Recent improvements in the Delft3D-model (e.g. inclusion of roller model and breaker delay concepts) combined with the implementation of the more advanced transport formulation of TRANSPOR2004, formed the impetus for an application of the upgraded Delft3D-Online model to investigate the effects of various nourishment designs on the nearshore morphology. The main purpose of this study was to illustrate the effect that various nourishment designs have on the nearshore morphology and to demonstrate the capabilities of the Delft3D-Online model in its present state. With the implementation of gradient boundary conditions (a so-called Neumann boundary condition) in Delft3D, Roelvink and Walstra (2004) demonstrated that a combination of tide, wind and wave-driven currents does not induce any visible boundary effects along the lateral boundaries. The obvious advantage of the Neumann boundary condition is that the model domain may now be focused more on the region of interest without having to worry about boundary effects which might negatively affect the model predictions. Furthermore, it was proven that Delft3D could be reliably run as a profile model by considering only one longshore grid cell. Based on the Roelvink and Walstra (2004) results, an innovative dual modelling approach was adopted in which Delft3D was applied as a 3D area model using parameter settings of a calibrated Delft3D 2DV profile model. This enabled a clear distinction between longshore effects such as leeside erosion, sediment trapping and cross-shore phenomena such as feeder effects. The intercomparison of the investigated nourishment designs has shown that the design in which the nourishment was constructed in the trough between the outer and inner bar has the largest positive influence on the sand volume changes in the inner bar region and inter-tidal beach area. However, the construction height only had a small effect for the remaining alternatives in which the nourishments were constructed against the seaward slope of the outer bar. The design in which the length of the nourishment was varied did not lead to a significantly increased or decreased trapping of sand behind the nourishment for the long and short designs. The volume of the nourishment did have a relatively significant effect; a reduction of 50% in the nourishment volume (per unit alongshore length) resulted in a 75 % reduction in the seaward migration of the MKL-position.
To further our insight into the morphological behaviour of shoreface nourishments, nourishment schemes may be evaluated in the field with the help of high resolution video monitoring techniques. A combination of Argus video imagery and inverse modelling techniques may be used to map bathymetric evolution in the surf zone. The Subtidal Beach Mapper (SBM-2DH) is a model that provides this type of bathymetric information. A state-of-the-art version of SBM-2DH can be used to conduct case studies on seasonal bed evolution from summer to the subsequent winter season as well as a case study on bed evolution at the storm time scale. This information can be used to analyse whether winter bathymetry differs significantly from the summer bathymetry. To analyse bed level variability during a storm in more detail based on video-derived bathymetries requires further improvement in the SBM-2DH model. Notwithstanding the latter conclusion, it is important to note that there is currently no alternative available for monitoring bathymetric change during a storm event. |
