Because large quantities of loose deposits were accumulated in gully after “5•12” Earthquake

the occurrence probability of gully debris flow is enhanced greatly in rainfall. However

due to the special conditions of provenance and disaster environment

the traditional theory cannot be used to expound the mechanism of gully debris flow and prevent the disaster effectively as ever after the event. Here

based on hydrology

the hydrological model of ground water table in loose deposits is established first. Then by aid of hydraulic theory

the change of hydraulic characteristics and the variation of hydraulic power with the rising of groundwater level are analyzed. Furthermore

the calculation formulas of hydrodynamic and hydrostatic pressure are also proposed. Finally

by applying infinite slope theory in calculation of sliding force

anti-sliding force and residual sliding force acting on each segment

the failure mechanism of accumulation body is illustrated clearly with an example. The result indicates the failure of loose deposits in rainfall is the result of increase of the hydraulic power with the rising of groundwater level. Those deposits which have small coefficient

gentle slope

narrow-deep gully and large basin would be easy to fail because of its high hydraulic pressure generating in rainstorm

and the failure modes can be divided into two types according to residual sliding force as pushing failure and hauling failure.