●Messages from Departments and Centers of NILIM

How Will We Survive the Era of Rising
Flood Disaster Risk and Uncertainty?
Koichi FUJITA
Director of the River Department

2. Not hard-to-soft displacement theory
In view of these challenges, people may consider
that now is the time to switch from relatively
expensive hard countermeasures (building disasterprevention facilities to improve rivers) to soft
countermeasures (measures other than building
facilities), but this is over-simplistic.
Constructing facilities to reduce the frequency ofinundation disasters is uniformly effective, including
for the flood plains where all people need protection.

RCM5
phase 2 model
RCM5後期モデル
Multipliersof each item
各項目の倍率1. The challenge, “increasing risk and uncertainty”
Figure 1 shows an example of a trial calculation toclarify the increase in torrential rainfall by global
warming till the end of this century, and how this will
increase the flood flow rate, required amount of river
improvements, and the possibility of flood inundationof land protected by levees. The three solid lines show
differences between climate change prediction models
used to estimate torrential rainfall.
According to this trial calculation, torrential rainfall
will increase by approximately 1 to 1.3 times, theflood flow rate by 1.1 to 1.4 times, and the possibilityof inundation (frequency) by 1.8 to 5 times. The
required amount of river improvements is an index of
the degree of river improvements (improving thecapability to safely carry a flood discharge byimproving the river course, or lowering the flood
discharge flow rate itself by flood regulation facilities)
that will be necessary in order to safely handle flood
discharge caused by a torrential rainfall of the target
scale of flood control planning (basic guideline toriver improvement). This will also be between 1.6 and3.4 times higher than it would be without globalwarming.
Even if torrential rainfall increases by 20% to 30%,etc., the two indices on the right side of Figure 1 willbe the multiplier orders. The nearer the items ofrequired quantity of river improvements andinundation potential are to countermeasure items, thegreater the impact of global warming. Although theprecision of climate change prediction models hasbeen greatly improved, because of its amplification
characteristic, the increase multiplier of the required
quantity of river improvements and inundation
potential based on global warming impact will have alarge range of estimation. Thus, there is considerable
uncertainty in the foundation for adapting to this andthe increased risk.

RCM5phase 1 model
RCM5前期モデル

Median value

GCM20
phase 2 model
GCM20後期モデル
Broken lines enclose 95% reliability※破線は計算年数が短いこsection
caused by small number ofcalculation
yearsとに起因する95%信頼区間

Torrential
豪雨量rainfall

Flood flow洪水流量
rate

Quantity of
河川整備
required river
必要量
improvement

95% reliability
section(Keywords) flood risk, adapting to climate change, uncertainty, disaster mitigation, regional development

Inundation

氾濫可能性
氾濫可能性
potential

Figure 1. Change of flood control related quantities
caused by global warming (The multiplier is the ratio of
the end of the century to the present time: the simple
mean value of 109 Class 1 river systems: Estimatedbased on climate change prediction models by theMeteorological Research Institute: RCM5 phase 1,
GCM20 phase 2, and RCM5 phase 2)1)

But once a flood exceeds the capacity of a facility, asevere disaster may result. On the other hand,assuming that various countermeasures such as
suitable evacuation systems based on warnings, orurban development considering evacuation anddamage mitigation have been carried out, it is not easy
to prevent severe damage. The important point is not
to pursue a single magic solution, but to pursue thebest mix of diverse countermeasures including both
hard and soft countermeasures.
3. What circumstances will be created?
Figure 2 shows the form of this “best mix.”
Facilities prevent disasters caused by the following
dangerous phenomena under external forcesconsidered for planning and design (phase on the rightside of the figure). If the scale of a dangerous
phenomenon exceeds this external force (including
cases where the handling capacity of a facility is
exceeded while the structure is being built) it is
impossible for a facility to completely prevent all risks.
Even under such circumstances, by ensuring good city
development using various methods, and by providing
methods of escape and survival, severe damage will
not occur (phase on the left side of the figure.)
Therefore, simply stacking the three building
blocks—facilities to prevent or mitigate disasters,
good city development, and methods of escaping orsurviving—for the phase on the left side of the figure,●Messages from Departments and Centers of NILIM
will not be sufficient. These must be firmly bondedwith each other. Bonding means that good city
development that considers how facilities prevent or
mitigate disasters is practiced, and that methods ofescape or survival which fully reflect suchdevelopment are provided.
4. Producing knowledge contributing to thecreation of circumstances
As research on technology policies to create theworld shown in Figure 2, it will be necessary toconsider three broad pillars. First, refining the method:
providing facilities. In addition, maintenance offacilities will be improved and rationalized whileclarifying the relationship with the way functions aredisplayed, information (including weather predictions,when real time or circumstances allow them) will beused to improve the functioning of facilities, therenovation of existing facilities will be expanded, andfacilities groups will be treated as systems and thecombinations will be optimized for the areas protectedfrom flooding.
Secondly, the ways in which damage occurs when adangerous phenomenon exceeds the handling capacityof a facility will be clarified more than in the past. For
each river system, the relationship of the scale(external force) of the dangerous phenomenon withthe damage which could occur will be clarified, andcharacteristic properties, speed, or points of an abrupt
increase in damage for example, or the existence oftipping points or peaking trends, etc. will be confirmed.
This requires a good knowledge of how each facility
functions and the state of the protected region.

様々な手段の活用・結集
Using and combining various
methods

Facilities to
防災・減災のprevent or
ための施設
mitigate
damageOccurrence of a dangerous phenomenon

exceeding that on the right
右を越える危険事象の生起
Occurrence of combined dangerous
複合的な危険事象生起
phenomena

Vitality of the globe

Regional
まちのつくり方
development method

Daily lif Daily life

しのぎ方

ふ
地だ
域んのの活暮力 ら
しVitality of the globe e

逃げ方
EscapemethodSurvival method

Facilities to
防災・減災のprevent or
ための施設
mitigate
damageAction of external forces for

計画・設計上の外力の作用
planning and design

Figure 2. What circumstances will we create in order tointelligently face flood disasters?

Thirdly, based on obtained knowledge andlong-term prospects for the way damage occurs, goodregional development including towns, and methodsof combining methods of escaping and survival will becompleted. This study will incorporate two-way
exchanges to provide feedback to improve ways thatfacilities functions. These three pillars will, acting

together, be introduced where countermeasures can becontinuously checked and expanded centered on
multilateral flood risk assessment efforts. The
Tsunami Disaster Prevention Regional Development
Law enacted in December of last year is an important
precursor to the framework of the policies shown inFigure 2. A study should be done on applying such
efforts to all flood disasters.
Needless to say, the daily lives of people are notintended to prevent or mitigate disasters. Their mainrole is normal daily life rooted in each region. How is
the world shown in Figure 2 harmonized with normal
daily life? Can any innovation allow harmonizationthat increases the vitality of a region? Answers tothese questions will require a deeper and broader
vision.
References
1) K. Fujita et al. (2012): Evaluating the impact ofclimate change on river management, etc., 21stcentury climate change prediction reform program:Research on predicting future changes of extreme
phenomena based on ultra-high resolution
atmospheric models, Report on ResearchAchievements, 2011.
2) K. Fujita (2011): Flood control technology systems
conscious of risk: prospects and challenges,NILIM Conference of 2011(http://www.nilim.go.jp/lab/bcg/siryou/tnn/tnn0655pdf/ks065511.pdf)