A double layer-averaged model for stratified sediment-laden flow in open channels

Abstract

Sediment-laden flows in open channels can be sharply stratified vertically, characterized by a double-layer flow structure composed of a subaqueous sediment-laden flow layer immediately over the bed and an upper clear-water flow layer. Typical examples include dam-break flows and reservoir sediment-laden flows featuring turbidity currents. In general, sharply stratified sediment-laden flows involve a number of physical factors, including sharp flow stratification, inter-layer exchange, active sediment transport, and substantial mass exchange with the bed. Double layer-averaged models are attractive in modelling such flows in connection to its vertical structure. However, existing double layer-averaged models have either partly or completely ignored the primary features of stratified open-channel sediment-laden flows and thus are not generally suitable. In the present thesis, a two-dimensional double layer-averaged model has been developed, explicitly incorporating the fundamental physical factors and therefore generally applicable for sharply stratified sediment-laden flows in open channels. First, the governing equations of the new model and the employed numerical algorithm are presented. Then, the model is applied to investigate mobile-bed dam-break flows due to instantaneous full dam break and progressive failure of a dike and landslide dams. Enhanced performance of the new model is demonstrated over the previous models. Most notably, it clearly justifies the physical necessity to incorporate sediment mass conservation. Next, the proposed model is applied to investigate reservoir sediment-laden flows featuring turbidity currents. The model is benchmarked against turbidity currents due to lock-exchange and sustained inflow. It is revealed that an appropriate clear-water outflow is favorable for turbidity current propagation, and also conducive to improving sediment flushing efficiency. As applied to prototype-scale turbidity current in the Xiaolangdi Reservoir in the Yellow River, China, the model successfully resolves the whole process from formation to recession. Following that, the hyperbolicity of the model equations is analyzed as related to dam-break flows and reservoir turbidity currents. The present model is demonstrated to preserve hyperbolicity and thus avoid Kelvin-Helmholtz instability. Computational tests for reservoir turbidity currents reveal that an excessive clear-water outflow would keep the turbidity current from being spoiled, and improves sediment flushing efficiency correspondingly.