The height of a high-standard embankment should be as prescribed by the Cabinet Order Concerning StructuralStandards for River Management Facilities, etc.

155

Explanation
Usually, the height of an embankment should be prescribed chiefly for two purposes: 1) to ensure an appropriate
allowance against a temporary rise in water level by wave, swell, and jump at times of flood in order to prevent
overflow less than the design high water amount; and 2) to ensure an appropriate allowance to meet various
needs such as river patrol in times of flood, security for flood control activities, countermeasures against woody
debris etc. High-standard embankments should not only fulfill the functions that normal levees have, but should
also be able to stand up to a flood action whose height exceeds the design high water amount.
Also, high-standard embankments are commonly developed not in succession but separately in many cases incombination with urban development, park improvement, farmland consolidation, etc. It is therefore necessary todevelop them so that their structures can be coherent with those of the normal embankments near them. Also,
even after a series of sections have been completed, cutting down their height is not desirable in terms of theircoherence with the height of other downstream and upstream embankments, so the high-standard embankments
will need to be as high as the height prescribed by the Cabinet Order Concerning Structural Standards for RiverManagement Facilities, etc.

9.3 Backwater sections of branches flowing into sections where high-standard embankment is to beinstalled
In the backwater section of branches flowing into the section where the high-standard embankment is to beinstalled, the excess flood countermeasures needed will nearly equal those at the confluence of the main river.

Explanation
In times of excess flood, when backwaters occurs in a branch flowing into the main river in an area where ahigh-standard embankment is to be installed, breaching of the dike of the branch by the backwaters will lead tothe same damage as that in the main river. In other words, the embankment of the backwater section of a branchfulfills the function of the embankment of a main river and must similarly be able to stand up to excess flood
action.
This, however, does not apply to such a section that can prevent backwaters from building up through facilities
to control backward flow, such as water gates etc. In this case, a water gate or sluice established as a facility toprevent backward flow fulfills the embankment function of the main river, so it must be able to bear the excess
flood action.
Also, when a backwater levee is applied, since it is assumed that the impact extent of excess flood can reach thepoint at which the main river high-standard embankment design water level (see 3.1.3 of Chapter 1 in the Designvolume of Technical Criteria for River Works [Proposed]) is about to catch up with the design water level of the
branch, in the branch facility planning, etc. it will be necessary to give sufficient consideration to the sections
beyond the backwater section.

9.4 Adjustment for projects associated with regional management
The high-standard embankment is based on normal land use, which is often developed in combination with156

urban development, park improvement, farmland consolidation, etc. It will therefore be necessary to makesufficient adjustments to suit any regional management projects along the river.

Explanation
A high-standard embankment is developed without acquiring land, except in cases in which it is necessary forriver administration, in anticipation that most of the site will be subject to normal land use. It is therefore
essential to develop the embankment consistently with the trend in urban development of the concerned area.
Thus, there will be a need for active involvement in coordination among the relevant organizations and bodies atthe preparation stage of planning for regional management. Urban development, especially, often takes a great
deal of time to be actually started, so it will be necessary to work with a coalition of local authorities’ city
planning departments to make a “basic plan for city development in the super-levee area” as a master plan for thehigh-standard embankment. The basic plan should decide on the river embankment’s master plan and theparticulars concerning the local regions to preferentially promote improvements on the basis of the city planningand urbanization trend in the target area.
Also, in terms of the area in which preferential improvement is decided on by the basic plan for city
development in the super-levee area, when local authorities formulate a “plan for urban area development along
the river” in relation to the high-standard embankment and urban development in consultation with riveradministrators, the high-standard embankment should be developed on the basis of this plan.

157

Basic plan for city
development in the
super-levee area

Plan for urban area
development along
a river

Plan for individualarea development

The basic plan will concern the integrated development of the high-standard
embankment and the urban area along the river
It is drawn up by committees set up in every prefecture or every river area
It is made on a conjoint basis at least between river administrators and local
authorities
It should specify the master plan for development along the river and areas inwhich the development should be promoted preferentiallyThis plan is drawn Up for areas suitable for urban development integrated witha high-standard embankment, it concerns areas in which preferential
development is prescribed by the basic plan for city development in the superlevee area
Local authorities should draw up the plan in consultation with riveradministrators
It should specify the section in which integrated development should beattempted, the land use policy, the installation of main city facilities, thepolicy of project execution, the plan for project implementation, etc.

This plan should be drawn up in corporation with landowners and constructors
of urban development project
IT should specify The longitudinal and cross-sectional profiles of the areas ofconcern, as well as the implementation schedule, cost burden, etc.

Fig. 2.1–2 Steps in drawing up the plan to enforce the high-standard embankment and urban development
in an integrated manner
References
1) Japan Institute of Construction Engineering. Manual for River Planning Consideration,. Ed. Japan Instituteof Construction Engineering. February, 2002.
2) National Association of Disaster Prevention. Basic Policy for Disaster Restoration to Protect Beautiful
Mountains and Rivers, June, 2002.
3) Japan Institute of Construction Engineering. Dynamic Revetment Design Method. Ed. Japan Institute ofConstruction Engineering. February, 1999.
4) Japan Institute of Construction Engineering. The Research and Countermeasures of Landslide aroundReservoir. General Edition. River Bureau of Ministry of Land, Infrastructure and Transport. July, 1997
5) Kashima Laboratory of Public Works Research Institute, Ministry of Land, Infrastructure and Transport
Japan. ‘River Mouth Improvement Theory [I].’ Public Works Research Institute Document 1395, May 1977.
6) Japan River. Revised Description and Cabinet Order concerning Structural Standards for RiverManagement Facilities, etc. Ed. Japan Institute of Construction Engineering, November, 2001.
7) River Study Room of Public Works Research Institute, Ministry of Land, Infrastructure and Transport Japan.
‘One Dimensional Analysis for River Bed Variation’ ISSN0386–5878, Public Works Research InstituteDocument 3099, March, 1992.
8) Japan Society of Civil Engineer. The Collected Hydraulic Formulas (1999 Version). Ed. Japan Society of158

Civil Engineers’ Hydraulic Council, Hydraulic Formula Revision Subcommittee. May, 2000.
9) Japan Institute of Construction Engineering. Manual for Urban River Planning: Steric River Facility Design
Edition. Ed. Japan Institute of Construction Engineering. April, 1995.
10) Foundation for Riverfront Improvement and Restoration. Project for Creation of Rivers Richly Endowedwith Nature in Medium and Small Size Rivers. Ed. Study Group of Project for Creation of Rivers Richly
Endowed with Nature in Medium-sized and Small Rivers. October, 2000.
11) Water Resources Environment Technology Center. A Discussion of Analytical Forms of the Environmental
Effects on the River Project Planning Phase. Ed. Exploratory Committee concerning Analytical Forms of
the Environmental Effects on the River Project Planning Phase (River Planning Division, River Bureau,
Ministry Land, Infrastructure and Transport Japan). December, 2002.

159

Chapter 2–2 Drainage Facilities for the Inner Water
Section 1 General Information
An inner water drainage plan shall be established in due consideration of the characteristics of the landside waterin the subject landside river basin and past disasters caused by landside water.

Explanation
An understanding of the landside water characteristics in the subject landside river basin and of past disasters
caused by landside water is indispensable for the establishment of an inner water drainage plan. Landside water
characteristics in the narrow sense mean the characteristics of the landside water phenomenon itself, but in the
broad sense they also mean the presumption of possible causes of the landside water flows. To fully elucidate thelandside water characteristics, it is necessary to not just determine the current characteristics but also their
historical chronological changes, as well as the causes of such changes. For example, if a constantly high outerwater level is the cause of landside water flooding, improvement of the landside river by using a self-flow levee
system for inner water drainage will not be very effective; channel improvement by the full-back or semi-backmethod or the drainage-pumping method will be much more effective. In this case, because the scale of landsidewater is regarded as depending solely on the landside river basin yield, the design landside water shall be that at
the time of heavy rainfall (yield) in the landside river basin, and a probability assessment can also be conductedby using the rainfall data for the landside river basin.
The damage caused by landside water will depend on the flood characteristics and asset distribution. For
example, if most of the damage is to farm products, the design scale may be reduced compared with that for an
urban area. If damage to ordinary houses takes place on the perimeter of the water-covered area and the depth ofsubmergence is small, it is possible to prevent damage to houses by slightly reducing the total submerged depth.
However, when there are several houses near the lowest point of ground in the water-covered area, it is
impossible to completely prevent damage by drainage-pumping; therefore, measures to improve theflood-resistance of these houses may be more advantageous in terms of investment effect. Also, in recent years,
damage due to landside water has occurred frequently in heavily populated large cities. This is the result of theoccurrence of flow volumes exceeding the design flow volume of facilities, in association with urbanization of
the basin, and also the occurrence of landside water along rivers in association with the construction andimprovement of sewerage systems. In addition, the Tokai flood disaster and the inundation of undergroundspaces in Fukuoka City, which were caused by short-term torrential rainfall exceeding the design scale, caused
tremendous damage owing to spill overtopping from sewerage systems. Therefore, for landside rivers in highly
urbanized basins, it is important to determine the design scale on the basis of prediction of changes in flow
regime, to consider the balance between sewerage systems and the level of river improvement, and to adjust the
progress of improvement.
If landside water covers an extensive area, then it will be necessary, in the process of planning, to differentiate
between the landside water removal zone (the area where landside waters should be actively drained) and theretarding zone (the area whose retarding function needs to be maintained by minimizing landside water drainage).
In this process, it is necessary to enhance the safety level of flood control of the entire basin while taking into
160

consideration flood damage to asset distribution in the region.
Descriptions of landside water surveys and the creation of landside water analysis models can be found in
Section 3 of Chapter 8 in the Research volume of the Technical Criteria for River Works (Draft). Refer to otherdocuments etc. in developing a specific plan for inner water drainage.

Section 2 Determination of Inner water drainage Method
The method of inner water drainage shall be selected from alternatives that are economically and socially
feasible and expected to bring results in terms of the topography; land use; drainage; improvement plans for thelandside river and main stream; plans for related projects; landside water flood characteristics; landside water
flood damage characteristics of the subject landside river basin; and the presumed flooding area, through acomprehensive assessment of the difficulties of facility management and the extent of damage in times of excess
flooding.

Explanation
The inner water drainage method shall be determined through the selection of alternative methods that are
applicable to the subject area and a comparative review of them.
The primary items to be considered in determining the inner water drainage method include the following:- Proportion of river basin in mountainous terrain- Bed slope of the landside river- Distance to the destination river- Discharge capacity of the destination river- Land use, depth of water, and filling time of the presumed flooding area.
In the comparative review of inner water drainage methods, the scale of facilities necessary for each treatment
method shall be determined and the necessary cost of the project and its beneficial and adverse effects shall beassessed through consideration of the social conditions of the subject area and difficulty of management, etc.
Anticipated damage at times of excess flooding shall also be taken into consideration.
The choice of inner water drainage method should differ depending on the characteristics of the subject landsideriver, and it is not necessary to fix on the choice of a uniform method. In the process of choosing a method,
landside river improvement planning needs to be developed simultaneously, and, in this sense, the method oflandside river improvement is included as part of the inner water drainage method. The inner water drainage plan
to be determined here shall include not only structural measures such as river improvement, installation of adrainage pump station, and installation of a retarding basin, etc., but also non-structural based measures such as
land use management and land utilization guides, as well as a combination of structural and non-structural
measures.
Inner water drainage methods are roughly classified as shown below.
1. Classification based on method of implementing measures(1) Structural measures161

(2) Non-structural measures
2. Classification based on measure implementation target area
(1) Measures for the discharge basin(2) Measures for a landside water area(3) Measures for the main stream
3. Classification based on drainage method(1) Regional outlet channel(2) Water-gate closure method
Figures 2.2–1 and 2.2–2 show these three classifications as they are examined in the actual process of landsidewater planning.

162

Classification
based onmethod of
implementationof measures

Classification
based on thetarget area of
measureimplementation

Classification
based ondrainagemethodMethods of inland flood control

Separation of discharge area (diversion channel, etc.)

Discharge area

Storage facilities such as retarding basin
River improvement (including channel reconstruction)
Diversion channel
Drainage basin
Naturaldrainage

Drainage pump station for diversion channel
Drainage pump station for secondary inland flood
Levee foot channel (sub-channel)

Structural

Inlandflood area

SiphonMeasures forsecondaryinland flood

River improvement (including channel reconstruction)
Drainage pump station (including pump for diversion channel)
Closing of
the floodgate

Diversion channel
Drainage basin
Overflow levee (inundating on agricultural land)

Main stream

River channel improvement
Drainage basin

Discharge area

Preservation of water retention function (discharge control byinfiltration facilities)
Guide and regulation of land use (conservation of green area, etc.)

NonStructural

Guide and regulation of land use (designation of inundating areaand disaster danger area, etc.)
Improvement of water resistance capability of houses and facilities(elevating ground, ring levee, relocation, etc.)
Inland flood area

Preservation of retarding function (preservation of agriculturalland, regulation of filling, etc.)
System of relief for flood disaster (water damage insurance, etc.)
Reinforcement of warning and evacuation system (including
hazard maps, etc.)

Fig.2.2-1 Classification of methods of inland flood control

In order to decide on the optimal inner water drainage method, different methods shall be examined on the basisof the classification shown in Fig. 2.2–1. Possible measures and specific measures shall be first investigated forinner water drainage method classification, and then a combination of these measures shall be investigated asshown in Fig. 2.2–2. After an examination of the scale of facilities for each measure, the finalized measure shallbe determined through comparative review in terms of effectiveness and economic efficiency, etc. It is desirable
that non-structural measures are developed through close cooperation and collaboration with relevant163organizations and bodies in order to avoid placing too much weight on structural measures.

Non-structural
measureStructural measure

Discharge area

Inland flood area

Natural drainage

Discharge area

Inland flood area

Closing of thefloodgate

Study feasibility of measures and of implementation method of measures for each classification

Study of combination of measures
Classification of
methods

Study of appropriate quantity of facilities

Comparison study of control method

Final decision on control method
Note1. Some of the classification above may not be applicable, depending on the subject river.
Note2. These measures can bring out expected effects individually or in combination with other measures .

Fig.2.2-2 Process of study of inland flood control

Section 3 Selection of Landside Waters for Examination
Several landside waters for examination shall be chosen in consideration of historical rainfall, outer water levels,and flooding, and depending on past damage and the availability of hydrological data. These should include theone that caused the largest recorded damage.

Explanation
The landside waters for examination are used for the following purposes:
1. Verification of landside water analysis models (landside water for verification)
2. Examination of inner water drainage method and facility scale (landside water for planning)
It is desirable to use the same landside water for both 1 and 2, but different landside waters may be used,
depending on the availability of hydrological data.

164

To examine the selection of an inner water drainage method, as landside waters for examination it is desirable toselect those waters that have various hyetographs and outer water hydrographs and a diverse relative relationshipbetween them. On the basis of this idea, four to six landside waters for examination, including the one thatcaused the largest record damage, are usually selected on the basis of historical data, the extent of damage, andthe availability of hydrological data. It is also important to select those landside waters that have caused the mostrecent damage in view of exemplifying the subject area. Past inner water drainage plans have taken the following
points into consideration in the selection of landside waters for examination:(1) Landside water that caused serious damage(2) Landside water with large amounts of rainfall in the landside river basin(3) Landside water with large amounts of rainfall in the landside river basin and a high outer water level(4) Landside water with a high flood level(5) Landside water with a long flooding time(6) Landside water for which there are sufficient hydrological data available(7) Other (subject floods for a main stream improvement plan, etc. or representative floods in the main stream,
etc.)

