A numerical model to simulate short-term beach and dune evolution

Sammanfattning: Sediment transport in the cross-shore (CS) and associated changes in the beach profile, especially during storms, have been topics of widespread concern. Since storms are often accompanied by high water levels and large waves, large quantities of sand from the beach and the dune are typically transported offshore, leading to severe beach and dune erosion, which threatens the integrity of buildings and infrastructure near the coast. With climate change, sea levels are expected to rise and storms are likely to grow in numbers and intensity, which further aggravates coastal flooding and erosion. The capability to quantify storm impact on the beach and dune is becoming increasingly important both for coastal engineers and managers. Thus, in this thesis, a new numerical model to simulate hydrodynamics, CS sediment transport, as well as beach and dune evolution under varying waves and water levels was developed. Particular focus was put on describing the response of the subaerial region of the profile, involving the foreshore, the berm, and the dune. A variety of modules, involving wave transformation, CS currents, mean water elevation, and CS sediment transport across the profile, by including relevant physics in combination with a set of theoretical and empirical formulas were included in the model. The theory employed in the new model was first calibrated and validated against data from the SUPERTANK laboratory, where the experimental cases selected encompassed several types of profile evolution, including berm erosion and bar formation, berm flooding and erosion, and offshore mound evolution. Good agreement was obtained between calculations and measurements, indicating that the model can produce robust and reliable predictions of CS transport and profile evolution in the nearshore. Then, the model was applied to two field sites, Cocoa Beach and Perdido Key Beach in Florida, USA, to simulate the evolution of a mound placed in the offshore exposed to varying non-breaking waves and water levels. In addition, several scenarios with different mound volume and location designs were investigated to indicate potential uses for the model. The results illustrate that the model can be used for providing guidance to the design of mounds in the offshore that is of great value in coastal planning and management, especially for beach nourishment. Finally, the model was applied to simulate the dune erosion during storms, where the wave impact theory was used for describing the impact of waves on the dune. Both laboratory data and field data were used for model testing. The results indicated that the model could reproduce the dune retreat rather well. Overall, the new numerical model could be a useful tool in practical engineering projects for predicting CS sediment transport and beach and dune profile evolution.