Flood Hazard Mapping Manualin JapanJune, 2005
Edited by

Flood Control Division, River Bureau,
Ministry of Land, Infrastructure and Transport (MLIT)

Translated byInternational Center for Water Hazard and Risk Management(ICHARM)

Introduction
Japanese natural conditions, such as its topography and climate, are severe. Besides, about
50 % of its total population and about 75% of its assets are concentrated in flood vulnerable
areas in the alluvial plains, which account for only 10% of its total land area. Furthermore, asa result of the recent trend of the nuclear family and the increasing urban population, more
people, even those who live in a flood-prone area, have no past experience of floods. As thepeople’s memories of floods fade over time, the awareness to the potential flood risks is alsofading away.
Consequently, there now is a higher risk that, once a levee breaks during a massive flood, notonly many lives and assets will be lost but also an unimaginable scale of social andeconomical confusion will occur. In recent years, torrential rainfall frequently causes flood
disasters, and a quite a few of them cause tremendous damage to the areas, such as
inundation in a large urban area and in underground facilities.
In order to mitigate those flood disasters, it is important to promote structural measures byconstructing flood control facilities such as levees. It is also important, however, to prepare
non-structural measures by improving ways to communicate disaster information and better
evacuation as well as by enhancing public awareness towards disaster prevention, since there
is always a possibility that a levee can breach if a flood exceeds its design capacity.
For these reasons, flood records were disclosed as a part of the comprehensive flood control
measures, which has been carried out since 1980. In 1991, the river council issued a report
entitled “Future Vision of River Development”. In this report, the council argued the need for
promoting non-structural measures including enlightening the public on the possible disasterrisks caused by floods, tidal waves, tsunami, mudslide, volcanic eruption and others, in orderto minimize the damage at the occurrence of a disaster. As a result of this report, the map offlood risk areas along the major rivers throughout Japan was created based on flood
simulations and disclosed from 1993 to 1994.
As these measures for warning possible flood risks have progressed, public demands formeasures to alleviate flood damages grew. Against the backdrop of such situation, “Promotionof Flood Hazard Mapping” and “Guidelines for Flood Hazard Mapping” (Manager of FloodControl Division, River Bureau, the Ministry of Construction) were notified to municipal
governments in 1994 to encourage preparation of flood hazard maps. The preparation has
been underway in the municipal governments with the support of the Regional Development
Bureaus and prefectural governments.
In 2000, just after the downpour in the Tokai area, the river council positioned flood hazard
mapping as very effective measures for disaster prevention and presented a report entitled“Future Role of Flood Prevention”, which underlined the need for active preparation anddisclosure of flood hazard maps. Based on this report, the Flood Fighting Act was partly
amended in June 2001, and the inundation risk area system was established. By thisamendment, rivers under the control of prefectural governments were also included in thescope of designation as a flood forecasting river. Besides, designation of inundation risk area
and its disclosure became mandatory for the flood forecasting rivers. In addition, municipaliii

disaster prevention plans are now required to include information for helping residents toevacuate quickly and safely, such as means of delivering flood forecast information andevacuation sites, by each inundation risk area. Municipal governments are also required tokeep their residents informed of such information. For these reasons, flood hazard maps werepositioned as a means to inform residents of information on flood risks and evacuation. Alongwith the partial amendment of the Flood Fighting Act, “Flood Hazard Mapping Manual” wasalso amended, and consequently flood hazard maps have been prepared and disclosed by 301
municipalities across Japan as of the end of fiscal year 2003. However, the progress is notnecessarily satisfactory.
In 2004, a series of downpour that occurred in many places across Japan revealed several
problems to be solved. To address such issues, it was decided to amend the Flood Fighting Act
so that the local capability of preventing disasters will be improved. The amendment waspromulgated in May 2, 2005, and went into effect in July 1 of the same year. By thisamendment, municipal governments are now required to designate inundation risk areas not
only along large rivers for which flood forecasting is conducted but along also major smalland medium-sized rivers. In addition, the municipal governments are required to keep theirresidents informed of means of delivering information on flood forecasts and evacuation sites
by distributing flood hazard maps that contain such information. The purpose here is toprovide residents with sufficient information on disaster prevention so as to help them act
appropriately at the occurrence of disaster. To this end, flood hazard mapping is now
considered to be even more important than ever.
This “Flood Hazard Mapping Manual” was prepared as a technical reference to provide
practical guidelines in flood hazard mapping, based on the amended Flood Fighting Act. I
hope this manual is helpful for all concerned to successfully prepare and disclose flood hazard
maps.

June, 2005
Manager of Flood Control Division, River Bureau, Ministry of Land, Infrastructure andTransportiv

Mapping Procedurev

1. Purposes of the flood hazard mapping manual
Flood hazard mapping aims to minimize flood damage as well as to promote non-structural
flood control measures which meet local needs and conditions so that it can help effective
operation of the inundation risk area system under the Flood Fighting Act, enacted in 1949.
The Flood Hazard Mapping Manual provides fundamental information to help prepare flood
hazard maps which display information about evacuation and inundation caused by levee
breaches and other reasons.

2. Definition
In this manual, a “flood hazard map” refers to a map that is prepared primarily to prevent
human damage by providing residents with inundation-related information, such as leveebraches and flood occurrences, and evacuation information in an easy-to-understand way,
primarily in order to prevent human damages. A flood hazard map must satisfy thefollowing conditions.
[1] Specifies inundation risk areas.
[2] Contains evacuation information.
[3] Prepared under the responsibility of municipal chiefs(including special wards; hereinafter the same)

3. Scope of application of this manualThis manual applies to the preparation of flood hazard maps in municipalities in whichinundation may occur because of levee breaches or other reasons.
.
4. Flood hazard mapping(1) The municipal governments prepare flood hazard maps based on their inundation risk
area maps, with the support of the central and prefectural governments.
(2) The central and prefectural governments actively support municipal governments’efforts to prepare flood hazard maps.

5. Necessary information for flood hazard maps
Information for flood hazard maps is divided into two kinds – general information andarea-specific information. General information is required to be included in every flood
hazard map while each local municipality can decide what area-specific information should
be included depending on its needs.
1) General information
General information refers to the minimum information that must be included in all flood
hazard maps, as inundation information and evacuation information.
- Inundation risk areas and types of damage- Evacuation sites- Dangerous spots along evacuation routes- Ways to deliver evacuation information such as flood forecasting, etc.

vi

- Sources of weather information, etc.
Refer to Item 6 below for points to remember when designating evacuation sites.
2) Area-specific information
Area-specific information refers to information that is specific in the target area and isuseful when residents evacuate. This kind of information is also helpful to promoteresidents’ awareness of flood disasters in normal times. Chiefs of local municipalities candecide which piece of area-specific information should be included in flood hazard maps.
<Information for evacuation>- Inundation information for other areas than inundation risk areas- Evacuation zones- Flood characteristics- Evacuation tips
- Information regarding evacuation recommendation, etc.
- Information regarding underground spaces- Information regarding facilities for disaster-vulnerable people- Other< Information for disaster education >- Generation mechanism of flood disasters, topography and types of flood- Information about possible risk of floods, types of damage, and past inundation- Meteorological information- Mental preparation for possible flood risks- Other relevant information

6. Points to remember when designating evacuation sites
Before designating evacuation sites, careful examination is necessary to ensure that thesites are free from the risk of inundation, landslide, storm surges and other disasters, basedon the information of inundation risk areas and landslide hazard areas. Also, dueconsideration should be given to temporary evacuation sites.

7. Wide-area evacuation plan
If an inundation risk area spreads across multi municipalities and therefore a large-area
evacuation is required, preparation of of a wide-area flood hazard map should be taken into
consideration based on a wide-area evacuation plan.

8. Reflecting opinions/suggestions from residents
Municipal governments should try to reflect as many residents’ opinions/suggestions as
possible in their flood hazard maps.

9. Coordination between flood hazard maps and municipal disaster prevention plans
Municipal governments should ensure coordination between their flood hazard maps andmunicipal disaster prevention plans.

10. Updating information in flood hazard maps
Municipal governments should appropriately revise their flood hazard maps when changes
are made in inundation risk areas or other information.

vii

11. Dissemination to residents
Municipal governments should disclose and distribute their flood hazard maps as soon as
possible and keep their residents informed of the maps so that it will be utilized effectively.

viii

Structure of this manualThis manual aims to give a concrete explanation of “Guidelines for Flood Hazard
Mapping” issued by the Flood Control Division, River Bureau, Ministry of Land,
Infrastructure and Transport, according to the 2005 amendment of the FloodFighting Act, so as to facilitate smooth and effective flood hazard mapping bymunicipal governments.
The primary aim of flood hazard mapping is to prevent human damage at theoccurrence of floods. Therefore it is important to provide residents withinundation/evacuation information in an easy-to-understand manner. In the
Guidelines for Flood Hazard Mapping, the minimum information on flood risks
and evacuation is positioned as “general information”.
On the other hand, information specific to local natural/social characteristics are
positioned as “area-specific information”.
In this manual, Chapter 1 describes how to prepare a flood hazard map which
covers all general information, following the preparation procedure step by step.
Chapter 2 describes area-specific information, which is important to examine
local characteristics.

ix

Contents

Chapter 1 Fundamentals of Flood Hazard Mapping ………………………………….1
Section 1 Purpose of the Flood Hazard Mapping Manual ……………………………….........2
Section 2 Definitions …………………………………………………………………………………7
1. Inundation Risk Area …………………………………………………………………………….7
2. Evacuation Information …………………………………………………………………………7
3. Responsible Body for Flood Hazard Mapping ………………………………………………..8
Section 3 Scope of Application ……………………………………………………………………...9
Section 4 Flood Hazard Mapping …………………………………………………………………11
1. Examination of Basic Issues ………………………………………………………………......12(1) Basic Conditions ……………………………………………………………………………..12(2) Preparing a Base Map ………………………………………………………………………12(3) Computerization of Flood Hazard Maps …………………………………………………13
2. Deciding Information to Be Included ………………………………………………………...14
3. Support to Municipal Governments ………………………………………………………….14
Section 5 Necessary Information for Flood Hazard Maps ……………………………………19
1. Inundation Risk Area and Types of Damage ……………………………………………….20(1) Inundation Risk Area ……………………………………………………………………….20(2) Types of Damage …………………………………………………………………………….20
2. Evacuation Site ………………………………………………………………………………….21
3. Dangerous Spot along Evacuation Route ……………………………………………………21
4. Ways to Inform Evacuation Information Including Flood Forecasting, etc. ……………24
5. Information Sources for Weather, etc. ………………………………………………………..25
Section 6 Points to Remember When Designating Evacuation Sites ……………………….27
1. Examining Appropriateness of Evacuation Sites …………………………………………..27
2. Evacuation Site Information Necessary for Flexible Evacuation …………………….....27(1) Wide-area Evacuation Site across Multiple Municipalities …………………………..27(2) Temporary/Emergency Evacuation Site ………………………………………………….28(3) Characteristics of Evacuation Sites ………………………………………………………28
Section 7 Wide-area Evacuation Plan ……………………………………………………………29
Section 8 Reflecting Opinions/Suggestions from Residents ………………………………….31
Section 9 Coordination with Municipal Disaster Prevention Plan ………………………….32
Section 10 Updating Information in Flood Hazard Map ……………………………………...33

Chapter 2

Area-specific Information …………………………………………………39

1. Information for Evacuation ………………………………………………………………………45(1) Inundation Information for Other Areas than Inundation Risk Areas ………………..45(2) Evacuation Zone ………………………………………………………………………………..48(3) Flood Characteristics ………………………………………………………………………….49(4) Evacuation Tips ………………………………………………………………………………...56(5) Information regarding Evacuation Recommendation, etc. ………………………………58

x

(6) Information regarding Underground Spaces ………………………………………………62(7) Information regarding Facilities for Disaster-Vulnerable People ………………………66
2. Information for Disaster Education ……………………………………………………………. 66(1) Generation Mechanism of Flood Disasters, Topography and Types of Flood …………66(2) Information about Possible Flood Risk, Types of Damage,and Past Flood Records ………………………………………………………………………..…70(3) Meteorological Information …………………………………………………………………..70(4) Preparation at Normal Times ………………………………………………………………71(5) Other Relevant Information ………………………………………………………………….73

xi

Figures and Tables
Fig.1

Workflow of Flood Hazard Mapping …………………………………………………………11

Fig.2
Fig.3

Support for Flood Hazard Mapping …………………………………………………………15
An Example of “Points to Remember” ………………………………………………………20

