Research Trends and Results

Promotion of Effective Inundation Countermeasures toPrepare for the 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

1. Introduction
In recent years, heavy rainfalls of 50 mm/h or more
and heavy rainfalls concentrated into short time spansof 10 minutes have occurred more frequently
throughout Japan. Although various cities have beendeveloping inundation countermeasures to prevent
inundation damage, looking at the long term, thecharacteristics of rainfall are changing, and existing
inundation prevention measures alone might not beable to handle the future increase of heavy rain.
Accordingly, the NILIM is studying thecountermeasures for the increase of heavy rain and isextracting problems on making the rainfall
countermeasure plan, considering all the above andimprovement measures.
2. Countermeasures to deal with the increase of heavyrain
In the result for the past year 1), 10-minute rainfall
intensity and 60-minute rainfall intensity of 5 or 10year probability are likely to increase, and theincreasing rates are estimated 1.3 or 1.4 times in 50years at most. Based on this result, we have made out
patterns about the rainfall scenario in the following
table with the view point of the effected appearance ofinundation countermeasures and practice potentialities.
Peak cut type countermeasures such as storm water
storage pipes and storm water reservoirs for flood
control, and countermeasures to increase the discharge
capacity such as supplementary pipes and by-passpipes were evaluated as being effective in every
Rainfall scenario. To the Rainfall scenario A, mutual
connections of pipe channels and replacing street inlet
covers with grating covers were also evaluated as
being effective. To the Rainfall scenario B, making
networks of large scale trunk sewers, newconstructions and additional constructions of storm
water pumps were evaluated as being effective.
Furthermore, we have made a simulation using two

virtual discharge areas of different discharge
dimensions in order to classify quantitatively thecontents of the inundation countermeasures pursuant to
the inundation caused in respective areas. It hasrevealed that peak cut type reservoir facilities are
effective to branch discharge areas and short time
heavy rain. To the heavy rain continuing over a long
period in trunk line discharge areas, it has revealed that
it is important to combine the countermeasure facilities
such as flow down facilities and reservoir facilities inorder to upgrade the whole discharge area.
3. Problems and improvements in making methods ofrainfall countermeasure plans
To study the combination of plural inundation
countermeasures to meet the future increase of heavy
rain, we consider that there will be increased need for
unsteady flow models. Accordingly we have
implemented the query survey and the like aiming at
24 local governments so as to file up the application
cases of the unsteady flow model and its merits anddemerits. We have also made a proposition on mostappropriate selection method of design in accordancewith the countermeasure study with the target of themethod using rational formulas and the method using
unsteady flow analysis models.
The error of the flow items in calibration such as total
flow rate, peak flow rate and waveform are one of thepoints of concern on using the unsteady flow models.
From now, we have scheduled to file up quantitatively
effects on the scale of the facility plan andcountermeasure effect caused by the error of the flow
items in calibration by means of the simulation using
the data of actual discharge area.
【Reference】
1) TECHNICAL NOTE of NILIM No.654 pp.21-36
http://www.nilim.go.jp/lab/bcg/siryou/tnn/tnn0654.htm

Table Setting of Rainfall scenarios
Objective rainfall
intensity

10 minutes
60 minutes

Characteristics of the rainfall overschemes

Assumed rainfallAssumed main inundation causes

Rainfall scenario A
Over scheme
Within scheme
Short rainfall continuation
Strong 10-minute rainfall intensity
Small total rainfall volume
Limited area rainfall

Rainfall scenario BRainfall scenario C
Within scheme
Over scheme
Over scheme
Over scheme Long rainfall continuation
 Long rainfall continuation Strong 60-minute rainfall intensity
 Strong 10- and 60-minute rainfall intensity
 Large total rainfall volume
 Large total rainfall volume
 Large area rainfall Large area rainfall
Heavy rain that passes over current sewerageFocused heavy rain such as the largest rainfall as
Local and short time focused heavy rain construction standard (60 minutes rain volume) ever that largely passes over current sewerage(such as typhoon).
construction standard
Flow down capacity shortage in the whole
Flow down capacity shortage of branch
Flow down capacity shortage of trunk sewers discharge area composed of trunk sewers andsewers
branch sewers