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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 a 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. • An improved numerical model simulates multiple debris flows after earthquake. • The parameters that control the magnitude-frequency of debris flows chang over time after the earthquake. • The post-earthquake debris flow activity is impacted by source material depletion, grain size coarsening, and revegetation. [ABSTRACT FROM AUTHOR]

Empirically derived rainfall thresholds of debris flows are used for regional-scale early warning. However, triggering rainfall intensities of post-seismic debris flows evolve with time, causing high false alarms since thresholds estimated by conventional methods ignore the uncertainty of non-triggering rainfall events. Based on 172 triggering rainfalls and 2396 non-triggering rainfalls from 2008 to 2013 after the Wenchuan earthquake, we analyzed the evolution of probabilistic rainfall thresholds for post-seismic debris flows using a Bayesian technique. We found, rainfall thresholds significantly decrease compared with pre-earthquake events initially and later tend to increase annually. Meanwhile, the triggering rainfall characteristics tend to gradually change from a short-duration high-intensity pattern to a long-duration and low-intensity pattern. We also checked the effect of antecedent precipitation on debris flows by defining an IET (inter-event time). Our results suggest the antecedent precipitation plays an important role in low-intensity long-duration rainfall-induced debris flows and has little effect on the short-duration high-intensity rainfall-induced debris flows. The characteristics of triggering rainfall for debris flows after the Wenchuan earthquake can be best reflected by IET = 7 h. By employing a Naïve Bayes algorithm, uncertainties of three rainfall threshold models, I-D (mean intensity-duration), I T -D (triggering intensity-duration), and I T -E (triggering intensity-cumulative rainfall) were investigated, and the I T -D model was found to perform best. The characteristics and evolution of debris flow rainfall thresholds over the years after the Wenchuan earthquake was best presented by the Bayesian probabilistic rainfall thresholds. We believe the results can be used to improve the precision of an early warning system for post-seismic debris flows. • We present probabilistic rainfall thresholds for debris flows post-Wenchuan earthquake using a Bayesian technique. • Antecedent precipitation is significant to trigger debris flows by low-intensity long-duration rainfall. • Thresholds significantly decreased post-earthquake, then increased annually, not yet returned to the pre-earthquake level. [ABSTRACT FROM AUTHOR]