Mass movements triggered by the Wenchuan earthquake and management strategies of quake lakes

The Wenchuan earthquake on May 12, 2008 triggered numerous avalanches and landslides and created many quake lakes. This paper studies the stability of mass movement deposits and quake lakes and the relation of the mass movements and quake lakes with riverbed incision by field investigations, satellite images and data analysis. The mass movements induced by the Wenchuan earthquake released different degrees of potential energy. Landslides which occurred within quaternary deposits released most of their energy and had the highest stability and safety post-event. The avalanche deposits along the Minjiang River had the lowest stability and are dangerous because the slopes are still steep. The essential cause of the mass movement was riverbed incision. From the viewpoint of river training and management, some landslides might be prevented if riverbed incision were controlled. Two strategies are applied to manage the quake lakes: (1) to preserve the quake lakes; and (2) to remove the quake lakes. The Tangjiashan quake lake water was drained out by scouring a spillway on the landslide dam from 740m elevation down to about 714m because there was a high risk of dam-break flooding, which endangered a million people in the downstream reaches, when 200 million m(3) of water had stored in the lake. For quake lakes with low risk of dam failure, the preservation strategy was applied. The continuous rising of the Qinghai-Tibet plateau has resulted in continuous river-bed incision in most streams in the earthquake-hit area. In general, the morphological process of incised rivers goes through four stages: (1) rapid degradation; (2) degradation and widening; (3) widening and resiltation; and (4) equilibrium. Most of the rivers in the earthquake-hit area find themselves in Stage 2. The landslide dams and the quake lakes may form knickpoints on the stream bed profile; the slope below it is steep but the slope upstream is remarkably gentler, which prohibits rivers from incising over the length of the landslide mass and the reach upstream of the quake lakes. Preservation of the quake lakes may create high resistance to flow and change the stream from being in stage 2 into stage 3, thus reducing the mass movement potential in a future earthquake event.