Relationship between grain composition and debris flow characteristics: a case study of the Jiangjia Gully in China

Debris flow occurs frequently (ten events on average per year) and displays a great variety of properties in the Jiangjia Gully (JJG) in southwest China. We find that the material of debris flow satisfies a universal grain size distribution (GSD) of P (D) = CD(-mu)exp(-D/D-c), and the parameters mu and D-c are closely related to the dynamical properties such as flow density, velocity, and discharge. A small mu implies a small porosity and possible high excess pore pressure in flow, reflecting high mobility and capacity of transportation, and a large D-c means a wide range of grain composition and hence a high grain concentration. A debris flow always achieves a state of certain mobility and density that can be well described by mu and D-c, and the parameters impose power law constraints on the fluctuations of debris flow surges. An upper limit of unit-width discharge is estimated as Q(u) = 1.25 D-c 1.12. Variation of GSD parameters also describes material exchanges between debris flow and streambed sediment. Intense incision or deposition occurs when remarkable difference of grain composition exists. As the GSD is satisfied universally, the results derived from JJG are expected to be applicable for evaluating the properties and peak discharge of a potential debris flow in other conditions.