Fig.4

Damage around a Broken Levee(Kariyata River, Nakanoshima, Niigata Prefecture, July 2004) ................................21

Fig.5

An Example of Indication of Dangerous Areas (Underpasses),
Landslide Hazard Areas, and Steep Slope Landslide Hazard Areas
in a Flood Hazard Map ………………………………………………………………………22

Fig.6

Inundation at an Underpass (Left – at normal times; Right – during flooding)(Sanjo city, Niigata Prefecture) …………………………………………………………….23

Fig.7

An Example of Route and Commonly Available Means ofDelivering Evacuation Information (Flood Forecast, etc.) ……………………………..24

Fig.8

An Example of Description of How to Deliver Evacuation Information(Flood Forecast, etc.) …………………………………………………………………………25

Fig.9

An Example of River Basin Map …………………………………………………………….26

Fig.10An Example of Wide-area Flood Hazard Map …………………………………………...30

Fig.11An Example of Designating Evacuation Zones based on School Districts …………..48

Fig.12

An Example of Description of the Relationship between Floodwater
Flow Rate and the Degree of Difficulty to Walk………………………………………….53

Fig.13

An Example of Description of Floodwater Arrival Time (Joetsu City) ……………….54

Fig.14

An Example of Description of Inundation Duration and Floodwater
Arrival Time …………………………………………………………………………………..55

Fig.15

An Example of Evacuation Tips ……………………………………………………………56

Fig.16

An Example of Evacuation Tips ……………………………………………………………56

Fig.17

An Example of Evacuation Tips …………………………………………………………....57

Fig.18

An Example of Evacuation Tips ……………………………………………………………57

Fig.19An Example of Description of Evacuation Preparation Added to an
Evacuation Order Description ……………………………………………………………..60

Fig.20An Example of Description of Voluntary Evacuation from Inner Flood,etc. Added to an Evacuation Order Description …………………………………………61

Fig.21

Floodwater Running into Hakata-Station Underground Mall (July 2005) ………….62

Fig.22

An Example of Description about Possible Risks in Underground Spaces
in case of Floodings …………………………………………………………………………..63

Fig.23Inundation around Hakata Station and at Underground Malls ………………………64

Fig.24

An Example of Description on the Time until Floodwater Reaches
Underground Stations ……………………………………………………………………….65

Fig.25Fig.26

Flood Occurrence Mechanism ………………………………………………………………67
An Example of River Basin Landform Classification Map(Tama River) ……………………………………………………………………………………69

Fig.27

An Example of Description of Past Floods ………………………………………………..70xii

Fig.28

An Example of Description of Preparation at normal times
for Future Floods ………………….71

Fig.29An Example of Description of Preparation at normal times
for Future Floods ………………….72

Table 1

Use of Flood Hazard Maps …………………………………………………………………..3

Table 2Major Forms of Flood Hazard Map ……………………………………………………….12

Table 3

Related Material List ……………………………………………………………………….16

Table 4General Information for Flood Hazard Maps …………………………………………...19

Table 5

Detailed Items of Area-specific Information …………………………………………….44

Table 6

Flood Types and Their Characteristics …………………………………………………..50

Table 7

Examples of Evacuation Order, etc. ………………………………………………………59

xiii

References

Reference 1

Inundation Risk Area System Improved by the Amendment of

the Flood Fighting Act ……………………………………………………………………...4
Reference 2

Tsunami/Storm Surge Hazard Mapping Manual……………………………….10

Reference 3 Map Use Procedures in Accordance with the Survey Law ……………………….14
Reference 4

Colors for different Inundation Depths …………………………………………….21

Reference 5
Reference 6

Inundation Information for Other Areas than Inundation Risk Areas ………..45
Source Information of Flood Records (Clearing House) ………………………….47

Reference 7

Changes in Runoff Patterns Caused by Basin Urbanization ……………………68

Reference 8 Operation at the Occurrence of Floods Beyond Designed Storage Capacity ….73

xiv

Chap.1 Fundamentals of Flood Hazard Mapping

Chapter 1

Fundamentals of Flood Hazard Mapping1Chap.1 Fundamentals of Flood Hazard Mapping
Sec.1 Purpose of the Flood Hazard Mapping Manual

1. Purpose of the flood hazard mapping manual
Flood hazard mapping aims to minimize flood damage as well as to promote non-structural
flood control measures which meet local needs and conditions so that it can help effective
operation of the inundation risk area system under the Flood Fighting Act, enacted in 1949.
The Flood Hazard Mapping Manual provides fundamental information to help prepare flood
hazard maps which display information about evacuation and inundation caused by levee
breaches and other reasons.

In recent years, torrential rainfall frequently causes flood disasters, and quite a few of themcause tremendous damage to the areas. In order to mitigate those flood disasters, it isimportant to promote structural measures by constructing flood control facilities such as
levees. It is also important, however, to prepare non-structural measures by improving ways
to communicate disaster information and better evacuation as well as by enhancing publicawareness towards disaster prevention, since there is always a possibility that a levee canbreach if a flood exceeds its design capacity.
For non-structural measures to function effectively, it is essential to inform residents in aneasy-to-understand way about evacuation procedures and inundation caused by levee
breaches and other reasons. Also, residents should be educated regularly about disasters tokeep aware of disaster prevention and be able to evacuate on their own decision. Sucheveryday preparation for disasters will enable residents to evacuate smoothly and quickly intime of emergency.
Flood hazard maps can be an extremely effective tool to promote non-structural measures. It
should be kept in mind, however, that for flood hazard maps to serve their purposes
effectively, they should be created based on the viewpoint of residents and proper selection of
information for residents’ quick and smooth evacuation. Selected information should be
displayed for residents to understand easily and quickly. It is also important to inform
residents on a daily basis about flood hazard maps and to educate them about how they canutilize in time of emergency (see Table 1).
This manual should be revised from time to time to include updated information.

2

Chap.1 Fundamentals of Flood Hazard Mapping
Sec.1 Purpose of the Flood Hazard Mapping Manual

Table 1Time

NormalUse of Flood Hazard Maps
Public sector- Implementation of flood hazard maps into
local disaster prevention plans and flood
prevention plans- Promotion of building a flood-resistantcommunity- Revision of evacuation sites/routes- Review of evacuation/rescue methods fordisastervulnerable
peopleincludingbedridden elderly people and physicallychallenged people
Developmentofcommunication
methods/systems for evacuation informationtime- Dissemination of knowledge about disasterprevention- Promotion of public awareness towarddisaster prevention- Nurture of voluntary disaster prevention
groups- Information provision, advice, and guidance
to flood fighting groups- Organization of flood fighting corps and firefighting corps- Promotion of disaster education andevacuation drills- Information provision (weather information,
flood forecasting, etc.)- Consideration for disaster-vulnerable people- Confirmation of inundation areas/depths andevacuation sites/routes in case of flooding

Residents
Residents should try to:Haveabetterunderstanding
about
past inundation andinundation risk in theirliving areas.
- Minimize damage by
preparing
emergency
goods in case of flooding.
Providedisastereducation and conduct
evacuation drills forthemselves.
- Choose land use andbuilding styles after
considering
possibleflood risk in their living
areas.

- Confirm evacuationsites/routes
andemergency goods.
- Decide voluntarily toevacuatebasedonweather information andflood forecasting.

Issuing of evacuation recommendation
Before / After
Evacuate to a best
evacuation
possible evacuation site
recommendation - Provision of evacuation information- Opening of evacuation sites
by a safest possible route- Evacuation guidance
based on evacuation
recommendation/order. If
not evacuating, residents
are expected to takewhatevermeasuresavailable for them toprotect their own lives.

3

Chap.1 Fundamentals of Flood Hazard Mapping
Sec.1 Purpose of the Flood Hazard Mapping Manual

Reference 1 Inundation Risk Area System Improved by the Amendment of the FloodFighting Act (FFA)

Designation of flood forecasting rivers (byDesignation
of
Water
level
information*2)
水位情報周知河川（ 国又は都道府県）
communication
rivers の指定(by nationor prefectures)*1)
洪水予報河川の指定（ 国又は都道府県）
nationor prefectures)

水防法第10条
FFA article 10
水防法第11条
FFA article 11

FFA水防法第13条
article 13

Notification *3)
toの通知
municipalities
and public of special
特別警戒水位、 周知の実施(国又は都道府県)
warning water-level (by nation or prefectures)

洪水予報の実施（ 国又は都道府県
Flood forecast(by nationor prefectures)）※気象庁と協議し、 協同で実施*cooperation
with Meteorological
Agency
水防法第10条
FFA article 10
水防法第11条
FFA article 11

水防法第13条
FFA article 13-Designation of inundation risk areas (by nation or prefectures)■浸水想定区域の指定（国又は都道府県）-Clear indication of inundation risk areas and predicted depth (by nation or prefectures)■浸水想定区域及び想定される浸水深の明示（国又は都道府県)-Notification of inundation risk areas/depth to municipalities (by nation or prefectures)■浸水想定区域及び想定される浸水深の関係市町村への通知 （国又は都道府県)
FFA article 14
水防法第14条

Implement ■浸水想定区域ごとに以下の事項を市町村地域防災計画に定める
the following in the local disaster prevention plan of each inundation risk area
・洪水予報等の伝達方法-Communication method of flood forecasting etc.
・避難場所その他洪水時の円滑かつ迅速な避難の確保を図るために必要な事項、障害者、sites
乳幼児その他の特に防災上の配慮を要す-Necessary・地下街等又は主として高齢者
information for securing evacuationand smooth and fast evacuation way
る者が利用する施設で当該施設利用者の円滑かつ迅速な避難の確保が必要と認-Names and locations of underground spaces and facilities for the old, handicapped and infant
められる場合にあっては、 これら施設の名称及び所在地
who need special care upon evacuation.
水防法第15条

FFA article 15Distributeprinted materials and take other necessary measures to inform■市町村地域防災計画に定められた上記事項を住民に周知させるため印刷物の配布その他の必要な措置を講じる
residentsof the above-mentioned points .
水防法第15条
FFA article 15*1) Flood forecasting rivers
According to Article 10, Flood Fighting Act, a flood forecasting river is a river that meets thefollowing:- Runs across two or more prefectures or has a wide basin area.
- Designated by the Minister of Land, Infrastructure and Transport as a river that may cause
a serious damage to the national economy if it floods.
- When the risk of floodind is recognized, the Minister of Land, Infrastructure and Transportmust, with the cooperation of the Meteorological Agency chief, identify and notify the water4

Chap.1 Fundamentals of Flood Hazard Mapping
Sec.1 Purpose of the Flood Hazard Mapping Manuallevel, flow rate and other conditions of the river to the governors concerned.
- Once the river floods, the Minister of Land, Infrastructure and Transport must, with thecooperation of the Meteorological Agency chief, identify and notify the water level, flow rate
and other conditions of the river, as well as inundation risk areas and their inundation depth,
to the governors concerned.
- The Minister of Land, Infrastructure and Transport must notify the public of suchinformation on the river by calling for the cooperation of mass media, when necessary.
According to Article 11, Flood Fighting Act, a flood forecasting river is a river that meets thefollowing:- Not designated as a flood forecasting river in the above by the Ministry of Land,
Infrastructure and Transport, but has a wide basin area.
- Designated by the governor concerned as a river that may cause a serious damage if it floods- When the risk of floods is recognized, the governor must, with the cooperation of the
Meteorological Agency chief, identify and notify the water level, flow rate and otherconditions of the river to the flood control/watermark administrator specified in theprefectural flood prevention plan.
- The governor must notify the public of such information on the river by calling for thecooperation of mass media, when necessary.
*2) Water level information communication rivers
According to Article 13, Flood Fighting Act, a water level information communication river isa river that meets the following:- Not designated as a flood forecasting river, but is a class-A river that runs outside the zone
designated by the River Law.
- Designated by the Minister of Land, Infrastructure and Transport as a river that may cause
a serious damage to the national economy if it floods.
- When the water level of the river reaches a specified special warning water-level, theMinister of Land, Infrastructure and Transport must notify it along with the water level, flowrate and other conditions of the river to the governors concerned.
- The Minister of Land, Infrastructure and Transport must notify the public of suchinformation on the river by calling for the cooperation of mass media, when necessary.
Also:- Not designated as a water level information communication river in the above by the
Minister of Land, Infrastructure and Transport, but is a class-A or class-B river that runs in
the zone designated by the River Law.
- Designated by the governor concerned as a river that may cause a serious damage if it floods- When the water level of the river reaches a specified special warning water-level, thegovernor must notify it along with the water level, flow rate and other conditions of the riverto the flood control/watermark administrator specified in the prefectural flood prevention
plan.
- The governor must notify the public of such information on the river by calling for thecooperation of mass media, when necessary.

