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.
Highlights
Concentrated mega-nourishments are kilometer-scale beach nourishments designed to mitigate beach erosion on decadal timescales, and the effects of climate change and associated sea-level rise [1]
In the case of a high-angle wave frequency equal to or greater than 80%, the feeding asymmetry has an irregular behavior at the initial stages due to the erosional hotspots that develop at the sides of the nourishment
The role of the wave angle and the initial mega-nourishment geometry on its decadal dynamics was confirmed with a morphodynamic numerical model based on the wave-driven alongshore sediment transport, which includes the cross-shore transport in a simplified way
Summary
Concentrated mega-nourishments are kilometer-scale beach nourishments designed to mitigate beach erosion on decadal timescales, and the effects of climate change and associated sea-level rise [1]. These mega-nourishments (hereafter MN) rely on the concept that a perturbation tends to diffuse, with erosion of the perturbation itself and accretion at the adjacent beaches due to alongshore gradients in sediment transport. A concentrated feeder nourishment named Zandmotor (hereafter ZM). With its 17 × 106 m3, the ZM is substantially larger than regular nourishments (1 − 2 × 106 m3 [2]). In 2015, a beach-dune nourishment of 30 × 106 m3 was built ∼100 km north of the ZM, and other mega-nourishments are being considered at several locations worldwide (e.g., Sweden, Belgium, UK, and USA)
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