Mechanics-Based Modelling Of Flood Risk Assessment And Application In A Floodplain Area

Climate change and strong human activities would increase the probability of extreme rainfall and flooding events.In addition,socil-econemic development has increased population and assets densities in floodplain areas.These two results would lead to more serious flood loss in floodplain areas,and the contradiction between people and floods would become more prominent.The Xiaolangdi Reservoir operation reduces the floodplain flooding chances in the Lower Yellow River(LYR),and however flow capacities in local reaches are still less than4000 m~3/s in the LYR.Therefore,the flood risk still exists in floodplain areas of the LYR,especially in the"secondary suspension"reaches.Floods will not only seriously endanger the Yellow River leeves,but also threaten the lives and properties of about 1.9 million people in floodplain areas.Therefore,the contradiction between socil-econemic developments and flood prevention is a major problem in floodplain areas in the LYR.In this paper,sketched physical experiments and two dimensional numerical modelling are used to investigate evolution characteristics and mechanics of widening processes in farm dike breach,to propose an algorithm for selecting optimal refuge and escape routes,to propose the indicator parameters and evaluation methods for evaluating flood risk of different submersed objects.Finally,a mechanics-based 2-D integrated model of modelling flood routing and evaluating flood risk were presented,and it was applied to the Lankao-Dongming floodplain area in the LYR.These results can not only enrich the research contents of river dynamics,but also provide technical support for the synergistic regulation and flood risk management in floodplain areas in the LYR.Major conclusions include:(1)The evolution characteristics and mechanics of farm dike breach width were studied using sketched physical experiments of widening processes in farm dike breach,and then an empirical formula to calculate breach width was proposed based on mechanical actions between water flow and soils.Experimental results show that the lateral widening mechanism in farm dike breach could be clarified by two aspects,including lateral erosion on the dike soil below the water level and cantilever dike soil collapse above the water level.The flow velocity at breach site increased first and then decreased.The water depth kept downward trend,but the decrease rate gradually became smaller.Water depths,breach velocities,discharges,and lateral widening rates increased with an increase of inlet discharge.An empirical formula to calculate the lateral widening rate was proposed based on the analysis of influenced factors and mechanical actions between water flow and soils,and the calculated widening process agreed well with the measured process.These results can provide experimental data not only for investigations into farm dike breach,but also for the validation of numerical models.(2)An algorithm of selecting optimal refuge and escape routes in floodplain areas was proposed.The influence factors including the shortest route,flood risk and escape speed for flooded people are considered in the algorithm.The determination methods of optimal refuge and escape routes under the conditions of a specific flood event and different flood events were proposed.The Dijkstra algorithm was adopted to derive the shortest routes.The two-dimensional hydro-sediment-morphodynamic module was used to simulate flood routing processes.At the same times,the flood risk for human body was evaluated using mechanical instability formular,and evacuation time was calculated by considering escape speed under different flow conditions.Thus,flood routing processes and evacuation movements could be synchronously simulated.The final escape moment was selected as the key factor of selecting optimal refuge and escape routes in a sigle flood routing process.Fuzzy Analytic Hierarchy Process was used to determin optimal refuge and escape routes under the condition of different flood events.(3)A mechanics-based integrated model of flood routing process modelling and flood risk assessment in a floodplain area was proposed.The modle integrat a two-dimensional hydro-sediment-morphodynamic module,a farm dike breach module,an optimal refuge and escape routes determination module,and a flood risk assesmenrt module.The two-dimensional hydro-sediment-morphodynamic module was used to simulate the processes of flood inundation in a floodplain area using a cell-centered finite volume method.A mechanics-based formula for the incipient velocity of a flooded human body of toppling instability was used to assest hazard degree,and the empirical formulas for flood loss of buildings and crops were adopted to evaluate their flood losses.Then the integrated modle was verified against three physical experimental data,with a favourable agreement being obtained between the calculated and experimental data.Further,the numerical results show that the calculated water depths with Manning coefficients equal to 0.05、0.08、0.12 and 0.16 were in good agreement with measured water depths under conditions of 3×3、4×4、5×5 and stagger model buildings arrangements.(4)The integrated modle was applied to the Lankao-Dongming floodplain area in the LYR.The flood routing process,optimal refuge and escape routes,and flood risk of flooded objects were calculated in a real flood event and in a hypothetical flood event.The widening process of farm dike breach and flood routing process were anlysed for the real flood event in 2003,especially for widening process of farm dike breach.It was found that the scour hole appeare firstly at about 10-40 m below the breach gate in evoluation process of farm dike breach.With the widening process of the breach,the range of the scour hole gradually expand towards the upper reaches of the breach gate.The water depth,flow velocity and discharge at the breach gate increased,with a final breach width of 61 m.It is in a good agreement with the actual observation data(58 m).Then the flood routing process and flood risk in the Lankao-Dongming floodplain area were anlysed in the hypothetical flood event in“1958”under the condition of Xiaolangdi Reservoi operation.The results show that the averaged flood loss rates of human bodys,buildings and corns would be 80%,80%and 45%at 144 h.The optimal escape routes were selected by modelling evacuation processes.The results indicate that 3 hours would be got if evacuees selecte the optimal routes.Finally,the optimal escape routes for Lankao-Dongming floodplain area were determined using FAHP(Fuzzy Analytic Hierarchy Process)under the condition of considering these two flood events.