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JWA's Counter-Measures against Floods in Kyushu
(One Example of Disaster Prevention Operation at Terauchi Dam)

July 9, 2017

When torrential rains hit northern Kyushu for two days from July 5 and 6, 2017, Terauchi Dam Office located on Sata River, a tributary of Chikugo River running in northern Kyushu, made the disaster prevention operation to minimize the damages downstream.
This operation depicts one of the typical examples of disaster prevention measures Japan Water Agency takes at the time of flooding.

Due to the continuous rainfall, tit reached 106mm between 15:00 and 16:00 on July 5 at Hashidate point, upstream of the river. Due to this rainfall about 888m3/s water inflow was recorded at Terauchi Dam reservoir.
Against this inflow, disaster prevention operation was conducted, namely about 99% of inflow, i.e. 878m3/s of water was stored in the reservoir to lower the river water level downstream. In concrete terms, at Kanamaru Bridge Water Level Observation Station, the water level reached 3.5m but if no prevention operation was taken, about 3.38m higher water level should have been measured and much severe damages could have been caused.

Details of the operation are illustrated in the following drawings and pictures.

Disaster Prevention Operation at Terauchi Dam

Location of Terauchi Dam (Northern Kyushu)
Location map

Max. water level

*Released info is on a preliminary basis and the values used may change upon further survey in the future.

Effect of lowering water level downstream

Location map

The effect of storage of inflow water to the dam, resulting in the smaller amount of discharge to the downstream (Kanamaru Bridge observing station)

(1) In case of no dam

Estimated max. water level 6.88 m
*Some margin of error included
(In the case that the banks of Sata River is higher and no dam is available)

(2)Max. river water level thanks to the storage of inflow water to the dam

Observed max. water level 3.50 m

The effect of inflow water storage at the dam reservoir (1)-(2)

River water level at Kanamaru Bridge About 3.38 m lowered

*This is the preliminary figure, and may change later.

Cross-section of Kanamaru Bridge water level observing station

Cross-section of Kanamaru Bridge water level observing station

Water storage at Terauchi Dam under disaster prevention operation

Normal condition

Reservoir level
EL. 121.50m
Before the start of disaster prevention operation
(at 10:00 on July 5)

EL. 111.09m

* approx. 10m lower than normal
Disaster prevention operation underway
(at 8:00 on July 6)

EL. 130.93m

Reservoir level increased by approx. 20m
Storage of approx. 11,700K m³ of water

Woody debris trapping at Terauchi Dam reservoir (at 7:00am July 6, 2017)

Location of photo shooting