A scaling distribution for grain composition of debris flow

Debris flow is composed of a wide range of grains. This study proposes a general form of grain size distribution, P (D) = CD−μexp (−D / Dc), which is satisfied well by various debris flows and by soils and sediments related to debris flows. The parameters μ and Dc are found to be related to debris-flow density in power laws. In particular, μ represents some characteristic porosity of soil in a natural condition and controls the variation of soils in developing debris flows; and Dc defines a characteristic size governing the sediment concentration. Field observations indicate that debris flows fall into a certain range of parameters (μ, Dc). Almost all debris flows have μ < 0.10, and most debris flows of high density have μ < 0.05. Moreover, experiments show that the exponent μ increases during soil failures under rainfall, providing an index varying in the course of debris flow initiation. Finally, grain size distribution is used to evaluate the properties of debris flows in different regions. The distribution provides a simple but quantitative method of predicting a potential flow through the source soils.