5th International Conference on Flood Management (ICFM5)
27-29 September 2011, Tsukuba-JapanNATIONWIDE ASSESSMENT OF THE DEGRADATION OF FLOOD DISASTER
MITIGATION EFFECTS OF RIVER IMPROVEMENT WORKS DUE TO GLOBAL
CLIMATE CHANGE
Osamu Itagaki1
Atsushi Hattori1, Koichi Fujita1, Shuichi Tsuchiya1
1 National Institute for Land and Infrastructure Management (NILIM), Ministry of Land, Infrastructure, Transport andTourism

Ministry of Land, Infrastructure, Transport and Tourism (MLIT) is struggling to implement adaptation
measures against floods under the global climate change, since its importance has been emphasized, forexample, in the policy report “Measures to adapt to climate change due to global warming that causes
flood disasters” by the Panel on Infrastructure Development, MLIT, submitted in June 2008.
Since Japan is much vulnerable to flood disasters because of the high density of land use and the deep
dependence on alluvial plains, the river improvement projects have been carried out toward the safetylevel prescribed in the river improvement framework plans, though the gap between the prescribed andachieved safety levels still exists. Under the global climate change, additional river improvement work is tobe needed for making up for the increased flood discharge, if we attempt to secure the same target safetylevel. It is important, therefore, to estimate this additional amount of river improvement work in comparison
with the amount of river improvement work planned in the current river improvement framework plan as a
key index of overall difficulties in coping with GCC impacts on flood disaster mitigation policies. How it will
be affected by the range of torrential precipitation projection due to the uncertainty over the calculation offuture climate change is also important for discussing the adaptation measures against floods under theglobal climate change.
The authors have estimated the median and the confidence intervals of the rate of increase of following
three values, flood discharge, amount of river improvement work, and probability of exceedance (forrepresentative flood discharge, e.g. 1/100 year flood discharge on the current river improvement
framework plan) for major rivers in Japan. For this estimation, the authors calculated the median and theconfidence intervals of the rate of increase of the annual maximum rainfall per day in each region from theresults of 20km mesh Global Circulation Model and 5km mesh Regional Circulation Model simulations
conducted by Meteorological Research Institute, Japan Meteorological Agency, MLIT. The estimation has
been conducted for two periods, one for about 30 years later from now, and another around the end of
21st century. The authors have drawn nationwide figures indicating the results above.
This estimation method provides one of the useful tools for capturing basic nationwide information
mentioned above, and it will contribute to the research and discussion on the specific adaptation
measures against floods. It has been shown that the rate of increase by GCC impacts and its confidence
interval of future rainfall are amplified as it is transformed into flood discharge, river improvement work,and probability of flooding. These results implicate that we should discuss the specific adaptation
measures considering the large uncertainty of the three factors.

International Center for Water Hazard and Risk Management (ICHARM)
under the auspices of UNESCO
Public Works Research Institute (PWRI)