The distributions and reserves of the loosen materials in debris flow valley takes great effects to the movement of debris flow, especially the ones will move toward downstream as debris flow by heavy rainfall. This paper takes Guojuanyan gully, located in Hongkou village, Du jiangyan city, as an example, through several times field investigations, making a distinguishing of loosen solid materials in the watershed. Then the reserves of the loosen materials had been calculated by the rupture of arcs method and the estimation model of critical accumulation rate. The comparison of the calculated results and minimum requirement of debris flow in this gully indicated that there were abundant of loosen materials lied in Guojuanyan gully and it was located at unstable state, the probability of arousing debris flow after heavy rain is very high and it maybe lasted for a long time. Therefore, this discrimination and calculation method could provide references to the layout of native engineering constructions as well as the forecast and defense of debris flow.
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