Section 4 Determination of the Probability Assessment Method
The method for probability assessment of the landside water scale in an inner water drainage plan should beselected in consideration of the characteristics of the landside water in the area for examination. Depending on
the availability of hydrological data, it should be chosen from among the following methods on the basis of anunderstanding of each method’s features:
1. Probability assessment based on the amount of rainfall in the landside river basin
2. Probability assessment based on the amount of rainfall over the landside water occurrence period
3. Probability assessment based on the amount of submerged water

Explanation
An examination performed to select a probability assessment method aims to accurately assess the annual
exceedance probability of the landside water scale, and it shall be conducted on the basis of the characteristics ofthe subject landside river basin.
The scale of the landside water is determined by the runoff in the landside river basin, the outer waterhydrograph, and the relative temporal relationship between them. Because a landside water probability is a
probability not for a single variable but for multiple variables, this complicates inner water drainage planning.
Also, because the runoff in the landside river basin, the outer water hydrographs, and the relative relationshipbetween them are determined not by natural conditions only, but also by human actions such as river
improvement, changes in land use, and construction of sewerage systems, this adds to the complication ofprobability assessment of the landside water scale. Therefore, in order to accurately determine the flood safetylevel of the landside water area and the proper facility scale, a probability assessment method that is suitable forthe study area has to be determined on the basis of the landside water characteristics, hydrological data, etc.
In an ordinary flood control plan, a method that calculates the design scale external force from probability165assessment of rainfall and conversion of this into a runoff is employed. In an inner water drainage plan, too, thedesign external force of a facility (channel, water-gate, sluice, etc.) that determines the flow capacity of thelandside river is determined on the basis of the peak discharge, and probability assessment of rainfall is usuallyused.
On the other hand, the scale of the existing landside water itself and the design external force of the inner waterdrainage facilities (drainage pump station, flood control basin, etc.) should be assessed by using the amount ofsubmerged water, not the peak discharge. Since landside water flooding is caused by multiple factors intricatelyintertwined with each other, there may be various methods for probability assessment of the amount ofsubmerged water. The most direct method for probability assessment is one that uses actual values of flood levelor the amount of submerged water for past landside waters. However, since such data usually has limited
availability, and the amount of submerged water varies under the same rainfall conditions owing to human action,in many cases this method cannot be used to assess the landside water scale probability.
Because a probability assessment based on the amount of rainfall over the landside water occurrence period
represents the amount of submerged water as determined from the amount of rainfall over this period (the periodof time during which the landside water area is presumed to be flooded), such an assessment can be applied if thelandside river basin is extremely small and the concentration time is short. However, because the application ofthis method requires data on outer water levels or data on opening and closing of the water gates, if such data is
not sufficiently available then the method cannot be employed, even if the landside river basin is small.
Probability assessment based on the amount of submerged water can be employed when sufficient data on theactual amounts of submerged water is available and there is no major change taking place in the basin andchannel of the landside river, or in the main stream, during the periods of time for which the statistics are
required. When there are major changes taking place in the basin and channel of the landside river, as well as in
the main stream during the time of collection of statistics, probability assessment based on actual amounts ofsubmerged water cannot be used, but probability assessment based on calculated amounts of submerged water
can be used. The calculated amount of submerged water is obtained from a landside water analysis model; thisanalysis requires outer water hydrographs and the amount of rainfall in the landside river basin for landsidewaters presumed to have caused the maximum amount of submerged water in each year of the time period used
to gather statistics.
Probability assessment based on the amount of rainfall in the landside river basin is a method that is employedwhen the above methods cannot be used owing to constraints such as insufficient availability of hydrological
data, and it is used relatively often because of the simplicity of the work involved. However, the time period forthe subject rainfall should be determined in consideration of the basin size, flood concentration time, flood
characteristics, and availability of hydrological data, etc.
Since the scale of landside water generally depends on three elements—the runoff of the landside river basin, theouter water hydrograph, and the relative relationship between them—the landside water probability of thesubject area may be represented more accurately through probability assessment using multiple variables that
represent these elements. There may be a weak correlation between the rainfall in the landside river reaches andthe outer water level in past landside water occurrences, for example, when the scale of the main stream basin at
the flow end is much larger than the scale of the landside river basin. In such cases, complex probability
166assessment using multiple variables may be more effective than probability assessment using a single variable.
Since the use of two variables is a practical limitation, it is necessary to determine two variables that represent
the landside water scale in consideration of the landside water characteristics of the subject area. In cases where
two representative variables cannot be extracted, however, this method is not very effective. For example, if thepeak outer water level and the rainfall in the landside river basin are used as the two variables, the occurrence
probability of the combination of these two can be obtained. However, the resulting probability does notnecessarily represent the scale of the landside water. This is because the scale of the landside water depends
heavily on the time lag between the outer water hydrograph and the rainfall in the landside water area.

Section 5 Determination of the Scale of Inner water drainage Facilities
The design scale of the inner water drainage facility shall be determined in consideration of various factors, suchas the importance of the landside water area, the actual damages caused by past landside waters, the balanceagainst the project scale of the main stream, and the balance against the project scale of neighboring landsidewater areas, etc. A facility scale that ensures a degree of safety corresponding to that at the project scale against
flood in the landside water area shall be determined.
However, the scale of drainage pumping, as a general rule, shall be determined on the basis of a cost-benefit
evaluation.

Explanation
If the plan uses a regional outlet channel, the scale shall be determined in the same manner as for general river
improvement.
If the plan uses a water-gate closure method, the scale shall be determined in consideration of various factors,
including the social and economic importance of the landside water area, the actual damage in past landsidewaters, the economic efficiency, the balance between the project scale of the landside water area and that of boththe main stream and the neighboring landside water areas, etc.
If it is impossible to construct an inner water drainage facility equivalent to the project scale from the beginning,
owing to constraints concerning consistency with the safety level of flood control of the main stream, balanceagainst the safety level of flood control of neighboring landside water areas, and financial constraints, etc.,staged construction shall be implemented.
1. Design scales of the landside river and inner water drainage facility
As described above, there is a significant difference in the damage structure between outer water and landsidewater. Although it is rational to determine different safety levels of flood control for outer water flooding andlandside water flooding, in establishing the design scale for an inner water drainage facility it is necessary to take
into due consideration the balance against the project scale of the main stream.
In addition, the design scale for the landside river channel and that for the inner water drainage facility do nothave to be the same.

167

2. Importance of the region
The importance of the region can be judged mainly from the land use of, and concentration of assets in, thelandside water area. In general, the asset concentration in agricultural areas such as paddy fields and uplandfields is lower than that of urban areas by two or three orders, and their damage potentials are very different.
Therefore, it is regarded as rational to change the design scale of the inner water drainage facility depending on
the land use of the landside water area. However, a probable flood area is rarely composed of a single land
category such as paddy field, upland field, or urban area; instead, it usually has land of mixed categories. In
addition, the predominant land category of the subject area for landside water removal is also different owing to
the distribution of these land uses by elevation.
Generally, when a landside water area is mainly urban, the target design scale of the inner water drainage facility
shall be more than 30 years; in the case of other land uses it shall be more than 10 years.
3. Damage prediction when floods exceed the design scale
Since there is a possibility of occurrence of landside water exceeding the design scale, it is desirable to conduct
damage prediction for such an occurrence, as required, in order to prevent serious damage.
If the necessary facility scale is very different, depending on the subject landside water, it is desirable toreexamine whether there is any technical irrationality in the relationship with the adopted probability assessment
method or in the process of expansion to the design scale. Also, it is necessary to examine the effects of flood
control of past eminent landside waters in determining the facility scale.
In the case of drainage pumping, however, examination from an economic viewpoint is important, and the scale
shall be determined, as a general rule, on the basis of a cost-benefit evaluation. For cost-benefit evaluation, refer
to "Chapter 20: River Economics Research" in the Research volume ofTechnical Criteria for River Works (Draft). In this case, the depreciation period for the machine and equipment
shall be determined to be 20 years.
Reference
1) Guide to Inner water drainage Planning: Supervised by Flood Control Division, River Bureau of the Ministryof Construction, Edited by Japan Institute of Construction Engineering, February 1995

168

Chapter 2–3 Multi-purpose Facilities
Section 1 General Information
1.1 Definition
Multi-purpose facilities include dams, flood control basins, weirs, channels for adjusting river flow, etc. withmultiple purposes related to flood control, water utilization, environmental preservation, etc.
1.2 Basics of multi-purpose facility planning
In planning a multi-purpose facility, necessity in terms of flood control, water utilization, and the environment;
economic efficiency; and ease and efficiency of facility operation and its impacts on natural and social
environments, etc. shall be examined.
1.3 Multi-purpose facility location
A multi-purpose facility shall be positioned at a location that ensures that the expected functions can be fulfilled
and, where necessary, storage capacity can be advantageously secured. The location shall be determined throughcomprehensive examination of different factors, such as the results of topographic, geological, andenvironmental surveys, the status of submerged areas, and economic efficiency, etc.

Explanation
A multi-purpose facility should be located at the most appropriate position selected in consideration of multiple
factors such as the results of topographic, geological, and environmental research, the status of submerged areas,and economic efficiency, etc. from among multiple candidate sites that are presumed to ensure the expectedflood control and water utilization functions as well as the necessary storage capacity. Environmental surveys areindispensable to gain an understanding of the status of healthy habitats for plants and animals around thereservoir and to preserve them, and to provide important information used for the selection of the reservoir
location.
Whether a single reservoir or a group of reservoirs is to be constructed shall be determined in consideration of
the sureness of flood control, combination of purposes, topographic and geological conditions, the environment
surrounding the reservoir, and economic efficiency, etc.
Because redevelopment of an existing dam through enlargement, combined use of existing dams, etc. may beable to achieve the expected flood control and water utilization effects with less cost and impacts on theenvironment compared with new dam construction, positive examination of the possibility of efficient use ofexisting dams should be promoted.
In view of effectiveness of flood control, it is desirable to determine a location that is close to the major flood
control area projected in the flood control plan and to use as few large capacity reservoirs as possible for flood
control. In developing a flood control plan by using a group of reservoirs, if the need arises from the viewpointof water utilization, then allocation of adjustment flow volumes to each reservoir shall be examined throughvarious combinations with water utilization purposes.

169

Section 2 Facility Basic Plan
2.1 Plan adjustment
In developing a multi-purpose facility plan, the manner of operation of the facility must be examined in order toavoid conflicts between purposes.

Explanation
In the use of a facility, especially a reservoir, flood control and water utilization initially conflict with each otherin terms of water management. For this reason, the flood control capacity shall be secured by setting a flood
season control level, and a water utilization plan must be established in consideration of this. In the case of amulti-purpose dam with a power generation facility, because the water used for power generation can be reused
for water supply and irrigation, etc., such uses rarely present conflicts of purpose. However, when the power
generation scale is large and the facility has a capacity for exclusive use for power generation, the capacity forother uses of water may be reduced, and adjustment between the multiple purposes will be necessary. Floodcontrol capacity, especially, must be secured with priority over other purposes, and a method that requires
overlapping of the capacity for preliminary release flood control and water utilization capacity must not be
adopted except in special circumstances. It is necessary to actively consider plans for the redevelopment ofexisting dams from a new point of view. Preservation of, and harmony with, the natural environment, etc. should
be considered.

2.2 Consideration of economic efficiency (cost-benefit analysis)
In the process of establishing a multi-purpose facility plan, the economic efficiency of each purpose should be
investigated. As a general rule, the total benefits obtained by project implementation must exceed the total costrequired.
An appropriate method of total benefit evaluation shall be selected in consideration of the project and thecharacteristics of the subject area, etc.

Explanation
In establishing a multi-purpose facility plan, examination from the viewpoint of technical possibility is anabsolute necessity. Examination not only from the viewpoint of rationality of reservoir use for securing thecapacity needed to meet demand, but also from the viewpoint of economic efficiency, is also necessary.
An appropriate analytical method shall be selected through consideration of the subject project and thecharacteristics of the subject area, and the reason for selecting the method must be clarified.

2.3 Multi-purpose reservoir planning
2.3.1 Evaluation of necessary capacity and its allocation
The storage capacity shall be allocated to various uses, including flood control and maintenance of normal
functions of the river water, as well as new uses of water, etc. for the effective use of limited facility sites in170

consideration of various factors such as demand for flood control, water demand, and the scales of thereservoirs, etc.

Explanation
The necessary capacity for each purpose shall be calculated in accordance with 2.3.2–2.3.7 of this section.
The calculations of the capacity necessary for maintenance of the normal functions of the river water and newuses of water shall be done by using information collected from existing hydrological data covering as long a
period as possible (basically 20–30 years, but approximately 10 years if the data are limited). As a general rule,the calculations shall be based on the flow regime at the time of the largest drought in 10 years (second-largest
drought in 20 years, or third-largest drought in 30 years).
In recent years there have been cases in which the storage capacity for water for melting, clearing, and removing
snow and the storage capacity for drought for mitigating the damage caused by extraordinary drought exceeding
the design scale have been determined.

2.3.2 Flood control capacity
The flood control capacity shall be determined in accordance with Section 3.1.3 of Chapter 2–1.

2.3.3 Capacity for maintenance of normal functions of river water (unspecified capacity)
The capacity for maintenance of normal functions of the river water (hereinafter referred to as "unspecified
capacity") shall be sufficient to secure "a necessary discharge for the maintenance of normal functions of theriver water (hereinafter referred to as ‘normal discharge’)".
The unspecified capacity shall be obtained by calculation of the excess and deficiency between the normal
discharge and the natural flow at the design control point.

Explanation
An unspecified capacity is a capacity necessary for the maintenance of the river and the maintenance of normal
functions of the river water, such as protection of vested water rights. It should be secured preferentially in a newwater-resource development project.

171O1 , O2 , O3 : Necessary volume of resupply

S 1 , S 2 , S 3 , S 4 : Possible storage volume

Flow rate

O2
S1

O1

S2

O3

S3

Flow rate at control point

S4

Recommending in stream flow

Mouth

Fig.2.3-1 Resupply plan diagram of recommending in stream flow

2.3.4 Capacity for irrigation
The capacity for irrigation shall be determined through calculation of excess and deficiency between the design
intake flow and normal discharge at the intake point and the discharge at the intake point after consideration ofstorage restrictions and other conditions for the reference dry year.
The design intake flow of irrigation water shall be determined in consideration of the necessary water volume
for each area and period of the replenishment zone and the vested water rights and effective rainfall of that zone,
and the necessary intake flow for the intake point shall be determined for each period.

Explanation
Irrigation waters include water for the growth of farm products, water for rice nurseries, and water for land
preparation; the amounts of water needed differ depending on the period of the irrigation season. Since the soil
properties, climate, kinds of crop, and growth period, etc. differ from area to area, the necessary water volume
differs, and it will be necessary to estimate the necessary water volumes by conducting surveys in field locations
over several years.
The indication for the water rights volume consists of the indication of the seasonal maximum water intake
volume (represented by m3/s) that controls the maximum water intake volume for each season and the total water
intake volume (represented by m3) that controls the total water intake volume. The necessary water volume istaken within these ranges. Consequently, as a general rule, calculations to determine the necessary capacity for
irrigation should be based on the necessary water intake volumes in the reference year (effective rainfall should
be deducted), not the seasonal maximum water intake volume obtained from the water rights volume.
Because it is necessary to make adjustments between the different water uses that are the subjects of themulti-purpose facility plan, as a general rule the plan shall be developed on the basis of a common reference dry
year.

172

Effective rainfall
Necessary volume for water use

Profile of water right

Volume ofwater intake

Irrigation season

Fig2.3-2 Intake water volume diagram

2.3.5 Capacity for municipal water
The capacity for municipal water shall be calculated by determining the necessary intake volumes for municipal
water for different periods, and by calculating the excess and deficiency against the flow regime at the intake
point after including the normal discharge and storage constraint conditions at the intake point in the reference
dry year.

Explanation“Municipal water” is a general term for the water used for water supply and industrial water. The necessary
capacity for municipal water shall be obtained from the necessary supply volume against the necessary intake
volume for a flow regime that gives priority consideration to the normal discharge.
In normal cases, the necessary intake volumes for municipal water, irrigation water, etc. shall be calculated in thesame manner.

2.3.6 Capacity for power generation
The capacity for power generation shall be determined in consideration of fluctuation in the use of electricity
and through adjustment with other purposes in such a way that efficient power generation is possible.

Explanation
The capacity for power generation should be determined in consideration of power demand and fluctuation in173

use of electricity and through adjustment with other purposes in such a way that economically efficient power
generation is possible.
When there is already a power station downstream in the river, since improvement in the flow regime at normal
times may cause increased power generation (increase in downstream), the effect of this also needs to beconsidered.
Calculations in a power station construction plan are based on discharge data for 10 or more years, and the
maximum high water level for power generation during a flood season must not exceed the control level. When
the water level is lowered to the control level before a flood season, ineffective release when release of morethan the turbine discharge is necessary shall be considered in advance. The capacity shall be determined in sucha way that storage can be started after the flood season and finished during the demand period.

2.3.7 Storage capacity for sedimentation
Normally the sediment deposition estimated for the next 100 years is used as the storage capacity for
sedimentation. However, the design sediment storage can be reduced in the case of a facility that flows sedimentfrom a flood spillway, one that removes inflow sediment in the reservoir, or other facilities for which special
measures have been implemented.

Explanation
It is desirable to estimate the sediment deposition of the reservoir from the sediment deposition of an existing
reservoir in a similar area. It should be determined in consideration of the sediment and erosion control plans forthe upper reaches, the size of the basin, the geology, and the forest physiognomy, etc. and in reference to theresults of calculation of the estimated sediment deposition. However, the design sediment storage can be reducedby the estimated sedimentation deposition for the next 100 years in the case of a facility that flows sedimentfrom a flood spillway, one that removes inflow sediment in the reservoir, or other facilities for which special
measures have been implemented.

174

Chapter 2–4 Water Quality Preservation Facilities
Section 1 General Information
A plan for the construction of facilities, etc. in a river, reservoir, or lake and swamp that are aimed at preserving
the water quality of a river, etc. shall be established on the basis of the current water quality and the natural andsocial environments of the basin and their historical transition. It shall also take into consideration consistency
with the various functions of the river related to flood control, water utilization, and the environment.

Explanation
Cooperation between the river administrator and relevant organizations and bodies, etc. in the basin is necessary
to develop multiple alternatives, conduct comparative reviews of them on the basis of the location where ameasure is to be implemented, the method to be used, and the expected effects, and finalize the measure.
In determining the measure, it is necessary to take into consideration the time lag before the measure will
actually take effect.
Multiple alternatives for the water quality preservation measures allocated individually to the river, reservoir,lake, swamp, and basin in view of role-sharing based on “Measures for the preservation of good water quality”(Explanation 4, 1.4 of Chapter 1) shall be examined in consideration of the characteristics of each measure. The
best measure shall be selected.
The basic procedure for selection of a water quality preservation measure is as shown below.
1. Selection of the location where the measure is to be implemented
The most polluted area, feeder river and discharge channel shall be selected on the basis of a change in water
quality in the flow direction and distribution of loading.
2. Selection of multiple alternative measures
Appropriate measures shall be selected in consideration of applicable conditions for the subject water quality
items.
3. Determination of the scale of measures
The scale for the selected alternatives shall be determined by comparing the predicted results and target values
for water quality preservation at representative locations.
4. Selection of a measure
The optimal measure shall be selected from among the selected alternatives and the facility installation shall be
determined through comparison of the locational factor, economic efficiency, and ease of maintenance, etc.
Facility installation needs to consider the condition of habitats for plants and animals and harmony with thesurrounding environment.

