Analysis of the Conditions of Blocking Rivers Formed by Tributary Alluvial Fans

The streams in Taiwan are characterized by steep slopes and fragile geology. Debris flows occur during the rainfall season and typhoon events in mountainous creeks. When debris flows in the tributary enter into the main stream, abundant sediment deposits at confluences could form alluvial fans. When the alluvial fans continue to expand, the river blockage may occur. Once the natural dam broke, it would cause serious flood disasters downstream. Therefore, the conditions for river blockage formation need to be analysed for disaster prevention. This study used the satellite images and Digital elevation model (DEM) to collect the types of alluvial fans which induce river blockage in Taiwan. We analysed the ratio of the channel width and slope (tributary to main channel), and the confluence angle to find out the conditions that are prone to river blockage. The results show that the river blockage is more likely to occur in places where the tributaries are steep, the confluence angle is 90 degrees, the main stream flow is small, and the channel width ratio is similar.

Keywords: Alluvial fan, Tributary confluence, Debris flow, River blockage