Satellite images were used to collect data on the confluences in Taiwan with considerable deposition for identifying river topography characteristics for different levels of river blockage. To define the levels of river
blockage, 113 events from 2008 to 2015 were analyzed using a geographic information system to extract topographic
factors, such as river width, slope, confluence angle, elevation, and watershed area. The results revealed that river width ratio and level of river blockage shared a significant, positive correlation, and river blockage occurs where the confluence angle approaches 90°. On the basis of these blockage characteristics, we forecast the potential debris flow torrent and potential areas for natural dams; these forecasts can be used to prevent debris flows into confluences during disasters.
Key Words: Tributary confluence, natural dam, level of river blockage