Morphological modelling in the vicinity of groynes: An extended application in Delft3D-RAM including tidal impact

The shoreline develops on several time scales. At some places along the coast the amount of sand in a cross-section decreases continuously and the shoreline shifts landward. This phenomenon is called structural erosion and is often caused by an increasing longshore sediment transport along the coast. When it is necessary to protect the coast, e.g. because the sea defense function of the coast is threatened, groynes can be applied to reduce the longshore transport. The erosion is then prevented locally. Nevertheless past the protected area leeside erosion occurs due to the full presence of the original sediment transport. In order to make a proper design for a shore protection with groynes, insight in the effectiveness as well as the side-effects is essential. In a previous study a model application was set up for the simplified case of a coast subjected to a uniform and stationary wave field in absence of the tidal impact. However, the tide does influence the morphological development, e.g. due to the displacing beaker zone, the drying and flooding of the beach and the presence of tidal currents. In this study the application is extended with the tide and an improved representation of the groyne within the computational modules is proposed. The applied computer model is Delft3D-RAM, a part of the Delft3D software-package, developed by WL Delft Hydraulics. The model is a process-based model that computes the 2dimensional current, wave and sediment transport field and translates these into bed level changes. The computational time stays limited due to the "Rapid Assessment of Morphology" (RAM) procedure. In the RAM-procedure, it is assumed that the local current and wave field do not vary much as long as the bed level changes are small. Within a certain period, defined as the morphological time step, the current and wave field are not recomputed, and the transports are simply adapted for the changed bathymetry. The RAM-procedure also includes a method to account for the development of the dry beach resulting from the morphological changes in the wet area. This approach, the "uniform beach profile method", redistributes bed level changes close to the waterline in such a way that the upper part of the beach profile shifts forward or backward while retaining its shape as initially predescribed. The groyne is implemented in the numerical grid using the "thin dam" option in the flow module and the "sheet" option in the wave module. The combination of two thin dams perpendicular to the shore and one connecting the two tips not only blocks the currents and sediment transports completely, but also efficiently separates the morphological development at both sides of the groyne. A set of three sheets is required to block the waves and prevent interpolation effects from the numerical wave grid to the flow grid. The proposed modelling of the groyne leads to satisfactory results. The model set up of the original application without tide is used as a starting point for the set up of an extended application including the tidal effects. Both the flow grid and the wave grid are refined around the groyne to obtain a more detailed representation of the local flow and wave field. In the extended application, the tide is implemented as a single sinusoid with a period of 12 hours. The propagation of the simulated tide corresponds more or less with the tide along the Dutch coast. The amplitude is taken higher than found in practice, namely 1.5 m. The application calculates the flow and waves iteratively to account for the interaction between the tide and the waves. The transports are then calculated throughout the tide and averaged over the tidal period. The bed level changes are computed from the averaged sediment transport field. Due to the computations over the full tidal period and an iterative calculation of currents and waves the application including tidal impact is computationally intensive.Hydraulic EngineeringCivil Engineering and Geoscience