Size-selective sediment transport and cross-shore profile evolution in the nearshore zone

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Srisuwan, Chatchawin
Work, Paul A.
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Cross-shore bathymetric evolution in the nearshore zone often leads to threatening consequences such as beach erosion and shoreline retreat that concern the coastal community. A new, comprehensive cross-shore morphodynamic model was developed that can be used to describe and predict these phenomena. The study included both physical and numerical models that were designed to focus on the influence of sediment size characteristics on the cross-shore sediment transport process. For a profile equilibrium timescale, three types of beach profiles with different sediment mixtures were simulated in a small-scale, random-wave flume laboratory using erosive, storm, and accretive wave conditions. Dynamic relationships between the sediment grain sorting and beach profile changes were found to be evident as size-graded sediment fractions tended to relocate to different energetic zones along the cross-shore profiles. Existing phase-averaged wave and circulation models were utilized together with several new intra-wave modules for predicting important hydrodynamic parameters that were validated using the experimental data. A novel, multi-size sediment transport model was formulated to compute individual transport rates of size-graded sediment fractions while accounting for their interaction and non-linear size dependencies. The model was coupled with a new grain sorting model that resolves cross-shore grain sorting and vertical grain lamination. Compared to a traditional modeling approach, the new comprehensive model proved to offer superior modeling accuracy for both profile evolution and sediment grain size change. The use of the model is most advantageous for a condition with intensive grain sorting, a common scenario on a natural beach profile. Equilibrium beach profile is also better simulated by the model as size-graded fractions are predicted to relocate to different zones where they could withstand local hydrodynamics. Other new components that also help improve the modeling capability include the terms for wave-breaking and bed-slope effects, wave-crest sediment flux, and acceleration-induced bottom-shear stress. Besides superior profile modeling accuracy, sediment size characteristics and their spatial and temporal variations are also a useful set of information provided by the new model.
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