5Chap.1 Fundamentals of Flood Hazard Mapping
Sec.1 Purpose of the Flood Hazard Mapping Manual

Water level information communication river

Flood forecasting river

Not predictable rivers of flood risks

Predictable rivers of flood risks
Announce flood warning officially as a reference ofProvide the water-level condition of present time as aevacuation using future water-level predict.

reference of evacuation.

Dangerous water-level

Publication of water-level

Reach in a given
period of time

Dangerous water-level

Flood warning

As reference ofevacuation

Reach in a given
period of time

Special warnig
water-level

Publication of water-level
Reaching
information

Warning water-level

Flood advisory

Designated water-level

Provide information based on special warning
water-level considering necessary time for evacuation.

*3) Special warning water-level
A water level beyond the warning water-level, which calls for a special warning against theoccurrence of a flood disaster.
(Article 13, Flood Fighting Act)

6

Chap.1 Fundamentals of Flood Hazard Mapping
Sec.2 Definition

2. Definitions
In this manual, a “flood hazard map” refers to a map that is prepared primarily toprevent human damage by providing residents with inundation-related information, suchas levee braches and flood occurrences, and evacuation information in aneasy-to-understand way. A flood hazard map must satisfy the following conditions:[1] Specifies inundation risk areas.
[2] Contains evacuation information.
[3] Prepared under the responsibility of the municipal chiefs. (including special wards;
hereinafter the same)

There is no clear definition of a “hazard map”. It is referred to in various ways as a “disasterprevention map”, “disaster prevention record”, “disaster map” and so on. Target disasters
include floods, landslides, earthquakes, volcanic disasters, and tsunamis. Hazard maps ofdifferent types are prepared in various organizations depending on their respective purposes.
Present hazard maps are a result of the accumulation of disaster research findings andrelated information, and the recent development in forecasting technology; yet, they are stillat the developmental stage and haven’t reached to the complete form. Hazard maps that havebeen disclosed so far were prepared independently in accordance with their respective targets
and purposes.
In the future, however, it is desirable to develop such individually unique hazard maps into acomprehensive one.
The flood hazard map described in this manual aims to help municipal governments to minimize
human damage by flood disasters. The manual describes a method for such governments to provide theirresidents, in an easy-to-understand manner, with inundation/evacuation information and otherinformation required for evacuation from inundation risk areas.

1. Inundation risk area
An “inundation risk area” is an area where inundation is expected to occur if the specific riverin the area floods due to rainfall (Flood control plans generally concern floods due to rainfall).
Such a area is designated by the central or prefectural governments in accordance with the
Flood Fighting Act and notified to the municipal governments concerned. Note that, however,inundation may occur in areas other than inundation risk areas in case of rainfall exceeding
the design rainfall, tributary floods, storm surge or inland floods.
2. Evacuation information
Evacuation information refers to information that is useful for residents to evacuate at theoccurrence of floods. Such information includes flood forecast communication methods and7

Chap.1 Fundamentals of Flood Hazard Mapping
Sec.2 Definition

evacuation sites, as well as information necessary to secure smooth and rapid evacuation.
3. Responsible body for flood hazard mapping
Flood hazard maps aim to help residents to evacuate at the occurrence of floods. Therefore
chiefs of municipal governments, who are responsible for disaster prevention in the area, arealso responsible for preparing and disclosing flood hazard maps.
Paragraph 4, Article 15, Flood Fighting Act specifies as follows:
The chiefs of municipal governments whose district areas include inundation risk areas are
required to familiarize their residents with the information that the municipal disasterprevention plans are required to include by Paragraph 1, Article 15, Flood Fighting Act, in
accordance with the MLIT Ordinance. For this aim, the municipal governments must takenecessary measures such as distributing pamphlets containing such information.

8

Chap.1 Fundamentals of Flood Hazard Mapping
Sec.3 Scope of application of this manual

3. Scope of application
This manual applies to the preparation of flood hazard maps in municipalities in which
an inundation may occur because of levee breaches or other reasons.

The “river” here refers to flood forecasting rivers and water level information communication
rivers that are designated by the central or prefectural governments in accordance with the
Flood Fighting Act. However, the content of this manual may also apply to other rivers that
may cause inundation damage.
The content of this manual is intended for the municipal governments whose district areas
include inundation risk areas designated in accordance with the Flood Fighting Act. However,other municipal governments may use this manual for preparing flood hazard maps if theirdistrict areas may suffer from inundation damage.
This manual does not consider inundation damage caused by tsunamis or storm surges.
However, if measures against such inundation damage are necessary because of respective
local conditions, it is desirable to give consideration to them separately first and eventually
incorporate them to develop comprehensive hazard maps.
The governments whose district areas include landslide warning areas are required toinclude information necessary for preventing landslide damage in their hazard maps. See“The Landslide Hazard Mapping Manual (tentative title)” for information to be included.

9

Chap.1 Fundamentals of Flood Hazard Mapping
Sec.3 Scope of application of this manual

Reference 2

Tsunami/Storm Surge Hazard Mapping Manual

津波・高潮ハザードマップマニュアル
津波・高潮被害を軽減するためには、従来からの海岸保全施設の整備とあわせ 、危険度情報の提供など
ソフト施策による住民の災害に対する自衛力を高めることが必要である。津波・高潮ハザードマップとは 、津波・高潮による被害が想定される区域とその程度を地図に示し、必要に応じて避難場所・避難経路等の防災関連情報を加えたものであり、 住民の避難や施設の必要性の検討などに非常に有効である 。
内閣府、 国土交通省及び農林水産省は「津波・高潮ハザードマップ研究会 （座長 ：河田 惠昭 京都大学防災研究所 巨大災害研究センター長）」を設置し、地方自治体によるハザードマップの作成・活用を支援するための諸課題について検討し、 津波・高潮ハザードマップマニュアルを策定した。

本マニュアルの特徴●津波・高潮ハザードマップの全国的な整備の推進を目指し、その作成目的 、整備主体・国・都道府県等の役割分担、利活用方策などの基本的考え方を明確化 。
●津波・高潮ハザードマップ作成に必要な標準的な浸水予測計算 、記載事項 、表現方法及び利活用方法などを記載。

マニュアルの構成
本編

第1章

津波・高潮ハザードマップの必要性と位置付け

第2章

津波・高潮ハザードマップの概要

参考資料

第3章

浸水予測区域の検討方法

参考資料1

時系列を考慮した
数値シミュレーションによる浸水予測
１． 津波浸水予測計算
２． 高潮浸水予測計算

第4章

浸水予測結果からの津波・高潮ハザードマップ作成方法
参考資料2

津波・高潮防災対策における津波・高潮ハザードマップの活用例

第5章

津波・高潮ハザードマップの周知、 住民理解、利活用等

参考資料3

関連ホームページリスト

Source: Tsunami/Storm Surge Hazard Mapping Manual

10Chap.1 Fundamentals of Flood Hazard Mapping
Sec.4 Flood Hazard Mapping

4. Flood hazard mapping(1) The municipal governments prepare flood hazard maps based on their inundation risk
area maps, with the support of the central and prefectural governments.
(2) The central and prefectural governments actively support municipal governments’ efforts
to prepare flood hazard maps.

Municipal governments prepares flood hazard maps by making use of information on
inundation risk areas, which is provided by the central and prefectural governments, anddisclose and disseminate the hazard maps to their residents. The maps should reflect therevision of their municipal disaster prevention plans and suggestions from the residents. The
maps should also be updated when necessary. The central and prefectural governments
actively support municipal governments to facilitate smooth preparation of flood hazard
maps.
“The Guidelines for Issuing an Evacuation Order and Preparing a Communication Manual”
and “The Guidelines for Supporting Evacuation of Disaster-Vulnerable People” are stipulated
in “The Report on Communication and Evacuation Support for Elderly People at the
Occurrence of a Torrential Rain” issued by the Cabinet Office. They are also useful for sortingout related issues.

Fig.1

国及び都道府県からの浸水想定区域図等に関する情報提供

ProvisionofInformation

Workflow of flood hazard mapping

Inspectionof basic contents
基本事項の検討
Alignment with local
disaster

記載項目の検討
Inspectionof entry

about

plans
市町村地域防災計画との整合inundation risk area
maps

from

the

prevention

Makingof flood hazard map
洪水ハザードマップの作成

central or prefectural
governments.

住民等からの意見の反映
Publicity公表・周知
announcement

Reflectionoffrom residents

記載内容の更新
Update of entry

11

needs

Chap.1 Fundamentals of Flood Hazard Mapping
Sec. Flood Hazard Mapping

1. Examination of basic issues(1) Basic conditions
Before actual preparation of flood hazard maps, basic conditions need to be identified basedon inundation records and evacuation situations at the time of past floods, inundation risk
areas and topography.
Flood hazard maps are intended for floods that may occur in inundation risk areas that aredesignated by the central or prefectural governments in accordance with the Flood Fighting
Act and then notified to the municipal governments concerned.
It is desirable that a flood hazard map covers the whole municipal district. However, it may
cover only a limited area that contains inundation risk areas and evacuation sites if theinundation risk areas account for only a part of the municipal district. On the other hand, if
an inundation risk area lies across multiple municipalities, the municipal governments
concerned may need to jointly develop a wide-area flood hazard map.
Information on inundation risk areas and evacuation sites/routes, which are basic elementsof flood hazard maps, are indicated on the map. It is because flood hazard maps are assumed
to be used at the time of evacuation. Therefore, it is desirable to examine how and how muchinformation shoud be displayed when deciding a form of flood hazard maps. Table 2 shows the
major forms of flood hazard maps.
Table 2Form

Map type

Pamphlet
type

Pamphlet +map

Major forms of flood hazard map

General description

Other

A0- to A1-size topographic- Can show the whole target
map with related information area in a single pageadded
- Can contain only a limited
amount of information
B5- to A4-size pamphlet map - Difficult to show the wholewith related information
target area in a single pageadded
- Can contain a large amountof information
B5- to A4-size pamphlet with - Can show the whole target
A0- to A1-size map insertedarea in a single page
- Can contain a large amountof information(2) Preparing a base map
The map scale should be 1:10,000 to 1:15,000 because, with a scale smaller than this, eachhouse can not be identified and therefore it is difficult to identify evacuation routes. It ispreferable not to adopt a map scale of 1:25,000 to 1:50,000 unless unavoidable, because it is
impossible to identify each house and road, which must be identifiable for evacuation, in
maps of this scale. The size of a base map should be approximately A0 to A1 for the map type.
However, A1 size is preferable because it is easier to handle.

