Modelling the evolution of debris flows after the 2008 Wenchuan earthquake

Strong earthquakes can trigger extensive landslides, generating abundant loose deposits that are prone to be remobilized as debris flows by subsequent rainfall events. The magnitude-frequency distribution of post earthquake debris flows often changes significantly over time. However, the key parameters controlling the post-earthquake debris flow activity and magnitude were poorly understood. In this study, we selected representative catchment in the Wenchuan earthquake-hit region to study the activity of debris flows from 2008 to 2020, as well as the changes in parameters due to material depletion, grain size coarsening and vegetation recovery. Samples were collected from the field in different years after the earthquake and were tested in the laboratory to obtain their geotechnical properties. A numerical model was improved by considering the changing properties to fit better to dynamic conditions after the earthquake. We found that the particle size of the loose material gradually coarsened, the shear strength showed an increasing trend, while the erodibility coefficient of loose material showed a decreasing trend. The evolution of these parameters controlled the magnitude-frequency of post-earthquake debris flows and also changed their behavior from slope failures in the first few years after the earthquake to concentrated surface runoff erosion (channel erosion) in later years. The results have an important implication for risk assessment and prediction of future debris flows in earthquake-prone regions.