Computational Investigation Of The Dynamic Processes Of Extreme Water-Sediment Hazards

Dam-break flows, glacier dam break floods and flash floods are typically extreme hazards which exist widely in the natural environment. The extreme flood events feature with rapid onset, difficult to predict, short time to effective response, and long term and catastrophic damage. Study about the mechanism of these processes can not only enhance the understanding of knowledge but also facilitate the risk mitigation and defense management.A well-balanced coupled2D shallow water hydrodynamic and non-capacity sediment transport model has been developed based on structured rectangular mesh. The OpenMP technique is used to parallelize the code. The computational efficiency has been largely enhanced. The model has also been extended to that based on adaptive structured non-uniform mesh. Stage gradient and sediment concentration gradient, which are important in cases involving rapid bed evolution and intense sediment transport, are incorporated in the adaptation indexes. A series of computational tests show that the proposed model has advantage in saving computing time80%to93%as contrasted against its counterparts based on fixed meshes, while similar accuracy is maintained. It should find wide applications in modelling large-scale shallow water flows featuring sediment transport and morphological changes.A one-dimensional coupled hydrodynamic and sediment transport model has been used to investigate the floods due to cascade dam break on the background of Yangtze River and its main tributary. This study contributes to the understanding of how and to what extent the hydraulics of a flood due to a cascade dam break differs from its counterpart due to a single dam break. Most notably, as compared to a single dam break, the occurrence of peak stage rise and/or advanced timing of cascade dam break floods have been revealed. Bed mobility is found to modify the flood processes (especially immediately following the dam break), the peak stage rise and advanced arrived time of the flood compared to the case of a single dam break. The larger the relative dam height of the cascade dams or initial reservoir water depth, the more the relative peak stage rises The quantitative results in the present study are dependent on the empirical formulations introduced for closing the computational model (i.e., those related to resistance and sediment entrainment for mobile bed cases) and the upstream, downstream and internal boundary conditions. Nevertheless, as the same closure formulations and boundary conditions are sensibly specified for both cases of a single and a cascade dam break, the finding of the present study holds, i.e., peak stage rise and/or advanced timing of the cascade dam break flood generally occur downstream compared against that of a single dam break. The heights and spacing of the cascade dams and/or initial reservoir water depths are primary factors. The present finding has significant implications for flood risk management. In practice, existing flood defense works may need to be redesigned and flooding alleviation schemes reformulated.The coupled2D hydrodynamic and sediment transport model is employed to simulate an ancient glacier dam-break flood in Altai Mountains. This may be the first time to apply the coupled hydrodynamic and sediment transport model to simulate both the flood routing downstream of the dam site and to investigate the geomorphologic effect of the Altai flood. The maximum stage profiles over fixed and mobile beds are in agreement with field observations. Specifically, the maximum stage profile over mobile bed is closer to the field data than that over fixed bed. This demonstrates that sediment transport and morphological evolution are very important in the megaflood, which should be taken into account properly.A full2D hydrodynamic model is proposed for rainfall induced flash flood. The model is first tested against two experiments. It suggests that the present model can model the overland flow well with or without infiltration at the laboratory scale. Then it is applied to model two real flash floods in Lengkou Catchment, Shanxi Province in China. It is found that the spatial distribution of rainfall is crucial for flash flood modelling even in the small catchment, which clearly necessitates refined rainfall gauging. Most notably, for a larger flash flood induced by heavier rainfall, the modelling results agree with observed data better despite the largely tuned model parameters, which clearly characterizes the paramount role of rainfall in dictating the floods. Based on the full hydrodynamic model, two real-time flash flood warning schemes are proposed incorporating with the hazard index. At last, the two schemes are applied to Lengkou cacthment as an example for flash flood warning.