The changes of the formation of debris flows after Wenchuan Earthquake were analyzed, including loose materials, micro-topography and hydrological condition. Analysis suggested that the total of the loose mass generated by the collapse and landslides reaches about 28×10^8m^3, which provides abundant materials for debris flows for a long term. The change of landform and especially the blockage of channel will increase the scale of debris flow. The conditions of the surface water infiltration, runoff and flow concentration are changed by the strong surface disturbance and large-scale destructive vegetation damage, which is beneficial to the formation of the erosion and flood peak easier. The great changes of the river basin constitute the conditional combination of the debris flow formation. Therefore, the intensity of post-earthquake debris flow increases rapidly and debris flow becomes one of the most serious risk source. The main characteristics of debris flows after Wenchuan Earthquake were as follows. The number of debris flow gully increased, a large number of debris flows occurred in gullies which had no debris flows before the earthquake. The critical rainfall decreased significantly. The events occurred at high frequency and in group. The the density increased by about 10% to 30% and transitional or viscous flows occurred in many gullies of turbulent flows. The discharge increased by about 50%~100%, beyond the results calculated by the current methods. The tendency of debris flow is that the intensity will grow and then devay, alternative with active and quiet periods. The first active period will last for 15 years. The trigger and control factor depends mainly on the supplies of loose material. The valleys that have rich materials and more than 5 km^2 of area will be the most dangerous in the following years.Accordingly, we proposed the strategies for mitigation of debris flow, including discrimination of potential debris flow and specification of countermeasures; risk analysis and management; calculation of magnitude and regular monitoring and warning system.
