RIVERBED VARIATION AROUND A WEIR IN A MOUNTAINOUS RIVER AND CONTROL OF SEDIMENTATION IN A FISHWAY

Abstract

The purposes of this study are to clarify observationally riverbed variation around a weir in a mountainous river and sedimentation in a fishway attached it during a fairly large flood and to grasp the influence of flood flow over the weir on sediment flush in the fishway. Both of the ordinary surveying by a total station and the photographic surveying from UAV (Unmanned Aerial Vehicles) by using image analysis softwares were carried out in order to estimate topographical changes in the riverbed and in the fishway deposits. Riverbed degradation occurred in the upstream reach from the weir, which became severer in the upstream direction, implying insufficient sediment supply from the upper reach to an equilibrium riverbed condition here while changes in riverbed topography is slight near the weir. During the flood, sediment deposits in pools of fishway were flushed away in the upstream ones while increased in several ones in the downstream part, indicating somewhat complicated influence of the flood flow on the flushing action in the fishway pools. A 2D shallow water analysis was applied to clarify the influence, and flood flow down the weir reproduction calculation of the analysis, it is pointed out the mutual influence between a flood flow and an overflowing water in the fishway.