Mathematical modelling of morphological changes and hyperconcentrated floods in the Yellow River Mathematical modelling of morphological changes and hyperconcentrated floods in the Yellow River

(CSC) " , " Het Lamminga Fonds " , and the Sino-Dutch collaboration project " Effects of human activities on the eco-morphological evolution of rivers and estuaries " (08-PSA-E-01) funded by the Royal Dutch Academy of Sciences (KNAW) and the Chinese Ministry of Science and Technology (MOST) within the framework of the Programme of Scientific Alliances between China and the Netherlands. Summary I Summary Hyperconcentrated flow/flood is a water-driven sediment transport phenomenon, which is characterized by high sediment concentrations between normal sediment-laden flow and debris/mud flow. In a hyperconcentrated flow, strong interactions exist between water flow, sediment, and river bed, which may not only change the flow rheological properties, but also affect the characteristics of sediment transport and bed deformation. In the lower reach of the Yellow River (China), hyperconcentrated flows/floods are frequently observed (with sediment concentrations > 200 kg/m 3) and primarily Newtonian, turbulent flows (van Maren et al., 2009a). This reach is characterized by two special, and possibly unique, hydrodynamic and sediment transport phenomena that are associated with hyperconcentrated flow: a downstream increasing peak discharge (at a rate far exceeding the contribution from tributaries) during hyperconcentrated floods, and a downstream decreasing runoff due to water diversions. Assuming spatially continuous diversions along a constant-width channel, previous studies suggest a longitudinally convex bed at the equilibrium state. However, the validity of a convex bed profile, for discrete diversions in natural channels of longitudinally varying width, remains to be justified. Also, such equilibrium analysis does not reveal the morphological time scale (MTS) associated with water diversions. Moreover, though many explanations have previously been proposed for the peak discharge increase, they have focused on only one possible mechanism (e.g., bed roughness change, bed erosion, floodplain influences) and no consensus has been achieved. The underlying physics still remain largely unknown. Research efforts are therefore needed to further investigate these two issues, which comprise the main work of the present PhD research and this thesis. For the study of the downstream peak discharge increase phenomenon, mathematical modelling is the main research method, together with the field data analysis. High-resolution morphodynamic modelling of complex fluvial processes, such as in a hyperconcentrated flood, has so far been limited by model accuracy or computational efficiency. In order to account for the strong interactions during the hyperconcentrated flood and to acquire accurate and efficient solutions in the field scale, a fully coupled morphodynamic model has first been developed …