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.
Effects of sediment bypass tunnels on sediment grain size distribution and benthic habitats
Four reservoirs in Japan and Switzerland with sediment bypass tunnels (SBT) as strategy against sedimentation were monitored to analyze their effects in terms of upto downstream morphological and biotic changes. Sediment grain size distribution (GSD), local bed characteristics, microhabitat abundance and invertebrate richness were analyzed. It was found that GSD at reservoirs with newly established SBTs are fine in the upand coarse in their downstream waterway due to lack of conveyed sediments in the past. Analysis of biotic data directly below the dams reveal that microhabitat richness is low and lentic species abundance is high compared to upstream, while these differences decrease further downstream. Microhabitat and invertebrate richness in the downstream adjust to the upstream values with increasing operation years clearly showing the positive effects of long-term SBT operation.
Sediment Transport Mechanism and Grain Size Distributions in Stony Bed
The riverbed elevation of stony-bed rivers is controlled by boulders and cobbles. The authors have determined the mechanism of riverbed variation, characteristic grain size groups, and riverbed stability in stony-bed rivers from the results of field experiments on straight channels conducted in the Joganji River . In this study, we analyzed the results of field experiments using a meandering channel of the Joganji River. We examined characteristic grain-size distribution in meandering rivers with boulders and cobble and effects of meander and bank revetment on riverbed elevation and grain size distribution. Furthermore, we determined dimensionless physical parameter governing the forming of stable crosssections in stony-bed rivers.