排砂バイパスの運用とダム下流環境の変化 --河床粗度と流水・止水ハビタットの多さに着目して--

Abstract

Effects of sediment bypass tunnel (SBT) on environmental recovery of the degraded channels below dams, which were suggested by Awazu et al. (2015), were further examined by aerial photos using a drone. Coarse bed materials such as boulders were more represented below than above the Koshibu Dam, where SBT had not been in operation yet. The coarse bed materials protruded into the water column or formed steps, which increased slow-flow areas below the Koshibu Dam. On the other hand, sand, gravel, and cobbles dominated below as in above the Asahi Dam, where SBT had been operated for more than 17 years. Although bed characteristics below Asahi Dam seem to have almost recovered to a pre-dam state, less representation of large cobbles and boulders may suggest limitations of SBT system to transport downstream full sizes of sediment especially for the coarsest class of the bedload materials.