旭ダム排砂バイパストンネルによる石礫の排出機構および下流河道の瀬淵の回復;旭ダム排砂バイパストンネルによる石礫の排出機構および下流河道の瀬淵の回復;Study on Flushing Mechanism of DAM Reservoir Sedimentation and Recovery of Riffle-Pool in Downstream Reach by a Flushing Bypass Tunnel
In the Asahi River, riffles and pools had been scaled down in the downstream reach of Asahi Dam due to sediment deposition in Asahi Dam reservoir. To improve this environmental problem, a flushing sediment tunnel was constructed so as to bypass Asahi Dam reservoir. In this study, first, the authors demonstrated the recovery of riffles and pools and the grain size distribution in the downstream reach by using the data of cross-sectional surveying, air photographs and field investigation. Next, authors’ two-dimensional model was applied to a series of flood events in the Asahi River. As a result, the flushing mechanism of Asahi Dam reservoir sedimentation, the recovery process of riffles and pools and the change in grain size distribution in the downstream reach were clarified by the model.
ANALYSIS OF TWO-DIMENSIONAL RIVERBED VARIATION IN CHANNELS WITH STONY BEDS
In a stony-bed river with wide grain-size distribution, boulders that would not move during a flood act as strong fluid-resistance element. In addition, sand and gravel often remain in the bed irregularities formed by large stones. A stony-bed river will remain in a static equilibrium condition when the river bed surface grain size distributions corresponding to tractive force of the flow are reached. However, conventional riverbed variation analysis does not account for these essential mechanisms in stonybed rivers. Therefore, we had developed a one-dimensional riverbed variation analysis method by considering the mechanism of riverbed variation in stony bed rivers. This method is generally capable of explaining water surface profile, quantitative bed variation, and bed material size distribution in stony-bed rivers. In this study, we developed the method of two-dimensional riverbed variation analysis by considering the mechanism of riverbed variation and bank erosion in stony bed rivers. We verified the applicability of the model with the field experimental results which was conducted in the meandering channel in the Jyoganji River.
THE ROLE OF STONES ON THE RIVER BED STABILITY IN STONY BED RIVERS
River cross sectional form had settled in the dynamic equilibrium form at the relationship between discharge, the river bed slope and bed material when river improvement is not progressing so much. But, river channels do not seem to satisfy the equilibrium condition of sediment transport due to river improvement work in progress. As the result, many problems have arisen on river management. Sediment transport became discontinuous by the dam and removing large size of bed materials as stones and gravel from rivers is mentioned. In this study, we describe the bed stabilization mechanism of the stony bed rivers found by the field experiment of the Joganji River and the difference in sediment transport mechanism of the stony bed river and the gravel bed river. Moreover, we insist on the problem on washing out of dam reservoir sediment and the necessity for the new bed variation analysis method in the stony bed rivers.