Interior Drainage Flood Control_03

The details of South URAWA Relief Sewer Second are in the following pictures1)2).


It is necessary to avoid other utility lines while not causing damage to structures such as bridge foundations in the downtown.
This leads the relief sewers to be placed deeper and deeper and the costs higher and higher.

1) http://www.city.saitama.jp/www/contents/1189057832553/files/minamiku1.pdf
2) Construction Works of South URAWA Relief Sewer Second, Brochure, Saitama City & Kajima Corporation

Interior Drainage Flood Control_02

In most cases, runoff needs to be drained to rivers by sewers.
In some, it is drained to lakes and seas.
Urban rivers lack flow capacity to accommodate all the urban runoff in strong rainfalls.
Urbanization has caused runoff increase and difficulty in expansion of capacity.
Many bridges with heavy traffic of cars and trains are already over the rivers.
Many houses and buildings are along the rivers1).

Those existing structures have to be relocated for expansion works.
It is almost impossible as negotiation with property owners is very difficult.
There is no open land for detention ponds.
Therefore, relief sewers are needed to detain runoff under the roads although costly.
The roads are only space available for runoff detention.
Under this situation, Saitama City constructed and completed a relief sewer in 2009.
It is called South URAWA Relief Sewer Second.
It has a capacity of 40,000m3 for storage.
It is 13 to 23m below ground, 4.75m across, and 2,260m long.
It runs busy downtown like this picture2).

1) http://www.yamaiko.com/touring/20070204.html
2) Construction Works of South URAWA Relief Sewer Second, Brochure, Saitama City & Kajima Corporation

Interior Drainage Flood Control_01

Urbanization has caused runoff increase.
The pictures show progress of urbanization at Westside of Omiya Station1).


In Saitama city, the runoff ratio has risen from 40% around 50’s to 60% in 2007.
In addition, climate change or urban heat island may have increased weather extremes.
Extreme rainfall events of 50mm/hr or over have increased in Saitama City.
In 10-year period from 1997 to 2006, the number was nine.
Out of this nine, four events were observed in 2005 & 2006 alone1).


Urbanization and extreme rainfalls have been raising the vulnerability to flood2).


1) http://www.city.saitama.jp/www/contents/1205126406056/files/keikaku01.pdf
2) http://www.city.saitama.jp/www/contents/1205126406056/files/keikaku03.pdf