175

Since the need to maintain any water quality preservation measures must be weighted heavily among
considerations, it is desirable to select a measure that is easily managed and has a low running cost.

Section 2 River Water Quality Preservation Measures
River water quality preservation measures include reduction of loading, separation of loading, securing of flow,
and reinforcement of the purification function of the river, etc. The measure shall be established by firstpreparing multiple alternative plans by combining these functions, and then through comparative examination
based on the location where the measure is to be implemented, the method, and the expected effects, etc. In
determining the measure, the target water quality, effects, economic efficiency, ease of maintenance, and impacts
on the environment, etc. shall be taken into consideration.

Explanation
The following points require consideration in a river water quality preservation measure:
1. When selecting a method for loading reduction, disposal and utilization of bottom sediment and sludge, etc.
also need to be considered on the basis of the water quality factors of the subject water and the properties of thatwater.
The facility location shall be one where loading can be efficiently reduced.
2. When selecting the method of loading separation, it is necessary to consider the impacts on the lower reaches.
3. When selecting the method for securing the necessary flow volume, it is necessary to consider the flow regimeof the raw water source, water quality, water utilization plans, geographical condition, and impacts on the riverenvironment downstream of the raw water transmission point.
4. When determining the measure for reinforcing the purification function of the river, a location that facilitatesefficient reinforcement of the purification function shall be selected in consideration of the longitudinal
distribution of water qualities, current velocity, and the condition of the bottom sediment, etc.
(1) Reduction of loadinga) Water quality preservation by river flow purification
When planning a water quality improvement measure that uses a river flow purification method, thehydraulic and water quality characteristics of the subject water body and the site for facility construction
shall be investigated, and the optimal method and scale for the characteristics of the water body shall be
determined.
When a river flow purification method is used, a purification method suitable for the hydraulic and water
quality characteristics of the subject water body shall be selected. The choice of a purification method
depends, for example, on the ratio of grains and soluble matter. Since a river flow purification method176requires sludge management, it is desirable to select a measure that facilitates sludge management andrequires a low running cost. The subject water volume, which is a key determinant of the size of thepurification facility, shall be determined on the basis of the goal achievement frequency, etc.
The site for facility construction should be thoroughly investigated, because the purification method andintake and release method depend on the location (landside area, high water channel, channel, etc.), area,
shape, height difference, and habitats for plants and animals.
The following chart shows major methods of river flow purification.

Reference: methods for purification of river water
Purification
methods

Purification
mechanism

Subject for
purification

FeatureContact oxidation Biological oxidation Organic matter,
SS, NH4-N
Precipitation

Decomposition of organic matter by
growth of microorganisms attached tocontact media such as grovel, plastic, etc.

Water
purification by
soil

Water purification by soil capability ofabsorption, decomposition and filtration,
and absorption function of vegetation

Filtration
Absorption

Organic matter,
Nutrient salts,
SS, Turbidity
Organic matter,

Oxidation pond

Biological oxidation Nutrient salts,
Precipitation
SS

Water
purification byvegetation

Absorbed Vegetation Organic matter,
Precipitation
Nutrient salts,
SS

Decomposition of organic matter andintake of nutrient salts by microorganisms
in the pond, and depositing of SS maters
owing to decreased water flow velocity
Uptake of nutrient by vegetation,
depositing of SS maters owing todecreased water flow velocity, andabsorption by soil. The vegetated area
perform as a habitat

Among various river flow purification methods, the contact oxidation method has been used most widely.
The contact oxidation method takes advantage of the self-purification function of the river, utilizing
filtration by contact media such as gravel and organic matter decomposition by attached organisms.
b) Dredging of bottom sediments
In bottom sediment dredging planning, the range and amount of dredging, the dredging method, and thetiming shall be determined, and a bottom sediment treatment and soil allocation plan shall be established.
In determining the range and amount of dredging, the nutrient salt concentration of the bottom sediment tobe dredged and the range and depth (amount) of dredging shall be determined on the basis of the results of abed material investigation and the relationship between nutrient salt concentration and the solution rate, etc.

177

The dredging method shall be determined depending on the hydraulic condition of the subject water bodyand the characteristics of the bottom sediment. When dredging, it is important to avoid cluttering the bottom
sediment as much as possible. Especially for bottom sediment that has an amount of harmful substanceexceeding the level for removal, a method that prevents its flow loss and dispersal shall be planned, and it
must be detoxified and allocated to a controlled disposal site.
In determining the timing for dredging, it is necessary to take into consideration harvest times for seaweeds
and fish, and the timing of odor occurrence from the bottom sediment.
In the process of establishing a plan for bottom sediment treatment and allocation, the method of treatment
shall be selected from among solidification at the stockyard, air drying, etc. and the final disposal site shallbe planned.
c) Water quality preservation measures in the basin
Measures implemented in the basin by the river administrator include river flow purification in an ordinary
river, etc.
(2) Separation of loading
Methods for loading separation include the use of a flow conservation channel.
A flow conservation channel aims to separate river water from polluted water.
In establishing a plan, an appropriate method of river water separation shall be determined in consideration of
the volume of polluted water inflow, the intake location for the water supply system, the presence of harmful
substances in the basin, and the water quality of the final effluent, etc. The subject area, route, and treatment
method, etc. shall be determined.
Since a flow conservation channel is a low-water channel that separates river water from polluted water within
the river channel, it is an effective water quality preservation measure for rivers where water for the water supply
system is taken from the lower reaches, as well as for rivers to which many branches and discharge channels
carrying polluted water are connected.
When water treatment is necessary in a flow conservation channel, the target quality of the final effluent watershall be determined on the basis of the water quality and standards of the receiving river, and a treatment method
suitable for the determined water quality items and target values shall be selected.
When it is necessary to flow polluted water downstream, the impacts at the release location shall be assessed anda water quality improvement measure shall be implemented as required.

178

(3) Securing of flow volume
The effects of securing flow volume include dilution of polluted water, improvement of dissolved oxygen, andreduction of detention time in the water body.
In planning the introduction of purification water, the necessary water volume for purification shall be
determined through investigation of the water quality and flow volume of the subject water body. It is also
necessary to balance the plan against flood and water utilization plans.
Methods of securing flow volume include the conveyance of purification water from another river, etc. and the
securing of maintenance flow discharge by using a dam, etc. Here, the securing of purification water by water
conveyance will be described.
In the process of planning, since the amount of purification water differs depending on the use of river water andthe status of the sewerage system, it is necessary to clarify the estimated effects at different times by thoroughly
considering the current state as well as future changes. In determining the water conveyance capacity of thesource river, use of the source river and preservation of the river environment shall be taken into consideration. It
is also necessary to confirm the discharge capacity of the channel that receives the purification water.
In determining the locations for the intake and the discharge outlet for water conveyance, it is necessary to takethe habitats of plants and animals into consideration.
Plans for construction or installation of a reservoir, diversion canal, lift pump, etc. need to be well balanced withfacility construction plans for flood control and water utilization. Facilities that can be used commonly for their
purposes shall be planned through adjustment.
Conveyance of sewage-treated water, seawater, etc. is effective for small urban rivers that do not have a reservoir
area nearby. The water quality of the supplying water body shall be improved, and, if necessary, treatment of theconveyed water should be considered.
(4) Reinforcement of the purification function of the river
At locations such as a tidal area, dam, or weir where water stagnates in a filling area, lack of oxygen prevents the
progress of organic matter decomposition. The oxidizing power of these locations can be enhanced by supplying
oxygen by aeration or by the use of a fountain.
When this method is to be adopted, the necessary scale shall be determined by investigating the hydraulic andhydrological characteristics of the subject water body, the status of the channel, and the facility installation site,
etc.
The addition of riffles, pools, bends, and other variations to a monotonous river channel is also an effective way
179

of preserving water quality, as is the construction of a vegetated zone along the river.

Section 3 Water Quality Preservation Measures for Reservoirs, Lakes, etc.
Water quality preservation measures for reservoirs, lakes, etc. include water temperature and flow control,
reduction of loading, separation of loading, and water conveyance, etc. A measure shall be determined on thebasis of multiple alternatives combining these functions and by conducting a comparative review of these inview of the location, method, and expected effects, etc. of the measure. In determining the measure, the target
water quality, effects, ease of maintenance, and impacts on the environment, etc. shall be taken into
consideration.

Explanation
Water quality preservation measures for reservoirs, lakes, etc. include those for water quality preservation of thereservoir area and those for mitigation of impacts on the downstream sector. Reservoirs, lakes, etc. include weirreservoir ponds and regulating reservoirs, in addition to reservoirs and lakes.
For reservoirs and lakes there are appropriate individual measures for cold water problems, problems oflong-term persistence of turbid water, and eutrophication problems. Some of these problems are effective forwater quality problems other than the expected water quality problem. For example, a selective water intake
facility used as a measure against a cold water problem is also effective for the problem of long-term persistenceof turbid water when it is used at a time of flood and after.
On the other hand, there are cases in which a measure for improving a certain water quality problem will affectsome other water quality problem. For example, if a selective water intake facility places too much emphasis on
the cold water problem and surface water intake, a turbid water mass that flows in at the time of flood may
remain in the reservoir during and after the flood, causing prolonged turbidity of the water. Inappropriate controlof water temperature and turbidity may worsen eutrophication problems.
For these reasons, selection of water quality preservation measures requires thorough examination from theviewpoints of measure combination and prevention of influence.
The following points need to be considered in the development of function-based water quality preservation
measures for reservoirs, lakes, etc.
1. Selection of a method for water temperature and flow control should be based on the vertical distribution ofwater temperatures, which indicates the thermal stratification of the reservoir. The reservoir turnover and the
diffusion of inflow water in the reservoir need to be taken into consideration.
The facility location shall depend on the shape of the reservoir, and its scale shall be determined on the basis ofan estimation of thermocline formation and its effects, as obtained from water discharge and meteorologicalconditions, etc.
2. Selection of a method for reduction of loading should be based on the water quality items of the water to be
purified and the water’s characteristics. Treatment, utilization, etc. of bottom sediment and sludge should be180taken into consideration.
A location that enables efficient reduction of loading shall be selected as the facility site.
3. In selecting a method of separation of loading, effects on the lower reaches should be taken into consideration.
A location where the facility length can be minimized shall be determined as the facility site.
4. In selecting a channel for water conveyance, it is necessary to take into consideration the flow regime, water
quality, water utilization plans, and geographical condition of the source river.
If other measures are necessary, an appropriate method should be selected on the basis of the concept describedabove.
1. Controlling the water temperature and flow
(1) Selective water intake facility
A selective water intake facility is applicable as a measure against cold water, long-term persistence of turbid
water, and eutrophication. In all of these applications, stratification of the reservoir is a prerequisite, and themethod of operation should depend on the temperature and water quality characteristics and turbidity distributionof the reservoir, as well as the water quality characteristics of the inflow water.
(2) Aerating circulation facility
An aerating circulation facility is applicable as a measure against cold water, long-term persistence of turbid
water, and eutrophication. A method, location, and scale, etc. that is suitable for the purpose of aerating
circulation shall be determined from the characteristics of the reservoir and water quality properties.
Greater effectiveness may be expected by combining an aerating circulation facility with a selective water intake
facility.
2. Reduction of loading(1) Measures for inflow water quality
A measure for inflow water quality is applicable as a measure against eutrophication and is effective for reducing
nutrient salts. Possible purification methods include vegetation purification and soil purification. The method,
location, and scale, etc. shall be determined from the water quality and amount of pollution loading of thepolluted tributary and in consideration of the subject flow volume, site condition, ease of maintenance, and cost,etc.
(2) Front reservoir
A front reservoir, which is a dam, etc. where water can be temporarily stored before it flows into a reservoir,
181

thereby precipitating nutrient salts, is applicable as a measure against eutrophication.
The facility scale (capacity) and location shall be determined on the basis of the characteristics of the nutrient
salt (ratio of dissolved matter to particle matter, etc.) and the characteristics of the inflow loading amount, as
well as the retention time necessary for nutrient salt precipitation and subject flow volume, and in considerationof the site condition, economic efficiency, etc.
(3) Bottom sediment dredging
Bottom sediment dredging, which aims to suppress oxygen consumption by bottom sediment and to prevent
nutrient salt elution, is applicable as a measure against eutrophication.
The procedure and considerations in bottom sediment dredging planning are the same as those for bottom
sediment dredging as a water quality preservation measure for a river (see Section 2: River Water Quality
Preservation Measures).
(4) Water quality preservation measures in the basin
Measures that the river administrator can implement include the following:a) River flow purification for non-classified rivers.
b) Installation of an “environment preservation zone” with a wooden corridor in the peripheral area of areservoir with markedly turbid water, eutrophication, etc.
3. Separation of loading
Construction of a bypass is applicable as a measure against long-term persistence of turbid water andeutrophication. Measures against long-term persistence of turbid water include a method that bypasses highly
turbid water contained in the feeder river, and a method that takes in clear water that flows in from the upstream
end of the reservoir and bypasses the reservoir (clear water bypass). To suppress eutrophication, phytoplankton
growth shall be suppressed by not letting nutrient salts, a phytoplankton growth factor, enter the reservoir.
In any such measures, the plan needs to be developed on the basis of the water quality and loading amountproperties of the feeder river, and in consideration of the reservoir operation and the water volume, water quality,and environment of the river downstream of the reservoir. A measure for sediment control at the time of flooding
needs to be considered.
4. Water conveyance
Water conveyance is applicable as a measure against eutrophication. This method increases the turnover of a lake,etc. and suppresses the growth of phytoplankton by conveying water from another water source. It is necessary
to take heed of the nutrient salt concentration of the conveyed water.
In planning a measure that uses a water conveyance method, the water quality and flow volume of the subject
182

water body and conveyed water shall be investigated, and sufficient examination shall be conducted so as tomaintain a balance between flood control and water utilization plans, before the water volume to be conveyed isdetermined.
Measures against coloration (red water, black water), accumulation and elution of harmful substances, etc.
include bottom sediment control (dredging, sand cover), and bottom layer DO improvement by aerating
circulation, etc.

Chapter 3 Installation Plan for Erosion and Sediment Control Facilities
Chapter 3–1 General Information
The installation plan for erosion and sediment control facilities, etc. consists of the installation plans for erosion
and sediment control facilities (drawn up on the basis of the erosion and sediment control master plan), landslide
prevention facilities (drawn up on the basis of landslide control master plan), steep slope failure prevention
facilities (drawn up on the basis of steep slope failure control master plan), avalanche control facilities (drawn
up on the basis of the avalanche control master plan), and comprehensive sediment disaster control facilities(drawn up on the basis of the comprehensive sediment disaster control master plan).
Devising installation plans for facilities entails the consideration of the effects on the natural environment
including the landscape that the intended facilities would cause. In a basin with obvious problems associatedwith sediment movement, such plans need to devised, in addition, from the viewpoint of comprehensive
sediment treatment over the entire sedimentary system.

Chapter 3–2 Installation Plan for Erosion and Sediment Control Facilities
Section 1 General Information
The installation plan for erosion and sediment control facilities drawn up on the basis of the erosion andsediment control master plan consists of installation plans for sediment yield control facilities, sedimenttransport control facilities, woody debris control facilities, and volcanic sediment and erosion control facilities.
The installation plan for erosion and sediment control facilities is based on the river system erosion and sedimentcontrol plan and is combined with the installation plan for sediment yield control facilities, and debris flow
prevention plan, which is combined with the plan for the installation of sediment transport control facilities.
Table 3–1 categorizes the locations of sediment yield and sediment transport and the types of erosion control
works to be implemented at those locations in accordance with the installation plans for sediment yield control
facilities and sediment transport control facilities, as part of the installation plan for erosion and sediment control
facilities.
In this revision, from the viewpoints of the structure and its function, the phrase “erosion control dike” shall be
adopted as the common designation for any facility installed across a torrent instead of the phrase “erosion

183control dam” as has been conventionally stipulated in the Manual for River Works in Japan (draft version). Thisis because of the recent rapid advances in the introduction and spread of erosion and sedimentation facilities withpermeable steel structures, in addition to the fact that many dikes were introduced before the publication of theManual for River Works in Japan (draft version).

Table 3–1 Main installation plans for erosion and sediment control facilities including types of works

Classification of installationplans for erosion and sedimentcontrol facilities on the basis of
the plans of river system erosion
and sediment control and debris
flow prevention

Locations of
sediment yieldand sedimenttransport

Types of works

Hillsides

Foundation works for erosion control
afforestation, planting works, hillside
slope reinforcement works, hillside
conservation works

Installation plan for sedimentyield control facilities
Riverbeds andriversides

Installation plan for sedimenttransport control facilities

Torrents andriversErosion control dike construction,
consolidation works, bed girdle
construction, anti-erosion works, torrentpreservation works
Erosion control dike construction,
consolidation works, bed girdle
construction, anti-erosion works, groyne
construction, torrent-preservation works,
training dike, sand-retarding basin works

Section 2 Installation Plan for Sediment Yield Control Facilities
2.1 Overview
The installation plan for sediment yield control facilities, which is based on the plans of river system erosion andsediment control and debris flow prevention, aims to install erosion and sediment control facilities for the
purposes of protecting hillsides, riversides, and riverbeds and preventing sediment yield. Care need be taken
during the installation of sediment yield control facilities to ensure that function of each facility appropriately
and adequately fulfilled.

