Morphodynamic developments in distributary channel networks: impacts on flood hazards in the Pearl River Delta and the Rotterdam Rijnmond channel network

River deltas are known to be threatened by sealevel rise, increased peak discharges and storm surge levels, trapping of sediment in reservoirs and engineering works. The present proposal aims to establish and understand how in delta distributary networks channel morphologies adjust to changing boundary conditions, and to quantify the impact of such adjustments on tidal propagation patterns and associated flood hazards. Two parallel PhD studies of the Pearl River Delta and the Rotterdam Rijnmond channel network are planned, based on joint hydrographic field campaigns, simulation of key morphological channel adjustments and hydrodynamic modelling of historical flow regimes. The field campaigns will be focussed on the division of water and sediment at tidally-influenced channel junctions, which have rarely been the subject of a comprehensive field study. In each of the two study areas, channel junctions will be monitored over a semidiurnal tidal cycle at neap tide and at spring tide, both during high flow and under low flow conditions. During field measurements, flow patterns, bedload and suspended load sediment transport, grain size distributions, and bed morphology will be monitored. Apart from conventional survey methods, bedload sediment transport will be estimated acoustically, using the so-called bottom tracking functionality of acoustic Doppler current profilers. The subsequent part of the planned research aims to understand mechanisms driving the dominant morphodynamic developments in the two study areas, synthesizing field measurements and simulations with morphodynamical submodels, nested in all-encompassing hydrodynamical models of the two delta regions. To yield a synoptic overview of the spatio-temporal changes in flood hazards, the hydrodynamic response to morphological adjustments under peak water level scenarios will be investigated. In a final stage, nonlinear interactions between tidal amplitudes, mean sealevel and river discharge will be analysed, to reveal the underlying physical mechanisms in detail.