Variation in Grain Size Distribution in Debris Flow

Grain composition of debris flow varies considerably from fluid to deposit, making it uncertain to estimate flow properties （e.g., density, velocity and discharge） using deposit as done in practice. Tracing the variation of grain composition is thus more important than estimating some certain properties of flow because every debris flow event consists of a series of surges that are distinct in properties and flow regimes. We find that the materials of debris flows, both the fluid and the source soils, satisfy a universal grain size distribution （GSD） in a form of P （D） = CD-zexp（-D/Dc）, where the parameters C, p and De are determined by fitting the function to the grain size frequency. A small At implies a small porosity and possible high excess pore pressure in flow; and a large D~ means a wide range of grain composition and hence a high sediment concentration. Flow density increases as 11 decreases or Dc increases, in a power law form. A debris flow always achieves a state of certain mobility and density that can be well described by the coupling of p and Dc, which imposes a constraint on the fluctuations of flow surges. The GSD also describes the changes in grain composition in that it is always satisfied during the course of debris flow developing. Numerical simulation using the GSD can well illustrate the variation ofμ and Dc from source soils to deposits.