Basic Planning

3

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

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

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

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

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

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

2.

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

3.

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

6

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

Sea areas

Land and water management

Transport

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

Hardware-based measures

Software-based measures

7

Urban

Chapter 1 Basic Policy
Section 1

General Information

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

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

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

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

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

Appropriate use of rivers
and maintenance of
normal water flows

Improvement andconservation of the riverenvironment

Construction of a healthy water cycle system Comprehensive sediment management

Fig. 1–1 Comprehensive planning

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

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

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

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

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

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

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

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

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

2.

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

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

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

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

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

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

Proper use of rivers and maintenance of normal water flows

2.

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

3.

Maintenance and formation of an aesthetically pleasing landscape4.

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

5.

Conservation of good water quality.

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

Proper use of rivers and maintenance of normal water flows

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

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

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

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

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

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

Conservation of[Protecting] water quality

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

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

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

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

18

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

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

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

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

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

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

2.

Basic river improvementa.

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

Design flood discharge at major locationsc.

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

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

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

The goals of the river improvement plan

2.

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

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

facilities to be builtb.

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

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

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

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

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

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

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

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

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

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

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

23

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

Fig. 2–1

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

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

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

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

Over 200

Class B

100–200

Class C

50–100Class D

10–50

Class E

Below 10* Inverse of the annual probability of excess

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

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

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

25

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

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

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

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

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

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

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

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

2.

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

3.

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

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

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

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

Fig. 2–2 Adjustment of rainfall durations
28

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

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

Fig. 2–3 Determination of the design flood

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

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

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

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

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

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

Possible difference between the actual and design flood scales2.

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

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

30

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

<Inlet time>
0.9

Mountainous river basin

2 km2:

30 min2

General built-up area

0.8River basin with very steep slopes:

2 km

20 minField and plain

0.6

Area with sewerage system

2 km2:

30 minPaddy field

0.7

Mountain

0.7

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

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

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

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

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

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

2.

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

3.

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

4.

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

5.

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

6.

Evaluation at each phase of project implementation.

1.

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

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

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

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

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

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

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

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

33

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

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

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

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

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

36

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

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

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

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

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

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

background.

Fig.2-4

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

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

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

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

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

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

40

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

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

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

41

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

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

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

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

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

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

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

4)

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

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

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

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

44One toseveraldays
Time scale

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

Short term

Medium term

Extent of centenarian(Return period of the design rainfall)

Time spanLong term

Limit rainfall of collapse
Rainfall

Unstablesediment is
not yielded
newly
significantly

Sediment yield of
unstable sediment

Sediment
amountby theclasses ofgrain size

Sediment
Discharge

Riverbed
fluctuation

Sediment
yield frowlarge scalecollapse

Sediment
discharge
to becontrolled

Fluctuation
to becontrolled

Conutermeasures
for sediment yield
field

Sediment discharge tobe controlled

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

Sediment yield anddischarge

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

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

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

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

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

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

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

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

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

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

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

2.2.6 Sediment generation control plan47

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

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

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

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

48

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

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

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

Mixed with cobbles (Disaster caused byflooding from successive flow)

Muddy water (Disaster caused byflooding from successive flow)

Fig. 3–2 Conceptual diagram of debris flow flooding

2.3.2 Design scale

49

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Explanation
Disasters associated with landslides occur under conditions particular to this nation, such as its characteristic
topography, geology, climate, and land uses. These disasters are roughly divided into two types: 1) direct
disasters, where the subject requiring protection is located on a landslide slope or within reach of a moving soil
mass generated by a landslide; and 2) indirect disasters, which affect areas upstream and downstream through theblocking of rivers by sediments or the secondary collapse of river-clogging sediments.
The aim of a landslide-prevention plan is to prevent or mitigate these direct and indirect landslide-related
disasters, and it should be developed from preliminary landslide surveys and the analyses thereof. The various
conditions of the landslide-prevention area, such as topography, geology, climate, land use, the status of thesubjects requiring protection, and the urgency of protection all need to be taken into account.
Since landslides generally occur on a large scale and consist of two or more movement blocks, it can take many
years are to complete construction works. On the other hand, because there are often many houses and public
facilities on the landslide slope and within reach of the moving soil mass generated by the landslide,
non-structural measures such as an appropriate warning and evacuation system are essential to cope with thethreat of the landslide being activated. For this reason, a landslide-prevention plan needs to take into
consideration not only structural measures such as construction and the improvement of landslide-prevention
facilities but also non-structural measures to ensure the establishment of a warning and evacuation system.
The plan must be kept consistent with the surrounding environment, relevant laws and regulations and regional
plans.
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Once a landslide-prevention plan is established, it needs to be properly reflected in the landslide-prevention
works master plan stipulated in Article 9 of the Landslide Prevention Act.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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Seacoastpreservation plan
Coastal environment
Living and growing
environment
Coastal landscape

