Protecting People From Flooding
Improvements make the natural retarding basin eveeb safer
The Tsurumi River has long been known as a raging river prone to repetitive flooding, and the Kozukue/Toriyama region where the multipurpose retarding basin is located has always served as a natural retarding basin for the river. By upgrading its natural functions with some human engineering, we were able to provide protection against the dangers of flooding to the surrounding communities as well as the communities further down the river.

The multipurpose retarding basin upgrade is being promoted as a joint project with the City of Yokohama. Effectively utilizing the limited amount of land available in this densely populated city, the basin improvements enable the site to serve flood control functions during times of flooding and to be used as an athletic park during regular times.

A retarding basin project for fllod protection
The Tsurumi River Flood Control Plan sets as its target the safe discharge of the rains of a flood of a magnitude seen only once in 150 years, but as shown in the flow rate distribution chart below, the current improvements to the river basin aim to allow it to safely discharge the amount of rainfall that occurred in the largest postwar flood (from the Kanogawa Typhoon of 1958) at a rate of 200m3/sec.

How do the mechanisms of a retarding basin work to prevent water from building up and lfooding?

A retarding basin regulates the amount of water that is allowed to flow down a river during a flood.
The retarding basin that regulates flooding on the Tsurumi River is encompassed by a surrounding embankment, and by excavating its interior, engineers have been able to ensure that it can hold a certain volume of floodwater. This basin can hold a maximum of 3.9 billion liters of water.

Water flows in over the overflow type levee built along the Tsurumi River.
Because the overflow type livee along the Tsurumi River are built about 3 m lower than the separating levees, they allow floodwaters to flow into the retarding basin. By temporarily storing the water, the basin prevents the water from causing massive flooding in the downstream communities of the Tsurumi River basin.

When the flood is over, the stored water is gradually discharged back into the Tsurumi River.
When the flood has passed, the water is allowed to flow through the spillway back into the Tsurumi River.

Flood risk preparations, creating a basin that is safe for residentes
If the same amount of rain that fell during the Kanogawa Typhoon in September 1958 were to fall again today, it would likely flood an area of 664 ha and cause flood damage for about 90,000 people. The estimated value of the damage would be around \400 billion. Once the comprehensive flood control measures, which include the Tsurumi River Multipurpose Retarding Basin, are completed, damage from the inundation of river waters will become preventable.

*This simulation predicts what will happen when the river basin measures and improvements to the sewerage system are complete.
*The results will differ depending on these conditions.