12

Chap.1 Fundamentals of Flood Hazard Mapping
Sec.4 Flood Hazard Mapping

If a topographic map on which a base map is based is old and does not sufficiently exhibit thecurrent conditions of topography, houses, roads, etc., the base map must be revised. Note thatcertain procedures may be required by the Survey Law or other laws to use a map for thebase map.
(3) Computerization of flood hazard maps
Flood hazard maps are printed on paper, but they should also be converted into electronic
data to facilitate disclosure via the Internet and updating of hazard maps. For this reason,
municipal governments should promote conversion of inundation risk area maps provided by
the central and prefectural governments into electronic data and should make use of suchelectronic data when preparing flood hazard maps.

13Chap.1 Fundamentals of Flood Hazard Mapping
Sec. Flood Hazard Mapping

Reference 3 Map use procedures in accordance with the Survey Law
In general, procedures based on the Survey Low are required for using or printing a map
developed at a public organization. For using a map developed by the Geographical Survey
Institute as a base map, onto which inundation risk areas are printed later, the approval foruse is required in accordance with Article 30 of the Survey Law.
Use of a map for flood hazard mapping will be approved without any difficulty because thepurpose is highly public.
Similarly, for using a map developed by national or local public organizations other than the
Geographical Survey Institute, an approval will be required in accordance with Article 44 of
the Survey Law.
Naturally, any approval is not required when a municipal government uses a map developedby themselves.
The Copyright Law Paragraph 10 stipulates that maps are included in copyrighted works.
For using a commercial map for flood hazard mapping, the municipal government mustobtain “permission for the use” according to Paragraph 63 of the same law or perform
necessary procedures for “use as a reference” according to Paragraph 32 of the same law.
2. Deciding information to be included
Municipal governments should give due consideration to decide what information items areto be included in their flood hazard maps in order to make them useful enough to ensuresmooth and rapid evacuation in case of floods.
Information for flood hazard maps is divided into tow kinds: “general information,” which arebasically required to be included, and “area-specific information,” from which local
municipalities decide what should be included depending their own conditions.
See Chapter 1 Section 5 for details of general information and Chapter 2 for area-specificinformation.
3. Support to municipal governments
Municipal governments are responsible for preparing flood hazard maps by making used ofinundation risk area-related information developed by the Regional Development Bureau
and prefectural governments. The diagram in Figure 2 shows the relationship between
municipal governments and river administrators in the Regional Development Bureau andprefectural governments. River administrators provide inundation information and various
materials necessary for flood hazard mapping to municipal governments. Based on suchinformation, municipal governments revise their local disaster prevention plans and prepare,
disclose and disseminate flood hazard maps.

14Chap.1 Fundamentals of Flood Hazard Mapping
Sec.4 Flood Hazard Mapping

Fig.2

河川管理者
River
manager
国及びorCountry
都道府県
prefectures

Support for flood hazard mapping

浸水情報の提供
Provisionof flood information
浸水想定区域
Inundation
risk area

浸水実績
Flood
record

外水はん濫
Outsidewater flood

内水はん濫
Insidewater flood
等etc

協力・支援

Cooperationand support
Review of local disaster

市町村地域防災計画の見
prevention plan
直し（ 法第15条第1項）

洪水ハザードマップの作成
Makingof flood hazard map(Law no.15, article 1)

市町村長
Local
chiefsLocal disaster
市町村防災会議
prevention council

洪水ハザードマップの公表・普及
Publicityand distribution of FHM

15

Chap.1 Fundamentals of Flood Hazard Mapping
Sec. Flood Hazard Mapping

Table 3 is a list of materials useful for examining basic issues and preparing a flood hazard map.
Table 3 Related material list

PurposeInformation to be identified
Background map
General map

Material
1:2500-scale
topographic
map
1:10000-scale
topographic
map

Base map

Inundation information
Evacuation information

Overflow,overtopping,
levee
break
point,
inundation
area,
Inundation
inundation depth
recordDamage state
Hourly water level andhourly rainfall at major
spots along the riverFloodanalysis
results(inundation area, flood
water depth, inundation
Inundation
depth, etc.)
prediction
Flood spread stateLongitudinalinundation
depth
Flow rate of flood waterBlocknumber,school
Evacuation
district,
neighborhoodzone
association, etc.
Calculation of Population by area
numberof Households by area
residents whowill need toevacuate

16

1:25000-scale
topographic
map
Materialforinvestigating
inundation
recordSame as above
Flood record

Prepared or
issued by
Municipal
government
Municipal
government,
Geographical
SurveyInstituteGeographical
Survey Instit
ute
Municipal
government,river
administrator
Same as aboveRiveradministrator

Inundation risk Riverarea,
flood administrator
hazard
area
map,
flood
analysismaterial

Relatedmaterial

Municipal
government

Census data
Municipal
Censusdata, government
house map
Municipal
government,
etc.

Remar
ks
Substitutable withurban
planChap.1 Fundamentals of Flood Hazard Mapping
Sec.4 Flood Hazard Mapping

Evacuation site
Type of public facilities(nurseryschool,elementary/junior
Evacuation sitehigh/highschool,university,
civic
hall,
assembly hall, gymnasium,
etc.)
Steep
slide
landslidehazardarea,
mudslide
hazard mountain stream
Road that has been closed
Dangerous spot because of floods
along
Area where landslide has
evacuation
occurred before
route
Underpass
BridgeMunicipal
disasterprevention plan
Relatedmaterial

Municipal
government
Same as above

Relatedmaterial
Past
flood
investigation
recordSame as above
House
map,
road ledger, etc.
Riverledger,etc.

Delivery route/method of
information(inundation
forecasts and evacuation
information)
Informationonunderground mall (location,
Information on
systemfordelivering
undergroundevacuation
information
malls, etc
from underground malladministrator)
Evacuation criteria
Evacuation
record(issuance
stateofEvacuation
evacuation order, issuancecriteria
notification
method,opening/reception state ofevacuation sites)
Populationofdisaster-vulnerable people
by area
Facilities
for
Type
of
facilities
for
disaster-vulnerdisaster-vulnerable people
able people(hospital, nursing home,facilities for physicallychallenged people, etc.)

Municipal
disasterprevention plan,flood
controlplan

Municipal orprefectural
government
Same as above
Same as above
Municipal
government,roadadministrator
Municipal
government,river
administrator
Municipal
government

Informationdeliverymethod17

Firedefense Undergroundplan, layout
malladministrator

Municipal
disasterprevention plan,flood
control
plan
Past
flood
evacuation
recordRelatedmaterial
Same as above

Municipal
government
Municipal
government

Municipal
government
Same as above

Chap.1 Fundamentals of Flood Hazard Mapping
Sec. Flood Hazard Mapping

Other information to be included in flood hazard map

Disasterprevention-relatedorganizati
onDisasterprevention
facilities

Medicalfacilities

Lifeline

Socialwelfarefacilities

Municipal facilities
Fire service facilities (firestation/branch station, fireservice corporation)
National
facilities(organizations related to
MLIT,
Meteorological
Agency, Defense Agency)
Prefectural facilities (local
office and civil engineering
office)Police organization (police
station/substation)
Disasterprevention
administration
radio
station, speaker, siren
Basefordisasterprevention
First-aid
station,flood
information
presenting
facilities
Water-level/rainfall
observatory
Emergency hospital
Healthcare center
Hospital and clinic

Municipal
overview,
municipal
disasterprevention plan,
telephone
directory
Same as above
Same as above
Same as above
Same as above

Municipal
government
Municipal
government
Municipal
government
Municipal orprefectural
government
Same as above

Municipal
disasterprevention plan,flood
control
plan
Same as above
Same as above
Same as above
Observatory
ledgerMunicipal
government
Same as aboveRiveradministratorRiveradministrator,
meteorologicalobservatory

Municipal
overview, housemap
Same as above
Same as above
Supply/disposal
facilities Municipal(water/sewer system, gas overview, housesystem,power
plant, map
Same as above
substation)
Communication
facilities(telephone station)
Nursing home, facilities for Municipal
overview, housephysicallychallenged map
peopleMunicipal
government
Same as above
Same as above

18

Municipal
government
Municipal
government,
NTT

Municipal
governmentChap.1 Fundamentals of Flood Hazard Mapping
Sec.5 Necessary Information for Flood Hazard Map

5. Necessary information for flood hazard maps
Information for flood hazard maps is divided into two kinds – general information andarea-specific information. General information is required to be included in every flood
hazard map while each local municipality can decide what area-specific information should
be included depending on its needs.
1) General information
General information refers to the minimum information that must be included in all flood
hazard maps.
- Inundation risk areas and types of damage- Evacuation sites- Dangerous spots along evacuation routes- Ways to inform evacuation information such as flood forecasting, etc.
- Sources of weather information, etc.
2) Area-specific information — See Chapter 2.

The principle purpose of flood hazard maps is to prevent human loss in flooding. For thatreason, it is important for the maps to be able to inform residents in an easy-to-understandway about inundation and evacuation. This manual lists up minimum information related toevacuation and risk in flooding as “general information". All flood hazard maps must include
the general information listed in Table 4.
Table 4General information for flood hazard maps

Type of information
・

・
・
・

・

Contents

Designated areas, inundation depth, types ofand damage(Coloring for different inundation depths should
follow that for inundation risk areas.)
Names, addresses, telephone numbers, etc. of
Evacuation sites
evacuation facilities
Dangerous
spots
along Mudslide hazard areas, steep slope landslide
evacuation routes
hazard areas, underpasses, etc.
Ways to inform evacuation Communicationroutes/means
for
flood
information including flood forecasting, water level information, evacuation
forecasting, etc.
order, evacuation instruction, etc.
Names and addresses, website addresses,
Sourcesofweathercell-phone website addresses, etc. of water
information, etc.
level/precipitation stations
Inundation risk
types of damage

area

19

Chap.1 Fundamentals of Flood Hazard Map
Sec.5 Necessary Information for Flood Hazard Map

1. Inundation risk area and types of damage(1) Inundation risk area
Among information items related to inundation risk areas, the area and depth of inundationmust be described in flood hazard maps. Coloring of inundation depth should be inaccordance with the inundation risk area map provided by the central or prefectural
governments (see Reference 4). There may be a case where, at around the confluence of anationally-administered river and a prefecturally-administered river, an inundation risk area
is designated along each of the two rivers. In such a case, examination will be required todecide either to prepare one flood hazard map covering both of the inundation risk areas or toseparately prepare a flood hazard map for each inundation risk area, depending on respective
local conditions.
(2) Types of damage
If a river levee brakes, flood water will flow out over land with such a powerful energy that it
may destroy houses. Therefore people who live near the levee must complete evacuation
before the levee breaks. Similarly, residents in areas where a considerable amount of flood
water flows into and accumulates need to rapidly evacuate. These information should be
included in flood hazard maps as “points to be remembered” so that residents are aware ofsuch issues in advance. Examples of “points to be remembered” are shown below (Figure 3
and Reference 4).

Early evacuation will save your life

If a levee breaks during flooding, houses near the levee may be destroyed or carriedaway by the onrush of flood water.
If you stay in your house in stead of evacuating, it may risk your life.
When there is a possibility of flood occurrence, try earlier on to collectinformation on rainfall at upstream as well as at your area and the water level ofthe river. Also pay attention to evacuation information notified by the municipal
government. Start evacuating as early as possible and be sure to complete
evacuation before a levee break occurs.

Fig.3

An example of “points to remember”

20Chap.1 Fundamentals of Flood Hazard Mapping
Sec.5 Necessary Information for Flood Hazard Map

Reference 4

Colors for different inundation depths

凡例

浸水した場合に想定される水深（ランク別）
0.5m 未満の区域
0.5～1.0ｍ未満の区域
1.0～2.0ｍ未満の区域

Depth range

Colorsample（CMYK）

Below 0.5m

Y50

0.5 to below 1.0 m

Y30，C10

1.0 to below 2.0 m

Ｃ20

2.0 to below 5.0 m

Ｃ40

2.0～5.0ｍ未満の区域(2.0 to below 3.0 m)

5.0ｍ以上の区域(3.0 to below 4.0 m)

Ｃ30，M10

浸水想定区域の指定の対象となる洪水予報河川(4.0 to below 5.0 m)

M20

5.0 m or higher

Ｃ20，Ｍ20(Source: “Inundation Risk Area Mapping Manual” prepared by MLIT River Bureau FloodControl Division in July 2001)

Fig.4

Damage around a broken levee (Kariyata River, Nakanoshima, Niigata Prefecture,
July 2004)(Source: “Flood Damage Report 2004” by Japan River Association, March 2005)

For unique topographical reasons, some areas exhibit flood characteristics which deserve
special attention, such as a fast flow rate of flood water and a fast increase rate of flood waterlevel. Flood hazard maps for those areas should include such information because it is very
effective to help residents to evacuate in an appropriate manner at an appropriate timing.
See Chapter 2 for details of flood characteristics of rivers.

