Reshaping of realistic beach nourishments under high angle wave conditions.

Bastiaan Huisman, Victoria Curto, Roderik Hoekstra, Matthieu de Schipper

Friday 3 july 2015

14:15 - 14:30h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling

Parallel session: 16B. Sediment - Transport

The impact of very obliquely incoming waves (i.e. larger than 45° with respect to the shore-normal) on the morphology of nourishments has been investigated in literature using either a one-line approach or quasy-2D models. Typically, these studies predicted downdrift nourishment migration and the generation of alongshore sand waves for coasts with persistent high-angle wave conditions. The size of the nourishments or the cross-shore profile shape applied in these studies is however much larger than what is common for the Dutch coast. This research therefore aims at exploring the reshaping mechanisms of beach nourishments of realistic Dutch proportions as a result of high-angle waves. To this end a Delft3D numerical model study was performed testing two conceptual nourishment configurations of 2 km long and a cross-shore extent of respectively 250m or 1000m. First, the numerical model was applied to provide a qualitative indication of the impact of high-angle waves by examining the spatial patters of wave height and direction as well as the initial alongshore sediment transport fields. These computations show that for the Dutch coast situation nourishments with a relatively small cross-shore perturbation (250m width and 2km length) are not likely to develop high-angle wave instabilities even for continuous very obliquely incoming waves. As the initial sediment transport patterns indicate that instabilities might be present for large nourishments (1km width and 2km length) the latter case was tested more rigorously with a morphodynamic simulation. These simulations show that indeed some lee side effects could be expected for continuous high angle waves. Further investigation with alternating low angled and high angled waves, revealed that the sequencing and frequency of the high angled waves can be of importance. Although large nourishments can migrate downdrift during high-angle wave conditions and generate some leeward erosion, a relatively short period with low-angle waves can wipe out the features of the leeward erosion.