| Summary: | In this thesis, the process-based numerical model Delft3D 4 (Deltares, 2021a,b) is used to investigate how tidal range and significant wave height affect, and potentially prevent, the development of river deltas within a range of idealised simulations. In order to reduce the parameter space and hence simplify analysis, focus is restricted to just the effects of wave height and tidal range, following the assumption that the effects of discharge are relatively linear and predictable—i.e. lower river sediment flux equates to reduced likelihood of delta formation. The primary objective of the work, therefore, is to determine if there exist limits of significant wave height and / or tidal range beyond which delta formation is prevented, and to elucidate the relevant processes where (if) this is found to be the case.
To this end, we investigate how delta development is affected under increasing values of wave height and tidal range, both independently and in combination, leading to the identification of four distinct regimes of delta formation. We then discuss the mechanisms by which delta formation is prevented in our simulations with the largest tidal ranges and wave heights, hence also identifying a fifth regime of delta-suppression. Each of these five regimes is qualitatively compared to representative real world examples of (non-)deltas, and the work is positioned against existing literature regarding the limits of delta formation and classification of deltas by process-dominance.
Finally, based upon the observation that waves and tides in combination act to functionally ‘diffuse’ sediments away from the mouths of rivers in the simulations, it is hypothesised that deltas do not form when such diffusion is of sufficient magnitude to prevent the formation of persistent deposits (and hence deltas) within the vicinity of river mouths. This hypothesis is tested via comparison to a 1D along-shore sediment diffusion formula with a source term representing river sediment discharge. This formula is found to match sediment distributions of the process-based simulations with increasing accuracy under the larger significant wave heights and tidal ranges modelled.
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