Research Trend & Results

Margins of forecasting the future increase in torrential rains
controlling adaptation to climate change
ITAGAKI Osam, Senior Researcher
KATO Takuma (Ph.D., Engineering), Researcher
Water Management and Dam Division, River Department
HATTORI Atsushi (Ph.D., Engineering), HeadRiver Division, River Department
YOSHITANI Junichi (Ph.D., Engineering), Research Coordinator for Watershed Management
FUJITA Kouichi, Director (Ph.D., Engineering)
River Department(Keywords)

climate change, torrential rains, flood control projects, increase rate, sensitivity analysis
many times of increase in insufficient cross section offlow in a river course necessary for safety flow down of
the water volume equivalent to the target level in thefuture compared to the present”. (See the increase rate offlow, increase rate of probability of flooding, and increase
rate of river development efforts in the figure.) 2) It isfound that as for the national average, the torrential rains
will increase 1.16 times, the river development efforts
1.88 times, and the probability of floods 2.87 times
greater than the present. In other words, it means even if
river development meeting the present target flow level iscompleted, the probability of flooding will increase 2.87
times, and required efforts will be 1.88 times to prevent
this. This resulted in that only about 0.16 times ofincrease in torrential rains leads to multiplying theprobability of flooding and efforts for river development
and therefore future situation may change drastically even
if the increase rate of torrential rains is within the marginstated before.
3. Measures supposing the future with margins
Thus, future forecasting is not easy. It will be required
to expand the practicable measures both in terms ofsoftware and hardware free from the limitation of existingmeasures for flood control by entering on the basin ofrivers. Moreover, it is important to appropriately use andcombine various measures so that their effects are
maximized, and reach the way of strategy planningwithout having regrets on the assumption the future is
uncertain even if prediction is wrong. In the River
Department, we are now examining for solving the issues
while analyzing flooding risks so that specific damagesby flooding for each river system and the effects ofmeasures can be evaluated.
<Reference> 1) MEXT: Innovative Program of Climate ChangeProjection for the 21st Century “Projection of the change in future
weather extreme using super-high-resolution atmospheric models, 2011
Study report, 2012. 2)Mr. Hattori and others: Macro-evaluation ofclimate change impact on flood control measures in Japan, Advances inriver engineering, Vol.18, 2012)

Increase rate of each item(Future)

1. General condition of river development for flooding
prevention and climate change
Even the safety level of flood control of the average
first-class rivers in Japan has been improved byaccumulated river development, it has not yet meet atarget level indicated in a river development policy. On
the other hand, land use is becoming densely therefore
even the same scale of flooding may cause furtherdamages. Furthermore, it is pointed out that torrential
rains may increase due to global warming. If torrential
rains occur more frequently while river development ismaking progress, it is easily supposed that the safety levelof flood control will not improve as before. Therefore, it
is required to think about how to adapt to the situation
while forecasting the increase in torrential rains in thefuture and enforce measures strategically. In that case,there is a margin in the forecast predicting a degree ofincrease in torrential rains, and it is being questioned how
to deal with them while looking at the margin (and thelarger margins just in case). In the River Department, thedivisions relating to flood damages are cooperating witheach other for addressing this issue. As a first step,
calculations using the latest forecast for climate change
were made concerning the degree of increase in torrential
rains and the rate of increase in the possibility of flooding
arising from it, and the scale of additional efforts for riverdevelopment in case of setting the equivalent target levelto the present level to improve the possibility. At thecalculation, we studied how much a future vision could
be changed when the rate of increase in torrential rainschanges a little, by giving the forecast results of torrential
rains some margins
2. Impacts of increase in torrential rains on floodcontrol
We calculated the increase rate of torrential rains
showing how much the largest yearly amount of rainfall
used for river development will increase in the future [atthe end of 21st century] compared to the present,
targeting the directly controlled 109 river systems basedon the four forecasts1) on future climate change. Looking
at the average of all river systems, forecast results are
ranging from 1.12 to 1.27 on average and a difference(confidence interval 95%) between the maximum and theminimum forecast is ranging from 0.02 to 0.11. Together
with such increase in torrential rains, the flow equivalent
to the target level will also increase. As the result, wecalculated “how many times of increase in the probabilityof flooding exceeding the river flow equivalent to a target
level in the future compared to the present” and “howGCM20 early period
GCM20 late period
RCM5 early period
RCM5 late period
Average of 4 climate models

HighestAverage
LowestFuture
When the increase rate of torrential rainsis large, the increase rate of other factors
is amplified.
豪雨量倍率が大きい場合、This trend
is applied especially to RCM
増幅率が増して伝播する。
late period
RCM後期は特にその傾向にある。

Whenthe margin of confidence interval
豪雨量倍率の信頼区間の幅
for the increase rate of torrential rains isが小さいほど、他の指標への*Values show the average of four climate models. smaller,that for other indicators変換で信頼区間の幅が小さい。

becomes smaller.

Increase rate of
torrential rains

Increase rate offlow peak

Increase rate of riverdevelopment
efforts

Increase rate ofprobability offlooding

Figure Impacts of torrential rains increase on indicators