Risk Analysis Of Mountain Torrents In The Yarlung Zangbo River Basin

As the main kind of natural disasters causing casualties in China,mountain torrents is a major obstacle to the economic development of mountainous areas.The system of floods prevention and mitigation in China is mainly based on non-engineering measures,supplemented by engineering measures.And the hazard assessment is an important part of non-engineering prevention and control measures.Although scholars at home and abroad have made a lot of research on the risk analysis of flash floods,there are some defects in the selection process of analysis indicators,such as subjective and fuzzy,and obvious differences about determination the weight.There are many and complex reasons for these shortcomings,and the study of floods hazard is also developing in the direction of overcoming these shortcomings.And limited by the difficulty of obtaining data and the discontinuity of disaster records,the regional mountain flood hazard analysis research is still relatively lacking in plateau area.Based on the 30m DEM,the boundary data of the small watershed in the study area is extracted according to ArcGIS hydrological analysis tools.And the small watershed is classified and analyzed by K-means method.Then,on the basis of detailed analysis the floods spatio-temporal distribution pattern and change trend in the Yarlung Zangbo River Basin,we select the preliminary indicators.And the evaluation index system of flash floods is determined quantitatively by using correlation analysis method and Boruta feature selection algorithm,the result of flood hazard assessment is obtained by applying DEA evaluation model.Finally,by calculating peak water levels of different frequencies in typical small watersheds,the risk of mountain flood disasters in typical small watersheds is evaluated and analyzed on the basis of flood submergence range.This paper provides a new idea for objective analysis of mountain flood hazard risk in plateau area by means of combining regional and typical small watershed.The main contents and research results are as follows:(1)K-means clustering method was used to classify the geometric and topographic characteristics of 16537 Small Watersheds in the study area.The results show that among the four types of small watersheds,the first type has relatively flat terrain and tends to be square,the second type has relatively steep terrain and long strip shape,the third type has flat terrain characteristics and tends to be long strip shape,and the fourth type has steep terrain and tends to be square shape.Moreover,historical mountain torrents are more distributed in the second and fourth small watersheds with steeper topography.(2)Analyses historical floods spatio-temporal patterns,it is found that the annual trend of mountain torrents in the study area was increasing,and the trend of mountain torrent disasters increased significantly after 2004.Among the four main secondary river basins,the Brahmaputra basin has the greatest increasing trend,while the torrents in the study area mainly occurred in June to August in summer,accounting for more than 90%of the total historical mountain torrents.(3)21 rainfall indicators,3 topographic indicators,soil types and land use,4 small watershed characteristics indicators and 3 human activities indicators were selected as the preliminary indicators of the study area.Through correlation analysis and the Bortua feature selection algorithm,the evaluation index system of mountain torrent hazard in the study area was constructed quantificationally,which was the maximum 10 points in 20 years.Clock precipitation,20-year maximum one-hour rainfall,100-year maximum 24-hour rainfall,10-year maximum 6-hour precipitation,multi-year average precipitation,elevation standard deviation,soil type,small watershed area,population density.(4)Applying DEA model to evaluate the hazard of flash floods in the study area,the results show that the proportion of areas with lower risk,low risk,medium risk,higher risk and high risk of mountain torrents in Brahmaputra is 85.29%,6.73%,4.56%,2.35%and 1.07%,respectively.The most dangerous areas are in Shigaze,Shannan and Lhasa,and the most widely distributed areas are in Renbu among county.Overall,the development of mountain torrent disasters in the study area is weak,but the concentration of mountain torrent disaster risk areas and human activities areas is high,which needs to be paid attention to.(5)Selecting the Chaba Township watershed in Renbu County as a typical small watershed,using mature methods such as SCS model,reasoning formula method and manning's formula,the flood frequency submergence range of each typical small watershed is obtained,and the range of mountain flood risk zone is delineated based on this.The results show that the area of dangerous area is 1.27 km2,the area of high-risk area is 0.269 km2,and the area of extremely high-risk area is 0.136 km2.