The Research Of Surface Uplift And Erosion Process Of Mianyuan River Drainage Basin After The Wenchuan Earthquake

Occurred in a few seconds of the Wenchuan Earthquake, the seismic wave energy spread from earthquake source to the surrounding,which produced large scale collapse and landslide in the Longmen shan area and a amount of loose deposits accumulated in this area. However, the frequent heavy rainfall of post-earthquake reproduced secondary geological disaster, for example: debris flow. All these geological disaster is not only caused a process of surface erosion, but also the change of surface geomorphology of landform.Mianyuan river drainage basin, which is located in northern region of the middle segment of Longmen shan, between in the Maowen-Wenchuan fault and Pengxian-Guanxian fault in the west and east direction, the Yingxiu-Beichuan fault through the study area at the middle part of basin from SW to NE. In the Wenchuan Earthquake, amounts of landslide area of large-scale landslide in above 50000m2 to 12 place, shallow landslide is widely distributed. The collapse of loose accumulation in the basin, which provided the source material for occurred frequently happened the debris flow geological disastersThe paper relies on the study of response process of collapse, landslide and debris flow in river driven by the Wenchuan Earthquake. Mianyuan river drainage basin as the research area, which priority focus on the tectonic uplift, co-seismic landslides and its sedimentary flux which driven by Wenchuan Earthquake, and the debris flow and its sedimentary flux which driven by post-earthquake heavy rainfall. Then calculated the unloading time of the loose accumulation. Using ArcGIS 10.0 software to extracted the geomorphic parameters, such as: the water system, topographic slope and aspect, which are used to study on the area of spatial analysis. In addition, through the field survey, determined the number of collapse, landslide and debris flow, the space distribution and the size of the deposit amount. Finally, combining with the basin landform evolution characteristics and analysis of the dynamic mechanism to comprehensive analysis. The author gets the preliminary insights as follows:(1) The evolution of the Mianyuan river drainage basin geomorphology was controlled by Longmen shan fault: From the view of geomorphological spatial morphology, topography slope, amplitude of landforms were all controlled by the fault. From view of river form, the river flow direction is also controlled by the fault.(2) The triggering seismic fault(Yingxiu-Beichuan fault) through the study area at the middle part from SW to NE. On the one hand, which caused the surface displacement. On the other hand, which provide the geological condition to the surface erosion.(3) Surface rupture of driven by the Wenchuan earthquake is the main factor to cause the flush flood and debris flow disaster, the surface rupture provided the source material to debris flow disaster. Regional heavy rainfall is inducing factors of landslide and debris flow disaster, all this reflecting the positive feedback affect between Wenchuan Earthquake tectonic process, erosion process and rainfall process.(4) Through the calculation, the volume of co-seismic surface uplift area of the study area is about 1.65×109m3, while the volume of co-seismic surface erosion area of the study area is about 1.03×108m3, which shows that the co-seismic uplift volume is more than the amount of co-seismic landslides volume, this implicated that the landform appeared new growth.(5) Rely on the empirical formula, according to 30% mass of the study area of co-seismic landslide can be portable, i.e. 757.1 x 108 kg. Under the Mianyuan river carrying capacity, and under the condition of fully carrying out co-seismic landslide material, which means that it takes at least 343 year that the co-seismic landslide material can be carried out Longmen shan, this also implicated that the co-seismic landslide material can be unloaded during a strong earthquake period, according to the theory of the crust equilibrium, the material of the crust surface is unloaded, which makes the material upwelling in the soft fluid below the Moho surface, and thus promotes the uplift of. Longmen shan. This is consistent with the steep topography of Longmen shan.