| Landslide is a very serious natural disaster,often causing casualties and property damage.In mountainous areas with steep terrain,slopes are prone to instability and form a large number of landslides.In valley areas,some large landslides are easy to block the river.It is inferred that the Qiaojia ancient landslide blocked the river,based on the analysis of the distribution characteristics and sedimentary characteristics of the sediments and the photoluminescence dating in the Qiaojia-Dongchuan section of the Jinsha River,and combined with the development of the ancient landslide in the study area.With the help of ArcGIS and remote sensing images,a detailed database of landslide disaster and influencing factors is established in this paper.Then the Frequency ratio-Analytic hierarchy process(FR-AHP)is used to evaluate the susceptibility of modern landslides and all landslides-including ancient landslides.The results show that the existence of ancient landslide has a great influence on the susceptible evaluation results.Therefore,it is necessary to carry out a detailed field investigation and evaluation of the stability of ancient landslide in the regional evaluation for a reasonable evaluation result.In conclusion,based on the investigation and analysis of the lake sediment,the ancient landslide and the modern landslide,the main conclusions are obtained and as follows:(1)The sediments of dammed lake in the study area are mainly distributed along the Jinsha River in Qiaojia-Dongchuan,about 50 km in length,concentrated in the range below 850 m,and a small amount is distributed in 900-950 m,showing obvious difference distribution in elevation.The sediments outcrop is about 10-35 m thick,are mainly grey,grey-yellow medium-coarse sand and gravel sand,and the light yellow and grey-yellow fine sand and sandy silty soil with good sorting are exposed in the Qiaojia,the horizontal bedding is very developed.And we collected 17 sediment samples,and studied the grain size characteristics and sedimentary environment of the sediments by analysing 4 particle size parameters-average particle size,standard deviation,skewness,peakedness,and the frequency curve,probability cumulative curve.It shows that The main sedimentary system widely distributed in the study area was lacustrine sediments in the dammed lakes formed by landslide.and the closer they are to Qiaojia,the more fine particles the weaker hydrodynamic conditions.(2)The formation of lacustrine sediments is closely related to the existence of ancient landslides.Qiaojia landslide is the largest landslide in the study area.It has obvious characteristics in remote sensing image and field investigation,the landslide trailing edge like a round-backed armchair,Jinsha River bends to the left,and the sliding surface is clear,the sliding surface is retained with scratches,and so on.Based on the analysis of the distribution of sediments and the location of ancient landslides,as well as the photoluminescence dating of sediment samples,the results showed that the sediments aged about 20 ka years are only distributed inQiaojia-Jitangsection.There is no distribution of lacustrine sediments and common dam break deposits around the valley of the lower reaches of Qiaojia.It is inferred that the Qiaojia landslide formed barrier lakes,and blocked the river twice in about 20 ka years and 70-100 Ka years ago with two stages of sediments in the study area.(3)With the ArcGIS and the remote sensing images,there are 3573 modern landslides,which were mainly distributed along the Jinsha River,the Xiaojiang and the large gully,which is closely related to the erosion of the river and the background of the regional structure.Then the influence factors of the landslide distribution are analyzed to set up a more detailed database of disaster catalogue and influence factors.Then,based on the landslide catalogue and the influence factor database,the Frequency Ratio method and the Analytic Hierarchy Process are combined to form the model of the susceptibility evaluation of the landslide disaster.The results of susceptibility showed that about 80% of the study area belonged to middle and low susceptibility,while only about 1.42% of the area was extremely susceptible,and most were located on both sides of Jinsha River valley on the south side of the study area.Finally,the area under curve(AUC)of the ROC curve was counted.The accuracy of the model is 0.827,which shows that the model is reliable.(4)By re-evaluating the susceptibility with all landslide(ancient landslide and mordern landslide),the results show that the giant ancient landslide is an important factor for the evaluation.Compared with the result of the modern landslide,the area proportion of the medium and low susceptibility zones has decreased,the proportion of the high was increased.The high-susceptibility zones are distributed in the side of the Jinsha River between the Qiaojia-Gele and Qiaojia landslide,and the AUC value is 0.755.The susceptibility of the area of the ancient landslide is obviously raised,therefore,it is necessary to evaluate the stability of the ancient landslide.However,the whole Qiaojia landslide is very stable with the unequal thrust method.The landslide susceptibility mapping is quite different from the actual situation.So,when evaluating the susceptibility of ancient landslides,we should carefully consider the risk of typical ancient landslides for more valuable evaluation results. |
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