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

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

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

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

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

67

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

Capability

・Wave over topping prevention

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

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

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

Utilization

・Swimming beach・Fishing
・Place for festive events

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

Other

Foundation of structures
such as embanlcments

Support strength of rock bed,scour rateand stability

PurposeProfection

Environment

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

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

while maintaining

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

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

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

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

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

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

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

70

Backshore
Backshorewidth

Nearshore

Foreshore
Foreshore
width

Revetment

Offshore

Nearshorewidth

Backshore height

H.W.L
L.W.L

Foreshore slope
Nearshore slope

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

71Run-up height < Crest height of the embankment

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

Design seashore profile

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

Design seashore profile

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

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

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

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

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

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

Largest measured ocean wave

2.

Ocean wave predicted from an abnormal weather model3.

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

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

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

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

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

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

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

75

Design seashore profile

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

Measuresare required

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

Measuresagainst storm
surge
Measuresare required

Evaluation of the current seashore
profile with the design tsunami.

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

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

Establishment of the seashore
protection measures

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

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

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

77

Fig.4-7 Flow chart of erosion control planning

78

Loss to submarine valley
Sediment supply from sea cliff

Retrogression of sea cliff

Sediment supply from riverLong shore drift

Long shore drift

Long shore drift

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

79

Sediment supply(small amount)Long shore drift

Erosion control measures
Sea cliff(Erosion)

Sediment
Discharge(small amount)

Decreasing of sediment supply

Wave

Break waterLong shore drift

Sedimentation

Long shore drift

Expansion of eroded area

Disruption of sediment transport

Incident wave

Break water

ErosionSedimentation

Formation of a shielded area

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

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

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

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

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

82

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

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

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

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

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

Protection
Constraints on construction
or improvement ofprotection facilities

Protection facilities
affect environment

Environment

Constraints on useof seacoast or
construction of
convenient facilities

Protection facilities
inhibit use of seacoast

Environment

Use

Use

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

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

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

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

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

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

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

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

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

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

89

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

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

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

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

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

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

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

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

92

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

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

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

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

93

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

105

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

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

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

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

Explanation
River management facilities such as river dikes and groundsills and seacoast preservation facilities such as
coastal dikes and beach nourishment facilities must possess the functions that are stipulated in the Cabinet Order
Concerning Structural Standards for River Management Facilities, etc. and the Criteria for Seacoast Preservation
Facility Construction, etc., and it is important for the management of rivers, etc. that these functions arecontinuously maintained without deterioration.
The planning and design of plans for the management of water, sediment, etc. and arrangement of facilities
should be carried out in such a way that the functions expected of the facilities can be fulfilled in considerationof the level of maintenance. It is important to conduct thorough monitoring through surveys and measuring
instruments and, from the monitoring results, to assess the functionality of the facilities to check whether thefacilities are fulfilling their expected functions.
The items, timing, and frequency of monitoring should be dependent on the purpose and expected functions ofeach facility. In general, though, monitoring should be conducted through periodic inspections, inspections at thetimes of floods and storm surges, and inspections after such events. Measurements that require specializedmeasuring equipment should be conducted more efficiently if such considerations are included in the design andconstruction of the facilities.
It is important that the results of monitoring be organized using predetermined formats for the assessment offacility functionality. If it is determined that the functions of the facility have deteriorated and are not satisfying
the expected standard, the results should be fed back into the facility plan (which may lead to reconstruction) or
maintenance so that proper functions of the facility should be achieved.
Reference
1) Hydrological Observation, 2002 Edition, Authored and edited by the Public Works Research Institute,
Supervised by the River Bureau of the Japanese Ministry of Land, Infrastructure and Transport. Published by theJapan Construction Engineers’ Association107