Erosion and sediment budget of the 2008 Wenchuan earthquake: A case study on Mianyuan River basin

Abstract

The 2008 Wenchuan earthquake of magnitude 6.5 caused a large number of avalanches and landslides at different scales. It is extremely significant to evaluate the sediment in the earthquake river basins. Along the 38 km long upper Mianyuan River 196 landslides and avalanches happened during the earthquake, which have formed 25 landslide dams and quake lakes. The total volume of sediment erosion due to earthquake was about 115 million m3, which is 75 times higher than the soil erosion in normal years. Only a part of the solid material could be transported by the river water flow as suspended load and bed load. The total volume of bed load deposit in the river and the quake lakes was 1.43 million m3. Moreover, the quake lakes had also trapped 0.12 million m3 suspended load. Only 0.18 million m3 of fine sediment had been drifted through the quake lakes and transported into the lower reaches of the Mianyuan River. The wide range of size distributions of sediment from earthquake erosion caused the extreme difference in the amounts of sediment erosion and transportation. Most of the sediment from earthquake erosion can be only transported for a short distance by landslides and debris flows. Less than 0.2% of the total volume of sediment from earthquake erosion may be transported into large rivers. Therefore, earthquake erosion has a little effect on the sediment transportation and fluvial processes in the large rivers.