Transformations in Exposure to Debris Flows in Post-Earthquake Sichuan, China

Abstract. Post-earthquake debris flows can exceed volumes of 1 x 10⁶m³ and pose significant challenges to downslope recovery zones. These stochastic hazards form when intense rain remobilises coseismic landslide material. We investigate the relationship between changing exposure and hazard of post-2008 debris flows in three gullies in Sichuan province, China. These were selected based on the number of post-earthquake check dams – Cutou (2), Chediguan (2) and Xiaojia (none). Using high resolution satellite images, we developed a multitemporal building inventory from 2005 to 2019, comparing it to 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 – 10⁴m³ (low), 10⁵m³ (high) and 10⁶m³ (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 through employing a statistical exposure model, adapted from a social vulnerability equation. Cutou's exposure increased by 64 % in 2019, Chediguan's by 52 % whilst only 2 % for Xiaojia in 2011, highlighting that extensive grey infrastructure correlates with higher exposure to extreme debris flows, but less so to smaller events. Our work suggests the presence of check dams increases the perception of exposure reduction downstream, however, ultimately produces a levee effect that raises exposure to large events.