Study On The Risk Evaluation Of Geoenvironmental Hazards In The Mountainous Areas Of Southern Ningxia In The Middle And Upper Reaches Of The Yellow River

The southern mountainous region of Ningxia(Ningnan Mountain District)is located in the middle and upper reaches of the Yellow River,and the fragile geological environment and frequent human activities have led to frequent geological hazards in the region.The study of geological hazard risk assessment is a focus of attention and a difficult area in the field.Therefore,the study of the evolution pattern and risk evaluation of geological hazards in the mountainous areas of Ningnan can provide a scientific theoretical basis for regional geological hazard prevention and control policies.In this paper,we analyze in detail the occurrence types,distribution rules and basic characteristics of geological hazards,and establish a regional geological hazard risk evaluation system.The main research conclusions are as follows:(1)The main types of geological hazards in the southern mountainous areas of Ningxia include unstable slopes,mudflows,collapses,landslides and ground subsidence.Among them,there are 892 unstable slopes,789 landslides,427 landslides,133mudslides and 7 ground collapses.Landslides,debris flows and landslides often occur in loess hilly areas,while ground collapses are mainly distributed in mining areas.(2)The slope,elevation,aspect,terrain relief,geotechnical type,fault,road,river,NDVI and land use type were selected as the susceptibility evaluation indexes,and the susceptibility evaluation was carried out by using the information volume method(IV)combined with random forest(RF)and XGBoost models,respectively.The ROC curve analysis was used to select IV+XGBoost as the optimal disaster susceptibility evaluation model.The evaluation results were:high hazard susceptibility,higher susceptibility,medium susceptibility,lower susceptibility and low susceptibility accounted for 17.50%,21.33%,23.10%,12.56%and 25.51%of the whole study area,respectively.There are 1,805 geological hazard sites in the high and higher susceptibility areas of geological hazards,accounting for 80.29%of all geological hazard potential sites.There are 309 geological hazard sites in the medium-prone area of geological hazards,accounting for 13.75%of the hidden hazard sites.There are 134geological hazard sites in low and low susceptibility areas,accounting for 5.96%of the total number of hazard sites.(3)The IV+XGBoost model with the best performance in susceptibility evaluation was selected to add the average annual rainfall index factor for geological hazard evaluation in the mountainous area of Ningnan,and the evaluation results were:high hazard area,medium hazard area,lower hazard area and low hazard area of geological hazards accounted for 16.91%,22.78%,18.36%and 41.95%of the whole study area,respectively.(4)This paper selects four indicators of namely population density,GDP density,road density,and cultivated land density,were selected as vulnerability evaluation indicators,and the combination of AHP-entropy weight method and fuzzy comprehensive evaluation method was used to calculate the weights for vulnerability evaluation.The evaluation results are:There are 41 townships are in the very low vulnerability zone,with an area of 10586.12km~2.There are 31 townships are in the low vulnerability zone,with an area of 6036.32km~2.There are 17 townships are in the medium vulnerability zone,with an area of 829.84km~2.There are 9 townships are in the high vulnerability zone,with an area of 2455km~2.(5)According to the evaluation model of"risk=danger×vulnerability",the risk evaluation of geological hazards in the mountainous areas of southern Ningxia was carried out by using the results of the evaluation of danger and vulnerability of geological hazards.The evaluation results are:the area of low risk zone and lower risk zone of geological disaster are 7644.20km~2and 4038.08km~2respectively,accounting for38.36%and 20.27%of the total area respectively;the area of medium risk zone is5825.76km~2,accounting for 29.24%of the total area;the area of high risk zone is2417.28km~2,accounting for 12.13%of the total area.According to the results of geological disaster risk evaluation,the area is divided into key management area,secondary key management area and general management area,and the countermeasures related to disaster prevention and control in the near and medium and long term are proposed.