The Risk Assessment Of Debris Flow In The Old Urban And Rural Market Town Of Shidian County,Yunnan Province

The old urban and rural towns are located on the right bank of the Boluo River,the first tributary of the Nujiang River in the southern part of Shidian County,Yunnan Province.The northwest side is characterized by deep-cut high-mountain landforms and many mudslides.The towns and surrounding villages are located in the mudslides.These mudslides have many characteristics of outbreaks.Since the 1990s,the Yunnan Provincial Government has carried out phased treatment of some mudslides around the towns and achieved good governance effects.Along with the rapid development of the social economy,the scope of the functional areas of the towns is expanding,and the mudslides may pose new threats to the towns and their planning areas.Therefore,the risk assessment of debris flow can provide a scientific basis for disaster prevention and mitigation in old urban and rural towns.Based on the geological conditions and the distribution characteristics of debris flow in the system research area,the formation conditions and formation mechanism of debris flow are analyzed.According to the distribution characteristics of debris flow debris in the ditch,the historical evolution of debris flow is analyzed.The FLO-2D software was used to simulate the movement characteristics of three gully debris flows in the study area at 20y?50y and 100y rainfall frequencies.The dangerous range of debris flow was predicted,and the dangerous zoning was carried out according to the two debris flow intensity parameters of flow depth and velocity..The population density and land cover type were selected as the main indicators,and the vulnerability assessment of debris flow was carried out,and the risk of debris flow was evaluated.The following main results have been achieved.(1)The study area mainly develops three large gully-type debris flow ditches,including the Manggeng River,the Luziyuan River,and the Great Stone Ditch,and three gully-type debris flows in the bengtang,Huafan and Shuanglonggou.The gully-type debris flow is generally a thin debris flow.The slope of the formation area is steep,and the formation area of the Luziyuan River and the Dashitouaogou debris flow mainly reveals the 3-5 section(?gn~3~?gn~5)strata of the Cambrian middle and lower Gongyanghe group,and the debris flow of the Manggeng River is formed.The area is mainly composed of the 2-4 section of the Gongyanghe Group(?gn~2~?gn~4),and its lithology is mainly gray-dark gray slate quartz sandstone,feldspar quartz sandstone,siliceous rock,mixed sandstone,etc.The occurrence of 305°?39°~50°,the development of the slope in the formation zone is reversed,and the slope rock mass is more prone to dumping and bending-deformation,forming a large-scale collapse-sliding deposit,which provides the main body for the formation of debris flow.source.In addition,the slate lithology in the slope is relatively weak,and it is easy to weather to form a residual slope.Under the action of heavy rain,slope erosion and shallow soil landslide are easy to occur,which is the secondary source of gully-type debris flow.The gully-type debris flow is generally a small viscous debris flow with a high development frequency.It is mainly developed in the 1 member(?gn~1)stratum of the Gongyanghe Group in the middle and lower Cambrian.Its lithology is gray-grey-green rhythmic slate with a small amount of fine sandstone containing feldspar metamorphic,and its lithology is weak.The Jidong fault zone is on the upper plate,the rock mass is broken,and the slope rock mass is easily weathered to form a thick residual slope.Under the action of heavy rain,slope erosion and shallow soil landslide are easy to occur,and a gully-type debris flow is formed.(2)The gully-type debris flow in the study area has multiple outbreak characteristics.According to the distribution characteristics of debris flow deposits in each ditch,the 4th stage of debris flow in the Manggeng River,the 3rd stage debris flow in the Luziyuan River,and the 2nd stage debris flow in the large stone ditch.According to the historical evolution analysis of each debris flow ditch,the scale of each debris flow ditch is gradually decreasing,indicating that the source in the basin tends to be stable,and the newly generated source is less.The Manggengi River has different degrees of outbreak in the annual rainfall season,which belongs to the high-frequency debris flow;the large stone ditch and the Luziyuan River have not seen the debris flow outbreak recently,which belongs to the low-frequency debris flow.(3)The numerical simulation of the activity of gully-type debris flow under different rainfall frequencies shows that under the natural conditions of 20-year rainfall frequency,the dangerous area of the Manggeng River,the Luziyuan River and the large stone ditch are about 32700m~2?79900m~2?11700m~2,respectively.The flushing amount is 7.46×104m~3?13.03×104m~3 and 2.35×104m~3 respectively.When the 50th year is reached,the dangerous area of the Manggeng River?the Luziyuan River and the large stone ditch are about 694000m~2?138000m~2 and 113700m~2respectively.The yields are 13.14×104m~3?28.98×104m~3and 4.01×104m~3 respectively.Under the 100-year rainfall frequency,the dangerous area of the Manggeng River,the Luziyuan River and the large stone ditch are about 124600m~2?312900m~2 and113700m~2 respectively,and the debris flow The flushing amounts were 32.89×10~4m~3?74.33×10~4m~3and 27.94×10~4m~3,respectively.The risk range of the Manggeng Terrier River decreased by 27,200m~2?28,500 m~2 and 32,300m~2 in the three recurrence periods after 20?50 and 100 years of treatment,which decreased by 83.2%,41.2%and 25.3%,respectively,and the amount of flushing decreased by 6.6×.104m~3,7.65×104m~3?9.45×104m~3,a year-on-year decrease of 88.4%?58.4%,and 40.8%,indicating that the treatment project has a good effect,but there are still some hidden dangers in the50-100-year rainfall conditions.(4)Two major evaluation factors,population density and land cover type,were selected to complete the vulnerability assessment of debris flow in old towns.The area with high vulnerability is about 0.83km~2;the area with moderate vulnerability is about 0.304km~2;the area with low vulnerability is 3.99km~2.They accounted for14.2%?6.78%and 79.02%of the area of the town.High school vulnerability accounted for 20.98%.(5)The risk of debris flow in the study area is dominated by medium and low risks.When the recurrence period is 100 years,the areas of high,medium and low risk zones are 9725m~2?230900m~2 and 284800m~2 respectively.The high-risk areas are mainly the county roads in the left bank residential area,the old city power plant and the town area.When the recurrence period is 50 years,there is no high-risk area,and the area of medium and low-risk areas is 126950m~2 and 103200m~2 respectively.When the recurrence period is once in 20 years,the area of the medium and low risk areas is36150m~2 and 589058m~2 respectively.The main threats in the middle and low risk areas under the three recurrence cycles are cultivated land and shrub land along the ditch..