The River and lts Basin Work Togeth er to Prevent Flooding
Functions as a roadway even during flooding
A main roadway was built in the retarding basin to connect the east, west, south and north sides of the basin. The road runs either along the top of an embankment or on an elevated roadway so that it can continue to be used even if water flows into the basin during a flood, with no negative impact on flood control capacity.

Environmental factors incorporated in embankment design
The embankments were designed to slope gently, not only for the wildlife that inhabit them, but to make them easy for local residents to enjoy. The Surrounding levee does not use a conventionally steep small-tier design, but the new more gradually sloped, one-layer-method.

IT improvements also planned
The multipurpose retarding basin has also been equipped with information devices that can detect and warn people about dangerous situations during a flood. River information display boards have been installed in three locations in the basin. They can be used to provide flood-related information to local residents quickly. Overflow conditions and other conditions within the basin are also monitored using closed-circuit TV cameras, water level sensors, and flow rate sensors. All of the cameras and sensors are networked using fiber-optic cable, and can monitor and convey information on flood threats in real time.

Tsurumi River Basin Information Center provides information on the entire river basin region
The national government, local governments, and civic groups have formed a partnership to create a jointly-managed river basin information center. The four-story information center building houses an exhibition area, classrooms, exchange lounge, and library, and can be used as a venue for exchanging information about the river basin and for local community exchanges, as well as for general study, environmental education, or disaster preparedness education activities.
Location
A 20-minute walk from JR Shin-Yokohama Station
A 5-minute walk from JR Kozukue Station

Flood of September 1982
 Urbanization heightens flooding danger
The Tsurumi River is a Class One river that flows from its source in Machida City, Tokyo through the Tama Hills, and down into Tokyo Bay at the river mouth in Tsurumi-ku, Yokohama. Rapid urbanization, along with the development of an arterial transportation network, has resulted in the elimination of rice fields and forests that naturally serve to hold rainwater and absorb it into the ground. There has thus been an increase in the amount of surface runoff flowing into the Tsurumi River, increasing the chances of flooding.
Main Runoff Records for Recent Years
Flood date Average two-day rainfall in the basin (mm) Water level (Sueyoshi Bridge) T.P. (m) Peak flow rate (Sueyoshi Bridge)
(m3/s) 		Comments Above ground flooding (buildings) Under ground flooding (buildings)
09/26/58
Kanogawa
Typhoon 		352.7 3.76 509 Flooding throughout the basin 16,991 48,766
06/28/66
Typoon No.4 		310.2 3.94 503 Flooding throughout the basin 15,422 51,741
09/09/76
Typoon No.17 		142 3.51 693 Flooding throughout the basin 1,343 3,481
10/22/81
Typoon No.24 		180.1 3.49 755 Flooding in areas without embankments 28 396
09/12/82
Typoon No.18 		218 4.02 1,051 Flooding in areas without embankments 1,491 4,056
09/18/91
Typoon No.18 		287.2 2.85 1,021 Inner water 59 52
08/20/94
Heavy rain front 		206.8 2.03 - Inner water - 11
09/21/96
Heavy rain front 		195.9 1.94 884 Inner water - -

River and River Basin Work Together
Comprehensive Flood Control Measures
The increase in floodwater flows that has accompanied rapid urbanization in this area is very difficult to contain through river improvements alone. Various measures are thus being taken to improve the water retention and retarding functions that allow rainfall to accumulate in and be absorbed by the river basin, and to develop communities that are highly resistant to flood damage. The Comprehensive Flood Control Measures represent an effort to deal with the river and the river basin as an integrated whole. The multipurpose retarding basin project is one of these efforts, and is being promoted as a major pillar of the river improvement project. To carry out the work that will make it possible for people to live safely in this area, the understanding and cooperation of each and every river basin resident is necessary.
Flood of September 1982

History of the Tsurumi River Multipurpose Retarding Basin Project
Jan 1985 Efforts to acquire land begin.
May 1889 Tsurumi River New River Basin Improvement Plan is formulated. (Sets the basic high water flow rate at Sueyoshi Bridge at 1,300 m3/s, and the planned high water flow rate at 1,050 m3/s.)
Jan 1993 Urban plan is decided (retarding basin, site parks, and urban planned roads).
Mar 1993 Parks projects are approved under the City Park Law.
Jan 1994 Groundbreaking ceremony (January 17). Site excavation works begins.
June 1994 Basic construction plan is revised. (Sets the basic high water flow rate at Sueyoshi Bridge at 2,600 m3/s, and the planned high water flow rate at 1,800 m3/s.)
Feb 1996 Construction on retarding basin perimeter embankment begins.
Mar 1996 Replacement work begins on supplemental roads (the Shin-Yokohama Motoishikawa Road and others).
Oct 1997 The International Stadium Yokohama is completed (work began Jan. 1994).
Oct 1998 Kanagawa National Sports Festival is held.
Mar 1999 Construction on Sawage gate begins.
Mar 2001 Construction on overflow type levee begins.
Mar 2002 Joint use of supplemental facilities (urban planned roads (Shin-Yokohama Motoishikawa Road and Ikuta Road)) begins.
Mar 2003 Use of Tsurumi River Multipurpose Retarding Basin scheduled to begin.

Tsurumi River Multipurpose Retarding Basin Specifications
Location: Kozukue-cho and Toriyama-cho in Kohoku-ku, Yokohama
Retarding basin area: 84 ha
Flood regulation volume: (Provisional channel) Retarding basin alone - 200 m3/s
(Planned channel) Retarding basin and improved water channel 700 m3/s
Total water storage capacity: 3,900,000m3
Planned high water level: T.P. + 8.573 m
Land use: Multipurpose use (e.g., parks)
Ground height in basin
North of Shin-Motoishikawa Road: T.P. + 2.5 m
South of Shin- Motoishikawa Road: T.P. + 4.0 m
Overflow type levee
Structure: Sides - steel mesh baskets, Top - asphalt facing
Height: T.P. + 6.6 m
Length:provisional 450 m planned 777 m
Separating levee
Height: T.P. + 9.773 m
Length:provisional 1,830 m planned 1,500 m
Surrounding levee Height: T.P. + 9.773 m
Length:provisional 2,260 m
Sewage gate
Location sTsurumi River right bank 13.6 km + 70 m
Height T.P. + 2.0 m
Normal drainage (conduit) B=1.8m ~ H=1.00m (slide gate)
Flood drainage (spillway) B=10.0m ~ H=7.77m (roller-type steel gate)
Supplemental facilities (roads)
Shin-Yokohama Motoishikawa Road: Elevated portion: B=32.0m L=728m
On-ground portion: B=32.0m L=975mIkuta Road:
Elevated portion: B=12.0m L=379m
On-ground portion: B=12.0m L=600m
Yokohama City Road No. 1 and 2: B=16.0m L=265m (140m, 125m)

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