Research Trends and Results

Efficient Inundation Countermeasures According toRegional Characteristics
OGAWA Fumiaki , Head; HASHIMOTO Tsubasa , Researcher
Wastewater System Division, Water Quality Control Department
Keywords: heavy rain, inundation countermeasure, regional characteristics
small-and-medium-size cities are promoting development
focused on main lines. The characteristics of storm sewer
development in big cities and small-and-medium-size
cities are summarized in the table below.

2. Damage by inner water inundation anddevelopment of storm sewers
An “inner water inundation” is a state wherein heavystormwater that falls on an urban area or farmland cannot
be drained into drainage facilities (sewerage, etc.) or
public waters, and inundates buildings, land, and roads.
From the inundation statistics 1) of 2001 to 2009, we
identified damage data for cases where no less than 50
structures were inundated by inner water inundations
above or below the floor level (“inner water inundationdamage”). We then organized the numbers of such cases
according to the population of the municipalities in which
they occurred. In addition, using the storm sewer
development area (FY2010) survey conducted by theMLIT, we calculated the ratio of developed area to thetotal area where storm sewers are planned ("ratio of stormsewer development"). The results of the calculation areindicated in the figure. In small-and-medium-size cities(population of less than 300,000), the ratio of storm sewer
development is smaller than that in big cities (populationof not less than 300,000), but the number of cases of
inner water inundation damage is small. In big cities
where a decline in stormwater storage penetration ismarked due to the progress of urbanization, area-wide
development of storm sewers—including branchlines—is required. In small-and-medium-size cities wherea certain level of stormwater storage penetration isexpected, the development of branch lines is notnecessarily required, therefore the selection andconcentration of target areas should be ensured. As aresult of analyzing the development of storm sewers fromFY2008-2010, for which a detailed explanation is omitted,
it was found that big cities are promoting area-wide
development of main and branch lines, while

人口規模*30万人～：政令指定都市と中核市を除く
Population size * 300,000-:
Excluding ordinance-designatedcities and core cities

2.570%
Number of cases of inner

内水被害件数water inundation damage
Ratio of storm sewer

雨水整備割合development

260%
50%

1.5

40%
30%

120%
0.5

～1万人
- 10,000

10,000-50,
1～5万人
000

50,000-100
5～10万人,0000%
100,000-30
10～30万人
0,000

city
Core中核市

Ordinance政令指定都市
designatedcity

030万人～
300,000-* *10%

Ratio of storm sewer development (%)
雨水整備割合（％）

Number of cases of inner water inundation damage per city (cases)
一都市あたりの内水被害件数（件）1. Introduction
In recent years, many regions of Japan have been hit
frequently by heavy rain with an hourly precipitation of
50 mm or more, or by concentrated heavy rain in a shortperiod of time such as 10 minutes. As the frequency ofheavy rains is increasing, concern is also increasing overthe decline in stormwater penetration due to the progressof urbanization, as well as the increase in inundation
damage potential due to the development of undergroundspace use. Cities need to advance urban stormwater
management measures efficiently and effectively within
their limited financial resources. In order to support
inundation countermeasures according to regional
characteristics, NILIM has organized various
characteristics, including inundation damage and thedevelopment of storm sewers, according to the size ofcities.

Figure: Number of Cases of Inner Water
Inundation Damage and Ratio of Storm Sewer
Development by Population Size
Table: Characteristics of Storm Sewer
Development by Population Size
TownscapeTrend of development
Inner water inundation
damageRatio of storm sewer
development
Necessity forstructural measuredevelopment

Big city (Population of notless than 300,000)

Small-and-medium-size city (Population of less than300,000)

Many urbanized areas areformed in area.
Development of main /branch lines in area

Town is formed around the terminal station (heavilybuilt-up areas are limited).
Concentrated on main lines (More selective /concentrated development than big cities)

Much

LittleHigh (51.1%)

Low (34.9%)

Development including
branch lines is necessary.

It is necessary to examine necessity for branch line
development while advancing monitoring andnon-structural measures (selection and concentration).

3. Inundation countermeasures in cooperation
with other (non-sewerage) projects
In order to promote efficient inundation
countermeasures in cooperation with non-sewerage
projects, we are collecting and organizing examples ofcases of such cooperation, including: effective use ofstormwater reservoirs for inundation control in rivers;
improvement of stormwater storage penetration usingparks, schools, space under elevated roads, and farmland;
support systems for development of stormwater storage
penetration facilities in development activities; and others.
In relation to this cooperation policy, we are going toevaluate their implementation methods and direct
solutions alongside regional characteristics such as
municipality population, urban area, suburban area, etc.
[Reference]
1) Inundation Statistics, 2001-2009, MLIT