Transformations in exposure to debris flows in post-earthquake Sichuan, China

Abstract. Post-earthquake debris flows can exceed volumes of 1 × 106 m3 and pose significant challenges to downslope recovery zones. These stochastic hazards form when intense rain remobilises coseismic landslide material. As communities recover from earthquakes, they mitigate the effects of these debris flows through modifications to catchments such as building check dams and levees. We investigate how different catchment interventions change the exposure and hazard of post-2008 debris flows in three gullies in the Sichuan Province, China. These were selected based on the number of post-earthquake check dams – Cutou (two), Chediguan (two), and Xiaojia (none). Using high-resolution satellite images, we developed a multitemporal building inventory from 2005 to 2019, comparing it to the spatial distribution of previous debris flows and future modelled events. Post-earthquake urban development in Cutou and Chediguan increased exposure to a major debris flow in 2019, with inundation impacting 40 % and 7 % of surveyed structures respectively. We simulated future debris flow runouts using LAHARZ to investigate the role of check dams in mitigating three flow volumes – 104 m3 (low), 105 m3 (high), and 106 m3 (extreme). Our simulations show check dams effectively mitigate exposure to low- and high-flow events but prove ineffective for extreme events, with 59 % of buildings in Cutou, 22 % in Chediguan, and 33 % in Xiaojia significantly affected. We verified our analyses by employing a statistical exposure model, adapted from a social vulnerability equation. Cutou's exposure increased by 64 % in 2019 and Chediguan's by 52 %, while Xiaojia's increased by only 2 % in 2011, highlighting that extensive grey infrastructure correlates with higher exposure to extreme debris flows but less so with smaller events. Our work suggests that the presence of check dams contributes to a perceived reduction in downstream exposure. However, this perception can lead to a levee effect, whereby exposure to larger, less frequent events is ultimately increased.