Biwako - Lake and Rivers

2. Measures at Biwako

2.3 Flood control at Seta River Weir

2.3.1 General water-level control (Target peak throughput in Yodo River system)

2.3.2 Lake water-level adjustment to delay upstream weir discharge during downstream peak flow

In accordance with an ordinance dated March 1992, during discharge, the water in Biwako is normally maintained at a level within 300 mm above B.S.L. (standard Biwa surface level). When there is a perceived risk of flooding, in anticipation of the need to stop discharge during downstream peak flow, water level may be decreased to -200 mm or -300 mm B.S.L. Furthermore, in times of drought, as part of a system including reservoirs along the Kizu River and Katsura River, to supply the household, industrial, and agricultural water needed by downstream users, the water level may be decreased by as much as -1,500 mm B.S.L. to supplement the flow of the Yodo River.

2.3.3 Operating the weir to adjust flow according to differences in downstream and upstream peak water levels

The risk of flooding along the mainstream of the Yodo River increases mainly when rain brought by typhoons increases the flow in one or more of the major upstream rivers: that is, the Uji River, Kizu River, or Katsura River. Flood flow from Kizu River is a particular concern. If the level of Biwako is allowed to rise to its maximum level, it gives time for the Yodo River flow to start to drop from the peak level. This typically takes about a day. Knowledge of this characteristic can be used at the weir and flooding of both Biwako and the Yodo River is managed.

Following rain, the water levels in the Uji River, Kizu River, Katsura River, and then the Yodo River begin to rise.
While the Biwako water level has hardly risen, the downstream flow is reaching its peak. Consequently, the weir restricts the outflow of water.
The flow downstream starts decreasing. Meanwhile the level of Biwako has been rising, so the weir gates are lowered to reduce the Biwako water level.

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