The initiation of debris flow at high slopes: experimental results

Debris deposit laying over an impermeable high slope surface (e.g. in steep streams and gullies) could develop a water sediment mixture flow (usually known as debris flow) when a high enough water flow is flowing over the sediment bed. In particular the rainfall first saturates the debris deposit and then mobilizes it by an overland flow. The mechanisms which make unstable the granular material layer in such a case were studied by experimental tests performed in a tilting flume filled with uniformly distributed granular material. The experimental results are compared with theoretical relationships on the occurrence of debris flow and with experimental findings pertinent to sediment transport phenomena. The material employed in the experiments was nearly uniform gravel of three different sizes with mean diameters respectively equal to d = 0.023, 0.029 and 0.034 m.