2. Evacuation site
Refer to Chapter 1 Section 6 for points to remember when designating evacuation sites.
3. Dangerous spot along evacuation route“Dangerous spots along evacuation routes” refers to places which may pose a risk to safe
evacuation of local residents.

21

Chap.1 Fundamentals of Flood Hazard Map
Sec.5 Necessary Information for Flood Hazard Map

Possible dangerous spots include landslide warning/hazard areas (mudslide hazard areas,
steep slope landslide hazard areas, etc.), roads that have been closed because of flooding, andunderpasses and ditches where flood water depth is expected to become deeper than that inother areas during inundation.
Dangerous spots to be included in flood hazard maps as general information are: places
where mudslide is expected to occur during flooding, and places where the conditions during
flooding will be so different from those in normal times that dangers are difficult to predict
during evacuation, which could result in endangering human lives.
Municipal governments whose district areas contain landslide warning areas should include,in their hazard map, information necessary for preventing landslide damage in addition to
information on landslide hazard areas. See “The Landslide Hazard Mapping Manual(tentative name)” for information to be included.

Fig.5

An example of indication of dangerous areas (underpasses), landslide hazard areas,and steep slope landslide hazard areas in a flood hazard map(Source: Yamagata city flood evacuation map, issued by Yamagata city on March 2004)

22

Chap.1 Fundamentals of Flood Hazard Mapping
Sec.5 Necessary Information for Flood Hazard Map

Fig.6

Inundation at an underpass (left – at normal times; right – during flooding) (Sanjo city,
Niigata Prefecture)(Source: MLIT Geographical Survey Institute website)

23

Chap.1 Fundamentals of Flood Hazard Map
Sec.5 Necessary Information for Flood Hazard Map

4. Ways to communicate evacuation information including flood forecasting, etc.
This section describes a communication route, from information sources to residents, andcommunication means of information on flood forecasts, flood water level, evacuation
information (evacuation orders/instructions).
In order to minimize flood damage, residents’ awareness toward possible disaster risks andappropriate evacuation at the occurrence of floods are critical. For this reason, it is importantto provide residents with information that keeps their risk awareness level and offers a goodbasis for their decision-making. Municipal governments must familiarize their residents withthe process and means of communication. Due consideration should be given on the means ofcommunication, such as using multiple means depending on information types and local
characteristics. In addition, it is important to clearly specify how to provide information todisaster-vulnerable people.
As shown in Fig.7, flood hazard maps should contain the process and available means of
information delivery.
Country and prefectures
Meteorological agency(river manager)

Local government

Possible Communication Tools
Phone
Community
Silen
OrallyLoudspeaker Mobile phone
wireless(moble, PHS)
Fire bell
(fire
mail
van
company etc) BanderolesystemFAX

Internet

Media(CATV, radio)

Residents

Fig.7

An example of route and commonly available means of delivering evacuation information (flood forecast, etc.)24Chap.1 Fundamentals of Flood Hazard Mapping
Sec.5 Necessary Information for Flood Hazard Map

How the evacuation and meteorological information is conveyed.

Media
agencies

Shizuoka city disasterprevention measurement
officeShizuoka-ken
Local office

MLIT
Shizuoka local
meteorological officeFire service
flood prevention
team

TV
radio

Shizuoka-ken

Loudspeaker van, silen
Multiple radio

Homepage

PC
Mobile phone

MLIT Homepage

Disaster prevention
information of river
http://www.river.go.jp

PC
Mobile phone

Residents
The way of evacuation
advisory and direction

Fig.8
etc.)

The way of flood warning(meteorological information)

An example of description of how to deliver evacuation information (flood forecast,(Source: Shizuoka city flood evacuation map [issued by Shizuoka city in Mach 2004])

5. Information sources for weather, etc.
It is important for municipal governments and residents to proactively try to collectinformation instead of only receiving information provided by the national and prefectural
governments. In particular, dissemination of the sources and means of related information,such as weather information, is important to enhance residents’ awareness of acting
independently and taking care of themselves. For this reason, the sources and means ofrelated information should be included in flood hazard maps. Including information on therainfall at the time of past floods and inundation in flood hazard maps is also helpful forresidents to understand possible flood risks under a cirtain rainfall condition in their living
areas.
It is also effective to craft river basin maps which include target municipalities and thelocations of rain/water gauge stations. Such maps will help residents to have better
understanding of the area that each target basin covers and the characteristics of each target
river, including the time period until upstream rainfall reaches a target municipality and therainfall at which the risk of disaster occurrence increases. In addition, since information on
upstream rainfall and river water levels is important for residents to identify the flood risk
and decide when to evacuate, it is recommended to include how to get such information inflood hazard maps.
The source and means of related information that may be added in flood hazard maps include
the following:25

Chap.1 Fundamentals of Flood Hazard Map
Sec.5 Necessary Information for Flood Hazard Map- Names and locations of rainfall/water level observatories (which should be included in flood
hazard maps as river basin key maps)- Addresses of website/cell-phone websites that provide rainfall and water-level data
- Local cable television channels- Frequencies of community FM radio and/or NHK radio- Addresses of websites provided by river administrators and/or local authoritiesThose living in Fukuoka city should pay
attention to the rainfall and water level at
upstream as well.
Rainfall/water-level information
Fukuoka city: http://bousai.city.fukuoka.jp/
MLIT: http://www.river.go.jp/

LegendPrecipitation station
Reference point
Water level gauge
Basin boundary
Municipal border

Fig.9

An example of river basin map

26

Chap.1 Fundamentals of Flood Hazard Map
Sec.6 Points to remember when designating evacuation sites

6. Points to remember when designating evacuation sites
Before designating evacuation sites, careful examination is necessary to ensure thatthe sites are free from the risk of inundation, landslide, storm surges and other disasters,
based on the information of inundation risk areas and landslide hazard areas. Also, dueconsideration should be given to temporary evacuation sites.

Evacuation sites are for residents to escape flood waters. Therefore it is important to clearlyspecify the names and locations of evacuation sites in flood hazard maps in aneasy-to-understand manner so that it will help residents to safely and appropriately evacuate
at the occurrence of floods.
Giving careful examination of evacuation sites during flood hazard mapping helps enhance
the awareness of disaster prevention among municipal personnel in charge of disasterprevention as well as residents in the area.
1. Examining the appropriateness of evacuation sites
In general, evacuation sites specified in municipal disaster prevention plans are often
designated mainly for earthquakes. Therefore examination should be given in advance toensure that they are free from the risks of inundation or landslide based on information on
inundation risk areas, landslide hazard areas and the structure of evacuation sites. Also,
such examination results should be reflected in municipal disaster prevention plans. It may
be helpful to remember that appropriate evacuation sites are not necessarily required to beoutside inundation areas; single-story or higher robust buildings in case of inundation of 50
cm or less, or two-story or higher robust buildings in case of inundation of around 2 m may be
used as appropriate evacuation sites as well.
Note that, however, examination must be given also to how to secure rescue and medial aid in case ofevacuation sites being isolated.

2. Evacuation site information necessary for flexible evacuation
The following points should be examined, if necessary, and add easy-to-understandinformation that helps residents to take flexible evacuation actions.
(1) Wide-area evacuation sites across multiple municipalities
When evacuation is required at a stage where the actual conditions of floods are not clear, it
is most desirable to evacuate to shelters outside the inundation risk area. In such a case,wide-area evacuation across municipal boundaries may need to be considered.
In such a case, municipal governments should develop a wide-area evacuation plan(wide-area flood hazard map) after identifying inundation conditions in all the areas
concerned and implementing coordination for sharing evacuation sites with the neighboringmunicipal governments. Refer to Chapter 1 Section 7 for points to remember when27

Chap.1 Fundamentals of Flood Hazard Mapping
Sec,6 Points to remember when designating evacuation sites

establishing a wide-area evacuation plan.
Ultimately, evacuation sites should be located in a wide-area flood hazard map, which covers
neighboring municipal districts as well, after consultation with the neighboring municipal
governments.
(2) Temporary/emergency evacuation site
When evacuation is required at a stage where the actual flood conditions are not clear or
based on the anticipated flow rate and depth of flood water, residents may need totemporarily evacuate to private facilities equipped with robust buildings, levees, or roads ifthey are higher than the predicted depth of flood water. Examination for securing such sites
may be required in some cases.
If there is not sufficient time for evacuation or the depth of flood water is too deep to evacuate,
it may not be appropriate to evacuate to pre-determined evacuation sites. In such a case,residents need to take appropriate actions flexibly depending on the situation, such as
evacuating to the second floor of their houses or to a nearby building. Adding such
instructions in flood hazard maps is important to prevent accidents from occurring during
evacuation.
Before adding information on temporary evacuation sites in flood hazard maps, municipal
governments should gain a permission of the administrators of private facilities regarding
the use of facilities in case of a disaster. An agreement should be made in accordance withrespective local conditions. Once the use of facilities is accepted, a letter of agreement should
be signed by both parties. In consultation for the permission of using such facilities as
emergency evacuation sites, municipal governments may take the initiative throughout theprocess. Another option is to allow residents to participate in the consultation, and in such acase, municipal governments can take full responsibility only for concluding the designation.
The need of flexible evacuation actions, such as temporarily evacuating to the second floor of
their houses or to a nearby building, should be instructed as “points to remember” in flood
hazard maps.
(3) Characteristics of evacuation site
Information on evacuation sites should contain the following items. It will be helpful forresidents to take appropriate evacuation actions.
- Whether a municipal official arrangement plan has been developed- Whether a relief material distribution plan has been developed- Whether a disaster information delivery plan has been developed- Timing of opening evacuation sites (primary evacuation sites, secondary evacuation sites,
etc.)

28

Chap.1 Fundamentals of Flood Hazard Mapping
Sec.7 Wide-area Evacuation Plan

7. Wide-area evacuation plan
If an inundation risk area spreads across multi municipalities and therefore a large-area
evacuation is required, preparation of of a wide-area flood hazard map should be taken into
consideration based on a wide-area evacuation plan.

A wide-area evacuation across multiple municipal districts may be required in the following
cases:- When an inundation area may extend a large area across multiple municipal districts andcooperation between municipal governments is essential- When most of a municipal district is inundated and few evacuation sites can be secured
within the district.
- When a municipal district is cut off into segments because of flooding and it is difficult toevacuate to an evacuation site within the district without crossing the river- When there is an area in a municipal district near which evacuation sites are not available
and evacuating to neighboring municipal districts is more appropriate
When either of the above holds true, it is desirable to consider preparing a wide-area flood
hazard map on the assumption that a wide-area evacuation will be required in case offlooding.
Information sharing among municipal governments is particularly important. Municipal
governments should take appropriate measures to provide smooth and sufficientadministrative services, including providing information on evacuees from neighboringmunicipalities to their original municipal government (See Figure 10).

29

Chap.1 Fundamentals of Flood Hazard Mapping
Sec.7 Wide-area Evacuation Plan

Fig.10 An example of wide-area flood hazard map(Source: “Fujimi city flood hazard map” issued by Fujimi city in July 2001)

30

Chap.1 Fundamentals of Flood Hazard Mapping
Sec.8 Reflecting Opinions/Suggestions from Residents

8. Reflecting opinions/suggestions from residents
Municipal governments should try to reflect as residents’ opinions/suggestions as possible
in their flood hazard maps.

