河川における土砂環境と砂州地形特性--排砂バイパスダムの上下流区間における調査事例--

Abstract

We measured riverbed topography in the upstream and downstream reaches of Asahi and Koshibu dams with sediment bypass tunnel (SBT) using drone. Aerial photos of a gravel-bar were taken with >70% overlap between images, which were analyzed to generate DEM data by a stereo photogrammetry. Bed profiles were longitudinally smooth in upstream reaches with natural sediment supply, while they were rough with irregular and step-like forms in the downstream reaches with least sediment supply. Riffles were cross-sectionally flat in the upstream reaches, while they were incised in the downstream reaches. In contrast, pools were cross-sectionally confined in the upstream reaches, while they were relatively flat in the downstream reaches. These differences in topography can be explained by gravel-bar shape that varies according to sediment condition. Drone and image analysis are suggested to be an effective and time-saving method to evaluated changes in gravel-bar topography and related flow patterns.