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

导流堰的堰基一般高于河底水位，以保持水位。因此，导流堰下游的河床被冲刷得非常厉害。这种冲刷在高水位时间歇性地发生，会造成建筑物和河岸的泥沙。拦河坝是由几个可控闸门组成的导流坝。可控闸门的门槛几乎可以调整到河床，因为进水水位是由关闭的闸门保持的。冲刷带来的麻烦是可以预防的。如今，日本新建的防波堤通常会根据规划的河床高度进行调整。在日本的大多数河流中，这一海拔比现在的河床低约2米。为了让更大的泄洪流量，设计了较大的横截面区。分析了这些拦河坝的问题，例如伊比河的冈岛拦河坝和Watarase河的太田拦河堤。当建造新的拦河闸时，拦河闸附近的河床被疏浚到基床的高程。由于疏浚区域往往太窄，洪水很容易将沙子和砾石沉积在堰塞坝的基床上。可控闸不能关，水不能引，为了不让洪水后的堤坝被埋，每年汛前都要把堰塞区周围的大片河床，通过疏浚、开挖，保持到堤坎高程。

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.