Numerical and physical modelling of different nourishment designs.

Alessio Giardino, Yorick Broekema, Jebbe van der Werf, Arnold van Rooijen, Michalis Vousdoukas

Friday 3 july 2015

14:00 - 14:15h at Mississippi (level 1)

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

Parallel session: 16B. Sediment - Transport

Although the construction of nourishments as coastal protection measure against erosion is becoming common practice in several countries around the world, the design and prediction of the morphological development of nourishments over time is often based on empirical rules and practical experience. This is mainly related to the uncertainties within the currently available numerical models in predicting with sufficient accuracy the hydrodynamics and sediment transport in the surf zone and beach where nourishments are generally applied. In this study, a novel modelling approach was used to simulate the developments of different designs of shoreface and beach nourishments. In particular, a 2DV version of the process-based Delft3D model was used to simulate the hydrodynamics, waves, sediment transport and morphodynamics along cross-shore beach profiles. To account for the effects of short-wave groupiness on the water motion and the accompanying generation of infragravity waves, the “Roller Model” in instationary mode was used (Reniers et al., 2004). Moreover, the spatial and temporal changes in bed composition were computed using multiple fractions and a multi-layered bed stratigraphy. This allows for simulating nourishments with different grain characteristics compared to the native beach sand. The model simulations were validated using a number of lab experiments carried out in the wave flumes of Hannover and Delft, under different erosive and accretive wave conditions. The model shows promising results, both in predicting the morphological development of the nourishments over time (Brier score = “excellent” for not-nourished cases and “good” for nourished cases), and the grain sorting along the beach profiles and adjacent to the nourishment locations. The effect of infragravity waves on the short-wave averaged sediment transport leads to larger peaks in the suspended sediment concentrations. This effect is found essential for a proper representation of the offshore and onshore sediment transport, and the morphological development in case of plane sloping beaches as well as nourished beach profiles.