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