Flume tests to study the initiation of huge debris flows after the Wenchuan earthquake in S-WChina

Abstract

In the Wenchuan area in the southwest of China, a huge amount of loose co-seismic landslide material was deposited on slopes during the Wenchuan earthquake of May 2008. These loose deposits formed the source material for rainfall-induced debris flows or shallow landslides in the years after the earthquake. On August 13, 2010, about 20 large debris flows were triggered by heavy rainfall in the area around the epicenter of the Wenchuan earthquake. Field reconnaissance revealed that the initiation of these post-earthquake debris flows was closely related to severe erosion of the loose deposits. Flume tests were carried out to study the initiation mechanism of these post-earthquake debris flows and the related influencing factors. The flume was instrumented with ten combination sets of suction and pore-pressure sensors. These sensors were accompanied by TDR probes to measure the soil water content. The flume has a length of 2.5 m and a width of 1.5 m. A series of 26 tests was conducted to study the influence of slope gradient and discharge on the initiation mechanism and scale of the debris flows. The amount of debris-flow discharge was obtained by collecting the washed out deposits every 20 s. The experimental results showed that the initiation mechanism of debris flows for gentle slopes and steep slopes was different. On steeper slope, incision by run-off water was very rapid initiating directly the start of a debris flow. The debris flow volume increased rapidly by a chain of subsequent cascading processes starting with collapses of the side walls, damming and breaching leading to a rapid widening of the erosion channel. At gentler slopes less intensive run-off incision caused an accumulation of material down slope, which, after saturation, failed as shallow slides transform in a second stage into debris flows. It was demonstrated that the slope was the dominant factor in controlling the scale of the debris flows while the effect of discharge on erosion and the size of the debris flows was less clear.