Numerous landslides occurred owing to the Wenchuan earthquake

leading to the accumulation of loose solid materials on valleys and slopes that trigger debris flows. The debris flows induced by rainfalls oc-curred frequently due to the presence of the wide-grade-unconsolidated soil. The loose soils that were dis-turbed intensively during the earthquake and mixed with rainfall after the earthquake became highly unstable and resulted in debris flows easily under storm rainfalls. It was found through the experiments of the infiltra-tion and erosion that the rate of the infiltration of the post-earthquake

mixed-grain-sized

unconsolidated soil was 1.5~6.0 times those of the similar soils in Jiangjia Gully

and resulting in the loss of finer particles in the soil and eventually the destruction of soil structure. The results of anti-scourability experiments show that the anti-scourability of the unconsolidated soil was weak due to the intensive disturbance it experienced

resulting in disintegration of soil agreggates and the soil was then eroded and transported away by surface runoff. The frequent occurrence of debris flows might be attributed to the strong rainfall along with the un-consolidated state of the soil. This phenomenon could keep occurring until the soil structure return to the o-riginal status before the earthquake. The critical rainfall for debris flow initiation may also change during this period.