Accumulation Characteristics Of Landslide Dam Formed By Diexi Earthquake In 1933 And Simulation Of Outburst Flood In The Upper Minjiang River

Due to the special geographical location and strong tectonic action in the upper reaches of Minjiang River,there are frequent earthquakes,landslides and other geological hazards in the region.After landslides occur on both banks of Minjiang River,the body of landslides falling into the river are easy to form a dam under the gravity,and then form secondary hazards such as a dam lake and outburst flood.The loss caused by secondary hazards such as outburst flood is much greater than the direct loss caused by geological hazards such as landslides.Therefore,the research on outburst flood is also one of the hot spots.In 1933,there is a strong earthquake of MS 7.5 occurred in Diexi area of upper Minjiang River on August 25.The strong earthquake induced many geological disasters such as landslides and collapses along the Minjiang River,and formed more than ten dams in Diexi area.The remaining barrier lakes are Dahaizi and Xiaohaizi.There are three large dams in the main channel of Minjiang River,which are Diexi Dam,Jiaochang Dam and the Yinpingya Dam.We studied the accumulation characteristics of three dams in Diexi area in 1933 by field investigation.Through collecting the data of the outburst flood occurred in Diexi in 1933,the flood model is established by numerical simulation method.According to the simulated flood area,it is suggested to the towns in the study area that we should establish the flood shelter to avoid the influence of the outburst flood.The main research results are as follows:(1)Based the field investigation,the location and the range of three dams are determined.Through the survey of the rest dam body,the profiles of three dams are described.It is found that the Diexi Dam in the downstream contains more fine-grained materials by the grain-size testing of the field samples,while the dam in the upstream contains less fine-grained materials.The results show that the flow from the dam of upstream brings out the fine-grained material from dam body to Diexi Dam.It explained a sedimentary process of the formation of the dammed lake and the grain-size characteristics of the dam.(2)According to the collected data of Diexi dam break event in 1933,the BREACH dam-break model combined with HEC-RAS flood model,HEC-RAS 1D?2D unsteady flow flood model about the Diexi event are established.The peak flow of the Diexi ancient-town dam by BREACH model calculated is 27970 m~3/s,the peak flow of HEC-RAS 1D unsteady flow flood model is 27710 m~3/s,the peak flow of HEC-RAS 2D unsteady flow flood model is 34033 m~3/s.The flood submerged area of HEC-RAS 2D unsteady flow flood model is the largest range of the three results from three models,but the shape of three flood submerged areas is basically the same.The results of three models confirm that the flood of Diexi event in 1933 scoured the Dujiangyan area.Using the information of flood marks in the collected data of Diexi event,the results of three models are verified,which shows that the flood submerged area is relatively correct.It shows that the flood submerged area of HEC-RAS 2D unsteady flow flood model is the closest to the marks of flood trace.(3)My study uses the numerical simulation method to reconstruct the outburst flood of Diexi event in 1933,and gets the inundation range of outburst flood in study area.Through comparing the scope of flood inundation range with the current distribution of cities and towns,we can find out the inundation situation of the downstream cities and towns,and provide reference value for the prevention of large-scale floods in the study area.