Troubles of Accumulated Sand aroud Diversion Dams to Adjust Sill Elevation to That of Planed Rever Bed

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The sill of the diversion weirs is usually higher than the level of river bottom to keep water level. So, the river bed downstream of the diversion weirs is extremely scoured. This scour occurs intermittently at high water, and can cause trobles of structures and river banks.A barrage is the diversion dam which consists of several controllable gates. The sill of the controllable gates can be adjusted nearly to the river bed, because the intake water level is kept by the closed gates. Troubles from scour can be prevented.Nowadays, the sill of newly constructed barrages in Japan is commonly adjusted to the elevation of planned river bed. This elevation is lower than the present river bed by about 2 meters in most japanese rivers. Large cross-sectional areas are designed to let larger dischage flow than the present.The problems with these barrages, for example, the Okajima barrage in the Ibi River and the Ota barrage in the Watarase River, have been analyzed. When a new barrage is constructed, river bed near the barrage is dredged to the elevation of the sill. Since the dredging area is often too narrow, a flood flow easily deposits sands and gravels on the sill of the barrage. Controllable-gates cannot be closed, and water cannot be diverted.In order that the sill will not be burried after flood, we have to keep river bed in vast area around the barrage to the sill elevation by dredging and mining before every flood season.