Based on soil samples from living debris flows in Jiangjia Gully and deposits of debris flows in various valleys and regions in China, a general form of grain size distribution,P(D) =CD^-μexp(-D/Dc),which fitted the soils of debris flows perfectly and universally was proposed.The values of μ and Dc were used to describe the nature of grain composition,where the power exponent represents some characteristic porosity of the soilin natural condition, and Dcdefines a characteristic scale for the range of grains.According to soil samples of debris flow from various sources,μ is less than 0.1,and for high density debris flow,μ is less than 0.05.The bulk density of debris flow increases with Dcin a power law. So the parameter Dc can depict the power of the transportation of debris flow. Finally, the parameters were used to evaluate the debris flows occurred in the Wenchuan earthquake area, which agrees well with the observations in Jiangjia Gully.
