Modelling mud-induced wave damping with Delft3D and SWAN-Mud
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Fluid mud is suspended cohesive sediment on top of the consolidated bed. The fluid mud is generated by an external force that liquefies the bed sediment. When the water in the overlying layer is in motion, it experiences energy dissipation due to the fluid mud. A large decrease in significant wave height was observed in field experiments for beaches with fluid mud compared to beaches without this mud (Wells & Coleman, 1981). Often this fluid mud forms mudbanks in front of a shore, alternating with areas without mud, the interbank area. These mudbanks are found, for example, in front of the coast of India or on the north coast of South America. This study aims to learn more about mud-induced wave damping as a function of mud and wave characteristics. To reach this aim, various idealized model simulations were set up in which mud and wave characteristics were varied in 1D and 2D; the model was then applied to a case study on the Suriname coast. SWAN-Mud is combined with Delft3D-Wave, an application from Deltares that works around SWAN-Mud and improves the model’s usability for more extensive and complicated experiments. Different formulations for dissipation of wave energy due to fluid mud are implemented in the model SWAN-Mud, but experience with these formulations in real-life test cases is limited. At the onset of this study, we corrected an error in the dispersion relationships embedded in SWAN-Mud. Furthermore, different bugs in the source code have been restored, which improved the model. For the DeWit method, a dissipation term had to be included in SWAN-Mud. In order to test different mud characteristics, experiments for a 1D setting (transects) were performed, based on Kranenburg (2008). A 2D experiment with an idealized mudbank was conducted for different wave heights, periods, and directions at the domain's boundaries and different wind speeds and directions. The same wave characteristics have been tested for the Suriname experiments as in the 2D experiments. These wave and wind characteristics were based on the ERA-5 model data set, representing summer, winter, and storm conditions. However, more practical scenarios have been conducted to find accurate results. For the wave environment, only one parameter, e.g., direction, was changed in these scenarios. After updating SWAN-Mud, results for the imaginary wavenumber (which measures the rate at which wave height exponentially decreases when the wave propagates through the domain.) correspond to analytical solutions of the dispersion relationships. With the updated model, we could observe the influence of the mudbanks on the incoming waves. Mud dissipation is highest for the waves with the largest wave height. However, a minimal wave period seems necessary for dissipation to happen. Compared to the scenarios without mud, the significant wave height is much smaller when the waves reach the shore. This implies that without these mudbanks, the coast is much more exposed to wave forcing and, e.g. erosion of the coast is more severe. The mud does not only dissipate waves with larger wave periods. Measuring the wave spectra over the mudbanks revealed that the mud dissipates a fraction of high-frequency wind waves generated in the domain. However, a large part of the high-frequency waves still reaches the shore. Nevertheless, the exact morphodynamics and hydrodynamics on and around the mudbanks remain unclear. The influence of the mudbanks on the wave environment and so the coastal morphodynamics is huge. The mudbanks play an essential role in protecting the hinterland of the mudbanks and are related to areas of coastal accretion. In contrast, the interbank areas are related to coastal erosion due to the high waves reaching the shore.