IMPACT OF RIVER BANK STRUCTURE ON SUBSTRATE CONDITIONS IN POOL-RUN SEQUENCE AND ITS FORMING MECHANISM

Abstract

Pool, where the depth is considerable and the velocity is moderate, is important for fish. Especially, riverbed materials are used for refuge of aquatic creatures. Riverbed material in run is also important for spawning. However, the forming mechanism of substrate condition in pool-run area has not been clarified yet. Through field measurements, it was clarified that riverbed material structure in pool depends on the river bank structure. Especially, in a scoured pool in front of the foot protection of the bank, cobble bed which is suitable for refuge is formed and gravels and sand deposit at the inner side of cobble. The forming mechanism of this sorting structure is as follows. In flood period cobble, gravel and sand deposit in pool. Then in descending period, when the dry riverbed appears above the water surface, gravel and sand in pool is washed out by the strong secondary flow. This mechanism could be validated by the flume experiment. Moreover, riverbed material structure in run is also affected by the sediment movement in pool. The sorting structure should be preserved by protecting in front of foot protection.