Research Trends and Results
Addressing Flood Damage Reduction under the Global Climate
Change Using Imagination and Organizing Technique to theFullest Extent
TORII Kenichi (D.Eng.),Director; FUKAMI Kazuhiko , Research Coordinator for Watershed Management
River Department
HATTORI Atsushi (D.Eng.), Head; ITAGAKI Osamu , Senior Researcher; KATO Takuma (D.Eng.), Researcher
River Division, River Department
Keywords: global climate change, flood damage reduction measures, flood risk
Measures considering the global climatechange are no-regret measures
When developing flood damage reduction measures toprepare for an increase in the scale and frequency ofheavy rains that may be caused by global climate change,
it is essential not only to prevent the occurrence of flood
disasters but also to minimize damage to the extent
possible. Systematic development of flood damage
reduction measures requires avoidance of several
potential pitfalls.
First, take steps to avoid over- or under-estimation offuture flood possibilities. These estimates are based on
the results of climate change simulations; bothcarelessness and optimism can skew the estimates.
Furthermore, when evaluating the effect of measures tocompare multiple alternatives for countermeasures,
attention should be paid to avoiding "insufficientconsideration," e.g., failure to identify positive effects
expected to mitigate disaster, and "insufficient caution,"e.g., failure to identify serious risk without recognizing
negative effects.
For example, imagine that we look back upon theaforementioned study of measures at a certain point oftime in the future. We may regret it if we commit any of
the aforementioned mistakes. We should use such
imagination constantly and examine measures so that wecan make the best efforts.
2.

Organize and grasp various cases that
could occur
As an effective approach to examine flood damage
reduction measures, we are studying a flood risk analysis
method that utilizes the relationship between themagnitude of flood damage and the scale of the flood, as
shown in figure. In calculating the direct damage in thewhole basin, as indicated on the vertical axis, the position
and the number of points at which levee breach could
occur is set based on the relationship between the flood
water level and the height of the levee. The number ofbreach points is not limited to one, as all possiblecombinations of levee breach points are listed. The figure
shows the maximum, minimum, and average values ofdamage in all these potential cases. In calculating thedamage, the scale of flooding was set to range from aflood that just exceeds the present discharge capacity up
to a large-scale flood with the annual probability ofoccurrence of 1/500.
When the results of such calculation are organized, as
shown in the figure, it is possible to grasp the increasing
tendency to damage according to the increase in flood
scale (drastic increase in damage, reaching a limit, etc.). It
is also possible to grasp the positive and negative effectsof countermeasures from the changes in maximum,minimum, and average values.
3.

Suggest proper measures depending on
the flood damage change tendency
The figure also shows an example of an assumptive
calculation for raising the levee height in the upstream,
not considering the basic theory of flood control, i.e.,raising the levee from the downstream. The negative
effect is clearly visible, i.e., it brings mostly a decrease in
the minimum value, brings an equivalent or an increase in
the average value, and brings an increase in the maximumvalue.
Other indexes, such as death toll, could be examinedon the vertical axis. It is also important to examine theincrease or decrease in damage in each local area, as wellas the whole basin. We are compiling a systematic
approach to examine these “no-regrets measures.”Direct Damage in the Basin (HM yen)1.

Afterleveeraising (1)(1) Maximum value

Before leveeraising (1)Width of damage(2) Average value

After leveeraising (2)After leveeraising (3)Before leveeraising (2)Before leveeraising (3)(3) Minimum value

1/30 yrs 1/50 yrs 1/80 yrs 1/100 yrs 1/200 yrs 1/300 yrs 1/500 yrs

Scale of Flood

Figure: Example of Flood Risk Calculation[Reference]
Climate Change Adaptation Research Group: Interim
Report on Climate Change Adaptation Studies, Technical
Note of NILIM, No.749, 2013.