Collapses-Landslides Susceptibility And Risk Assessment In The Riparian Zone Of Xiaowan Reservoir,Lancang River

The Xiaowan hydropower station,as the“dragon head”meg reservoir which located in the middle mainstream of Lancang River,have resulted in a series of geohazards during its building and operating.After the fully impoundment of the Xiaowan Reservoir(XWR),an artificial riparian zone with a vertical height of 60 m were formed due to the water level periodically changes from 1180 to 1240 m.The complex lithology,steep terrain,sparse vegetation combined with the alternation of drying and wetting in the riparian zone have reduce the stability of reservoir bank,and finally leads to bank collapses and landslides which seriously threaten the dam safety as well as the lives and property of people around the reservoir.Collapses-landslides susceptibility and risk assessment in the riparian zone would helpfully for local government and power station management department to prevent disasters and reduce damages.Thus,this study investigated the collapses and landslides occurred in the riparian zone of the XWR obtained from remote sensing imagery interpretation together with the field investigations.Furthermore,analytic hierarchy process(AHP)method with 9 factors including the disaster density,disaster development area,lithology,slope,aspect,elevation,precipitation,max fluctuated water–level and earthquake intensity was used to to assess the Susceptibility of collapses and landslides.Then,field surveys of the reservoir area were conducted,the type of elements exposed to hazards was determined,and the vulnerability was assessed.Finally,based on the results of collapses-landslides susceptibility and vulnerability assessment,the risk of collapses-landslides in the riparian zone was assessed.The present study mainly achieved the following conclusions:(1)three major categories including hazards history factors,terrain and triggering factors are selected after comprehensively considering the assessment indicators,the analytic hierarchy process(AHP)is utilized to evaluate the collapses-landslides susceptibility,which was divided into four levels,i.e.,extremely high,high,medium and low.AHP results indicated the susceptibility area for extremely high,the high,the medium and the low is 15.9 km~2,31.3 km~2,6.6 km~2 and25.7 km~2,respectively.(2)The main hazard-effected body of the research area is the population and the material economy.Considering the temporal and spatial difference of human activity and physical exposure distribution,three periods were selected,January-May,June-September and October-December.After that,the vulnerability assessment of three periods are conducted.The results showed that the Kongquedu section has the highest population and material values during all three periods,and the vulnerability value is also the largest;the population has great differences in three periods.In the first and third periods when the reservoir area is maintaining at a relatively high-water level,and together with the inter-annual travel law of neighboring residents,broading and ferry transport in this two peroids is very busy.Due to the extremely high density of population,Kongquedu section suffered heavy losses of people and material when encounters hazards.The extremely high-risk areas for all three periods was 3.8 km~2,accounting for 4.1%of the total area of the riparian zone.The area of high-risk regions in each of the three periods is quite large,and the largest exists during October-December with an area of 36.3 km~2.(3)Based on the assessment of susceptibility and vulnerability,the hazards risk in the riparian zone of the Xiaowan Reservoir at different periods were evaluated.The results demonstrated during January to May when the water level falling period,the extremely high-risk area was 3.8 km~2,while the high-risk area was 29.1 km~2,the medium-risk area was 24.1 km~2 and the low-risk area was 35.9 km~2;During the period of rainfall concentration(June-September),the extremely high risk area also was 3.8 km~2,but the high risk area was 29.1 km~2,medium risk area was 42.8 km~2 and the low risk area was18.6 km~2;From October to December,when the reservoir was operated at high water level,the area for extremely high risk,high risk,medium-risk and the low-risk regions were 3.8 km~2,36.3 km~2,18.5 km~2 and 34.4 km~2,respectively.The present study supports our preliminary investigation results of the collapse and landslides hazards in the riparian zone of the Xiaowan Reservoir,and further presents the risk zoning results in the same study area.The results of this study would usefully for the hazards governance in the Xiaowan reservoir and also helpfully for hazards prevention and mitigation in other reservoir areas.It also provides an alternative approach for reservoir bank stability assessment in the riparian zone which formed by hydropower station operation in the alpine gorges.