Explanation
The installation plan for sediment yield control facilities is devised appropriately giving due consideration toworks in relation to erosion and sediment control such as hillside preservation works, erosion control dikes,
torrent-preservation works, etc. at sources of sediment yield, e.g. hillsides, riverbanks, and riverbeds.

184

2.1 Hillside preservation works
2.2.1 Overview
Hillside preservation works aimed at controlling floods consist of hillside works and hillside conservation
works. The former works stabilize slopes by cutting and filling or by earthwork construction on denuded lands
or bare slopes to prevent or reduce the triggering or aggravation of surface erosion or shallow slides by the
introduction of a vegetal cover while the latter works boost the function of the former by nurturing the
introduced vegetal cover.
The hillside preservation works consist of foundation works for erosion control afforestation, planting works,
and hillside slope reinforcement works.

Explanation“Denuded land” refers to land that is bare as a result of hillside collapse.
“Bare slope land” refers to hillside slopes on which the vegetation has entirely or partly vanished or declined.
In view of the fact that persistent sediment yields from denuded lands or bare slopes are contributory in causing
landslide disasters at downstream sites during flood, hillside preservation works are considered very important inconjunction with erosion control against flood.
The degree to which hillside preservation works could control the triggering or aggravation of shallow slides
generally depends, in terms of general construction, on the extent of foundation; in terms of vegetation, it
depends on the extent of the roots’ ability to reinforce the soil. Thus, evaluation of the effects of hillsidepreservation works on deep-seated collapses or landslides is a future task.

Fig. 3–1 System of hillside preservation works

2.2.2 Hillside works
Hillside works are categorized into three groups: 1) foundation works for erosion control afforestation tostabilize hillside slopes and/or to prevent slope erosion; 2) planting works on hillsides to prevent or reduce thetriggering or aggravation of sheet erosion or shallow slides by introducing a vegetal cover on denuded lands orbare slope lands; and 3) hillside slope reinforcement works to increase the shear resistance of the slope as awhole against collapse by carrying out concrete crib works or reinforced concrete insertion works on denuded
lands or hillside slopes that approaching or at the verge of failure. Utilizing a single measure or a combination of
the aforementioned techniques sediment yield shall be controlled; Topography, geology, soil, climate, andvegetal cover in the design area and its surroundings and the effects of intended facilities on the existing erosion

185

control facilities shall be sufficiently researched to determine the appropriate type of work/s needed. In
particular, while deciding vegetal cover, sufficient consideration must be given on the compatibility with thesurrounding vegetation.

Explanation
1. Foundation works for erosion control afforestation
Foundation works for erosion control afforestation are aimed at stabilizing hillside slopes by cutting, fitting, orestablishing small check dams and to prevent slope erosion due to overland flows by constructing channel works.
Laying of foundations for future planting or hillside slope reinforcement works at the site in question is intended.
2. Planting works on hillsides
Planting works on hillsides are aimed at introducing vegetal cover in the region of interest. It includes hurdle
works, terracing works with seeding, and simple terracing works that introduce a vegetal cover and thereby toprevent the topsoil from moving. The vegetation type shall be carefully chosen in such a way that it is integrated
with the surrounding vegetation, in consideration of its secular change.
3. Hillside slope reinforcement works
Denuded land or hillsides that are at the verge of failure and where stabilization works are deemed necessary
with immediate effect or where the triggering or aggravation of slope failure cannot be prevented or reduced byfoundation works for erosion control afforestation or planting works alone, hillside slope reinforcement works
can enhance the resistance of the slope as a whole against collapse by carrying out concrete crib works orreinforced concrete insertion works on hillsides.
A vegetal cover cannot be introduced naturally on a steep denuded slope, for instance, as the topsoil frequently
moves. In such a case, generally, after the slope has been stabilized (mainly by the use of foundation works byerosion control through afforestation to control the movement of the topsoil), vegetal cover is introduced byplanting works on the hillside. On the other hand, if the site borders on a slope and has to be stabilized as soon as
possible, then hillside slope reinforcement works are introduced.
In landforms with poor soil but relatively gentle slopes such as bare slope lands, planting works are the primary
type of remedial measure in many cases.
These types of works, which are often used in combination on sites with colluvial soils, should be properly
combined in the actual planning. In the actual planning on hillsides with large sediment yields, such as on eroded
areas bordering torrents, the combination of hillside works, erosion control dikes, and torrent-preservation works,
for example, in which an erosion control dike aimed at stabilizing the foot of the mountain is used as thefoundation works for erosion control afforestation, is sometimes introduced.

2.2.3 Hillside conservation works
Hillside conservation works following hillside works shall promote proper growth of vegetation on a hillside
slope, aiming to prevent the triggering and/or aggravation of surface erosion or collapse or to increase theabatement function of the vegetation.

186

Planning of hillside works shall include guidelines for conservation of hillside in accordance with its purposeand on the workflow.

Explanation
Since the vegetal cover to be introduced by hillside planting works—unlike concrete structures—require a
specified period of time to exert its influence, it is important to stipulate guidelines on the same.
Usually, the first hillside planting works cover the bare land with plants or pioneer trees (soil-improving trees) toprevent the surface soil from moving or eroding and to form the growing foundation of a forest. After thisinitiation of disaster prevention activity by the hillside conservation works, the identified vegetation is cultivatedin such a way that it can harmonize with the surrounding natural vegetation.
Moreover, when the introduced vegetation is remarkably different from the surrounding vegetation at a sitewhere hillside works are executed, or when the greenbelt is not expected to be affected by pest control orsediment controls because the introduced trees are homogenous with the surrounding vegetation, then, at a point
when the colony has grown to a certain level, the type of tree or forest may be changed to increase the functionof the hillside works as deemed necessary.

2.3 Erosion control dikes
An erosion control dike as a sediment yield control facility is one that aims to 1) prevent or reduce the triggering
or aggravation of hillside collapse by mountain foot fixation; 2) prevent or reduce riverbed longitudinal erosion;or 3) prevent or reduce the runoff of unstable sediment accumulated on the riverbed.
While planning to construct the facility, its scale and structure should be selected according to its purpose.
An erosion control dike as a sediment control facility should be arranged in consideration of its expected effects,
the topography and geology of the design site, and the stability status of the debris.
Thus, in principle, the dike should be arranged in the stream just beneath a hillside at the verge of failure in thecase of 1); in the case of 2, just beneath a vertical erosion area; and in the case of 3), just beneath unstableriverbed sediment

Explanation
While planning for the installation of sediment yield control facilities, in many cases the erosion control dikes
are intended to control sediment transport as well as sediment yield. The function of the erosion control dike,aimed at mountain foot fixation, is to accumulate sediment at the upstream side of the established dam; this inturn raises the riverbed and fixes the mountain foot to prevent the hillside from collapsing or stop a hillside
collapse from spreading. The erosion control dike aimed at controlling longitudinal erosion accumulates
sediment on the upstream side of the established dam to prevent longitudinal erosion of the riverbed.
The erosion control dike, aimed at preventing unstable sediment accumulated on the riverbed from discharging,accomplishes its purpose by establishing an erosion control dike.
The scale of the erosion control dike for longitudinal erosion control or of the one for controlling the runoff ofinstable sediment accumulated on the riverbed can be estimated by riverbed variation calculations or by187

hydraulic model experiments. The conditions necessary for these riverbed variation calculations or hydraulicmodel experiments, such as the time variations of flow and sediment runoff, the parcel size distribution of
sediment accumulated on the riverbed, etc., need to be properly established. It is desirable that erosion control
dikes be established at locations exposed with bedrock on the riverbed or riverbank; this ensures the security ofthe structure and, particularly, the prevention of basement corrosion and wing bank runoff.
Moreover, the decision on whether to establish a single erosion control dike or a cluster of low dikes in sequence
depends on the characteristics of the sediment yield type and the difficulty of execution and maintenance at thedesign site.
Erosion control dikes are classified by their types, structures, and materials, which should be selected on thebasis of their surrounding situations or economic efficiency. They are divided into two types: permeable andimpermeable. In terms of the structure, they are further divided into two types: gravity and arch. Concrete, steel,and soil–cement are mentioned as materials. Moreover, it should be kept in mind that the permeable type is notalways suitable for erosion control dikes as debris production control facilities in terms of the functions requiredof such dikes, which should take into account sediment yield type, the topography, and the geology of the area.

2.4 Consolidation works
Consolidation works are facilities that are intended not only to stabilize the riverbed by controlling riverbed
longitudinal erosion or preventing the riverbed sediment from moving again, but also to prevent or reduceriverbank erosion or collapse. Additionally, they function to prevent foundations, such as anti-erosion works,
from being scoured and to protect them.
The following shall be considered in selecting the places where consolidation works should be installed:
1.Consolidation works shall be set up locations under threat by riverbed degradation.
2.When they aim to protect the foundations of structures, they shall be established in the lower reaches of thesestructures.
3.In places with riverside erosion, collapse, and landslide, they shall, in principle, be established in the lower
reaches.

Explanation
The height of consolidation works is, commonly, less than about 5 m.
Consolidation works function to control the occurrence of debris flow by preventing the riverbed from being
lowered by the tractive force of the flow and arresting unstable sediment from moving. They also prevent andreduce erosion and collapse of the riverbanks by preventing lowering of the riverbed, making the riverside slopes
milder, and controlling turbulent flow.
When a section is encountered with problems such as riverbank erosion or collapse and extensive longitudinalerosion is, a combination of consolidation works shall be formulated and carried out in order to stabilize theriverbed and banks.

1882.5 Riverbed girdles
The riverbed girdle is a facility that is intended to prevent longitudinal erosion.
It is set up at the downstream side of a single consolidation work and at a place where the interval between
consolidation works is large or where longitudinal erosion had taken place or is likely to take place.
The riverbed girdle shall be planned in such a way that its crest is as high as the computed height of the riverbed
so as to eliminate the gap between the bed and the girdle.

2.6 Anti-erosion works
Anti-erosion works are facilities which prevent riverside erosion and collapse.
They shall be set up in places such as water-collision fronts, in which riverside erosion or collapse has takenplace or is likely to take place because of debris movement or flow, or at locations where mountain foot fixation
or erosion prevention is required.

Explanation
Since anti-erosion works may make the waterfront environment monotonous, it is desirable to increase thenatural appearance of the waterline by keeping the range over which the works are established to the necessary
minimum.

2.7 Torrent-preservation works
Torrent-preservation works are facilities that are intended not only to prevent riverside erosion and collapse bycontrolling turbulent flow or drift currents in the torrent across a mountainous plain or fan, but also to prevent
riverbed or bank erosion by regulating the longitudinal slope. The torrent works are a combination of
consolidation works, riverbed girdles, anti-erosion works, groynes etc.
The torrent-preservation works shall be planned to install riverbed girdles and groynes, and carry out
consolidation, ,and anti-erosion works, as deemed necessary, taking advantage of natural landforms such as
unconstrained and constrained areas preserving the various torrential spaces and ecosystems and apply natural
sediment control functions

Explanation
The slope of the stream bed of a torrent is determined by the flow discharge: that is, the flow velocity, the water
depth, and the resistance of the stream bed. Thus, the design slope of the stream bed upstream of consolidation
works should be determined by considering the occurrence of erosion and sediments, in addition to theabove-mentioned factors, and then referring to the dynamic and statistical equilibrium gradients of thedischarged sediment. On the other hand, in the actual preparation of torrent-preservation works, it is necessary toproperly locate the erosion and sediment facilities only in the places where they are required to be set up, while
taking advantage of the natural landscape. Note that the channel consolidation works dealt with in the Manual
for River Works in Japan (draft version) are included in the torrent-preservation works because of the suitable
nature of their purposes and functions.

189

Section 3 Planning for Sediment Transport Control Facilities
3.1 Overview
A plan for the installation of sediment transport control facilities should prescribe the installation of such
facilities for the purpose of controlling sediment that is discharged in sediment transport sections. This planningshould be done on the basis of the river system sediment and erosion plan, and the debris flow measures.
In the actual planning of the sediment transport control facility installation, the purpose of each facility should
be made clear so that each function can be brought into play.

Explanation
Sediment transport control facilities include erosion control dikes and sand-retarding basins to hold and regulate
sediment yields.
A plan for the installation of sediment transport control facilities prescribes the proper installation of thesefacilities in the sediment transport sections.

3.2 Erosion control dikes
Erosion control dikes for sediment transport are facilities designed to: 1) control or regulate sediment discharge;or 2) hold or dissipate debris flow. There are two types: impermeable and permeable. In the actual planning, thefacility’s type, scale, and structure should be selected according to its installation purpose. An erosion control
dike for sediment transport should be located in an effective place, such as in a constrained stream whose
upstream width is large, or in a lower stream just beneath the confluence of the bank tributary; the decision
should be based on the expected effects of the erosion control dike and on the topography of the design site.

Explanation
In planning for the installation of sediment transport control facilities, erosion control dikes are often planned forthe purpose of controlling sediment yield as well as sediment transport.
An erosion control dike that aims to prevent sediment discharge can accomplish its purpose effectively bystoring sediment discharge equivalent to its capacity. However, if in the planning stage the capacity is anticipatedto be lost by advanced sedimentation, it is necessary to prepare debris-exclusion works in advance.
Because a large amount of sediment inflow into the sediment area of an erosion control dike makes the slope ofthe stream bed gentler, and consequently the bed width greater, than it would be in the absence of an erosion
control dike, sediment is accumulated temporarily by any slope (dynamic equilibrium gradient) that is steeper
than a stable slope (static equilibrium gradient). By this means, an erosion control dike that aims to regulate
sediment discharge also performs its part effectively. Moreover, a permeable erosion control dike that aims tocontrol sediment discharge can do so, especially at the peak of discharge, if large grain-sized stones are fixedwith a grid and the flood is dammed back. Additionally, the use of a permeable erosion control dike can ensure
the consistency of a stream by the cross-section of the part through which water flows.
When it is full of sand, an erosion control dike that is intended to hold debris flow and dissipate its energy can do190this by temporarily accumulating sediment into a sediment holding area via the action of a slope that is steeper
than a stable slope. On the other hand, although the dike directs debris flow into the holding area, this holding
capacity is lost when advanced sedimentation occurs. Thus, if such a loss is anticipated in planning, the holding
capacity will have to be restored by debris-exclusion works. On the other hand, if the slope of the stream bed is
mitigated and consequently debris flow is transformed into bed load transport, then an erosion control dike canalso dissipate the energy of the debris flow. Fundamentally, a permeable erosion control dike that is intended tohold debris flow and dissipate its energy can hold debris flow by filling up the permeable part through which thewater flows with the debris itself.
It is desirable to establish an erosion control dike in a place with bedrock on the riverbed and banks in order toensure the security of construction, and especially to prevent the foundations of the dike from being scoured andthe wing bank from running off. Additionally, the choice between a single erosion control dike and a cluster of
erosion control dikes will depend on the characteristics of sediment transport in the area and the difficulty of
execution or maintenance.
Erosion control dikes are classified according to type, structure, and materials, all of which should be specified
on the basis of the surrounding environment at the design site, the economic efficiency of each option, etc.
Erosion control dikes are of two types: permeable and impermeable. Their structural types are also divided into
two types: gravity and arch. Their materials include concrete, steel materials, and soil–cement.
In principle, permeable erosion control dikes should not be located in such places that require foot fixation.

3.3 Consolidation works
See Chapter 3–2, section 2, 2.4

3.4 Riverbed girdles
See Chapter 3–2, section 2, 2.5

3.5 Groins
Groins are facilities that prevent riverside erosion and collapse by controlling the direction of running water or
restricting the width of a channel. Their function is not only to accumulate sediment by dissipating the energy ofrunning water, but also to protect the riverbanks.
In principle, groins should be set up in turbulent flow sections without a steep slope on the stream bed, such as in
the lower reaches of a torrent, debris-cone areas, fans, etc. Even in the upper reaches of a torrent, however, theyshould be established in places such as at the foot of an area of denuded land, as the occasion demands, toprevent the extension of collapse due to the impact of running water.

191

Explanation
When a groin is set up on one side of a river (say, at the foot of an area of denuded land), because the other side
can often serve as a water-collision front it is necessary to be aware of the situation on the other side.

3.6 Anti-erosion works
See Chapter 3–2, section 2, 2.6.

3.7 Sand-retarding basin works
Sand-retarding basin works are facilities used to widen part of a river by excavation and to thereby accumulate
debris to control transported sediment. They should be commonly established on the downstream side of the exitof a valley, where space for the accumulated sediment can be ensured. On the other hand, although they should
be arranged with an erosion control dike on the upstream side and a consolidation work on the downstream side,
they should be combined properly with low water channels, training levees, erosion control greenbelts, etc.

Explanation
When woody debris threatens to flow out from sand-retarding basin works, it is necessary to consider installing awoody debris control facility, for example, by adding a riverbed girdle at a dam downstream to catch woody
debris.
Notwithstanding the debris flow control function of debris-exclusion works, such works are generally referred toas sand-catching works.

3.8 Torrent-preservation works
See Chapter 3–2, section 2, 27.

3.9 Training dikes
The training dike is a facility for safely training debris flow in its lower reach to prevent it from directly hitting
the preservation target in time of flood. In the absence of a training dike, the debris flow is usually caught andaccumulates upstream of the preservation target. Under more difficult topographical conditions, however, a
training dike should be established to provide a space that allows the debris flow to safely accumulate in its
lower reach. Initially, establish an erosion control dike or sand-retarding basin works to catch the debris flow.
Subsequently, establish the training dike as an addition to the previously built facility. In principle this is done by
excavation, in order to train the debris flow into the space in which it can be safely accumulated.
Additionally, when the site conditions make it difficult to excavate, a training levee can be established to control
the direction of debris flow and safely train the debris downstream.

