| Longxi River catchment in Longchi Town, Dujiangyan City, Sichuan Province,sustained damages from landslides and debris-flows during Wenchuan earthquake in2008. The earthquake led to co-seismic landslides and subsequent rainfall induced48debris-flows in Longxi River catchment on August13th,2010. For the sake of riskclassification,12typical debris-flows in study area are chosen for simulated byFLO-2D numerical model. On the basis of sediment yields, the residual sedimentvolume in the12gully was determined to be42.3×106m3by comparison of digitalelevation models (DEMs). This study aimed to review the change of hazard zone afterdams were built in the gully on the basis of simulation results obtained by using themodel to provide references for future disaster prevention and resident evacuationplans.The FLO-2D model is a two-dimensional flood routing model that can simulateflows over complex topographies and roughness on urbanized alluvial fans.Hyperconcentrated sediment flows, such as mudflows and the transition from waterflows to fully developed mud and debris flows, can be simulated as well.FLO-2Droutes a flood hydrograph using the full dynamic wave momentum equation toaccurately predict the area of inundation. The fluid viscous and yield stress terms areaccounted for in the model for hyperconcentrated sediment flows.In the process of simulation, based on the typical model of flood amplificationprocess, using the integral mathematical methods derived water flow process line.First of all, the total flow is derived by rain water, that is, through the rain and thebasin area determine the corresponding storm lengths of T, then according to therainfall time T and the basin area watershed WP, deduce the total water flow in thescope;Set up five kinds of return period in each debris flow numerical simulation(20%,5%,2%,1%,0.5%). The procedure for determining the depositional extent ofdebris flow is given as follows：1. Assign the location of debris-flow water-collectingposition based on the channel partition and landslides interpretation results. We takethe water-collecting position as the area of debris-flow formation in the main channelor influent. This position is located in the sediment source areas and can make full collection of rainfall in the around area. The location of water-collecting position canbe obtained from field investigation and Aerial images.2. Prepare input data whichcan be grouped into the categories of geometry, hydrology and sediment. Geometricdata include DEMs of watersheds with a resolution of5m by5m, channel andfloodplain roughness coefficient n values which can be referred to from the FLO-2Duser’s manual and other data such as channel geometry. Hydrologic data includerainfall data and input data for the water flow process line which has been derived.Sediment data include yield stress, dynamic viscosity, specific gravity, volumetricsediment concentration and resistance parameter K decided by previous studies andthe survey data.3. Produce an inflow hydrograph for a simulated water-shed using themodel has been derived. In addition, hydrological analysis, including rainfallfrequency analysis and design storms, can be done in this step.4. Proceed withdebris-flow simulation by assigning all other required input data and the resultingdebris flow hydrograph. From this, the inundated area of debris-flow can be obtained.Based on the result, the potential debris-flow hazard zone for a study creek can bedelineated.In the simulation which return period is20years(disasters of August13th,2010),the total scale of Debris flow accumulation area is215550m2, the sediment volumewas approximately950000m3;Among them, more than half of the flow depth are1-3m, maximum flow velocity was about7.22m/s, an average velocity on the maximumflow velocity map is about0.92m/s. Use the field survey data and imageinterpretation data to checking the Accuracy of simulation results, Accuracy isbetween60%~87%. It shows that the simulation results are reliable.The hazard assessment (natural conditions) in the study area is based on thedebris flow influence intensity and return period. The factors of debris flow influenceintensity are the max deposit depth and flow velocity which obtained from numericalsimulation. The total scale of Debris flow hazard zone is529775m2in study area.High hazard areas are mainly distributed in central and near the river downstream ofAccumulation zone.Considering the conditions of debris flow mitigation engineering in Wenchuanearthquake zone. Maliu gully and Shuida gully were selected for dam-simulated byFLO-2D numerical model, to review the change of hazard zone after dams were builtin the gully. There are5dams were built to prevent debris flow in each gully. In orderto achieve the better contrastive effect, two conditions have been design: Engineeringwas not damaged and Engineering was damaged. The correction of the terrain had been done before the numerically simulated. In the first condition, due to the role ofthe dam, the scale of Shuida gully accumulation area was reduced47%, deposit depthdecreased obviously in two gullies. Due to landslide, there are many huge rocks in thebasin. Field investiga-tion shows that most of the concrete dams destroyed bymassive impact when debris flow in Wenchuan area. In the other condition, In orderto reflect the dam failure process, Simplified Levee Failure Model of FLO-2D wasbeen chosen. In this model, dam failure process was permitted to design on the basisof field investigation, The main parameter for the model are Dam crest elevation,Dam failure base elevation, Dam failure width, Vertical failure rate and Horizontalfailure rate. For the convenience of hazard analysis, according to dam damagedcondition, each gully8times simulation is designed.The final simulation results show that in the event of damaged dam,accumulation zone area significantly greater than natural conditions, about1~1.5times. At the Moment of dam damage, the maximum velocity of debris flow wasImprove about3.4~4times after the dams; the maximum discharge of debris flowwas Improve about2~6times after the dams. The results conform to the fieldinvestigation, It shows that the Simplified Levee Failure Model of FLO-2D is suitablefor the research in the study area.The hazard assessment (engineering conditions) in the study area is based on thedebris flow influence intensity, return period and damaged condition. The damagedcondition divided into two cases: Only the first dam damaged; Whole mitigation damsdamaged. By the two cases, we can define a change interval for the damage of dams.Finally, it has overlay by the hazard zoning when the dams not damaged. The resultsshow that the total discharge smaller the control effect is better. On the whole, themain change of hazard zone is high hazard range reduced, middle hazard range andlow hazard range increase. |
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