天竜川下流域における濁水の量的・質的変化と砂州地形特性の関係について

Abstract

Long-term persistence of turbid water has become an environmental issue in downstream reaches in a lot of dams. Turbid water causes degradation of river landscape, load of water treatment process, burden on the hydraulic power plant and degradation of biodiversity. In this paper, we focus on sandbars as a place of purification of the turbid water. We evaluated the influence of sandbars on qualitative and quantitative change of turbid water in the Tenryu river. Regarding quantitative change of turbid water, turbidity in the river water at downstream of the sandbar showed a 52.8% reduction by drifting through the survey section of 23.7km in total length. However, the concentration of suspended solids in the river water was not significantly reduced, and fluctuated between 3.4 and 5.0 mg L-1. As for qualititative change, proportion of dam POM in SFPOM at downstream of sandbar declined as downstream and varied between 0.01 and 0.94. The reduction rate slope of Dam POM was higher in the double row bars section than the point bars section. Therefore, it is necessary to evaluate the turbid water purification function with attention to various characteristic values of sandbar.