Dynamic Characteristics And Quantitative Risk Assessment Of Catastrophic Debris Flows In Wenchuan Earthquake-stricken Area

The risk assessment is one of the most effective soft measures in natural hazard prevention,and the base of hazard risk management,which can be contribute to the construction planning and prevention measure making.In recent years,according to the development of physical model and numerical simulation,quantitative risk assessment has been focus in the field of geohazard research.However,because of the complicated geohazard mechanisms,various elements at risk,and incomplete research theories,the effective study system for geohazard quantitative risk assessment has not been formed.In this article,quantitative risk assessment system for debris flow in Wenchuan earthquake area will be established from debris flow mechanisms and element features,which can promote the geohazard mitigation and other natural hazard risk assessment.The research structures,conclusions and creative points are as following:?1?To propose a critical concentration-based real-time rainfall data method for predicting the initiation of a debris flow.Firstly,in order to calculate the water volume of whole catchment,the physical process of surface runoff and infiltration has been described quantitatively by integrating rainfall series segmentation criterion,Jury-Horton extended infiltration model and Arc Hydro tools together;Secondly,to acquire sediment volume of debris flow initiation region by utilizing infinite slope theory,RS image interpretation and field investigation;At last,to set up base for analysis of debris flow transportation process,a large number of characteristic parameters in initiation area will be computed quantitatively according to Takahashi critical concentration model,including the occurrence time,initial debris flow volume and sediment volume concentration.?2?To establish static equilibrium-based debris flow entrainment mechanism model.The instability mechanism of channel sediments can be illustrated comprehensively by considering the stress enlargement,strength degradation and material basis mechanism,which is helpful for analyzing debris flow formation and transportation properties.What's more,the main factors in the above-mentioned process are following shear stress,gravity stress,pore-water pressure and sediment-volume change.This model can illuminate the channel erosion mechanism by applying the USGS flume experiment.?3?To describe the outburst mechanism of barrier dam using BREACH model.The dam break induced discharge enlargement effect has been explained scientifically by contrasting the different period of hydrographs.Moreover,by integrating this BREACH model with above-mentioned entrainment model,the dynamic mechanism and important parameters of debris flow in earthquake-stricken area have been demonstrated systematically.?4?To determine the key controlling parameters for FLO-2D numerical simulation accurately.Firstly,the simulation inflow point was be set at the channel dividing point between erosion and deposition process.Secondly,the debris flow hydrograph of whole watershed was be built by referencing the dam failure hydrograph.Thirdly,the total sediment volume and debris flow concentration were determined precisely by analyzing the initiation and dynamic process.?5?To elaborate the run-out and deposition characteristics of giant debris flows basing on numerical simulation quantitatively.The simulation accuracy was be assessed by using the integrated accuracy index?,in which the area and volume were considered simultaneously.In addition,the damage capability of debris flow was substituted with the intensity index I_DF?v²d?,which were theoretical basis for debris flow vulnerability and risk assessment.?6?To establish the building and road vulnerability empirical functions for debris flows in earthquake-stricken area.Firstly,the building vulnerability function which were based on debris flow intensity index I_DF and loss exceedance-probability was be fitted by empirical statistical method from catastrophic debris flow case in Wenchuan earthquake area.The main procedures included the representative building definition,loss class determination and curve/function fitting.This model would be suited to China's actual conditions,because it avoid the difficulty and inaccuracy in the building loss value determination;Secondly,the responding mechanism between destroyed road and debris flow physical process was be described quantitatively by introducing Winter's road vulnerability curve.What's more,the road classification and loss type criterion were illuminated clearly in order to be applied to Wenchuan earthquake area.?7?To complete debris flow quantitative risk assessment for significant elements at risk in earthquake-stricken area.An integrated theory system and technique procedure for debris flow risk assessment was be established.In this study,various debris flow future scenarios were built basing on different return periods of rainfall and prevention measures,in which the debris flow dynamic characteristics and mitigation plans in earthquake-stricken area were considered.Moreover,expected loss values of representative buildings and roads under different debris flow conditions were assessed accurately by combining FLO-2D numerical simulation,elements at risk database and corresponding vulnerability functions.Taking Yangling catchment for example,after building various debris flow scenarios which included 10,20,50,100 rainfall return period and prevention projects respectively,four methods of expressing debris flow risk were stated as following:1)the whole loss class,amount and value of buildings in debris flow deposition area;2)the loss class and probability of single building for 78#building;3)the loss class and loss probability of a planning construction site;and 4)the whole loss class and amount of roads at risk.Finally,the cost-benefit analysis was completed by contrasting various expected loss values whether considering this prevention measure.It should be noted that,the above conclusions are applicable to the Wenchuan earthquake-stricken area,and application in other regions or other type debris flows needs further validation.Limited by research theory and sample size,some possible deviations are inevitable during applying this debris flow quantitative risk assessment system.Even so,the results are expected to provide a reference to research in natural hazard risk assessment and its engineering application.