Research On Hazard Assessment Of Multigeohazards In Tianshui Basin

In recent years,about 20,000 geohazards occur in China every year,of which collapse,landslide,and debris flow account for more than 95%.Although the overall trend of geohazards in China is declining,with the continuous enhancement of urbanization,the degree of damage caused by geohazards is also increasing year by year.There are about 30% of the total geohazards occurring on the Loess Plateau of China each year,but they occupy only about 6% of the Chinese land area.Tianshui City is located in the southwestern part of the Loess Plateau.It is a sub-core city of the Guanzhong Plain City group and an important node city in the Silk Road economic belt.The Loess hilly-gully regions are widely distributed in the territory.The Weihe River and its tributaries are spread throughout the whole area.The gullies are vertical and horizontal,and the geo-environment is fragile.It is one of the cities with the most serious geohazards in China.On July 25 th,2013,Tianshui City encountered cluster geohazards due to a heavy rainstorm,which caused tens of thousands of houses to be damaged,25 deaths or disappearances,and direct economic losses of 8.275 billion yuan,accounting for more than 80% of the annual direct economic losses caused by geohazards.The situation of geohazards prevention and control is very severe.Therefore,this thesis chooses Tianshui City as the study area and uses data-driven statistical models(information,weight of evidence,and logistic regression)to carry out multi-geohazards hazard assessment research.The thesis reveals the development and distribution of geohazards and the causative factors,constructs the evaluation index system of multi-geohazards,and draws the hazard zoning maps of single geohazards and multi-geohazards.The main results achieved are as follows:(1)The geohazards are mainly distributed in the loess hilly area of the study area,and the bedrock mountain areas in the east are basically underdeveloped.The geohazards are closely related to rivers,topography,vegetation,and faults in the study area.The geo-environment in the territory is fragile,local rainstorms occur frequently,and the ability to resist geohazards is weak.This leads to more active predisposing factors for geohazards.(2)The type of landform,the distance to the mainstream,the distance to the Qh fault,the average annual rainfall,and the peak acceleration of the earthquake constitute the common factors of the single geohazard and multi-geohazards hazard evaluation index system for collapse,landslide,and debris flow.Slope,elevation,and land-use type are indicators of landslide hazard assessment.Slope and vegetation index are indicators of collapse hazard evaluation.Distance to tributaries,vegetation,topographic location index,and collapse-landslide density constitutes evaluation factors for debris flow geohazard.Slope,vegetation,and distance to tributaries are indicators of multigeohazards hazard assessment.(3)The best models for the hazard assessment of landslide,collapse,and debris flow are the information model,the information model,and the logistic regression model.The evaluation accuracy is 0.766,0.826 and 0.859,respectively.The best model for multi-geohazards hazard assessment of collapse,landslide,and debris flow is the information model,with an evaluation accuracy of 0.781.The best models for single geohazard and multi-geohazards hazard assessment meet the applicability conditions.(4)The area of landslide high-hazard zones accounted for 33.0% of the total area of the study area,and the number of landslides accounted for 73.7%.They are mainly located in the loess gully hilly area with sparse vegetation in the low to middle altitudes,the Weihe River and its tributaries Xihe River,Huluhe River,Sanduhe River,Niutouhe River,Yingchuanhe River,Dongkehe River Basin.The area of collapse high-hazard zones accounted for 31.9% of the research area,and the number of collapses accounted for 84.7%.The spatial distribution pattern is the same as the landslide hazard zoning map.The area of debris flow high-hazard zones accounted for 31.6% of the total area,and the number of mudslides accounted for 86.3%,mainly distributed in the mainstream and tributary valleys in the territory.The high-hazard zones of multi-geohazards are the same as landslides and collapses.They are mainly distributed in the Loess hilly area with sparse vegetation in the middle,along the Weihe River and its tributaries.