Debris flows and mitigation systems after the 2008 Wenchuan earthquake

Abstract

Poorly sorted loose granular materials deposited by the coseismic landslides in the 2008 Wenchuan Earthquake provided abundant source material for debris flows to occur during saturation and agitation by rainfall as well as surface and groundwater flow. Debris flows in southwest China are concentrated in the annual monsoon season when substantial precipitation occurs and have resulted in catastrophic damage to downstream regions. With debris-flow susceptibility increasing in regions affected by mega-earthquakes, mitigation systems are considered as critical infrastructures, the performance and resilience of which are vital to the local communities. In this paper, we present 1) the analysis on rainfall intensity-duration (I-D) thresholds, Rainfall Index (RI), Rainfall Triggering Index (RTI) and characteristics of the debris flows that occurred in the gullies located in three watersheds affected by the Wenchuan Earthquake, and 2) the evaluation of the performance of both conventional and some novel mitigation systems deployed in these gullies. Sharp decreases in the associated rainfall thresholds were found in the gullies after the earthquake; however, the level of increase in the rainfall thresholds is highly site- and system-dependent. Most of the existing debris-flow mitigation systems exhibited fair performance yet insufficient resilience to the impact. Notwithstanding the large amount of source materials available to be mobilized in debris flows and the intense surface run-off, many conventional mitigation structures required constant maintenance after each event and even reconstruction after the monsoon season.