Geomorphic impacts of rapid sediment-laden flows through computational modelling

Outburst floods are one of the most catastrophic natural hazards for populations and infrastructure. They are usually generated from storm runoff, rapid melting of glacial ice or man-made and natural dam breaks, such as river dikes, volcanic debris dams and landslide damns etc. Such high-magnitude sudden onset floods generally comprise of an advancing intense kinematic water wave that can induce considerable sediment transport. Therefore, the exploration and investigation of sediment-laden outburst floods cannot be limited solely to water flow but must also include the flood-induced sediment transport. Understanding the complex flow-bed interaction process in large (field) scale outburst floods is still limited, not least due to a lack of well-constrained field data, but also because consensus on appropriate modelling schemes has yet to be decided. In recent years, attention has focussed on the numerical models capable of describing the process of erosion, transport and deposition in such flows and they are now at a point at which they provide useful quantitative data. Although the "exact" measure of bed change is still unattainable the numerical models enhance and improve insights into large outburst flood events.
In order to model and better understand heavily sediment-laden flows and resulting geomorphic impacts, this thesis adopts a layer-based conceptual model which separates the system into an active bed layer, a water sediment mixed sheet flow layer and a suspension layer. Correspondingly, a layer-based hydro-morphodynamic model is proposed fully considering both bedload and suspended load based on shallow water theory. The model system is primarily composed of a combination of three modules: (1) a hydrodynamic module; (2) a sediment transport module; and (3) a morphological evolution module. In the thesis, firstly, a robust hydrodynamic model is proposed and tested including addressing the source terms and wetting/drying issues for application to irregular beds. Then based on the robust hydrodynamic model, a layer-based morphodynamic model is developed and solved numerically by an advanced second-order Godunov-type finite volume method. A series of theoretical and experimental tests are applied to validate the model in terms of both hydrodynamic and morphodynamic aspects. The results of these tests show that the developed models can predict the hydrodynamic and morphodynamic process effectively with good agreement with theoretical and experimental results. To demonstrate a real application, a full-scale volcano-induced jökulhlaup or glacial outburst flood (GLOF) at Sólheimajökull, Iceland is reproduced by the proposed model. The simulation of the sediment-laden outburst flood is shown to perform well, with further insights into the flow-bed interaction obtained from the modelling output. These results are beneficial to flood risk management and hazard prevention and mitigation.