Long-term performance of mega-nourishments: Role of directional wave climate and initial geometry

Abstract

Concentrated mega-nourishments, built as coastal protection measures for decadal time scales, are intended to diffuse, that is to erode and to supply sand to the nearby beaches and dunes by profiting of the natural drivers. Here, we aim to quantify the role of the wave incidence angle and initial geometry on the long-term evolution of mega-nourishments, in particular the Zandmotor (the Netherlands), using a nonlinear morphodynamic model for large-scale shoreline dynamics. Synthetic wave climates based on measurements are created to systematically vary the wave conditions. Simulations show that mega-nourishment diffusivity decreases linearly with an increasing percentage of high-angle waves (>45° incidence with respect to the global shoreline orientation in deep water). For wave climates with more than 80% of high-angle waves, erosional hotspots develop at the sides of the mega-nourishment. In unimodal high-angle wave climates, hotspot growth rates are large and alongshore migration of the nourishment can increase up to 40 m/year. The role of initial geometry is studied by creating synthetic nourishments that vary in shape, asymmetry, and volume. Slender initial shapes are less diffusive than wider shapes. The initial asymmetry influences the asymmetry in sand feeding to adjacent beaches throughout 50 years, which for symmetric nourishments is controlled by the imbalance in the wave climate. Smaller nourishments than the Zandmotor are more diffusive. This work contributes to a more general understanding of mega-nourishment evolution and may serve as guide for future mega-nourishment design.