Occurrence and Flow Mechanism of Flashflood

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Debris flow is a major cause of sediment related hazards in mountain streams. Typical debris flow where sediment grains are evenly dispersed throughout the entire depth has been studied the most. The criteria for identifying debris flow prone torrents are based on the research results from typical debris flow. Debris flow that is not typical, so called immature debris flow where a layr of water is seen near the surface may occur and cause hazards smilar to those of the typical debris flow. Here we call the immature debris flow a flashflood. We think flashfloods more commonly cause debris flow hazards than the typical debris flow. Some examples of flashflood disasters are introduced first. The occurrence conditions and flow mechanisms are studied through flume experiments. The domain of the occurrence of the flashflood coincides with that of immature debris flow or of the intermediate flow happening between bedload and debris flow. Next, the change of flow when the flashflood arrives at the channel with a reduced gradient is also observed through flume experiments. The change of overall sediment concentraion is measured. The results are useful and in fact necessary for building simulation models of depositional processes of debris flow on alluvial fans.