Modelling and understanding multiple roles of sediment transport in floods

Outburst floods are characterised by being sudden, short-lived and containing high energy. Such kinds of outburst floods generally comprise an intense advancing water wave which can induce considerable sediment loads. Therefore, it is necessary to incorporate sediment transport effects in flood models, instead of just considering water flow. In this paper, a layer-based morphodynamic model is presented. Of course, a model can never represent all the features in the real world exactly; the model here is mainly developed for sheet flow. The model system is composed of a combination of several modules: 1) a hydrodynamic module governed by 2D Shallow Water equations involving sediment effects; 2) a sediment transport module controlling the mass conservation of sediment; 3) a bed deformation module for updating the bed elevation due to erosion and deposition. For applications, a small-scale dyke breach due to flow overtopping and a large-scale volcano-induced glacial lake outburst flood (GLOF) are simulated by the computational model. Modelling results indicate that sediment transport plays multiple roles in the hydraulic events. In some cases, it delays the onset of flow, changing the flow hydrograph. Nonetheless, in other cases, the flood propagation is accelerated due to the incorporation of sediment transport; conversely, the powerful outburst floods induce sediment into motion and cause rapid geomorphic change of river bed, e.g. millions of cubic meters of sediment are eroded and re-deposited for the volcano-induced GLOF event.