Abstract
Many landslides occurred on mountain slopes of the Umi River basin in Fukuoka Prefecture, because of heavy rains on July 19, 2003. Most of the landslides changed into the debris flows and then moved down the mountain valleys. They caused significant sediment deposition at the check dams and then changed into the flood flows in the downstream reach. As a result many houses and roads were inundated along the river. In order to measure the disaster we have to predict the hydrograph of the debris and flood flows.In the present study we develop a runoff analysis method of the debris flows produced by many slope failures and investigate the effect of the slope failures on their hydrograph. Firstly, the river basin is divided into two parts; one is mountain slopes and the other river channels. Secondly, a kinematic wave method is used for the runoff analysis from the mountain slopes. This method considers sediment erosion on the slopes without landslides and rapid sediment supply on the slopes with landslides. Finally, a dynamic wave method is also used for the runoff analysis from the river channels.We find that slope failures increase peak discharge of the debris flows. However little dependence of sediment runoff on the slope failures can be found in the downstream reach.
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