May 15, 2018: Field excursion to Maoxian and Diexi for the old huge landslide dams and recent landslides
May 16, 2018: Field excursion: the reconstruction after the Wenchuan earthquake, Hongchun gully, Niujuan Gully and Qipan Gully
Profile of Qingping Area Wenjia Debris Flow
A giant debris-flow event occurred on August 13, 2010, which caused catastrophic losses by its huge destructive power even more than two years after the Wenchuan earthquake. Moderate rainfall started at 18:00 h on August 12, then developed into a heavy rainstorm at 22:30h, which lasted for almost 2h. Just when the rainfall decreased at 1:00 h on August 13, the rock slide deposits were transported by river flood, which then turned into a debris-flow, which buried the Qingping town near the mouth of the gully at the confluence with Mianyuan River. The Wenjiagou rock slide, only 3.6 km away from the main seismic Yinxiu-Beichuan fault, occurred in the tectonic unit of the Taiping Decken in the footwall zone of the central Longmen Mountain fault. This rock slide was the second biggest mass movement triggered by the earthquake. The volume of Wenjiagou rock avalanche deposits is approximately 7.26×107m3 and the maximum thickness is 150m (Figure 1).
Figure 1. Location of Wenjiagou gully; the aerial photograph is from May 18, 2008 (Q4=Quaternary, E=Cambrian, D=Devonian and C=Carboniferous); The photo of Wenjia watershed at the right bottom is from August 10, 2008.
Before the 8.13 debris flows, the conventional design specifications were used. However our knowledge and design codes appeared to be insufficient for the debris flows occurred after the earthquake. The mitigating constructions in the Wenjia catchment were completed before the 8.13 debris flows. The major mitigation measures consisted of 19 check dams along the gully, with a height of 3 m–6 m. They had a top width of 1.5 m, and a bottom width of 4.45 m–6.4m with an embedment depth of only 2 m. These check dams proved to be useless during the 8.13 debris flow disasters. Moreover, failure of the last dam at the exit of gully, with a length of 84m and 6m in height, enlarged the damage of the Qingping Town. Eyewitnesses described how the debris gathered behind the last dam, increasing finally the impact of the amount of material after the failure of dam. The hyper-concentrated flow formed a huge wave, destroying instantly the housing and infrastructures, such as roads and bridges. Figure 2 shows the situation after the destruction of the dam. The calculated volume of solid materials was 1.92×104 m3 based on a return period of 50 yr. While, the actual debris discharge during the 8.13 events was 70×104 m3, which is 36.5 times larger than the calculated value.
Figure 2 River channel and Qingping town
The 8.13 catastrophic debris flows remind us that these debris flows are significantly different in distribution, initiation and their consequences from those developed before the earthquake or in non-seismic areas. The conventional vision and experience to control the debris flow hazard in areas suffering strong earthquakes are not suitable any more. We have the following suggestions for prevention works of debris flows, which are especially suitable for the Wenchuan Earthquake region.