Formation Condition Analysis And Risk Assessment Of Mountain Torrents And Debris Flows In Houshan Small Watershed Of Sanming City,Fujian Province

Debris flows in Fujian province are mostly dominated by low-frequency debris flows.In case of negligence in prevention,it will cause tremendous damage and loss to the residents and buildings in the small watershed and at the gully.For the small watershed of mountains around the town of Sanming City,Fujian province,not only the debris flow disaster but also the flood disaster as an important natural disaster need to be taken into account.This paper analyzed the data of the flood,land disasters and the topography of the mountains around the city small watershed in the east of Sanming City,Fujian province through field surveys.It explored whether the 20 gullies in the mountains around the city small watershed of Sanming City have the basic conditions for the occurrence of debris flow.In the meantime,it evaluated the susceptibility of debris flows in the 20 gullies and calculates the design flow of floods and debris flows in related basins.Additionally,the overflow of downstream culverts in the case of floods and debris flows due to heavy precipitation once in 50 and 100 years was analyzed to assess the possibility of disasters.The main research contents and results of this paper are as follows:(1)Field survey and field survey were carried out to measure the slope on both sides of the gully,slope landslides on both sides of the gully and loose solid materials in the gully.There are a lot of slope soil from both sides of the slope in the gully basin of Sanming City,which may provide material source for the occurrence of debris flow.(2)By means of field survey,engineering geological mapping and other survey methods,the source type and blockage of driftwood in the debris flow gully are found out,the municipal drainage culvert is measured,the mountain flood trace section is measured,and the design flood peak discharge is calculated by the reasoning formula of Fujian Province.In this paper,through obtaining rainfall data and field mountain flood peak survey,it is concluded that the return period of rainfall in the study area from May 16 to 18,2019 is 50 years.At the same time,the recurrence period of flood is 50 years(3)Based on the analysis of the basic characteristics of debris flow development,quantitative analysis of debris flow susceptibility in Houshan small watershed is carried out.Fifteen main factors,such as collapse,landslide and soil erosion,loose material reserves along the gully,blockage degree of the gully,longitudinal slope of the gully,slope gradient of the gully bank,and cross section of the trench in the sand producing area,are used to analyze the susceptibility of debris flow in Houshan small watershed.In the study area,the debris flow of 20 gullies in the study area is not easy to occur except 1 gully,and the other gullies are slightly prone to debris flow(4)Risk assessment of mountain torrent and debris flow in small watershed: through calculation of design discharge of flood and debris flow,and comparison of flood flow and debris flow flow flow of culvert,and considering trees with culvert,the risk of watershed is analyzed.The flood at the lower reaches of Houshan small watershed in Sanming City is all discharged by municipal culverts.The judgment basis of blocking conditions and flow capacity,and the judgment method of mountain flood and debris flow risk are proposed.