The primary purpose of flood hazard mapping is to prevent human damage by providing
residents with disaster-related information, such as a levee break and floods, in aneasy-to-understand manner. For this reason, appropriate measures should be taken to collectopinions/suggestions from residents and to reflect local characteristics, such as local
circumstances and past experience of disasters, in flood hazard maps.
Where such measures are appropriately taken, the process of flood hazard mapping will help
municipal officials involved in disaster prevention and residents to gain better understandingof inundation characteristics in the area and to improve their awareness of disasterprevention.
Some of the ways to gather suggestions form residents are as follows:- Disclosing related information to residents- Holding briefing session to resident representatives (community association chiefs, etc.)- Consulting academic experts who know well about local circumstances and disasterprevention- Establishing a flood hazard map examination committee made of academic experts,
Regional Development Bureau staff, prefectural and municipal officers, flood fighters, firefighters, voluntary disaster prevention group members, flood control-related agency staff,
local representatives, etc.
- Conducting questionnaire and/or interview to residents- Other

31

Chap.1 Fundamentals of Flood Hazard Mapping
Sec.9 Coordination with Municiple Disaster Prevention Plan

9. Coordination between flood hazard maps and municipal disaster prevention plan
Municipal governments should ensure coordination between their flood hazard maps andmunicipal disaster prevention plans.

Municipal governments should make revisions to their municipal disaster prevention plans
based on inundation/evacuation information and disaster prevention-related issues that are
identified in the process of flood hazard mapping. Also, municipal governments should ensure
coordination between their flood hazard maps and municipal disaster prevention plans.
“Revision of municipal disaster prevention plans in accordance with a partial amendment of
the Flood Fighting Act” (Act No.165 of 2001, effective October 31, 2001) issued by the Fireand Disaster Management Agency may be helpful for the purpose above.
In addition, municipal flood control plans, which stipulate fundamental issues for local flood
control in accordance with the Flood Fighting Act Article 32, may form a part of the municipal
disaster prevention plans (Windstorm and Flood section). Therefore flood control
administrators should ensure coordination between flood hazard maps and flood control
plans.

32

Chap.1 Fundamentals of Flood Hazard Mapping
Sec.10 Updating Information of Flood Hazard Map

10. Updating information in flood hazard map
Municipal governments should appropriately revise their flood hazard maps whenchanges are made in inundation risk areas or other information.

The flood hazard map should be appropriately revised in either of the following cases:- When changes are made in inundation information, such as the designation of inundation
risk areas- When changes are made in the municipal disaster prevention plan, such as a new
designation or change of evacuation sites- When an improvement is required at the stage of dissemination to the residents or actualuse of the map in case of flooding, even if no change is made in those two cases above.
When flood hazard maps are revised, residents should be promptly notified about therevision.

33

Chap.1 Fundamentals of Flood Hazard Mapping「Examples」

Examples of Flood Hazard Maps
(The examples are flood hazard maps which are close to the ones made based on
the concept of “gerenal information.”)[Case 1] Shizuoka city flood evacuation map (flood hazard map)
(Shizuoka city, Shizuoka Prefecture) ------------------------------------------------------- p. 35[Case 2] Nakano ward flood hazard map (Nakano ward, Tokyo) ------------------ p. 37

34

Chap.1 Fundamentals of Flood Hazard Mapping「Examples」[Case 1] Shizuoka city flood evacuation map (flood hazard map) (Shizuoka city, Shizuoka
Prefecture)
Basic conditions
Form: Map type, double-side printing
Paper size: A1
Inundation information: inundation risk area map and the past record of inundation caused
by inland floods
Characteristics: Evacuation sites in the inundation risk area are indicated as well. Predicted
inundation depth at each evacuation site is shown in the evacuation site list.

Larger view is shownin the next page.

(Source: “Shizuoka city flood evacuation map” issued by Shizuoka city in March 2004)

35

Chap.1 Fundamentals of Flood Hazard Mapping「Examples」

36

Chap.1 Fundamentals of Flood Hazard Mapping「Examples」[Case 2] Nakano ward flood hazard map (Nakano ward, Tokyo)
Basic conditions
Form: Map type
Paper size: A1
Inundation information: the inundation risk area map issued by Tokyo Prefecture
Characteristics: Easy-to-understand flood hazard map consisting of inundation informationand evacuation site information

Larger view is shownin the next page.

(Source: “Nakano ward flood hazard map” issued by Nakano ward in September 2002)

37

Chap.1 Fundamentals of Flood Hazard Mapping「Examples」

38

Chap.2 Area-specific Information

Chapter 2

Area-specific Information

39

Chap.2 Area-specific Information

5. Necessary information for flood hazard maps
Information for flood hazard maps is divided into two kinds – general information andarea-specific information. General information is required to be included in every flood
hazard map while each local municipality can decide what area-specific information should
be included depending on its needs.
1) General information — See Chapter 1.
2) Area-specific information
Area-specific information refers to information that is specific in the target area and isuseful when residents evacuate. This kind of information is also helpful to promoteresidents’ awareness of flood disasters in normal times. Chiefs of local municipalities are
responsible for deciding which area-specific information should be included in flood hazard
maps.
<Information for evacuation>- Inundation information for other areas than inundation risk areas- Evacuation zones- Flood characteristics- Evacuation tips
- Information regarding evacuation recommendation, etc.
- Information regarding underground spaces- Information regarding facilities for disaster-vulnerable people- Other<Information for disaster education>- Generation mechanism of flood disasters, topography and types of flood- Information about possible risk of floods, types of damage, and past inundation- Meteorological information- Mental preparation for possible flood risks- Other relevant information

In this flood hazard mapping manual, information about natural and social characteristics ofthe target area is referred to as “area-specific information”. This kind of information includes
two sub-types of information, namely, “information for evacuation” and “information fordisaster education”. As the names tell us, the former helps residents to evacuate smoothly
and quickly, and the latter is helpful in educating residents and enhancing their awareness of
disaster prevention in normal times.
The chief of each local municipality can decide which pieces of area-specific information
should be displayed on their flood hazard maps after closely examining the main purpose of
their preparing flood hazard maps. Local characteristics to consider typically include naturalcharacteristics such as flood characteristics of rivers in each community and social
characteristics such as residents’ experience of floods and the level of the community’smaturity. Before choosing pieces of information for flood hazard maps, it is necessary to40

Chap.2 Area-specific Informationconsider such area-specific information including possible risks during flooding, residents’awareness of disaster prevention, and the current situation of the community.
The following are useful viewpoints in considering natural and social characteristics specific to each
area.

41

Chap.2 Area-specific Information<Useful viewpoints regarding natural characteristics>- Flood characteristics of rivers
Topography and inundation are very closely linked to each other like the two sides of thesame coin. Inundation affects topography; on the other hand, topography also affects how
inundation occurs. Area topographies are roughly classified into three types based on thetypes of inundation:a) Expanding type: Flooding water expands from a levee breach to the inland area.
b) Flowing-down type: Flooding water flows down in the inland area along the river course.
c) Retention type: Flooding water accumulates in a certain area.
Which pieces of information (the flow velocity of flooding water, the propagation velocity todownstream areas, the ascension rate of inundation, reservoir filling time, etc.) to be included
in flood hazard maps should be judged after considering types of inundation and the size ofinundation areas.
- Area-specific inundation characteristics
Area-specific inundation characteristics particularly refer to types of inundation an area
suffers for area-specific topographical reasons. This concept classifies areas into two types,
namely, the areas that are possible to be inundated by landside water and the areas that arenot.
The areas prone to landside water inundation may be inundated even before levee breaches.
Thus, local municipalities should decide on inclusion of information about inundation causes
and inundation areas by landside water while considering possible influence of landsidewater on residents’ safe evacuation.

<Useful viewpoints regarding social characteristics>- Past flood experience
In recent years, there seems a tendency that areas or residents that have little experienced
with past flood disasters suffer severer damage. Considering such tendency, local
municipalities should decide on inclusion of information about flood characteristics of local
rivers; how to make an evacuation decision upon evacuation recommendation: generation
mechanism of flood disasters: possible risk of flooding; and how to understand weather
information.
- Local communities
In recent years, quite a few communities are found to be still immature and have notnurtured a strong bond among their residents. Considering that, local municipalities should
decide on inclusion of information about tips for evacuation (inviting neighbors whenevacuating, etc.) and facilities for disaster-vulnerable people.
- Underground spaces, etc.
42

Chap.2 Area-specific Information

The existence of underground spaces (underground malls and other underground facilities
available to the general public) varies from area to area.
Local municipalities with underground spaces should identify their exact locations andconsider whether their flood hazard maps should include information about possible risk
which people should be aware of when in such spaces during flooding.
Table 5 shows detailed items of information described so far.

43

Chap.2 Area-specific Information

Table 5Type

Detailed items of area-specific information

Information to be included in flood hazard maps
- Inundation information in other areas Inundation area
than inundation risk areas
Date of occurrence(inundation record [levee break point,
inundation area], inundation forecast,
Cause of floods
inland flood, inundation-prone area, etc.)- Evacuation zone

Evacuation zone
Flow rate of flood waters

Information for evacuation

Flood propagation speed- Flood characteristics of rivers

Inundation depth
Increase rate of inundation depth
Inundation duration
Points to remember when evacuating- Evacuation tips
- Information regarding
recommendation, etc.

Conditions for issuance of evacuation
evacuation order/instructions
Details of evacuation order/instruction
and action guidelines

Name- Information regarding underground
LocationspacesFlood risk
- Information regarding facilities for Namedisaster-vulnerable people
Location
Information for disaster education

Floodgeneration
mechanisms, Flood generation mechanisms
topography and types of flood
Process of levee breakDamage record
Rainfall state- Information about possible risks of Inundation state
floods, types of damage, and past
Damage state
inundation
Photographs of flood damages
Evacuation state during past floods- Meteorological information

Details of weather forecasting/warning- Mental preparation for possible flood
Mental preparation at normal times
risks

44

Chap.2 Area-specific Information

1. Information for evacuation(1) Inundation information for areas other than inundation risk areas
Inundation risk areas are designated by the central or prefectural governments in accordancewith the Flood Fighting Act and notified to the municipal governments concerned. Besides
inundation risk areas, however, there are also other areas assumed to have higher flood risks(inundation-prone areas, inland flood hazard areas, etc.) based on past inundation records
and local characteristics. Adding information on such areas in addition to inundation risk
areas in flood hazard maps is helpful for residents to safely evacuate.
Note that, however, the following issues should be remembered when using such inundation
information.
- Use of inundation records
Due consideration should be given to decide whether to use multiple flood/inundation records
collectively or to use only major flood/inundation records separately before actually adding
inundation area/depth records in flood hazard maps.
- Use of flood analysis results
Flood analysis results may have been adjusted by the grid square method. In such a case,re-adjustment of results according to actual topographies is necessary before adding the data
to flood hazard maps. Refer to “The Inundation Risk Area Mapping Manual” issued by the
MLIT River Bureau Flood Control Division in July 2001 for details of re-adjustment.
Reference 5

Inundation information for other areas than inundation risk areas
Process of inundation prediction by simulation

Details
For all the rivers administered by the central government
Disclosure of “flood hazard throughout Japan, hazard areas were identified by simulation
with the design rainfall (an amount of rainfall assumed tomaps”
occur approximately once in 100 to 200 years) and disclosed in
1993-94“flood hazard area maps”. This information is available as“flood simulation results” via CD-ROM and the Internet.
Disclosure of “inundation Intended for the flood forecasting rives, in accordance with the
Flood Fighting Act. Inundation areas and inundation depths
risk area maps”
After the amendment of the are shown on maps with a scale of 1:10000.
Flood Fighting Act in July
2001
Item

45

Chap.2 Area-specific Information

Classification

Acquisition
methodTraceinvestigation

Major
information

Description
Aerial photographs
taken during flooding
help
identify
inundation areas andofdamage.

