MONITORING AND EVALUATION OF SEDIMENTOLOGY ON RESERVOIR TO ESTUARINE TIDAL FLAT FOR ASSESSMENT OF DAM REMOVAL EFFECTS

Abstract

The Arase Dam on the Kumagawa River in Kumamoto Pref., Japan, has been subsumed under removal project as the first high dam removal in Japan, - a process slated to influence the downstream sedimentology. During the first stage of the removal, in April 2010, the dam’s gates were opened. In this study, we investigated sediment characteristics on the river and tidal flat and estimated the changes in sediment outflow using a one-dimensional uniform bed load simulation. A remarkable level of sediment coarsening was identified on the surface of the tidal flat. The deposition rate on the tidal flat, which was estimated by vertical profiles of sediment size distribution and bed load flux, corresponded to the opening operation of the dam’s gate. The bed load simulation of the Arase Dam’s reservoir shows that the gate opening could increase the sediment outflow.