Dynamics of large-scale shoreline perturbations

Shorelines around the world are rarely smooth and they can present undulations and cuspate shapes. On the one hand, human actions can cause shoreline perturbations via beach nourishments, which in turn perturb the wave field that drives the morphological changes. On the other hand, there can be natural perturbations in the coastal system due to positive feedbacks between the wave forcing and the evolving bathymetric contours. In this thesis, the dynamics of mega-nourishments and shoreline sand waves are investigated. A morphodynamic model based on the wave-driven alongshore sediment transport, including cross-shore transport in a simplified way and neglecting tides, is improved and applied to the Zandmotor mega-nourishment on the Dutch Delfland coast. The model is calibrated with the bathymetric data measured from January 2012 to March 2013 using measured offshore wave forcing. The calibrated model reproduces the evolution of the shoreline and depth contours until March 2015. The modelled coastline diffusivity during the 3-yr period is of 0.0021 m^2/s, close to the observed value of 0.0022 m^2/s. In contrast, the coefficient of the classical one-line diffusion equation is 0.0052~m$^2$/s. Thus, the lifetime is predicted to be of 90 yr instead of 35 yr. This difference is attributed to the role played by the 60% of oblique waves in that climate. The dynamics of mega-nourishments are further investigated by designing analytic mega-nourishments with different asymmetry, shape and volume. It is found that narrow initial shapes are less diffusive than wider shapes and that the smaller nourishments are more diffusive than the bigger ones. Also, it is found that the initial asymmetry can influence the asymmetry in feeding capacity to adjacent beaches throughout 50 years. The mega-nourishment is also forced with wave climates of different obliquity percentages. Its diffusivity decays linearly with increasing obliquity and for very oblique wave climates (more than 80%) hotspot areas are formed at the sides (due to high-angle wave instability). The growth rate of the erosion hotspots is especially high for unimodal wave climates, which also makes mega-nourishments to migrate alongshore at rates of 40 m/yr. Kilometric-scale shoreline sand waves have been observed in the northern flank of the Dungeness Cuspate Foreland (southeastern coast of U.K.). They consist of two bumps separated by embayments with a 350-450 m spacing. We have analyzed 36 shoreline surveys of 2~km length using the Discrete Fourier Transformation (DFT), from 2005 to 2016, and seven topographic surveys encompassing the intertidal zone, from 2010 to 2016. The data set shows two clear formation events, which are correlated with moments were the wave energy of high-angle waves is dominant over the low-angle waves. Also, a linear stability model based on the one-line approximation is applied to the site. It predicts accurately the formation moments, with positive growth rates in the correct order of magnitude for wavelengths similar to the observed ones. All these results confirm that the shoreline undulations in Dungeness are self-organized and that the underlying formation mechanism is the high-angle wave instability. The two detected formation events thus provide a unique opportunity to validate the existing morphodynamic models that include such instability.