192

Explanation
Planning of the training dike must include an examination of the grain size of the discharged sediment in case thedebris accumulates within the training dike, causing overflow or flooding.
On the other hand, when the design debris flow can be sufficiently treated on the upstream side of the planned
facility, normal torrent-preservation works should be planned.

Section 4 Planning the Installation of Woody Debris Control Facilities
4.1 Overview
The plan for the installation of woody debris control facilities, which is based on the woody debris control plan,should properly locate woody debris control facilities in accordance with the planned amount of woody debris.
The plan should also consider woody debris behavior, corresponding to debris occurrence and its flow
morphology, in the section in which woody debris is formed in response to variations in sediment yield andtransport.
Facilities are roughly divided into woody debris occurrence control facilities that are intended to prevent theoccurrence of woody debris and woody debris trapping facilities that are intended to trap woody debris into
torrents so that it cannot flow out to their lower reaches.
Additionally, the woody debris control facilities should be consistent with any erosion and sediment works
installed as part of the plan for the installation of sediment yield control facilities or the plan for the installationof sediment transport control facilities.

4.2 Woodendebris control facilities
4.2.1 Facilities for the control of wooden debris
Facilities for the control of wooden debris should control and reduce the wooden debris that occurs withsediment by protecting hillsides, riversides, and riverbeds to prevent sediment yields. They should be sited inareas that are potential sources of sediment and wooden debris.

Explanation
Facilities for controlling wooden debris should include hillside preservation works established in areas likely to
produce wooden debris and sediment, such as on denuded lands; hillside preservation works, erosion control
dikes, consolidation works, and anti-erosion works established in stretches of the river where debris flow occurs;
and torrent-preservation works and anti-erosion works located in stretches thorough which mainly river debris istransported by traction.

4.2.2 Wooden debris trapping facilities
Wooden debris trapping facilities to hold woody debris flowing with the sediment should be established on
hillside slopes where fallen trees have accumulated or in torrents where debris and wooden debris flow

193

downstream. Additionally, in the actual preparation of a wooden debris trapping facility, consideration must be
given to the fact that the trapping function in the debris-flow stretch of the river will differs from that in the
traction stretch.

Explanation
Wooden debris trapping facilities catch debris and wooden debris in one lump in the debris flow stretch but
separately in the traction stretch.
Facilities for trapping wooden debris include log boom works established on hillsides to prevent fallen trees
accumulated on the hillsides from entering torrents; erosion control dikes and partly permeable erosion control
dikes established in the debris flow stretch; sub-partly permeable erosion control dikes of the impermeable
erosion control dikes established in the traction stretch; and log boom works and permeable erosion control dikes
placed in the lower reaches of sand-retarding basin works.

Section 5 Planning Volcanic Sediment and Erosion Control Facilities
5.1 Overview
Planning for the installation of volcanic sediment and erosion control facilities, which is done on the basis of the
volcanic sediment and erosion control plan, should be aimed at preventing and alleviating sediment disasters
caused by rainfall and volcanic action in volcanic sediment and erosion areas. The planning should specify thework type, work method, and facility scale of the design facility, considering the characteristics of the sediment
movement phenomenon and the regional plan of the target site.
In terms of the debris flow caused by rainfall etc., installation of facilities should be prepared according to theplan for installation of sediment yield control facilities (see section 2 of this chapter), the plan for installation of
sediment transport control facilities (section 3), and the plan for installation of wooden debris control facilities(section 4).
A plan for the installation of volcanic mudflow control facilities should be prepared to deal with volcanic
mudflow directly caused by volcanic action. At the time of an eruption, when emergency measures must be
taken, it may be necessary to use an unmanned construction system to install debris-exclusion works (such as anerosion control dike, a training dike, or a sand-retarding basin) that will remove the debris within the sedimentarea.

Explanation
Facilities for the control of volcanic sediments and erosion are planned according to the volcanic sediment anderosion plan and include erosion control dikes, torrent-preservation works, consolidation works, riverbed girdles,
anti-erosion works, sand-retarding basin works, and hillside works.
In terms of the targeted lava flows and pyroclastic flows, the actual planning should combine properly, if
necessary, some volcanic sediment and erosion control facilities, which should individually have such functions
as runoff control and training. The decision to make these inclusions will be made on the basis of the scale of thephenomenon, the efficacy of the artificial controls, and cost effectiveness.
1945.2 Planning for volcanic mudflow control facilities
Installation of volcanic mudflow control facilities should be planned for the stretch where volcanic mudflow islikely to flow down and accumulate. In the actual planning, some of the following facilities should be properly
combined: sediment yield control facilities that prevent volcanic mudflows from developing through erosion;
sediment transport control facilities that trap discharged debris or reduce peak flow amounts; and other facilities
that function in training and wooden debris trapping.
Additionally, the plan for installation of volcanic mudflow control facilities should be set up in accordance withthe plan for the installation of sediment yield control facilities (section 2 of this chapter), the plan for the
installation of sediment yield control facilities (section 3), and the plan for the installation of wooden debris
control facilities (section 4).

5.3 Planning for lava flow control facilities
The plan for the installation of lava flow control facilities should be established according to the scale of the lava
flow, the efficacy of any artificial controls, and cost effectiveness. If necessary, it should properly combine someof the sediment and erosion control facilities that function in runoff control, flow direction control, and trainingof lava flows.

Explanation
Runoff control of lava flow means the reduction of the amount of lava flow through the storage of lava.
Thus, it is necessary to prepare an erosion control dike that is intended to store lava flows in the river adjacent tothe section where the lava flow discharges. On the other hand, a sand-retarding basin should be prepared in thestretch where the lava flow is discharged and accumulates.
Lava flow direction control involves artificially controlling the flow direction of the lava. Additionally, lava flow
training involves directing lava flows to areas that is far enough not to pose any danger to local residents. For
flow direction control and training, training dikes are the main items referred to.

195Chapter 3–3 Planning Landslide Control Facilities
Section 1 General Information
The plan for the installation of landslide control facilities should specify the installation of landslide control
facilities for the purpose of maintaining security against disasters due to landslides. It should be based on thelandslide control plan.

Explanation
Landslides often consist of separate movement blocks that interact with each other. In planning the installation oflandslide control facilities, it is necessary to prioritize measures taken to deal with each block in accordance withthe movement range and stability of the block; the interaction among blocks; and the position and importance ofthe preservation target as the situation demands. It is also necessary to efficiently improve the stability of thetarget landslide area.
Since the preliminary survey cannot always show the whole picture of the landslide, the project can be reviewed
by gathering further information whenever necessary.

Section 2 Planning Landslide Control Facilities
2.1 Basis of planning landslide control facilities
The plan for the installation of landslide control facilities should specify the proper installation of each facility toprevent landslide disasters, while considering the effect of each facility in accordance with the scale, occurrence,and mechanism of the landslides.

Explanation
The scale of the plan for the installation of landslide control facilities, which is generally shown by the design
safety factor, is calculated by stability analysis of every unit of the movement blocks, which are supposedly
moving as one. In the actual determination of design safety factors, it is necessary to comprehensively consider
the phenomenon and scale of the landslide, the significance of the preservation target, the level of the disasterthat will supposedly be caused by the landslide, the level of emergency created by the landslide, and so on. In
many cases, however, design safety factors are calculated on the basis of the present safety factors that are
assumed in accordance with the movement state of the landslide. Thus, it must be noted that the design safety
factors represent only those safety factors that have been improved by prevention works and do not always
represent the stability of the slope after construction.
The stability analysis can be used to determine the scale of the landslide control facility through the proper
procedures, in accordance with the characteristics of the landslide (plane shape, slide plane shape, movement
state).

2.2 Selection of construction methods
In the plan for the installation of landslide control facilities, construction methods should be chosen in196

consideration of the scale, occurrence, and movement mechanism of landslide, the situation of the preservation
target, and the economic efficiency of the method. The plan should properly combine control works andprevention works.

Explanation
A landslide is a complex phenomenon caused by a combination of causative factors such as topography, geology,
soil properties, etc., and by inducements such as climatic conditions (e.g. rainfall and thaw), ground waterconditions, artificial works such as cutting, earthquakes, etc. Thus, an effective plan for the installation oflandslide control facilities should perform an adequate preliminary survey and show the cause, mechanism, andoccurrence of the landslide, the type of preservation target, and the positional relationship between the landslide
and the preservation target. From this information, the plan should, in turn, select the proper prevention works
and specify the installation of the facilities in the proper positions. Especially, in the actual selection of
construction methods, the plan should sufficiently consider the relationship between rainfall, ground water, andlandslide movement; the scale of the landslide; the properties of the landslide soil clods; the velocity of thelandslide, block section; the position of the control work; and the emergency control method. The timing ofexecution of the chosen works should be judged from the occurrence mechanism and movement state of thelandslide, the level of influence of artificial inducements, etc.
Landslide control facilities are divided into control works and prevention works:
1.

Control works aim to mitigate or stop the landslide movement by changing natural conditions such as thelandslide topography and groundwater situation.

2.

Prevention works aim to stop landslide movement by using the resisting power of the establishedconstruction.

In the actual selection of control works and prevention works, the following points must be noted:
1.

In planning, the characteristics of both control works and prevention works should be reasonably combined.

2.

The main construction methods should be groundwater drainage works, landing embankment works, andearth removal works. When stabilization of movement blocks is attempted to directly protect houses or
public facilities, pile works or anchor works should be introduced.

3.

If landslide movement continues, in principle, control works should not be conducted in advance ofprevention works. In other words, they should never be introduced until prevention works have mitigated or
stopped the landslide.

Section 3 Control works
The control work plan should be prepared in such a way that landslide movement can be effectively controlled
by changing the natural conditions such as the topography, geology, and groundwater.

Explanation
The control works include surface water drainage works, groundwater drainage works, earth removal works,197

loading embankment works, and erosion control works through river-related constructions. On the basis of thecharacteristics of each work, the plan should also designate the optimum site and number of facilities needed tocontrol the landslide.
1.

Surface drainage works (channel works and infiltration control works)
These works aim to prevent landslides from being induced by infiltration and re-infiltration within or outside
the landslide area, for example by rainfall, surface water, and water from wells, ponds or channels.

2.

Shallow groundwater drainage works (closed conduits, open conduits, horizontal drilling works)
These works drain shallow ground water to prevent it from being supplied near the slide surface of a slope.

3.

Deep groundwater drainage works (side-boring works, drainage-well works, drainage tunnel works)
These works drain deep ground water to reduce the pore water pressure (groundwater level) near the slide
surface.

4.

Earth removal works (cutting works)
In principle, these works are set up at the head of the landslide to decrease the ability of the land to slip off(landslide sliding power). It is necessary to pay attention to changes in typographic conditions so as not to
induce further landslides.

5.

Loading embankment works
In principle, these works should be set up at the end of the landslide to increase resistance to landslide
sliding power. As in the case of earth removal works, it is necessary to be careful of changes in the
topographic conditions that could induce further landslides.

6.

Erosion control works through river-related constructions (slope protection works, erosion control dikes, andanti-erosion works)
These works are aimed at preventing the erosion and collapse that may result from the presence of runningwater (e.g. river water or rainwater) and may induce landslides. The sediments trapped by the erosion
control dikes established in the stretch of river just beneath a landslide site can prevent erosion and collapse
at the landslide end. The effect is expected to be the same as that of loading embankment works.

Section 4 Prevention works
Prevention works should include structures that are safe against sliding force so that landslides can be prevented
by their resisting power. They should be planned to have sufficient effects on clod movement.

Explanation
Prevention works include pile works, shaft works, and anchor works. It is necessary to estimate the proper
positioning and number of these works to prevent a landslide, on the basis of the characteristics of each type ofworks.
1.

Pile works
These works aim to directly resist the sliding force of the landslide through the added shear resistance andbending resistance of the piles, which are inserted into unmoved ground.
198

2.

Shaft works (caisson pile method)
Shaft works involve the excavation of a shaft 2.5 to 6.5 m in diameter that reaches a solid foundation. The
concrete pile is then poured in place. When it is difficult to ensure the designated design safety factor
because the landslide has a large amount of sliding force but there is a good solid foundation, these works
should be set up.

3.

Anchor works
Taking advantage of the tenacity of steel materials fixed within the unmoved ground, these works aim toresist the sliding power of a landslide. They should be set up in a position in which their detaining and/or
fastening effects can be produced effectively.

199

Chapter 3–4 Planning for Steep Slope Failure Prevention Facilities
Section 1 General Information
The plan for the installation of steep slope failure prevention facilities should be set up on the basis of the steep
slope failure prevention facility plan, for the purpose of ensuring security against disasters caused by steep slope
failure.

Explanation
Steep slope failure prevention works are classified into construction works that are conducted directly on slopes
to improve their degree of safety and construction works that attempt to mitigate disasters in the prevention
target, preventing or reducing the debris produced at the target even if slope collapse has occurred.

200

Section 2 Planning Steep Slope Failure Prevention Facilities
2.1 Basis of planning steep slope failure prevention facilities
The plan for the installation of steep slope failure prevention facilities should be properly arranged in accordancewith the scale and phenomenon of the estimated collapse, for the purpose of preventing the damage that could becaused by failure.

Explanation
To improve the degree of safety of slopes by using steep slope failure prevention facilities, it is necessary to
remove unstable clods and decrease the force of collapse or sliding, or to add resistance to collapse or sliding.
Examination of the slope stability level is the most fundamental factor in estimating collapse forms anddesigning control works. In the actual evaluation of slope safety level, the following items must be noted:
1.

Examination of the degree of safety, based on comparisons with conventional experience, the actual
conditions of the slope, and the standard slope gradient

2.

Examination on the basis of anticipated collapse forms by on-site surveys

3.

Examination of safety levels by stability analyses

The design safety factor determined by stability analysis is based on in the content of Chapter 3–3, section 2,“Planning the Installation of Landslide Prevention Facilities.”
To prevent and mitigate sediment access to the prevention target, debris that has fallen from slopes needs to bereceived safely and surely.
It is necessary to keep a close watch on the condition of the foundations during construction. If phenomena
unexpected in the planning phase occur, measures to change the initial planning must be instituted promptly as
the occasion demands.
Any countermeasure works performed in the construction stage should be done in such a way as to notconspicuously decrease the degree of safety.

2.2 Selection of construction methods
The plan for the installation of steep slope failure control facilities should consider the causes, forms, and scalesof the anticipated collapses, the situations of the prevention targets, and the economic efficiency of the works.
The plan should properly combine construction methods to control slope collapse or sliding by changing natural
conditions such as the topography, geology, and groundwater state at the site, and construction methods toprevent collapse by providing resistance from structural objects.

Explanation
In the selection of control methods, various work types should be combined as the occasion demands. Whenconditions such as the topography, geology, and types of housing in a series of sites are not identical, it isnecessary to select a construction method suitable to each of the characteristics of the section, even if the section201

is short, while sufficiently considering the aspects of the slope.
In particular, because slopes requiring such steep slope failure prevention works are often near houses, theplanning should not just ensure the security and durability of the work sufficiently, but should also consider
harmony with the surrounding environment.

202

Chapter 3–5 Planning for Avalanche Control Facilities
Section 1 General Information
The plan for the installation of avalanche control facilities should specify the installation of avalanche control
facilities in such a way as to ensure security against avalanche disasters. It should be based on the avalanche
control plan.

Explanation
Avalanche control facilities are classified into prevention works to establish structural objects within the sectionof avalanche occurrence to prevent the occurrence of avalanches, and protection works that establish objects inthe avalanche path or in sedimentary areas to protect the preservation targets from avalanches that have justoccurred.
The plan for the installation of avalanche control facilities should consider the effects of each facility and specify
its proper installation properly.

Section 2 Planning for Avalanche Control Facilities
2.1 Basis of the plan for avalanche control facilities
In planning for the installation of avalanche control facilities, the facilities should be properly arranged toprevent disasters caused by avalanches, in accordance with the anticipated scales and characteristics of theavalanches.

Explanation
For avalanche control facilities to prevent the occurrence of avalanches, resistance to deposited snow movement(creeping or gliding) on a slope should be added.
In order to prevent avalanches from reaching the prevention targets, it is necessary to divide the energy of anavalanche that has occurred, dissipate its speed, guide its direction, and finally stop it.
The actual planning should consider the effects of each facility in accordance with the type, occurrence site, andreach-range of the anticipated avalanche, on the basis of the results of preliminary surveys. The aim is to makeeach facility suitable to the situation of the preservation target.

2.2 Selection of construction methods
The plan for the installation of avalanche control facilities should select construction methods in consideration of
the causes, form, and scale of the avalanches, the situation of the preservation target, the economic efficiency ofthe method, etc. It should properly combine prevention works and protection works.

Explanation
Prevention works are classified into occurrence prevention works that prevent deposited-snow movement
phenomena (creeping or gliding) from starting on a slope, and snow cornice prevention works that prevent the203formation of the snow cornices that causes avalanches.
In turn, the protection works are classified into interception works that intercept the avalanche on its way to theprevention target, energy dissipation works that disperse the energy of the avalanche to reduce its speed, andguidance works that change the course of the avalanche.
In installing avalanche control facilities, proper construction methods should be chosen in consideration of
security, durability, installation possibility, and the surrounding environment, in accordance with the types,
occurrence areas, and reach-ranges of anticipated avalanches and on the basis of the results of previous research.
In the actual selection of avalanche control facilities, although priority should be given to the examination of
prevention works that prevent avalanches from occurring just in the occurrence areas, other methods can be
combined as the occasion demands.
In the actual planning of the installation of avalanche control facilities, it is desirable to consider the surrounding
environment in terms of environmental measures, landscape measures, and so on.

