Changes in Debris-Flow Susceptibility after the Wenchuan Earthquake Revealed by Meteorological and Hydro-Meteorological Thresholds

After the 2008 Wenchuan earthquake (M w 7.9), an increase in rainfall-induced debris flows was initially observed in the earthquake-hit region (Sichuan, China). In the following years, the frequency of debris flows gradually reduced, indicating a recovery of stability of debris deposits originated from the earthquake. To assess these dynamically changing conditions, empirical thresholds have been identified to predict post-seismic debris flow occurrences with two approaches: a meteorological approach based only on precipitation characteristics, and a hydrometeorological approach that also considers the hydrologic conditions before the onset of rainfall. Both used the available record of precipitations and debris flows that occurred in the years 2008-2015 in Wenchuan county. Hydrometeorological thresholds for debris flows have been identified at the catchment scale by assessing the water balance with a simplified lumped hydrological model, based on the Budyko framework, and using catchment water storage as a proxy of the moisture state of the slopes. The results indicate that both meteorological and hydrometeorological thresholds allow catching the progressive recovery of stability of the debris deposits. In particular, the assessment of water balance at the catchment scale highlights the role played by the hydrological processes affecting the slopes, leading to the definition of reliable and robust thresholds. The hydrometeorological approach is thus recommended to define thresholds for early warning of debris flows at the catchment scale.