AerialphotographfloodsInundation record
Topography information

A map showing the
areas
where
inundationhasoccurred in the past.
For
riversadministered by theInundation
central or prefectural
record map
governments, someInterview
inundation
record
maps
have
been
disclosed since 1985
onan
as-neededbasis.
A
contourmap
showing topography,
groundelevation
distribution and theLand condition
locations of facilities
map
for the purpose ofidentifying
disaster-related land
Readingcharacteristics
geomorphologic
A map convertedclassificationfrom a land condition
maps
map to focus onriver-related
Flood
information
topography
regarding flood risk
classification
areas along rivers
map
administered by thecentral government
Literature
research

Readinggroundelevation
distribution
maps

Groundelevation map

Japan
Map
Center,Book stores

Regional
Development
Bureau,Geographic
Survey Divisionof
Geographic
Survey Institute,
Japan
Map
CenterA
1:50000
map National
Diet
indicating
ground Library,elevation/undulation Geographic
at 1m intervals
Survey Divisionof
Geographic
Survey Institute

Inundation record and topography information

46

Source ofinformation
Regional
Development
Bureau,Prefectural
government,
Municipal
government
Regional
Development
Bureau,Prefectural
government,
Municipal
governmentChap.2 Area-specific Information

Reference 6

Source information of flood records (Clearing House)(Source: MLIT website [Designation/disclosure state of inundation risk areas along majorrivers])Flood information that has been disclosed is available on the MLIT website (URL:
http://www.mlit.go.jp/river/saigai/tisiki/syozaiti/index.html), where you can check thedesignation/disclosure status, information sources, and contacts for inquiries of the following
items:- Inundation records- Flood hazard maps
Information disclosed on the Internet is equipped with a link to show details.

47

Chap.2 Area-specific Information(2) Evacuation zone
Specifying evacuation zones should be done in a unit which is familiar to residents, such as
residential district (blocks, etc.), school district, or neighbor association. Such zoning iseffective to facilitate mutual cooperation among residents during evacuation and at
evacuation sites. (See Figure 11.)
It is desirable to collect opinions from residents to decide whether to include the areas orbuildings in the evacuation zones in the following cases:- When the area is not designated as the inundation risk area but has a higher risk ofisolation when surrounding areas are inundated.
- When the area suffers inundation below floor level (below 50 cm).
- When the floors of a robust, high-story building are considered to escape inundation.

－

：小学校区

●番号：避難場所

Fig.11

An example of designating evacuation zones based on school districts(Source: “Shirane hazard map” issued by Niigata city in March 2005)

48

Chap.2 Area-specific Information(3) Flood characteristics
Flood types can be classified into the following three categories based on local topographic
conditions and flood water behaviors during flooding; “diffusion type”, “flowing-down type”
and “retention type”. Each type has characteristics in the flow rate of flood waters, speed ofpropagation to downstream areas, inundation duration, and increase rate of inundation
depth at water storage areas. Notifying residents of flood characteristics of a river, as shownin Table 6, is very effective to encourage residents to take evacuation actions on an
appropriate timing. Therefore such information should be added in flood hazard maps if thetarget river has particular flood characteristics.
Note that flood water behaviors considerably vary depending on various factors including
river-bed gradient and the existence of driftwood in addition to the flood types. It is
important to provide appropriate information specific to each river in an appropriate manner.
For details of flood water behaviors and levee break characteristics of rapid rivers, refer to“The Guidelines for Specifying Inundation Risk Areas along Rapid Rivers” (issued by MLIT
Hokuriku Regional Development Bureau in October 2003).

49

Chap.2 Area-specific Information

Table 6

Flood types and their characteristics

diffusion type

Flood type

Retention type

Flowing-down type

Tedori River

Asakawa River

Chikuma River

50

General characteristics
Points to remember[General characteristics]- Often seen in flood plains with a large
low-lying area, such as alluvial fans,natural levee areas, and deltas.
- Flood waters spread widely.
- The flow rate of flood waters is relatively
slow except around a levee break spot.
[Points to remember]- Adding information on flood water
arrival time in the flood hazard map will
be helpful for residents to identify anappropriate timing to evacuate.
[General characteristics]- Often seen in flood plains with a large
land gradient or with a small flatland,such as valley bottom plains.
- In most cases, flood waters flow downwith a large depth and high speed. It may
generate such a large energy that can flow
houses away.
[Points to remember]- Adding information on the floodwaterflow rate and energy amount to begenerated in the flood hazard map will be
helpful for residents to identify the risks
in evacuating on foot and the possibility ofhouse collapse.
[General characteristics]- Often seen in a flood area surrounded byhills or natural levees.
- Inundation duration is relatively long.
[Points to remember]- Information on the increase rate ofinundation depth will be helpful forresidents to identify the water level atwhich point evacuation is no longer
possible.
- Adding information on inundation
duration in the flood hazard map iseffective to enhance residents’ awarenessto possible flood risks.

Chap.2 Area-specific Information

1) Energy generated by floodwaters
Floodwater energy is generally calculated with the depth and flow rate of floodwaters. Theenergy to be generated will become large if either of the elements is considerably large evenwhen the other is relatively small.
It is desirable to add information on the floodwater energy in flood hazard maps as a degreeof difficulty to walk in floodwaters, as shown in Fig.12. It helps residents understand possible
dangers caused by floodwaters and decide whether they will be able to safely evacuate onfoot.
In general, the flow rate and depth of floodwaters tend to become larger around levees and in
steep areas. In such places, a considerable amount of energy will be applied to inland
facilities and houses during flooding, and there are risks that houses may collapse or be
washed away. Therefore it is important to add such information in flood hazard maps to warn
residents that they must evacuate as early as possible in case of flooding.
Note that municipal governments should make use of inundation risk area information
available from the Regional Development Bureaus and prefectural governments when
examining the issues above.
2) Propagation speed of floodwaters (floodwater arrival time)
Indicating the assumed propagation speed of floodwaters is effective to encourage early
evacuation. It should be indicated clearly by, for example, classifying the target area into“within 30 min” “30 min to 1 hr” and “1 to 2 hrs” sections, as shown in Fig.13.
Floodwaters that have overflowed in the upstream may flow down through inland areas.
Therefore it is important to indicate the floodwater arrival time from each of levee breakpoints so that residents are aware of the inundation characteristics in their living areas, suchas the floodwater arrival time.
3) Increase speed of inundation depth
Specifying the predicted increase speed of inundation depth in flood hazard maps is very
effective to encourage early evacuation, because residents can understand when evacuation
on foot becomes difficult after an inundation starts.
4) Inundation duration
Indicating predicted inundation duration in and around the living area, as shown in Fig.14, is
helpful to identify the need of evacuation and to predict the prospects of restoration. Adding
predicted floodwater arrival time by each levee break point, as shown in Fig.14, will be
helpful for residents to identify how long it will take floodwaters overflowing in the upstream
to reach their living areas through inlands. It is also effective to notify residents of how far
they should evacuate.
Information items described above are calculated by each grid square in flood simulation.
When the grid square method is taken, smaller grid squares enable more detailed
51

Chap.2 Area-specific Information

descriptions.
With the isopleth method, accurate descriptions of the above information are impossible
because the relationship with ground elevation is not clear, unlike water depth. Still, it ispossible to describe it by the isopleth method with a reasonable, if not perfect, accuracy bytaking topography and other elements into account. Therefore consideration should be given
to employment of the isopleth method.

52

Chap.2 Area-specific Information

Flow velocity and difficulty of on-foot evacuation
Flow velocities are flood
flow rates expected inresidential areas in case oflevee breaches along theHime River due to heavy
rain.

Water depth > 0.8m: On-foot evacuation is difficult.

Difficultyofon-foot
evacuation is evaluated inrelation to flow velocity
and water depth.

Water depth < 0.8m: On-foot evacuation is possible.

Water depth > 0.8m: On-foot evacuation is difficult.

LegendOn-foot evacuation is difficult.
On-foot evacuation is possible.
Water depth > 0.8m
Water depth < 0.8m
Max. flow velocity (m/sec)
Mesh size: 250 x 250m
Difficulty of on-foot evacuation
e.g. At the water depth of 0.5m, on-footevacuation is possible up to the flow
velocity of 0.7m/sec.
On-foot evacuation is difficult.

On-foot evacuation is possible

Fig. 12 An example of description of the relationship between floodwater flow rate and thedegree of difficulty to walk(Source: “Himegawa river flood hazard map” [Ome-cho, Itoigawa City, March 2001])

53

Chap.2 Area-specific Information

How fast can the floodwaters reach your area?

This

figure

shows

how

fast

the

floodwaters may spread over areas
when levee breaches occur along therivers of Seki, Hokura and Yashiro.
Most riverside areas may be flooded
in 30 minutes. Residents living near thelevees or in low lands should take extracare.
The figure also shows major roads
that may be flooded over 50cm.

Flood arrival times & flood-risk roads
In 30 minutes
In 1 hour
In 2 hours
Major roads
Roads to be flooded over 50cm
Emergency heliports

Fig. 13

An example of description of floodwater arrival time (Joetsu City)(Source: Joetsu City Seki River flood evacuation map [Joetsu City, June 1996])

54Chap.2 Area-specific Information

Fig.14 An example of description of inundation duration and floodwater arrival time(Source: Flood hazard map for Shonai River and Shinkawa River [Nagoya City, June 2002])

55

Chap.2 Area-specific Information(4) Evacuation tips
Describing points to remember during evacuation and everyday preparation for possible
disaster risks in flood hazard maps is effective to ensure safe and smooth evacuation.
The following are sample evacuation tips:- Neighborhood cooperation
Keeping good communication and mutual assistance among neighbors are important tominimize disaster damage in the area (see Fig.15).

Fig.15 An example of evacuation tips
(Source: Sapporo City flood hazard map [Sapporo City, July 2004])- Evacuation support for disaster-vulnerable people
Quick delivery of information is required for disaster-vulnerable people including the elderly,
the physically/mentally challenged, and infants so that they can evacuate early enough to
avoid damage. For this reason, it is important to describe the means of communication todisaster-vulnerable people in flood hazard maps, as shown in Fig.16.
Evacuation information (warning, etc.) will be provided by & obtained from:
Mr./Ms. (name) (Vice-president of the neighborhood association); also by the (name) care center.
*Hearing-impaired people must be informed in person.
Other:(Contact information of care insurance company, etc,)
Evacuation shelter

Evacuation site(assembly hall)
Evacuation
supporters’ house

Watch out for manholes
when it’s raining hard.
Evacuation
supporters’ house

Watch out forflooded areas

Fig.16 An example of evacuation tips
(Source: “Guidelines for evacuation support for disaster-vulnerable people” [Cabinet Office,March 2005])

56

Chap.2 Area-specific Information- Risk of evacuating by car
In general, it becomes difficult to drive in an inundation depth of 30 cm or more. Vehicles may
float in floodwater or passengers may be locked in a power window car when an inundation
depth reaches 50 cm or more. For these reasons, evacuation by car is dangerous. In addition,
driving vehicles are harmful to other people because they may block traffic of emergency
vehicles or cause waves that disturb walking evacuees. Therefore it is important to add
information on the risks of evacuating by car in flood hazard maps (see Fig.17).
Fig.17 An example of evacuation tips
(Source: Sapporo City flood hazard map [Sapporo City, July 2004])- Need of evacuation in group in comfortable clothes
Describing in flood hazard maps the need of evacuating in comfortable clothes and withothers, such as family members and neighbors is important to prevent accidents during
evacuation.

Fig.18An example of evacuation tips

(Source: Sapporo City flood hazard map [Sapporo City, July 2004])- Need of flexible evacuation actions
If there is not enough time to safely evacuate or if inundation depth is already too high, it is
not necessarily appropriate to evacuate to a designated evacuation site, but may be better tostay upstairs at home or evacuate to a nearby safe building. Describing the need of flexible
evacuation actions in flood hazard maps is important to prevent accidents during evacuation.