2.3 Prevention works
Prevention works should be set up in occurrence areas to prevent avalanches from occurring.

2.4 Protection works
Protection works should be set up in avalanche paths or sediment areas to protect the preservation targets fromavalanches that have just occurred.

204

Chapter 3–6 Planning Comprehensive Sediment Disaster Control Facilities
Section 1 Basis of Planning for Comprehensive Sediment Disaster Control Facilities
The plan for the installation of comprehensive sediment disaster control facilities should determine the proper
location of sediment and erosion facilities, landslide prevention facilities, steep slope failure prevention
facilities, etc. It should be based on the comprehensive sediment disaster control plan and should be aimed atpreventing and reducing sediment disasters that may convergently occur.

Section 2 Basis of Planning for Urbanized Foothill Greenbelt Facilities
In the plan for the installation of urbanized piedmont greenbelt facilities, sediment and erosion facilities,
landslide prevention facilities, steep slope failure control facilities, etc. should be properly located to promote
the development and conservation of trees as per urbanized piedmont greenbelt development projects.

Explanation
As prescribed by urbanized piedmont greenbelt development projects, the plan for the installation of urbanizedpiedmont greenbelt facilities should determine the conservation guidelines that will be used to control thedevelopment of the greenbelt by law. The plan should prevent the greenbelt from being neglected and should
maintain healthy trees, specifying items associated with the development of erosion control facilities andfacilities for sediment yield and transport control. These will include hillside works and erosion control dikes,
landslide prevention facilities, and steep slope failure control facilities. In the actual installation, it will benecessary to make a point of maintaining healthy trees above all else.
Facilities must be located in harmony with the conditions of the environment and landscape that are prescribedby urbanized piedmont greenbelt development projects.

205Chapter 4 Planning Coastal Protection Facilities
Section 1 General Description
Coastal protection facility plans should specify the type, scale, and arrangement of coastal protection facilities inthe seacoast preservation project.
Planning for the installation of coastal protection facilities must meet the three purposes of coastal protection,
coastal environment development and protection, and proper coastal utilization by the public.
In coastal protection facility installation planning, it is essential to consider the continuity of the seaside in its
coastal direction from the viewpoint of comprehensive sediment control, making the seaside fulfill its functionsof coastal protection, environmental conservation, and utilization.

Explanation
The facilities, which are classified into “erosion control facilities,” “storm surge protection facilities,” and“seismic protection facilities” for the purposes of description, must be comprehensively considered in coastal
protection facility planning, because erosion, storm surges, and seismic disasters can be intricately intertwinedwith each other on some coasts.
Because conventional coastal protection facilities have placed stress on disaster-prevention measures, sea
embankments have sometimes prevented access to the seaside, gentle slopes built to ensure access havesometimes promoted the disappearance of coastal vegetation, and detached breakwaters or wave-dissipation
works have sometimes spoiled coastal landscapes.
In planning for the installation of coastal protection facilities, coastal protection, environmental conservation,and utilization must be harmonized in respect of the coastal environment and utilization in the seacoastpreservation project. It is therefore necessary to consider the continuity of the seaside in a coastal direction fromthe viewpoint of comprehensive sediment control.
Above all, sandy beaches function in disaster prevention by mitigating external wave forces and preventing
waves from entering continental areas. Additionally, since it promotes seawater purification by its zoobenthos or
aeration action, which provides good habitats for living creatures, beach sand is also considered an importantfactor in terms of the coastal environment. Planning for the installation of coastal protection facilities must
preserve and utilize natural beaches and not damage their functions.

Section 2 Erosion Control Facilities, etc.
2.1 Overview
Erosion control facility arrangement planning should determine the allocation between sediment transport
control facilities and beach nourishment in order to ensure the parameters of the design beach profile andmaintain them in the long term, while specifying the fundamental conditions for a sediment control facility tofulfill its required sediment transport control function.

206

Explanation
Erosion control aims to ensure the parameters of the design beach profile and maintain them in the long run. In
projects for the installation of coastal erosion control facilities, etc., sediment transport control facilities aim tocontrol sediment transport rates and beach nourishment for the purpose of ensuring the design coastal form andsupply sediment inflow, which is insufficient compared with the sediment discharge.
This section will specify the conditions needed to select the most suitable combination of ideas about theallocation between sediment transport control facilities and beach nourishment and for the facilities to fulfill
their anticipated functions.
The conditions required for the stability of sediment transport facilities, etc. will be specified in the calculation
section.

2.2 Allocation between sediment transport facilities and beach nourishment
The allocation between sediment transport facilities and beach nourishment should be specified by considering
the economic efficiency, environment, and usage properties, the wave extinguishing effect, and the feasibility ofseaside preservation after taking measures to rectify or mitigate the discontinuity or change of sediment
dynamics.

Explanation
In specifying the allocation between sediment transport facilities and beach nourishment, it is necessary to
determine future sediment dynamics. To do this, the cooperation of relevant organizations and bodies across thebasin and coast beyond the jurisdiction of each coastal manager is required.
To examine the approaches to improving sediment dynamics with the relevant organizations and bodies, it isnecessary to elucidate a grain size group and the amount of sediment supplied from rivers on the basis of past
sediment transport rates and the texture of the bed materials when stable. When the amount of sediment supplied
from a river has decreased because of gravel quarrying, river bed excavation, and sediment flow prevention by adam or erosion control dike, measures should be considered to ensure an adequate amount of sediment is
supplied to the coastal system from the perspective of consistent management of the entire sedimentary system.
When the continuity of coastal drift is hindered by coastal structures such as jetties, breakwaters, and landfill
sites, it is necessary to consider sand bypasses etc. When coastal erosion is caused by gravel quarrying within asediment transport system, the coastal gravel-quarrying site should be relocated to a marine area or at theterminal end of the sediment transport system. On the other hand, because sediment of a particular grain sizegroup dredged from channels and composing the shoreline has fundamentally the same effects on the seaside as
the gravel from quarrying in the sea, it should be considered reusable as a material for beach nourishment.
When coastal erosion is caused by coastal landforms such as submarine canyons, social infrastructure such as
existing coastal structures, and works such as gravel quarrying that were conducted in the past, the estimatedincrease in the amount of sediment as a result of the improvement of sediment dynamics cannot make up for the
sediment shortage and therefore sometimes cannot maintain the parameters of the design beach profile. In thiscase, the design beach profile should be maintained by the combination of sediment transport control facilities
and beach nourishment.
207

Figure 4–1 shows the sediment transport rate distribution with a combination of sediment transport control
facilities and beach nourishment. In plan (a), the sediment transport, which originally flows from left to rightwithout imbalance (with the sediment transport rate leveling off), is blocked by a coastal structure. As a result,on the downstream side the sediment transport rate is decreased and imbalance and erosion are caused. In plan(b), the sediment transport rate is decreased by the presence of the sediment transport control facility and so canbe equivalent to a reduced sediment transport rate. In this case, since the imbalance of sediment transport rateoccurs downstream of the section where development of the sediment transport control facility occurs, the wholesediment transport system will have to be controlled by a sediment transport facility in the future. In plan (c), a
reduced sediment transport rate is ensured by (dynamic) beach nourishment. In plan (d), a sediment transportfacility is combined with (dynamic) beach nourishment, and its facility scale can be smaller than in (b) and its
beach nourishment rate less than in (c). With various options like these, the combination balance between
sediment transport facilities and beach nourishment that is suitable for the characteristics of the design areas
should be specified in accordance with the lifecycle cost of the sediment transport facilities, the maintenance costof the beach nourishment, and the characteristics of the preserved beach.
When the parameters of the design beach profile cannot be temporarily maintained during the design period, orwhen the sediment transport rate cannot reach the equilibrium state at the last point in time of the design period,
the temporal improvement of sediment dynamics by (dynamic) beach nourishment should be also considered.

Controlled sediment transport

Sediment
transport rate

Sediment
transport rate

Block

ErosionStability(a) Before implementing measure

Block

Stability

ErosionStability(b) Measures taken by installation of a sedimenttransport control facility
Controlled sediment transport

Beach nourishment

Sediment
transport rate

Block

Sediment
transport rate

Stability(c) Measures taken by (dynamic)
nourishment of beach

Beach nourishment

Block

Stability(d) Measures taken by a combination of sediment transport
control facility and (dynamic) beach nourishment

Fig.4-1 Conceptual diagram of sediment transport distribution by the pattern of measures against beacherosion208

2.3 Sediment transport control facilities
2.3.1 Selection of facilities
The sediment transport control facilities should be selected on the basis of their required performances in
sediment transport control; their effects on the coastal environment or utilization; their economic efficiency; andthe constraints of construction, etc.

Explanation
Sediment transport control facilities are detached breakwaters, artificial reefs, and headlands (including jetties).
Sediment transport control systems such as detached breakwaters or artificial reefs should dissipate the waves,cause diffraction of waves from the opening, and control the coastal sediment transport rate via the resulting
tombolos that form from the effects of dissipated or diffract waves. However, it is necessary to pay attention to
the decreased rate of transport of the sediment flowing into the lower reaches and the movement of sedimentfrom the surrounding coast to the back of any detached breakwater due to the waves diffracted at its edge. The
sediment transport control system of a headland should transform the beach surrounded by a headland into astabilized shape in which the sediment transport rate can be proportioned to the inflow sediment rate,
consequently controlling the shoreline regression rate. The jetty is an auxiliary work to control seasonal
shoreline changes.
The detached breakwater, the artificial reef landscape, and the coastal environment and utilization will be
described in “Explanation” in Section 3.4 “Wave-dissipation facilities.” The establishment of headlands may
transform a straight beachline into a bow-shaped one, although headlands, established with a wide interval
between them, have few effects on the landscape. Additionally, since the headland itself is likely to be a large
construction, sufficient consideration must be given to its establishment.
In terms of economic efficiency, as evaluated only by the initial construction cost, headlands are generally themost cost-efficient on a long beach, as are detached breakwaters on a pocket beach (a bow-shaped beach whose
sediment balance is closed because the ends are surrounded by capes etc. that prevent coastal drift sands frompassing through them) or a comparatively short beach. In the evaluation of economic efficiency, headlands need
to be compared in terms of not only other construction forms with the same functions of coastal protection,
environmental conservation, and utilization, but also their lifecycle costs. Thus, it is not appropriate to evaluate
them by only each construction cost at the initial stage.
Their constraints of construction depend on their foundations or the wave conditions. On poor ground or
homogeneous fine sand ground, the foundations need to be taken as far as the detached breakwater, whose damsite weighs a great amount.

2.3.2 Detached breakwaters
The plane configuration, dam crest height, and structure of the detached breakwater should be designed inconsideration of beach nourishment. The parameters of the design beach profile should be ensured.
In planning a detached breakwater, it is necessary to consider the coastal environment and utilization.

209

Explanation
Detached breakwaters are classified into continuous levees and discontinuous ones by their landforms and into
permeable levees and impermeable ones by their permeability to waves. Generally, impermeable anddiscontinuous levees with easy construction constraints are often used in expectation that a tombolo can beformed behind them. With continuous levees the formation of a tombolo cannot be expected; nor can dissipationof the coastal sediment transport rate by shoreline changes. Additionally, with continuous levees, it is necessary
to pay attention to the deterioration in water quality on the landward side due to lack of seawater exchange.
Impermeable levees are used when a wave-extinguishing effect higher than that achieved with permeable levees
is required.
The sediment transport control effect of a detached breakwater depends on structural parameters such as theheight and width of the dam crest, the depth of execution, the width of the opening, the distance from the shore,etc.
In the actual examination, the Design Manual for Detached Breakwaters should be referred to.

2.3.3 Artificial reefs
The plane configuration and dam crest depth of an artificial reef should be designed in such a way that theparameters of the design beach profile can be maintained, taking into account beach nourishment.
In designing the reef, its effects on the coastal environment and its utilization should be considered.

Explanation
The plane configuration and dam crest depth of an artificial reef should be determined in anticipation of its effectin decreasing the sediment transport rate by making circulation flow behind it, as well as its effect in decreasing
wave action.
The effect in decreasing sediment transport rate by decreasing wave effects is the same in the case of a detachedbreakwater. The wave decreasing effect can be specified by the width and depth of the dam crest. To make water
circulation flow behind an artificial reef, it is necessary to pay attention to the balance in the proportions of the
opening width, the dam length, and the distance form the shore. In order to widen the opening, it is necessary toensure the powerful effect of extinguishing waves (making the dam crest depth shallow). For the resulting
sediment transport control effect to be evaluated, a 3-D model for beach profile change, by which the changes inflow conditions can be considered, or a model for shoreline profile change, by which wave changes can beproperly considered, can be used. However, it is desirable to regulate and properly arrange the opening or levee
width in accordance with the shoreline changes at the site.
In the actual examination, refer to the Design Manual for Artificial Reefs.

2.3.4 Headlands
The plane configuration and dam crest height of a headland should be designed so that the parameters of thedesign beach profile can be maintained, on the basis of beach nourishment.
In the actual design, the coastal environment and its utilization should be considered.

210

Explanation
Figure 4–2 shows the sediment transport control mechanism of a headland. The longshore direction component
among wave energy fluxes contributes to the longshore sediment transport rate. Under natural conditions, theshoreline is stabilized because the inflowing sediment transport rate Q0 is balanced with the transport sediment
rate Q, which is determined at the relevant site (refer to Fig. 4–2 (a)). On such a beach, when the inflowing
sediment transport rate decreases from Q0 to Q, headland works are introduced as a countermeasure. In otherwords, the headland works incline the shoreline by α to decrease the sediment transport rate Q that occurs at thesite from Q0 to Q1 and balance it with the inflowing sediment transport rate Q1 (Fig. 4–2(b)). Since a long beach,divided by headlands, has a stable profile in each section, the shoreline regression rate can be dispersed and its

Q

Shore direction
componentLong shore drift(Q = Q0 )θDw irecav ti
e i on
nc of
id
en
t

Long shore
componentD
w irecav ti
e i on
nc of
id
en
t

maximum value made smaller.

Q0
Shoreline
Headland(a) Wave direction and long shore drift rate

Q = Q1 ←Q = Q0
Long
Q ← Q0 d shor
rift e
Sh
o re
lin
ef
θ
o rm
ed
by α
hea
dla
nds

Q < Q0

Headland(b) Erosion control by headlands

Fig.4-2 Mechanism of long shore drift control by headlands
The sediment transport control effect of a headland is determined by the dam length and dam crest length of theheadland. Figure 4–3 shows the relationships among the dam lengths of the headland, its execution interval, theshoreline position at the upper side of sediment transport between the headlands, and the shoreline profile
formed between the headlands, when no (static) beach nourishment is introduced. When no (static) beachnourishment is introduced, since sediment movement occurs between headlands, the shoreline is retrograded at
the upper side of sediment transport. The larger the permissible regression width is (i.e. the wider the space ofthe design beach profile compared with the present one), the wider the interval between the headlands and thelonger the dam length. On the other hand, when the shoreline cannot meet the parameters of the design beachprofile with a certain interval between the headlands, the necessary dam length of a headland and the parametersof the design beach profile should be ensured by beach nourishment.
In many cases, the height of the dam crest of a headland commonly serves as the backshore height. The side-damsite of a headland, which prevents the longshore current that occurs between headlands from changing into an
off-shore current and consequently prevents sediment from running off, forms an interception to mitigate
fluctuation of the shoreline. A small jetty is sometimes adopted as an auxiliary work to control a local fluctuation211

of shoreline.

Direction of wave incident
Shoreline formedby headlands
installed longinterval

Shoreline formedby headlands
installed shortinterval
Initial shoreline

When the interval of headlands is short
When the interval of headlands is long

Fig.4-3 Headland interval and shoreline regression rate

2.4 Beach nourishment
Beach nourishment is classified into the (static) beach nourishment that is necessary to ensure the parameters ofthe design beach profile and the (dynamic) beach nourishment that is necessary to maintain the parameters of thedesign beach profile in consideration of the effects of the sediment transport facilities.
In the actual planning for beach nourishment, its effects on coastal protection, environmental conservation, andutilization should be considered.

Explanation
Beach nourishment is classified into (static) beach nourishment to ensure the parameters of the design beachprofile and (dynamic) beach nourishment to increase the inflow sediment rate and maintain the design beachprofile. For example, in terms of headlands, (static) beach nourishment is the one that is needed initially toensure the shoreline profile of the design beach profile at the upper side of a headland (Fig. 4–4 (a)). The inflow
sediment transport rate that is needed to maintain this shoreline profile is decreased from the sediment rate Q0,
necessary for the initial shoreline, to Q1. On the other hand, (dynamic) beach nourishment should continuously
make up the difference, Δ Q = Q1 – Q, between the inflow sediment transport rate Q and the longshore sedimenttransport Q1 that is required to maintain the shoreline profile on the upper side of a headland. Moreover, if theshoreline profile of the design beach profile is formed by headlands and by (statistic) beach nourishment in sucha way that the sediment transport rate is Q2 = Q (Fig. 4–4(b)), then (dynamic) beach nourishment will become
unnecessary.

