Research On Mechanical Parameters And Process Simulation Of Debris Flows In The Areas Affected By The 2008 Wenchuan Earthquake

Debris flow is a devastating natural phenomenon in mountainous areas,which causes human losses and damage to infrastructures.The main triggering factors of debris flow are rainfall and earthquake.On 12 to 14 August 2010,heavy rainstorms occurred in the Sichuan province of South-West China which were affected by the 2008 Wenchuan Earthquake,induced a series of catastrophic debris flows,resulted in considerable loss of lives and infrastructure.Therefore,it is appropriate to investigate the actual mechanism and dynamic run-out characteristics of debris flow in detail and develop the guidelines for numerical simulations,particularly in the mountainous area like Wenchuan.The Longchi Basin is considered as case study because the area is most susceptible to debris flows by intensive rainfall.The main objective of this research is to evaluate the rheological behavior of debris flow by experiments while assessment of debris flow run out and its entertainment behavior using FLO-2D and Mass Flow models.The main characteristics and development status of debris flows in the Longchi basin have been investigated by fieldwork.Soil samples from the riverbed and sources area were collected for extensive laboratory testing.A series of laboratory testing,which includes soil indices test and rheological parameters of debris flow containing granular materials with size finer than 0.075mm have been obtained using a rotational rheometer(Physica MCR).The rheological behavior has been examined at different ranges of volumetric concentration.These results showed the values of shear stress and viscosity have been increasing exponentially with the increase of solid volumetric concentration.In FLO-2D,analysis of debris flow mobility was carried out using Bingham rheological model while Mass Flow follows the guidelines of the Coulomb and Voellmy rheological model.In this research,these programs have been compared according to their fundamental theories,input-output data,computational algorithms,and results by simulating debris flow in the study area.The simulation results of FLO-2D and Mass Flow were compared based on their observed data.The results of this study obtained through the FLO-2D and Mass Flow model also showed that the average depth and volume of debris flow deposit is relatively smaller than the actual range of debris flow.The sensitivity analysis of input parameters were carried out in the FLO-2D model.The model is found to be highly sensitive to solid volumetric concentration C_vand rheological parameters(yield stress and viscosity).Root Mean Square Error(RMSE) were conducted to verify the accuracy and performance of models.The RMSE for FLO-2D and Mass Flow model were 4.389 and 4.094,respectively.The validity of the FLO-2D and Mass Flow versus ground truth was statistically tasted with Receiver Operator Characteristic(ROC) approach.The accuracy values obtained are 0.89 and 0.91 respectively for the FLO-2D and the Mass Flow model,which showed a strong positive correlation.The best performing model in simulating the debris flow was selected based on the lowest RMSE and highest ROC values.Finally,the hazard map of debris flow was developed,and the resulting hazard map revealed that the high-hazard area is relatively large.In contrast,the medium and the low-hazard regions are relatively small in the debris flow gully selected from the Longxi river basin.Furthermore,the hazard map can supply a reference for debris flow control,land use planning,and disaster mitigation design in the future.Based on the "8.13" debris flow characteristics in the Longchi area,these models can be used in the future to predict the accumulation range and run out features of other debris flows at home and abroad.