| Baishuizhai gully is located on the left bank of the Minjiang River in Nanxin Town,Mao County,Sichuan Province.It is a typical multi-stage gully-type debris flow gully.The“5.12”Wenchuan earthquake caused a certain increase in the solid source in the gully,and a large-scale debris flow broke out under continuous heavy rainfall on July 10,2013.The facilities of the Baishui Huanglong I Hydropower Station and the Baishui Hydropower Station and the house in the gully brought a big hazard.Once the large-scale debris flow breaks out again in the ditch,it will also pose a potential threat to the safety of life and property of 25 households and 110 people in the Gewo group and the safe operation of the downstream Tongzhong Power Station.Therefore,carrying out the risk assessment of debris flow is of great significance to the treatment of the debris flow and disaster prevention and reduction.Based on the on-site investigation of Baishuizhai gully and the relevant geological environment and rainfall conditions in Maoxian area,this paper analyzes the type,scale,frequency and damage of debris flow,and studies the formation conditions,development and movement characteristics of debris flow.Based on the analysis of the danger and development trend of debris flow,the FLO-2D software was used to simulate the sedimentation process of debris flow and the risk assessment was carried out.The main research results were obtained.(1)The Baishuizhai gully watershed is fan-shaped,with an area of 75.2km~2.There are 1 main ditch and 6 branch ditch.The main ditch is 12.8km long,and the average longitudinal slope is 66~472‰,which is generally“steep-slow”.The total reserves of debris flow in the ditch reached 27044.84×10~4m~3,and the dynamic reserves were 770.16×10~4m~3,of which landslide deposits accounted for 95%of the total provenance.(2)The gully is a multi-stage old mudslide ditch.According to the accumulated landform,there have been four large-scale mudslides in history.The amount of the first-stage pale mudslide is 1000×10~4m~3,the phase II is 250×10~4m~3,the phase III is75×10~4m~3,and the IV phase is 33×10~4m~3.The scale of modern debris flow is relatively small.In 2013,the size of debris flow was 18×10~4m~3.In 1995,the size of debris flow was smaller.After the Wenchuan earthquake,the debris flow activity was slightly enhanced and the frequency was the intermediate frequency.(3)The modern debris flow in Baishuizhai gully is a thin debris flow with a fluid gravity of 1.55t/m~3.Under the three kinds of rainfall frequencies(P=5%,P=2%,P=1%),the debris flow peaks of Baishuizhai gully Mouth are 317.64,454.53 and549.35m~3/s,respectively;the total debris flow is 30.18,43.18,and 52.19×10~4m~3,respectively;the total amount of solid materials washed out was 10.06,14.39,and17.40×10~4m~3,respectively.(4)The gully still has the conditions of terrain,source and other conditions for the recurrence of debris flow.Under appropriate rainfall conditions,it is possible to re-explode the debris flow.According to Liu Xilin’s risk evaluation formula,the risk H of the debris flow is calculated to be 0.71,which is highly dangerous.The maximum risk prediction range s of the debris flow accumulation area calculated by the empirical formula is 0.583146km~2.(5)The FLO-2D software is used to simulate the movement process of debris flow.Under natural conditions,when the rainfall frequency is P=5%,2%,1%,the debris flow area of the debris flow is 55600m~2,150832m~2 and 247200m~2,respectively,and the square volume is 15.86×10~4m~3.49.92×10~4 m~3 and 88.46×10~4 m~3,the total amount of material punched out of the groove is 3.2×10~4 m~3,18.2×10~4 m~3 and28.6×10~4 m~3,respectively.After the project was treated,the debris flow area of the debris flow decreased by 30%,38%,and 46%,respectively,the volume decreased by41%,49%,and 53%,and the total amount of material that rushed out of the ditch was reduced by 69%,42%,and 46%,respectively.(6)The mud depth,mud depth and velocity product in the simulation results were selected as the basis for risk classification,and the Baishuizhai debris flow danger zone was divided into high,medium and low danger zones.Under natural conditions,when the rainfall frequency is P=5%,the areas of high,medium and low risk zones are 1.2816km~2,0.8080km~2 and 0.9344km~2 respectively.When the rainfall frequency is P=2%,the areas are 2.2416km~2,0.9056km~2 and0.8768km~2 respectively.When the rainfall frequency P=1%,the areas are 2.6816km~2,1.008km~2,and 1.2864km~2,respectively.After the project is controlled,when the rainfall frequency is P=5%,the areas of high,medium and low risk zones are 0.9696km~2,0.6832km~2,and 0.5488km~2respectively.When the rainfall frequency is P=2%,the areas are 1.0960km~2,0.7744km~2,and 0.5680km~2 respectively.When the rainfall frequency P=1%,the area is 1.2672km~2,0.5808km~2,0.7760km~2.Before the comparison,the total area of the dangerous area decreased by 27.19%,39.40%,and 52.01%,respectively.The area of the high-risk area decreased by24.34%,51.11%,and 52.74%,respectively,indicating that the barrier project played a significant role in disaster reduction,but the debris flow hazard still exists. |