212

Direction of wave incident

Q1 ←Q0Static nourishment

Q < Q0Initial shoreline

ΔQ = Q1 − Q
Rate of dynamic nourishment(a) Dynamic and static nourishment
Extension of headland when
(dynamic) nourish mend can
not be expected

Q2 = Q

Direction of wave incident

Static nourishment

Q < Q0Initial shoreline(b) Sand beach nourishment by static measure only

Fig.4-4 (Static) and (Dynamic) sand beach nourishment
The sand-grain diameter of the materials for beach nourishment should be as large as, or a little larger than, thatof the sand that is native to the site. After beach nourishment, fines run off into the sea and make it muddy, somaterials with the smallest amount of fines possible should be selected.
(Dynamic) beach nourishment should be set up at the upper side of the sediment transport if possible. Just after
beach nourishment the sediment is discharged offshore until stability is obtained (Fig. 4–5). When beachnourishment is executed too far offshore, the amount of sediment that is discharged offshore is increased. To
avoid this, nourishment should be executed in the longshore direction. Since the amount of sediment that isdischarged in the longshore direction depends on sea phenomena, it is desirable to ensure sufficient beachnourishment at the early stage. The time and amount of the materials that are supplied later should be decided
according to cost-effectiveness and the available amount of materials, as well as the state of the coastline.
Furthermore, the dam crest height of the (dynamically) nourished beach should be lower than the backshoreheight, assuming that the materials for beach nourishment should be discharged when the waves are high.

213

Cross-section of
nourished beachCritical water depth for
Stable cross-section
sediment movement byafter beach nourishment wave

Cross-section before
beach nourishment

Fig.4-5 Early discharge of beach nourishment materials

In examining a nourished beach as a countermeasure against erosion, it is necessary to consider the amount andmethod of transport of materials for stable supply. Sand bypasses (Fig. 4–6 (a)) and sand recycling (Fig. 4–6 (b))
are mentioned as effective methods of supplying the materials for beach nourishment.

214

Incident wave

Sand bypass (Sand replacement)

Long shore drift

Long shore drift(a) Sand bypass

Prevailing wave
Deposition

Sand extraction
Sediment flow
Decreasing of
sediment discharge

Sand replacement
Beach erosion

Sand feeding

Sand feeding(b) Sand recycling

Fig.4-6 Variation of (dynamic) beach nourishment

Section 3 Storm Surge Protection Facilities
3.1 Overview
Planning for the installation of storm surge protection facilities should specify not only the allocation between
dikes or revetments and wave-dissipation facilities but also the basic conditions for getting the best
disaster-prevention performance out of these structures.

Explanation

In storm surge protection projects, the use of distributed protection methods that combine a number of facilities
is becoming the norm.
This chapter will select the most suitable combination from the existing ideas for multiple allocations among215

dikes or revetments and wave-dissipation facilities and will then specify the conditions for optimizing thedisaster-prevention performance of these combinations.
The conditions for the stability of a facility and others will be specified according to the design edition of theManual for River Works (Draft).

3.2 Allocation of dikes or revetments and wave-dissipation facilities
The allocation of dikes or revetments and wave-dissipation facilities should be established based on their safety;
economic efficiency; constraints of construction, effects on hinterlands, waters, and beaches; and the status of
the neighboring coast.

Explanation

In planning a storm surge protection facility, it is necessary to comprehensively consider the attenuation of awave that occurs when it goes up the slope of a dike or revetment, as well as the attenuation caused by awave-dissipation facility.
Generally, the higher the dike, the greater its disaster-prevention effects. In some cases, however, it is desirable
to make the dike as low as possible from the viewpoints of coastal utilization, land use of hinterlands, landscapes,
etc. Additionally, the fishery can be lost and the beach width decreased by increasing the allocation of awave-dissipation facility. Thus, the combination of dikes or revetments and wave dissipating facilities should be
decided on after these options have been examined sufficiently in terms of their related coastal environment andutilization.
Although the liner protection method by a combination of dikes or revetments and wave dissipating facilities
was conventionally used in many cases, it spoiled the landscape and the beach access, and the non-point
protection method that combines dikes or revetments, offshore wave-dissipation facilities, and beachnourishment is often used now. The use of a combination of multiple facilities is tough and profitable from theviewpoint of disaster prevention.
The combination of dikes or revetments and wave dissipating facilities offers a variety of measures for disasterprevention. These disaster-prevention measures are compared from the viewpoints of economic efficiency,
coastal environment and utilization, and effects on society to select the most suitable project.

3.3 Dikes or revetments
3.3.1 Types of dikes or revetments

In choosing the types of dikes or revetments, it is necessary to consider the natural conditions, the level ofimportance of a hinterland, the neighboring coastal prevention facilities, the status of the land or water surface
use, etc.

Explanation

The types of dikes or revetments can be classified into upright-type breakwaters, mound-type breakwaters, andcomposite-type breakwaters, according to the gradient of their front slopes. The upright-type breakwaters should216have a gradient of under 10%, and mound-type breakwaters a gradient of over 10%. A breakwater with agradient of over 30% is called a gentle slope-type coastal dike. In composite-type breakwaters upright-type
construction such as a caisson or block is placed on the top of a mound-type structure such as a rockfill mound.
Types of dikes or revetments are shown in Figure 4–7.
The conditions generally appropriate to each type are given below.
1.

Upright-type breakwatersa.

Used when the foundation ground is relatively solidb.

Used when the site for a breakwater cannot be easily prepared

Since the land side of an upright-type breakwater can sometimes be separated from the seashore, stepladders or
ramps have to be set up for access if necessary.
On the seashore, where the foot of the dike is usually exposed to the waves, because the foot of the dike is likely
to be scoured by reflected waves, it needs to be protected.
2.

Mounted-type breakwatersa.

Used when the foundation ground is relatively weakb.

Used when the site for the breakwater can be easily preparedc.

Used when there is a strong need for utilization of the beachMounted-type

Gentle-slope type
(Step revetment)

Upright type

Upright type

Composite typeFig.4-7 Types of dikes

217Since the site dike is generally at the beach, the dam base usually covers part of the beach, thus threatening thecoastal vegetation.
In terms of the structure of the dike, mound-type breakwaters have an advantage over upright-type breakwaters
against waves. If they are constructed on sandy ground near the shoreline, however, erosion and waves are likely
to change the foundations of the dike, or reflected waves are likely to scour its foot, so the foot of the dike must
be protected. On the other hand, since the foot of the dike is covered by weeds and is likely to be slippery,
threatening to make access to the beach difficult, the dike must be well maintained.
When utilization of the beach is the aim, some of the sections of mound-type breakwaters should be set up toallow lines of access by beachgoers and also to suit the slope situation.
3.

Composite-type breakwatersa.

Used when the foundation is built in the sea and construction is difficultb.

Used when the site cannot be easily preparedc.

Used when the foundation ground is not very solid

3.3.2 Normal line of dikes or revetments

The normal line should be drawn so gently that it can run parallel to the coastal landform.

Explanation

The coastal landform is commonly harmonized with natural conditions such as the waves or the sediment supply.
Thus, drawing the normal line parallel to the coastal landform will lessen its effects on the seashore and thus
disperse the waves so that they do not concentrate on a particular point. In contrast, the normal line drawn
non-parallel to the coastal landform will transform the beach along the dike, lose part of the sandy beach, andcause wave concentration, so it is necessary to take note of the way to set up a normal line.
When the foot of the dike is under the water, it is likely to be affected by the waves and scoured by reflected
waves. Thus, it is necessary to examine a normal line inside the landside area and to consider the need to protect
the foot of the dike.

3.3.3 Gradient of Foreshore

The gradient of the front slope should be established by taking into account the stability of the dike and its
effects on the coastal environment and utilization.

Explanation

By making the gradient of the foreshore gentler, access to the beach is improved and the runup height reduced,
so that even if the height of the dam crest is lowered, the dam base can be widened, sometimes leading todisappearance of the beach. On the other hand, when the toe of the slope is underwater, reflection of the waves

218

occurs constantly, not only increasing the possibility of scouring but also causing weed overgrowth, whichmakes the toe slippery and obstructs access to the seashore.
Therefore, in the actual planning, it is necessary to consider the position of the toe of the slope.

3.3.4 Design runup height and design wave-overtopping rate

The design runup height and the design wave-overtopping rate should be properly determined in considerationof the effect of a wave-dissipation facility when the design tide level and the design wave work on a beach thathas the parameters of the design beach profile.

Explanation

The wave runup height varies with the wave height, wave period, the sea-bottom form, the form and water depthof the dike, and the presence of wave-dissipation facilities. Additionally, evaluation of the wave-overtopping rateis also conducted if necessary.

3.3.5 Freeboard

Freeboard should be allowed in dikes or revetments.

Explanation

In the specification of the height of the dam crest of a dike or revetment, freeboard should be allowed on thebasis of the design tide level, the design wave, and the uncertainty of settlement. It should be also specified
appropriately in consideration of the social and economical importance of its hinterland, sea phenomena, andtopographical conditions.

3.3.6 Height of the design dike or revetment

The height of the design dyke or revetment should be made up of the design runup height and the freeboard. In
other words, it is made up of the height whose wave-overtopping rate is less than the allowable overtopping rate,
and the freeboard.
The design dike or revetment height should be estimated by considering the economic efficiency, the land use
situation of the hinterland, the coastal use situation, the height of the neighboring dike or revetment, etc.

Explanation

The dam crest height of a dike or revetment varies depending on the parameters of the design coastal profile, thescales of the wave-dissipation facilities, and the gradient of the slope of a dike or revetment, which are combined
to offer many options. Generally, as the scale of a wave-dissipation facility is made larger, the height of the dam
crest of a dike or revetment is made lower. The low dam crest is favorable to the landscape or coastal utilization
but costs more. On the other hand, if the height of the dam crest of a dike or revetment is made larger, the coastal
use or environment in the landside area may deteriorate. Additionally, changing the height of a series of dikes orrevetments because of the construction of wave-dissipation facilities etc. is not psychologically desirable for the219

community. Thus, a certain dam crest height of a series of dikes along the same beach should be determined after
a comprehensive consideration of the regional situation.
When the dam crest height needed for the dike or revetment to protect the beach is not socially and economicallyfavorable, the parameters of the design coastal profile and the presence of wave-dissipation facilities or works
must be reexamined.

3.4 Wave-dissipation facilities
3.4.1 Types of wave-dissipation facilities

The type of wave-dissipation facilities should be selected based on the necessary wave-dissipation effects, theireffects on the coastal environment (including the landscape) and its utilization, economic efficiency, constraintsof construction, etc.

Explanation

The wave-dissipation facilities include wave-dissipation works, detached breakwaters, and artificial reefs.
The wave-dissipation works exert their wave-dissipation effects by wave reflection at the front of the slope, theattenuation of wave energy due to turbulence that occurs as the waves pass through the inside of the dike, or thedecrease in wave runup height due to the roughness of the slope.
Since part of the incident waves are reflected or the wave energy is decreased as the waves are passing throughthe dam site, detached breakwaters can exert wave-dissipation effects. A wave that flows in from the opening isdispersed by diffraction, and thus the wave height is lowered.
On artificial reefs, the wave height is lowered because the waves are breaking on a slope at the offshore side oron the crest; it is also lowered by the dispersion of wave energy that occurs as the broken waves pass over theshallow-water dam crest.
The wave-dissipation effects of the wave-dissipation facilities are represented by the permeability rate—that is,
the ratio of the height of the wave that has passed over the crest to the height of the wave that was in front of it.
The wave runup height can be calculated by converting the height of the wave after it has passed, back into the
deepwater wave height.
When the scale of a facility is made larger, the load on a dike or revetment is relatively reduced and its effect on
the inland area or access to it is also decreased. At the same time, however, the load on the coastal environment
and utility can be also increased, which is considered unprofitable from a cost standpoint in many cases. Thus,
the question of whether the scale of a facility should be enlarged must be comprehensively considered.
On the other hand, when the sea floor on which the foundation of a wave-dissipation facility is established islowered by erosion, the wave-dissipation effect is also lowered and the purpose of disaster prevention cannot be
accomplished. Thus, the presence or absence of erosion must be confirmed as well.
Meanwhile, artificial reefs do not spoil the landscape, because they are submerged facilities. However, withwave-dissipation works and detached breakwaters, especially of the block type, in which the dam crest is
constantly above the water, the deterioration of landscape is more likely to come into question. Thus, a new typeof detached breakwater that has a foot and performs better in the landscape than the conventional types has beendeveloped.
220

In terms of coastal environments, artificial reefs create shallow areas in the sea, which, in turn, produce undersea
forests or habitats for natural life forms. Additionally, detached breakwaters sometimes offer habitats for living
things that dwell around rocks. Because most of the effects of seacoast preservation facilities on ecosystems
remain unknown, it is necessary to properly monitor and understand the changes that occur in the coastal
environment after their construction.
In terms of coastal utilization, since an artificial reef with a wide and shallow dam site is likely to obstruct trafficand have detrimental effects on the fishery, careful monitoring is required. Detached breakwaters restrain theutilization of the sea surface. Wave-dissipation works obstruct access to beaches.
In terms of cost-effectiveness, evaluated from only the initial construction cost, detached breakwaters are
generally the most profitable. The cost-effectiveness of each facility should be examined not only in terms of thestructure, each of which should have the same functionality in terms of coastal protection, environmental
conservation, or utilization, but also the lifecycle cost. Thus, it is not appropriate to evaluate facilities only by the
initial construction cost.
The constraints of construction depend on the conditions of the ground or waves. For example, on poor groundor homogeneous fine sandy ground, it is necessary to build foundations when a detached breakwater, whose damsite weighs a greater amount, is selected.

3.4.2 Detached breakwaters

The plane configuration, dam crest height, and structure of a detached breakwater should be specified such thatthe necessary wave-dissipation effect can be ensured when the design tidal level and the design wave work on abeach that has the parameters of the design beach profile.
The detached breakwater planning should consider the effects on the shoreline, flow conditions, and coastal
environment and utilization.

Explanation

Detached breakwaters are divided into continuous and discontinuous levees by landform, and into permeable andimpermeable levees by wave permeability. In many cases, permeable and discontinuous levees, whose
constraints of construction are commonly less, are used in anticipation that a tombolo can be formed behindthem. With continuous levees, it is necessary to take care of water pollution caused by water level rises orseawater retention at the shore side. Impermeable levees are used when a wave-dissipation effect higher than thatprovided by permeable levees is required.
The sediment transport control effect of a detached breakwater depends on parameters such as the height andwidth of the dam crest, the depth at which the breakwater is established, the dam length, opening width, distancefrom the shore, etc.
For the actual examination, see the Design Manual for Seacoast Preservation Facilities.

3.4.3 Artificial reefs

The plane configuration and the dam crest height of an artificial reef should be specified such that the necessary221wave-dissipation effect can be ensured when the design tidal level and the design wave work on a beach that has
the parameters of the design beach profile.
The artificial reef planning process should consider the effect of the reef on the shoreline, the flow conditions,and the coastal environment and utilization.

Explanation

The wave-dissipation effect of an artificial reef is controlled by parameters such as the depth and width of thedam crest, the depth of the dam foot, the width of the opening, etc.
The mean water level rises on the artificial reef, where incident waves are forcibly broken. As the depth of thedam crest becomes shallower compared with the height of the incident wave, the percentage rise also becomes
higher, with the mean water level rising by 10% to 20% of the height of the incident wave on the shore side of
the artificial reef. Thus, in the actual planning of wave runup height, it is necessary to consider the percentage
rise in water level on the shore side.
In the process of configuration of the plane of an artificial reef, although it is desirable to make the dam site
continuous so that wave height distribution cannot occur in a coastal direction, the percentage rise in mean waterlevel can be more efficiently controlled by establishing an opening. The width of the opening needs to be
specified so that the wave height behind the dam site cannot exceed that of the dam site.
When the basic type of artificial reef is selected, the interaction among those parameters should be considered
carefully so that the reef’s required function of wave dissipation can be fulfilled most efficiently as a whole. For
the actual examination, see the Design Manual for Artificial Reefs.

3.4.4 Wave-dissipation works

The cross-section of wave-dissipation works should be specified in such a way that the required
wave-dissipation effect can be ensured when the design tidal level and the design wave work on a beach that has
the parameters of the design beach profile.
Planning for the wave-dissipation works should consider their effects on the coastal environment and its
utilization.

Explanation

The wave-dissipation effects of wave-dissipation works are determined by parameters such as the width of thedam crest, the height above the water surface, the depth of establishment, etc.
However, wave-dissipation works should not be applied, if possible, since they not only obstruct access to thebeach but also spoil the landscape and environment. When it is economically or socially difficult to have anyother option, however, there is no choice but to apply these works for the purpose of lowering the height of thedam site.
To specify the parameters of the wave-dissipation works, see the Design Manual for Seacoast Preservation
Facilities.

222

Section 4 Tsunami Control Facilities
4.1 Overview
The plan for the installation of tsunami control facilities should not only decide the allocation among dikes orrevetments and tsunami breakwaters but also develop the basic conditions for getting the required protection
performance out of dikes or revetments or tsunami breakwaters while considering the coastal environment andits utilization.

Explanation

This section aims to select the most suitable combination from the multiple allocation options of dikes orrevetments and wave-dissipation facilities.
For these structures to perform their required functions, the structural conditions must be properly developed.
The conditions related to the stability of the facilities etc. are established according to the Manual for River
Works in Japan (Draft).

4.2 Allocation among dikes or revetments and wave-dissipation works
The allocation among dikes or revetments and tsunami breakwaters should be decided in consideration ofparameters such as their stability, cost-effectiveness, constraints on construction, effects on hinterlands, sea
areas, and beaches, and the status of their neighboring beaches.

