Quantitative Study On Debris Flow Activity In Earthquake Area Based On Geomorphic Information Entropy

The Wenchuan earthquake that occurred on 12 May 2008 greatly disturbed the originally fragile geological environment and induced more than 560 million landslides in the mountainous areas.A large number of lanslides and collapses provided rich loose material sources for debris flows.At the same time,the rainfall threshold of the debris flow in the strong earthquake area has dropped sharply..The debris flow in the strong earthquake area is still in a relatively active state in 13 years following the Wenchuan earthquake.How to predict and evaluate the activity of the debris flow in the strong earthquake area is still an important topic at present.A large number of studies have shown that the activity of debris flow in the strong earthquake area is closely related to the topography,hydrological conditions,and coseismic landslide deposits.However,most of the pre-existing models used a single factor as a control variable,and an evaluation model with a single case was proposed to analyze the activity of debris flow,and the evaluation results lack a certain degree of objectivity.At present,the research on dynamic prediction and evaluation of multi factor debris flow activity is still lacking..To establish a universal,reliable and scientific debris flow activity evaluation model in strong earthquake areas,an evaluation system was estabished through regression analysis based on the source of collapsed landslides,landform information entropy and rainfall index,which can objectively reflect the actual activity state of the debris flow in the earthquake-affected area.This work can provide a reasonable scientific basis for the engineering planning.To solve the above problems,this paper selected two typical strong earthquake areas in Wenchuan County and Qingping Town of Mianzhu County with different material source conditions,geological environment and rainfall conditions as the study areas.Quantitative parameters were collected based on the database of multi-temporal landslide source,rainfall data and topography data in the study area to study the evolution characteristics of debris flow activity in the strong earthquake area and the law of the post-earthquake debris flow activity.The main research results are as follows:（1）By calculating the geomorphological information entropy of the 35 watersheds in the study area,it is found that the geomorphological evolution stages of the watersheds in the study area are quite different:4 watersheds in the study area are in the early stage of the prime of life;17 watersheds are in the middle of the prime of life;12 watersheds are in the end of the prime of life,and 2 watersheds are in old age.The results show that the development of watershed in the study area is mature and the evolution stage of the watershed is mainly prime of life and without juvenile,which provides favorable topographical conditions for the occurrence of debris flow after the earthquake.（2）Through statistical analysis of the 6 high-precision interpretation data of collapse and landslides in 35 debris flow basins,Although there are new landslides under the action of external conditions,the total sliding material source is still showing a decreasing trend.At the same time,it was found that the material supply capacity of at different distances to the main channel has certain differences.When the distance ratio is in the range of 0.1-0.4,the activity of the landslides is higher than in other ranges.By analyzing the evolution law of the landslide source,the spatial distribution of the landslide source is related to the degree of participation of the landslide source,and finally the volume and density of the source of the landslide source that can actually participate in the debris flow activity are obtained by calculation.（3）Based on previous studies,this model combines the material source density of the landslides that actually participate in the debris flow activity based on the information entropy of the watershed geomorphology to perform quadratic regression to establish a multi-period discriminant model of debris flow activity.（4）By fitting the ratio of the debris flow activity discriminant model to the corresponding rainfall index,a prediction and evaluation model for the debris flow activity in the earthquake area was established.The formula is as follows:Z_nn=（61.92×H²-37.93×H+6.5）×0.2925×(I₁₀/I_nn)^5.0304 where,H is the landform information entropy;I10 is the rainfall index in 2010;Znn is the critical line of the source density of landslides in 20nn year（unit:m3/m2）;Inn is the rainfall index in 20nn year.Compared with other evaluation models,the test results show that the this model is relatively more accurate when evaluating the debris flow activity in the earthquake area.The evaluation model has selected multiple areas for testing and verification to make this evaluation model useful and universal.（5）The equation of 12 typical debris flow basins in the study area between time and the density of landslide source was established,and the debris flow activities of the above basins were predicted with three representative rainfall indexes.By comparing the four types of watersheds,it can be found that the recovery rate of the debris flow basins in the middle and late stages of geomorphological evolution is slower than that of the other two evolution stages,which provides a reliable reference for future debris flow mitigation projects.