Analysis Of Disaster Characteristics And Risk Assessment Of Debris Flow At Miancu Gully In Mao County

Miancu gully,located on the right bank of Min River in Nanxin Town,Mao County,is a typical multi-stage outbreak of Valley-type debris flow.Affected by the Wenchuan Earthquake and rainstorm,a huge debris flow disaster broke out in Miancu gully in the early morning of July 3,2011,causing 8 deaths and burying more than 20houses.It also led to the explosion and leakage of chlorine storage tanks in a chemical industry enterprise in the industrial park,which caused uncomfortable reactions to many people in the area.After the"7.3"debris flow,the relevant departments of the Ministry of Land and Resources have carried out corresponding engineering treatment of the gully.However,due to the large amount of loose material sources in the gully area,if large-scale debris flow disasters occur again,it may still pose a serious threat to the lives and property of the residents.Therefore,it is of theoretical significance to study the disaster characteristics and risk assessment of the Miancu gully debris flow,to study the law of the disaster and the movement of the debris flow after the earthquake,and to plan the disaster prevention and mitigation.Through data collection and field investigation,based on the geological environment background,this paper studies the formation conditions and development history of debris flow.Makes a detailed analysis of the formation mechanism and disaster characteristics of the"7.3"debris flow.Use FLO-2D numerical simulation software to simulate the movement process of"7.3"debris flow.Finally,combined with mathematical models and numerical simulation methods,evaluate the risk of Miancu gully debris flow.The main conclusions are as follows:(1)With high mountains and steep slopes at Miancu gully basin,the drainage area is 64.5km~2,and the relative elevation difference is 2730m.There are one main ditch and three branch ditches.The length of the main ditch is about 16.4 km,and the average longitudinal slope is about 165‰.The Proterozoic biotite granite,Silurian phyllite and Cambrian sandy conglomerate are the most widely distributed.The Maowen fault runs across the basin,and is affected by tectonic activity and earthquake.There are many collapse disasters in the valley.A large number of landslides triggered by Wenchuan Earthquake have led to a sharp increase in the amount of debris flow in the area,the total amount of debris flow provenance in this area is 1373.3×10~4m~3,and the dynamic reserves are 173.1×10~4m~3.(2)At least three large-scale debris flows occurred in Miancu gully during the geological history.The I stage of debris flow is the largest,with the volume of material scouring out of the gully reaching 400×10~4m~3.Later,the scale of the II and III stages of debris flow is smaller and smaller,which are 20×10~4m~3 and 15×10~4m~3.The gradual reduction of the size of the ancient debris flow indicates that the debris flow at Miancu gully has experienced at least one cyclical change from formation-development-recession-stopping in history.(3)Earthquakes affect the three basic conditions for the formation of debris flows in the area.In the early morning of July 3,2011,a huge debris flow disaster broke out in Miancu gully.Earthquake and heavy rainfall were the main causes of the debris flow outbreak.Before the outbreak of debris flow,there are two large-scale barrier lakes in the main ditch.When debris flow occurs,rainfall washes away the BH31 accumulation body and starts to form debris flow.And it causes instability and collapse of two barriers,resulting in the flow velocity and flow of debris flow being amplified more than two times.It is concluded that the dam failure provides407.2m~3/s peak flow and nearly 2×10~4m~3 water source for the debris flow,which is the main reason for the enlargement of the scale of the debris flow disaster.(4)The movement process of"7.3"debris flow is numerically simulated by FLO-2D software.The catchment point is set at the dam break of No.1 dam body,and the peak flow line of flood at the dam break is fitted by nearly four times parabola method.The simulation results show that the deposit fan volume of debris flow is about 10.12×10~4m~3,the maximum deposit depth is 3.4m,and the maximum flow velocity is 8.2m/s,which is consistent with the field investigation.(5)The evaluation results based on the mathematical model of single ditch debris flow risk assessment are as follows:under natural conditions,the risk of Miancu gully debris flow is high.FLO-2D software was used to simulate the debris flow in Miancu gully under different rainfall frequencies before and after treatment.The product of the simulated mud depth,mud depth and flow velocity was used as the criterion to divide the debris flow into dangerous zones.The comparison of dangerous areas before and after treatment shows that the high-risk areas of Miancu gully debris flow are reduced by 53.1%,43.8%and 32.5%respectively under three kinds of rainfall frequencies;the middle-risk areas are reduced by 49.1%,41.5%and 35.2%under three kinds of rainfall frequencies;and the low-risk areas are reduced by 21.8%,35.5%and 31.7%under three kinds of rainfall frequencies,respectively.After treatment,the risk of Miancu gully debris flow has been obviously controlled.