Quantifying the long-term evolution of post-earthquake debris-flow activity in the epicentral area of the 2008 Wenchuan earthquake, China

Abstract The 2008 Wenchuan earthquake triggered widespread coseismic landslides and supplied large volumes of loose material on hillslopes and channels, resulting in persistent post-earthquake debris flow activity near the epicentral area. Quantifying the long-term evolution of this activity is important for geohazard assessment, but remains challenge because debris flows are jointly controlled by sediment supply, topography, and rainfall. In this study, a catchment-scale framework was developed to evaluate debris flow activity intensity in 44 catchments affected by the Wenchuan earthquake. Multi-temporal landslide inventories from 2008 to 2019 were used to calculate landslide sediment supply potential (LSSP), which integrates landslide areal density, spatial distribution indices, and channel gradient. LSSP was then combined with representative hourly rainfall intensities from documented debris flow triggering events to derive a debris flow activity intensity index (AI DF ). The results show that active landslide area decreased from 122.05 km² in 2008 to 30.55 km² in 2019, while recovery areas expanded progressively. LSSP increased sharply immediately after the earthquake and then declined over time, with a rapid decrease during 2008–2011 followed by a slower adjustment phase. Nevertheless, sediment supply in many catchments remained above the pre-earthquake level in 2019. AI DF showed an overall decline after its post-earthquake peak, although temporary increases occurred during major rainfall events. Comparison with documented debris flow events indicates that AI DF captures the main temporal pattern of observed activity. The fitted long-term trend suggests three stages of post-earthquake debris flow activity: an active period from 2008 to 2013, a recession period from 2013 to 2048, and a relatively stable period after 2048. These results indicate that debris flow activity in the Wenchuan earthquake-affected area may persist for several decades, mainly controlled by the gradual depletion and redistribution of landslide-derived sediment under rainfall forcing.