zMessages from Departments and Centers of NILIM

Implementing River Projects Adapted toClimate Change

PhotoYAMAMOTO Satoshi, DirectorRiver Department(Key words) Climate change, flood control, risk assessment, adaptation
1. National land susceptible to flooding
In Japan, about 1/2 of the total population and 3/4 of
total assets are concentrated on alluvial plains
covering only 10% of the national land. The landsurrounding three large bays (Tokyo Bay, Ise Bay,Osaka Bay) which adjoin the three large metropolitan
regions, includes zero-meter belts of land below sealevel, occupied by 4.04 million people. And Japan is
located at the eastern end of the Asian monsoon zone,
one of the world’s heaviest rainfall zones, where it is
exposed to the risk of the approach and landfall oftyphoons. Japan can, for these reasons, be described asland susceptible to direct attack by flooding, sedimentdisasters and storm surge disasters.
To overcome such national land conditions, theJapanese have worked unremittingly to implement
flood control measures such as building series oflevees and constructing dams and other flood
regulation structures, achieving great improvements insafety from floods. But the state of provision of flood
control facilities remains at a low level of only about
60% of the present targets.
On the other hand, a poetic term for the beauty ofJapan’s mountains means “purple hills and crystal
streams”, giving the strong impression of a beautiful
country endowed with abundant water, and the annual
average rainfall in Japan is about 1,700mm, which isabout double the worldwide average. But nevertheless,rainfall per unit of population in Japan is low at 1/3 ofthe world average, so Japan does not possess abundantusable water. Japan has, therefore, responded byproviding water resource development systems, but inrecent years, annual rainfall has fluctuated widely andyears of extremely low rainfall have tended to occur,
arousing renewed fears of a decline of the safety of thewater supply and of droughts.
Social factors include the declining population, afalling birthrate and aging of society, the concentrationof the population and assets in large metropolitan
regions, and deteriorating regional economies. Among

farming and fishing villages, so-called villages at theviability limit are increasing, mainly in mountainousregions. Such villages are gradually losing their ability
to maintain functions they need to protect theirresidents from natural disasters, and it is assumed thatwhen a severe disaster does occur, they will suffer
unprecedented damage. In Japan, a country susceptible
to flooding, droughts, storm surges, and other natural
disasters, finding ways to respond to intensifying risk
caused by climate change is an important challenge
facing future management of the national land.
2. Increase of risk caused by climate change
Severe natural disasters which can be associatedwith the impact of global warming are occurringaround the world. Examples include flooding triggeredin various parts of Japan by concentrated rainfall inYamaguchi Prefecture in 2008 and by the recently
increasing landfall of typhoons, and the terrible
disaster in the southern U.S. caused by storm surges
produced by Hurricane Katrina in 2005. In 2007, theFourth Report of the IPCC predicted prolonged
climate change and severe damage caused by thistrend.
Japan has prepared concrete estimates of the impactof increasing rainfall during torrential rainstorms, thedecline of the flow rate of melting snow etc., inregional and drainage basin units. For example, thedegree of flood control safety (annual probability ofexceedance) which is a goal under present plans, hasdeclined by between 40% and 70% according to many
estimates, and it is reported that in Hokkaido and in
Tohoku in particular, there are river systems where it
has fallen to about 30%.
And if it is assumed that in the zero meter zones
around the three large bays, the average sea level willrise 59cm which is a predicted upper limit shown in
the IPCC report, the population living in zero-meterzones will increase about 50%.
In order to survey the impact of and adaptation tozMessages from Departments and Centers of NILIM
climate change, it is essential to appropriately assess
the social and economic impacts of changes ofexternal forces. In Japan, systematic studies have notbeen conducted, so we must quickly carry out suchstudies.
3. Basic concept of adaptation
The fact that both adaptation and mitigation are vitalin order to respond to global warming is not only
written in the Fourth Assessment Report by the IPCC,
it is also an awareness shared worldwide. But
awareness of this point is low in Japanese society
where, discussions are liable to be biased toward
mitigation. In Japan, a country particularly susceptible
to flood disasters, it is vital to increase awareness ofthe need for adaptation.
And floods, sediment disasters, storm surge
disasters etc. which are intensified by climate change,
are assumed to vary widely in scale, so it is difficult toprovide total protection from all such disasters. It istherefore important to adapt to climate change by
clarifying to what extent structures will provide
protection from flooding, then based on the results,
study measures to restrict runoff in river basins,measures to mitigate damage by submersion ofinundation of the land, and measures to restore
damaged structures and help damaged regions recover,and in these ways, achieve the goal of “zero victims”
of flooding of all scales likely to occur.
And in the Tokyo region and others where corefunctions are concentrated, it is essential to takepriority action to avoid paralysis of the nation’sfunctions and to minimize damage. And according toregion, flood disasters may occur in many river basinssimultaneously, so it is necessary to plan for a wide
area response encompassing more than a single riverbasin.

To study research on climate change, flood disasters
caused by the change of the weather which is thepremise for the study, and the impact on society andthe economy must be assessed as disaster risks, andthe results represented in a form easily understood by
the public and by concerned organizations. Whendealing with flood disasters, it is possible to include
flooding of every scale which could occur by
analyzing the forms of inundations in the river basin toevaluate the flood risk posed by each form ofinundation. It is vital to show the results visually as arisk map. Assessing flood risk is important in thesense that it not only reveals an existing state ofsusceptibility, but also permits a comparison of thepresent susceptibility with a case where adaptation has
been introduced and the clarification of the results.
(2) Minimization of inundation damage
In order to prepare a flood control plan byevaluating the results of calculation of the risks ofinundation by flooding caused by climate change,
comprehensive measures combining measures in river
basins (for example, setting flood control safetyaccording to land use, and controlling inundation bybuilding backup levees etc.) are studied to compensate
for inadequate flood control measures on rivers. When
proposing such measures, a classification is madebased on conditions on each river (not only naturalconditions such as the scale, gradient, etc. of the river,
but social conditions which include population, landuse, state of concentration of assets etc. in the riverbasin) in order to propose patterns of adaptation suited
to each set of conditions.
To also minimize the harm to residents of the river
basin, basic studies of the best way to improve
evacuation guidance systems, to guide land use, and toprovide flood insurance which economically provides
for the risk (risk financing).

4. Responding to growing risk
So at the National Institute for Land andInfrastructure Management, an organization including
the River Department, Water Quality Control
Department, and the Research Center for Disaster Risk
Management has jointly established the ?Climate
Change Adaptation Research Headquarters?, which
undertakes research on rivers and coastline
management to respond to climate change as its
priority challenge. Specifically, it is now undertaking
the following research projects.
(1) Assessment of risk caused by climate change

5. To implement adaptations to climate change
It is essential for Japan, as a nation susceptible toflooding, to clarify assessments of risk under theimpact of climate change and to propose appropriate
and effective responses. And at the same time it isJapan’s duty to share information of advanced
technologies in this field with countries around theworld. In either case, there are time restrictions on theprocess of proposing adaptations to the impact ofclimate change, so we wish to quickly undertake
research.