57

Chap.2 Area-specific Information(5) Information regarding evacuation recommendation, etc.
Municipal governments should identify conditions under which residents should startevacuating and criteria for issuing an evacuation recommendation based on information
available from organizations concerned as well as information that they collect by themselves.
Describing the issuance criteria of evacuation recommendations in flood hazard maps will
help residents evaluate circumstances during disaster and start evacuating at an appropriate
timing.
It is desirable to describe under what conditions evacuation recommendations (including
evacuation recommendations/orders for disaster-vulnerable people), which are stipulated inmunicipal disaster prevention plans and flood control plans, will be issued. In addition,actions to be taken after an issuance of a evacuation recommendation should be described.

Table 7 summarizes the issuance conditions of evacuation recommendations/orders andactions to be taken by residents, based on “The Guidelines for Developing Evacuation
Recommendation Issuance/Notification Manual” (the Cabinet Office, March 2005). Figs.19
and 20 shows an example of description of evacuation recommendations.

58

Chap.2 Area-specific Information

Table 7 Examples of evacuation order, etc.
(Source: “Guidelines for Developing Evacuation Recommendation Issuance/Notification
Manual” [the Cabinet Office, March 2005])
Conditions for issuance
Actions to be taken by residents- A stage where disaster-vulnerable - Disaster-vulnerable people,
people, especially those who take especially those who take longer
longer time for evacuation should
time for evacuation should start
start evacuating; the risk of human
Information on
damage is expected to be rapidly evacuating to a designatedevacuation site. (Those who areevacuation
increasing at this stage.
preparation
to support them should start(intended forsupporting their evacuation.)
disaster-vulnerable- Residents other than the above
people)should start preparation forevacuation, including contacting
other family members andpreparing emergency goods
Evacuation
recommendation

Evacuation order- A stage where residents who canevacuate by themselves should also
start evacuating; the risk of humandamage is clearly high.

- Residents who can evacuate bythemselves should also startevacuating to a designatedevacuation site.

- The risk of human damage isextremely high, judging frompremonitory phenomena and theurgent situations at the time.
- The risk of human damage isextremely high, judging from local
characteristics, e.g. the area islocated right next to levees- Human damage has occurred

Residents
who
startedevacuation upon the evacuation
recommendation should complete
evacuation as quickly andsecurely as possible.
- Residents who have not started
evacuation should immediately
start evacuation. If there is notenough time to safely evacuate,
they should take whateveractions possible to be safe.

* Unexpected things may occur under natural phenomena. It is not necessarily appropriate toevacuate to a designated evacuation site; it is sometimes better to stay upstairs at home orevacuate to a nearby safe building, depending on the circumstances.

59

Chap.2 Area-specific Information

Fig.19An example of description of evacuation preparation added to an evacuation order
description(Source: Yamagata City flood evacuation map [Yamagata City, March 2004])Note) Municipal governments should give due consideration to decide what information should be included in the flood
hazard map, referring to Table 7.

60

Chap.2 Area-specific Information

Fig.20An example of description of voluntary evacuation from inland floods etc. added to an
evacuation order description(Source: Tosa City flood evacuation map [Tosa City, July 2000])Note) Municipal governments should give due consideration to decide what information
should be included in the flood hazard map, referring to Table 7.

61

Chap.2 Area-specific Information(6) Information regarding underground spaces
Since underground malls are closed spaces, once inundation starts, the water level rises
particularly fast, and therefore there is a considerably high risk of flood accident. In addition,
people in underground malls are often too late to recognize the flood risk because it is difficult
for them to gain information on weather forecasts, river conditions and others. Furthermore,
there are higher risks during evacuation from such places because floodwater catchments
often get in the way of evacuation routes, which may cause serious and fatal damage.
Therefore, when various underground facilities available for the general public exist in an
inundation risk area and when there is a need to ensure smooth and rapid evacuation in caseof flooding, the names and locations of the facilities must be specified in flood hazard maps.
If flood hazard maps become too crowded with such information to see other inundation data,
pamphlets specifically designed for underground facilities may be prepared separately anddistributed to residents.
If underground facilities are inundated, safe and quick evacuation from such places is moredifficult than evacuation on the ground. Indicating the possible flood risks at undergroundfacilities with photographs and illustrations, as shown in Figs.21 and 22, is helpful to ensure
safe and smooth evacuation from such places.

Fig.21 Floodwater running into Hakata-station underground mall (July 2005)(Source: “Flood Damage Report 2003” by Japan River Association, March 2004)

62

Chap.2 Area-specific Information

It is dangerous
to be in underground malls when storm rain and floodning.
豪雨・洪水時に地下街等は危険で

Fig.22

An example of description of possible risks in underground spaces in case of floodings(Source: “Possible Risks in Underground Spaces in Case of Inundation” by the Japan Building Disaster
Prevention Association, June 2000)

63

Chap.2 Area-specific Information

Figure 23 is a sample map that indicates the locations of facilities whose undergroundsections were inundated and facilities whose ground and underground sections were bothinundated. (This map is not used in a flood hazard map, but may be useful as a reference.)

Inundation around Hakata Station due to the June 29 storm

Fig.23Inundation around Hakata Station including its underground malls(Source: “Investigation material for 6.29 Hakata-station-area inundation investigation coordination
meeting”)

64Chap.2 Area-specific Information

Fig.24

An example of description of the time until floodwater reaches underground stations

What will become of the subway?
The chart and tables below show how fast the floodwaters may reach each station after
levee breaches occur along the Shonaigawa River.
Flood reaches in:
1 hour
1~2 hours
2~3 hours
3~4 hours
4~6 hours
Over 6 hours
No flooding

What will become of
the underground mall?

(Source: Flood hazard map for Shonai R iver and65

Shinkawa River [Nagoya City, June 2002])

Chap.2 Area-specific Information(7) Information on facilities for disaster vulnerable people
When there are facilities for disaster vulnerable people (i.e. those who need support in case ofdisaster), such as the elderly, the physically/mentally challenged and infants, in inundation
risk areas, and when there is a need to ensure smooth and rapid evacuation in case offlooding, the names and locations of the facilities should be included in flood hazard maps.
If the flood hazard map becomes too crowded with such information to see other inundation
data, pamphlets specifically containing information on those facilities should be preparedseparately and distributed to residents.
Examining detailed information other than the names and locations of facilities for disaster
vulnerable people is useful for municipal governments to establish evacuation plans. Whenexamining such information, it is desirable to refer to the “The Guidelines for Supporting
Evacuation of Disaster Vulnerable People” prepared by the “study panel on informationdelivery and evacuation support to the elderly, etc. in case of downpour or other disaster”.

2. Information for disaster education(1) Generation mechanism of flood disasters, topography and types of flood
Describing basic information on floods, such as inland floods, the factors and process of levee
break, and local topography, in flood hazard maps is effective to enhance awareness topossible flood risks and understanding of local flood characteristics among residents.
1) Flood generation mechanism
Figure 25 is an example describing the generation mechanism of floods caused by continual
rain or downpour and the flood generation process in urban areas. Inclusion of suchdescription in flood hazard maps helps residents to predict the occurrence of flood in theirliving areas based on weather information and to evacuate on an appropriate timing.
Other useful information on floods includes the relationship between the precipitation in the
upstream area and the river water level in the living area, rainfall intensity, and differencesin runoff patterns caused by different land uses, as shown in Reference 7. Describing suchinformation in flood hazard maps is very effective because it will provide a good basis forresidents to take voluntary actions.

66

Chap.2 Area-specific Information

Flood by torrential rainfall
集中豪雨による洪水の発生過程

Flood by long rain
長雨による洪水の発生過程

都市部による洪水の発生過程
Flood in urban areas

Fig.25 Flood occurrence mechanism(Source: “Kanto Region Flood Handbook 2003” by MLIT Kanto Regional Development Bureau
River Administration Division, October 2003)

67

Chap.2 Area-specific Information

Reference 7

Changes in runoff patterns caused by basin urbanization

2000

0

10

Rainfallin June 1966
S41.6実績降雨20

Calculated flood discharge in

Precipitation (mm/hr)

current situatiin(Urbanization 84%)40
雨量(mm/hr)

1500

50

1000

Flood discharge in 1966

雨量
S41( 市街化率25%)
現況(市街化率84%)(Urbanization 25%)60

末吉橋地点洪水流量(m3/s)

30

70
500
80

90

100

0
0
6/28

2

4

6

8

10

12

14

16

18

20

22

0
6/29

2

4

6

8

時刻
Time

Changes in runoff patterns caused by basin urbanization (along Tsurumi River)
The above graph indicates that the flood discharge increases and the peak flood
discharge appears earlier as development in the basin proceeds.
Note that the overflow from river channels is not taken into consideration in the analysis
above.

68

Chap.2 Area-specific Information

2) Topographic characteristics identified by river basin landform classification maps
Most plains in Japan which are prone to floods are depositional plains formed by erosion,transportation and deposition of sediment by streamflow or shore current. Sedimentation
occurs mainly during flooding. Alluvial fans, natural levees and deltas are formed by naturalforces applied repeatedly during floods. Since those natural forces, such as floods, which
creates landform do not drastically change, if levee break or flooding occurs again, inundation
tends to affect the landform basically in the same manner as when the previous inundation
shaped the present-day landform. For this reason, it is possible to predict the state andcharacteristics of floods by gaining understanding of topographical distribution in a certain
area. Topographical characteristics can be clarified by classifying an area by its form,
structure and formation period. Basin landform classification maps (see Fig.26) and land
condition maps are useful to this end, enabling identifying the relationship between
topographical characteristics and floods.
Currently it is also possible to identify micro-topography based on precise altitude dataacquired by airborne laser scanning.

Fig.26

An example of river basin landform classification map (Tama River)(Source: “River basin landform classification map” [the Ministry of Construction Kanto
Regional Construction Bureau, Geographical Survey Institute, 1977])
69

Chap.2 Area-specific Information(2) Information about possible flood risks, types of damage, and past flood records
Information such as possible flood risks, types of damage, and past flood records are also
helpful in enhancing residents’ awareness toward floods and reminding them of past flood
experience. Figure 27 is an example carrying information of such kind. The figure includes
records/photos on damage, rainfall, inundation, and evacuation. Inclusion of the relationshipbetween presipitation and food types in flood hazard maps will also useful if there are pastrecords available about such relationship.
Fig.27

An example of description of past floods(Source: Yamagata City flood evacuation map [Yamagata City, March 2004])）(3) Meteorological Information
Information such as rainfall and the relationship between the upstream and downstream
water levels as well as weather information and warning issuance criteria will be helpful forresidents to predict the probability of flood occurrence in their living areas and to evacuate
early enough.

70

Chap.2 Area-specific Information(4) Preparation at normal times
Figures 28 and 29 are examples showing in advance what we should prepare and what weshould do when a flood occurs. Including such information in flood hazard maps is effective toenhance residents’ awareness to the need of mutual assistance in the community and amongneighbors during evacuation. When preparing such information, local characteristics should
be taken into consideration.

Fig.28An example of description of preparation at normal times for future floods

71

Chap.2 Area-specific Information(Source: Yamagata City flood evacuation map [Yamagata City, March 2004])

Fig.29An example of description of preparation at normal times for future floods(Source: Yamagata City flood evacuation map [Yamagata City, March 2004])

72

Chap.2 Area-specific Information(5) Other relevant information
1) Information on the operation of flood control facilities
Describing dam discharge, opening/closing of the floodgates of drainage basins, the operationof inland water drainage pumps in flood hazard maps in an easy-to-understand manner is
helpful to enhance residents’ understanding of the roles and operations of flood control
facilities.
Reference 8 Operation at the occurrence of floods beyond designed storage capacity of a dam

Store floodwaters

Regular operation
The dam size isdeterminedtocope with thedesignflood
discharge.

Design max. outflow
discharge

Max. inflow

Flood volume

Operation during flooding over the dam’s design storage volume

A case of design flood
Max. inflow

If the flood exceeds
the design storage
volume, the dam
discharges
thewater at a rate notexceedingtheinflow to ensure
the dam’s safety.

Discharge
up to the inflow

Design max. outflow
discharge

Storefloodwaters
within flood control
capacity

Flood volume

A case of flood exceeding
design flood73