●Research Trends and Results

Effective inundation countermeasures to prepare forthe future increase of heavy rain
YOKOTA Toshihiro, HeadSHIGEMURA Hiroyuki, Senior Researcher
HASHIMOTO Tsubasa, Researcher
Water Quality Control Department, Wastewater System Division(Key words) Increase of heavy rain, inundation countermeasure, unsteady flow model

2. Studying countermeasures to deal with theincrease of heavy rain
We used annual maximum rainfall intensities from1960 to 2009 at 57 meteorological observatories
nationwide to predict the increase rate of
10-minute/60-minuterainfallintensity
with5-year/10-year probability during the next fifty years
in each observatory. The results showed a trend for
10-minute/60-minute
rainfallintensitywith
5-year/10-year probability nationwide to rise to a
maximum of between 1.3 and 1.4 times the present
levels (95 percentile values) fifty years in the future.
Based on this, inundation simulations were performed
for three districts to evaluate the impact on existing
inundation countermeasures of the future increase ofheavy rain, revealing a case where it is predicted thatthe area inundated to a depth of 20cm or more will
increase between 4% and 9%. The results also suggestthat in a case where the rainfall drainage capacity is
insufficient over a wide area, or in a case where local
inundation is caused by the drainage capacity of abranch channel, it will be necessary to takecountermeasures appropriate considering the cause ofinundation in each district.
In the future, we will universally evaluate theimpact that the increase of heavy rain will have on
inundation countermeasures to suggest specificmeasures according to the characteristics ofinundation.

1.00
95パーセンタイル値
95
percentile value
95パーセンタイル値

0.75

Percentile

パーセンタイル
パーセンタイル

1. Introduction
In recent years, heavy rain of 50mm/hr or more has
fallen frequently in various parts of Japan1). Measures
to prevent disastrous inundation are being taken by
local governments, but it is considered possible that in
the long term, as a result of change of the rainfall
characteristics, present inundation countermeasuresalone might be unable to handle heavy rain with
5-year probability or 10-year probability. So theNILIM is conducting studies to clarify future heavy
rain increase trends and countermeasures to deal withthis increase, and abstracting problems related to theenactment of rainfall countermeasure plans andstudying improvement measures.

0.50●1010minutesminutes
10分間
60
minutes
60分間○ 60 minutes

50パーセンタイル値
50
percentile
50パーセンタイル値
value

0.25

0.00
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8
毎年最大降雨量の増加率
Increase
rate of rainfall intensity

1.0

降雨強度の増加率

Figure 1. Distribution of Increase Rate of
10-minute/60-minute Rainfall Intensity at 57
Observatories Nationwide (5-year probability)
3. Survey of rainfall countermeasure plan
enactment methods
In order to deal with the future increase of heavy
rain, it is becoming increasingly important toefficiently enact rainfall countermeasure plans bycombining several inundation countermeasures using
an unsteady flow model. But in many cases, because
objective judgment standards for setting the scale ofcountermeasure facilities have not been generallyintroduced, an unsteady flow model is only used as anevaluation tool to discover operation methods andtemporary countermeasures for facilities designed
applying previously used rational formulas.
Based on this situation, we collected informationabout cases of the enactment of rainfall
countermeasure plans for about 20 cities nationwide,
and are now abstracting problems with rainfall
countermeasure plan enactment methods and studying
improvement measures.
[Reference]
1) Ministry of Land, Infrastructure, Transport andTourism, Sewerage and Wastewater Management
Department website(http:/www.mlit.go.jp/mizukokudo/sewerage/crd_sew
erage_tk_000117.html)