Explanation

It is necessary to determine the tsunami runup height, while at the same time comprehensively considering theattenuation of the tsunami that occurs when it runs up on the slope of a dike or revetment, as well as theattenuation by a tsunami breakwater.
Since the tsunami that has reached the highest-ever water level is set as the design tsunami in many cases, onethat has a very high water level can be also targeted, depending on the landform. Therefore, to protect this by adike alone, the dam crest inevitably becomes very high, possibly obstructing access to the beach or spoiling theenvironment. Thus, the height of the dam crest can be lowered by using a combination of a dike with awave-dissipation facility such as a tsunami breakwater, etc.
When revetment improvement is expensive because the bay mouth is small or the coastline long, the introductionof a tsunami breakwater is worth considering. Since this breakwater would prevent the seawater fromexchanging within the bay, however, it is necessary to address water quality deterioration.
The combination of dikes or revetments and wave-dissipation facilities can provide a number of ideas fordisaster prevention. It is necessary to choose the most suitable prevention idea from among these, while
comparing them from the perspectives of cost-effectiveness, coastal environment and utilization, and social
effects.

223

4.3 Design tsunami runup heightThe design tsunami runup height should be properly determined by considering the effect of a tsunami
breakwater when the design tidal level and the design wave work on a beach that has the parameters of thedesign beach profile.

Explanation

The types of dikes or revetments, alignment, and freeboard; the heights of dikes or revetments; and the gradientof the foreshore should follow the description in 3.3.
If the maximum water level at the position of a revetment can be estimated on the basis of the height of a pasttsunami mark, which would have been affected by complex ground landforms and constructions such as houses,
their hydraulic effects should be sufficiently considered. In conducting numerical simulations of tsunamis, thevalidity of the calculations must not be confirmed without comparing the results with the heights of the pasttsunami marks.

4.4 Tsunami breakwaters
Tsunami breakwaters should be planned properly and individually.

Explanation

Tsunami breakwaters cannot be generalized, since they are large-scale facilities. If anything, they should beplanned separately by using the latest technology.
References
1) National Association of Sea Coast. Design Manual for Artificial Reefs. Supervised by Seacoast Division,
River Bureau, Ministry of Construction, 1992:92.
2) Japan Society of Civil Engineers. Design Manual for Seacoast Preservation Facilities. Editorial
Subcommittee of Coastal Engineering Committee, 2000:582
3) Seacoast Division, River Bureau, Ministry of Construction. Design Manual for Detached Breakwaters, 1986

224

Chapter 5 Planning for the Installation of Information Facilities
Section 1 General Information
Planning for the installation of information facilities should be conducive to the implementation of river,sediment and erosion, and seacoast projects and the sophistication of river administration systems etc. It should
include the development of a system for efficiently gathering, managing, and commoditizing hydrologicalinformation such as present and past rainfall, water level, and water quality; disaster prevention information in
the form of images etc. and other disaster information; basin information related to river utilization etc.; andgeneral public opinions.
Planning should ensure the certainty, reliability, rapidity, and interactivity of information facilities used to issue
evacuation warnings so that these warning facilities can be properly arranged by proper methods. Notably, for
information related to landslide disasters, a system that allows interactive messages between community
members and administrative organizations should be developed.

Explanation

In the preparation of projects for rivers, sediment and erosion control, and seacoasts, hydrological observation
data of past floods and droughts and environment data are necessary as basic sources. It is necessary toaccumulate time data for each flood, annual day-unit data, short-time rainfall data every 10 minutes, etc. ashydrological observation data, and to maintain their quality. Additionally, the data should be basically developed
as GIS systems that allow multiple indications to be connected by maps, in order to conduct runoff analyses orflood simulations with typographical or construction data or to facilitate indication-processing descriptions. The
data management method must then be specially determined and the performance of information-gathering
instruments improved, for example, by introducing an automatic updating system.
On the other hand, from the viewpoint of management of rivers etc., it is especially important to gather
information on both the present situation and future predictions to determine what countermeasures should betaken in a time of disaster. Thus, to gather, develop and provide river information, the system needs to be formed
to meet a need from the viewpoints of the relevant organizations and bodies and to share necessary information
provided by them.
Above all, in the case of landslide disasters, which are likely to involve human lives, because the discovery of
premonitory signs and communication are the keys to reducing damage, it is necessary to construct a system thatallows community members and administrative organs to interactively communicate.

Section 2 Collecting Information and Ordering, Provisioning, and Commoditizing the
System
In the preparation of systems for collection and ordering by the offering and sharing of information about rivers,
sediments and erosion, seacoasts, etc., the following items should be examined:
・ Development of a system for gathering information, data, etc.
・ Development of a database whose quality is ensured
・ Construction of a network for sharing information and data.

225

Explanation

Information- or data-gathering instruments, such as CCTV, have been introduced in recent years to gather image
information. Information outlets and backslope sliding and debris detectors that work with flood protection andevacuation on warning, as well as hydrological information equipment (water gauges, rainfall recorders, radarrain gauges), have conventionally been used for river management. It is necessary to effectively arrange these
items depending on the purpose and to then gather information.
For the information to be offered to and shared with the general public, it is important that the quality of the data
be maintained. Thus, the information acquired must be examined on the assumption that it will be available to
the general public and its quality must be maintained. In order to share information with the general public, it isnecessary to develop the system into something more available to the public by computerizing the collected
information or data. Furthermore, it is also important to foster an environment in which opinions are exchanged
more actively, by sharing information with the general public.
In addition to conventional general public lines and dedicated wireless services, dedicated fiber-optic cables andsatellite connections are useful as methods of communicating and sharing information and data. It is important toconstruct an effective information-sharing system by forming a network for any purpose between the relevantorganizations and bodies.

Section 3 System for Gathering Information, Data, etc.
In addition to the system for gathering hydrological information such as rainfall and water levels, image
information and information on construction-operating conditions etc. are useful as countermeasures in times ofdisaster. Thus, it is necessary to examine the development of the disaster-information gathering system,
including CCTV, sensors, information outlets, etc. Moreover, in terms of landslide disasters, it is also necessary
to examine the development of mudflow detecting sensors and landslide monitoring instruments, which arerequired to construct a system for prompt evacuation on warning.

Explanation

In areas that have been frequently hit by flood damages in recent years or to which flood protection will have to
be selectively applied, the establishment of CCTV, water level sensors, etc., needs to be examined.
To confirm the safety within a river course at the time of dam discharging, the establishment of CCTV should beconsidered in places where there may be considerable delay between the warning point and the hazard location,such as in the basins of mountain streams or in places that are likely to have blind spots.
Meanwhile, it is important to develop slope-sliding detection sensors that can be interlocked with flood-fighting
works, information outlets by which information can be sent from disaster sites, and optic-fiber networks.
In particular, in cases where a secondary disaster may be caused by slope collapse etc., it is necessary not only toconstruct a warning and evacuation system but also to establish debris flow sensors or landslide monitors that
can assess sediment movement.

226Section 4 Development of a Quality-assured Database
It is necessary to formulate a system for dividing the basic data required by the general public into real time data
(quick estimation) and accumulated data (established estimation) and to then develop the data and offer them inthe form of a database. Since a system for gathering and providing real-time data is important for mitigating
disasters in time of flood, it is necessary to take the speed and certainty of the system into consideration. In
terms of accumulated data, quality maintenance and availability to the general public should be considered.

Explanation

In setting up river, sediment and erosion, and seacoast projects, data on the hydrological observation of previous
floods, droughts, landslide damage, environments, etc. are basic necessities. Also, the planning should provide
for accumulation of time-stamped data for each flood and annual day-unit data, maintaining their high quality. To
reduce damage in times of flood, it is important to promptly provide the general public with both situation
reports of hydrological observation data (such as rainfall rate, water level, etc.) and information on forecasts. In
this way, because some items of hydrological information that are offered take priority over others, depending on
their purpose, it is necessary to construct a database from the different viewpoints of real-time data (quick
estimation) and accumulated data (established estimation).
In gathering and developing the real-time data, it is especially important to gain data on the present situation andfor the future, in order to determine which countermeasures should be taken in times of disaster. In gathering andproviding the real-time data, it is necessary not only to satisfy needs from this viewpoint but also to have asystem by which information can be shared with medical agencies.
In terms of accumulated data (e.g. hydrological observation data), it is necessary to accumulate time data on eachflood and annual day-unit data, while ensuring their quality. In terms of availability to the general public, thedata must be basically developed as a GIS system. It should allow runoff analysis or flood simulation interlockedwith typographic or construction data, or multiple indications interlocked with maps to makeindication-processing easy to explain. Then, attempts should be made to improve the performance of
information-gathering instruments; for example, the introduction of an automatic updating system should beattempted and a specialized data management method should be devised.
The basic approaches to the development of a river GIS are given below.
a.

Basin and river base maps
The development of basin and river basement maps of 1/25,000 and 1/2,500 is attempted so as to gather on
GIS the data needed for flood simulation, runoff calculation, and analysis of hydrological observations orenvironmental information. To transform the basement maps into 3D imaging, the following procedures arefollowed: 1) feed in the 50-m mesh elevation data that have been developed to make hazard maps; 2) inmapping inundation risk area of different return periods by using a laser profiler, feed in the finer meshelevation data; and 3) in an area where micro-landform elevation data are to be gathered by laser profiler(e.g. a low-lying area within a city that requires coercive drainage by pumps and extremely precise
simulation of flooding with or without a dike), feed in the micro-landform elevation data gathered by laser
profiler.
227

b.

Data on the present status of the river
In terms of river longitudinal/cross section, construction parameters and drawings, disaster situation, etc.,
construction diagrams updated during the building, maintenance or repair of a structure, and diagrams of
permitted structures, should be developed in such a way that they can be computerized as CAD data andmanaged on GIS connected to CALS/EC.

c.

Hydrological and water quality data
The real-time data (quick estimation) or accumulated data (established estimation) related to hydrologicaland water quality data should be developed in such a way that they can be shown in comparison with thepast data on GIS.
On the other hand, since the use of weather radar data plays an important role in real-time emergency
management (for example, in flood forecasting), the quality of the data should be unified nationwide. A
system should be developed so that weather data that is revised by the analysis of ground rainfall gauge data
can be offered in real time. The accumulation and development of past weather radar data are conductedaccording to the plan.

d.

Data related to river environment information
Data should be developed in such a way that the variation with time of information related to important
habitats and growing environments of plants and animals can be described on GIS. This makes it possible tograsp the overall environmental characteristics of the target river, its particular attributes, and its important
habitats, all of which are conducive to river improvement and environmental management. (Suchmanagement includes the preparation of plans, the construction process, maintenance and management,
conservation of habitat, permission for occupancy, etc.) It is necessary to support the activities of community
members in river environmental conservation.

e.

Dam data, hydrological observation data related to dams, environmental information related to dams, etc.
Dam data such as storage capacity and outflow discharge, hydrological data related to dams, andenvironmental information are developed in such a way that they can be indicated in comparison with pastdata on GIS.

f.

Data related to water cycle information
To create a register of water utilization, the data should be developed in such a way that they can be
represented on GIS.
In basins where, despite particularly high levels of water utilization, some problems with the construction ofa sound water cycle remain unsolved, information related to the water cycle (such as the type of system andthe amount of intake and drainage), as well as the data mentioned above, should be developed. For that228

purpose, it is necessary to standardize the data structure and to construct a framework for sharing theinformation among relevant organizations and bodies.
g.

Information related to areas prone to landslide disaster
Information about debris-flow-prone torrents, landslide-prone places, and places prone to steep slope failure
should be developed.
At the same time, it is desirable to develop data on the peripheral topography, geology, and natural
conditions such as vegetation; social conditions such as housing and public facilities; past disasteroccurrence situations; erosion control design areas; erosion control facilities, etc.

Section 5 Construction of a Network for Sharing Information and Data
The construction of high-speed networks such as ultrafast large-capacity fiber-optic networks should be
examined among the relevant organizations and bodies for disaster prevention. Commoditization ofdisaster-prevention information and the mutual exploitation of the latest data need to be attempted using
high-speed networks. It is also necessary to construct a system by which the relevant organizations and bodies
can promptly and effectively deal with disasters in a time of disaster. Additionally, the information should be
widely provided to the general public over the Internet etc.
The provision of real-time data is important for the purpose of reducing damage in times of emergency. The
system should be such that real-time data can be promptly, precisely, and clearly provided to the general public
and the relevant organizations and bodies. Moreover, it is necessary to construct a system that can provide
information from multiple media while working with the relevant organizations and bodies, on the assumptionthat communication may be blocked in time of disaster.
Accumulated data should be conveyed to members of the general public by providing them with maps ofprobable flood-prone areas or hazard maps so that they can prepare themselves for an emergency on a daily
basis. At the same time, it is also important to provide both daily data on water levels and water quality, and on
the status of the river environment and its utilization, and past data, so that the general public can take advantageof this when carrying out various activities. Thus, the information service needs to be conducted by a system thatis available to them.

Explanation(Emergency)
From the viewpoint of reducing damage in a time of disaster, it is particularly important to promptly andprecisely determine the rainfall, water level, and other river information, such as the river course status, and toshare future prediction information among the relevant organizations and bodies. In gathering, developing, andproviding river information, it is necessary to satisfy needs from the viewpoints of the relevant organizations andbodies and to construct a system by which the information can be shared among these bodies.
Therefore, it is effective to construct a high-speed network such as an ultra large capacity optic-fiber network229

that can transmit image information and the results of flood simulations to the relevant disaster-prevention
agencies.
Additionally, the high-speed network should be constructed in consideration of security against virus entry,
backup, the effective arrangement of servers, etc.
Since a fast information service is important to prevent disasters and reduce the damage they cause, riveradministrators, erosion control implementation bodies, and seacoast administrators have to develop a real-timedata information service system by which they can grasp the information and provide it to the general public.
For the information service, it is important to indicate the present situation and predicted values by processing
them into what is easy for the general public to understand. Additionally, since this information needs to beprovided by a variety of media, such as those in related municipalities, mass media, the Internet, information
bulletin boards, recorded telephone messages, etc., the system must be prepared from various perspectives on aregular basis.
In terms of network construction, backup for emergencies is also considered.
(Ordinary times)
For smooth evacuation and flood protection in times of disaster, information on inundation-hazard areas, thebehavior of flood flows, volcano hazard maps, etc. must be conveyed to the general public through the Internet.
On the other hand, there is a common view that rivers are habitats, places for walking and sport, and importantfactors in forming the cultural climates of communities, as beautiful natural environments that change fromseason to season. This common view is very important for conducting proper river administration. Additionally,
to support these various public activities, it is necessary to develop a system to provide people with related
information, considering the roles of rivers as areas for civil activities or events related to rivers andenvironmental education. Moreover, it is also necessary to supply the public with daily observation data onhydrology, water quality, and the environment; this will help the general public to monitor or participate in riveradministration and will encourage them to embrace research activities.

230

Afterword by Translators
This English version of the technical note is the translation of "The Japanese Ministry of Land, Infrastructure,
Transport and Tourism’s Technical Criteria for River Works: Practical Guide for Planning" published on July 15,
2008. The translation was jointly done and published by the River Division of the River Department of the
National Institute of Land and Infrastructure Management (NILIM) and the Disaster Prevention Team of theInternational Centre for Water Hazard and Risk Management (ICHARM), Public Works Research Institute(PWRI) with permission of the Japan River Association. Through the process of editing the initial draft preparedby a translation agency, we made considerable efforts to find and use internationally understandable
terminologies and expressions instead of literally translating those only used in Japan. In general, however, we
tried to put the original Japanese sentences into English as faithfully as possible with few additional technicalexplanations. In this sense, manual users should know that to gain full understanding of the contents, they may
need other background knowledge which is not described in this version.

We hope that this English version of the technical note will be used internationally in a variety of situations, forexample, as a textbook for trainees and a reference to improve river management practices.

River Division, NILIM, MLIT
Disaster Prevention Team, ICHARM, PWRI

翻訳者あとがき
本資料は、平成 20 年 7 月 15 日発行「国土交通省 河川砂防技術基準 同解説 計画編」を社団法人日本河川協会の許諾を得て、国土技術政策総合研究所河川部河川研究室と土木研究所水災害・リスクマ
ネジメントセンター防災チームが共同で翻訳したものである。翻訳にあたっては、翻訳会社による一次翻訳を元に必要な修正を加える形で進めた。その際、日本の河川行政を知らなければ理解できない
独自の単語やフレーズは日本語表記に囚われず、適切な英語表記に置き換えることとした。ただし、原文の追加説明のための新規文の挿入などは行わず、できるだけ原文に忠実に翻訳した。

この翻訳が、研修テキスト等として活用されることを通じて、読者の国の河川行政の参考になるところがあれば幸いである。

国土交通省国土技術政策総合研究所河川部河川研究室
土木研究所水災害・リスクマネジメントセンター防災チーム

国土技術政策総合研究所資料
土

木

研

究

所

資

料

TECHNICAL NOTE of NILIM,No.519
and PWRI, No.4132
February 2009

編集・発行

©国土技術政策総合研究所©土木研究所

本資料は、社団法人日本河川協会より英語訳版の発行に関す許可を得て、出版されたものである。

本資料の無断転載・複写は､著作権法上での例外を除き､禁じられています。

本資料の転載・複写の問い合わせは〒305-0804 茨城県つくば市旭１番地
企画部研究評価・推進課 TEL 029-864-2675