On sandy coastlines dunes provide a barrier of protection from strong environmental forces that can naturally occur during storm events including storm surges that expose the dunes to large waves. A set of laboratory experiments were used to investigate the morphological processes during the erosion of a steep dune face under bichromatic wave conditions for two different mean water level elevations, corresponding to storm surges and waves that collide with or overwash the dune. In the collision regime, episodic slumping due to the undercutting of the dune resulted in sudden erosional events followed by long periods of wave-driven reshaping at the dune toe. In the overwash regime, dune erosion was faster and occurred at a more consistent rate. Small scale bedforms (ripples) measured during the overwash test evolved in height faster and to greater overall heights than collision test while bedform lengths were not affected by the change in water level. A numerical model, XBeach, was calibrated to examine the ability to predict erosion of the steep dune due to waves in the two water level regimes. XBeach was not able to recreate the spatial variability of the significant wave height profile from the laboratory measurements; however, mean velocities were in good agreement with observations suggesting that bed shear stress is well estimated. During mobile bed simulations of erosion in the two regimes, the model was in agreement with measured dune erosion after initial adjustment. XBeach was very sensitive to several parameters that control the rate of erosion including the critical avalanching slope under water, the threshold water depth and the sediment transport formulation. The model did not perform well at predicting erosion rates until these parameters had been modified. Overall, XBeach performed better when simulating dune erosion in the overwash regime than the collision regime. / Thesis (Master, Civil Engineering) -- Queen's University, 2014-04-01 14:54:35.257

Wave energy has a great potential in many coastal areas thanks to a number of advantages: the abundant resource, the highest energy density of all renewables, the greater availability factors than e.g. wind or solar energy; and the low environmental and particularly visual impact. In addition, a novel advantage will be investigated in this work: the possibility of a synergetic use for carbon-free energy production and coastal protection. In this context, wave energy can contribute not only to decarbonising the energy supply and reducing greenhouse emissions, but also to mitigating coastal erosion. In effect, wave farms will be deployed nearshore to generate electricity from wave energy, and therefore the leeward coast will be exposed to a milder wave climate, which can potentially mitigate coastal erosion. This thesis aims to determine the effectiveness of wave farms for combating coastal erosion by means of a suite of state-of-the-art process-based numerical models that are applied in several case studies (Perranporth Beach,UK; and Xago Beach, Spain) and at different time scales (from the short-term to the long-term). A wave propagation model, SWAN, is used to establish the effects of the wave farm on the wave conditions. The outcomes of SWAN will be coupled to XBeach, a costal processes model that is applied to analyse the effects of the milder wave conditions on the coast. In addition to these models, empirical classifications and analytical solutions are used as well to characterise the alteration of the beach morphology due to the presence of a wave farm. The analysis of the wave farm impacts on the wave conditions and the beach morphology will be carried out through a set of ad hoc impact indicators. Parameters such as the reduction in the significant wave height, the performance of the wave farm, the effects on the seabed level and the erosion in the beach face area are defined to characterise these impacts. Moreover, the role played by the key design parameters of wave farms, e.g. farm-to-coast distance or layout, is also examined. The results from this analysis demonstrate that wave farms, in addition to their main purpose of generating carbon-free energy, are capable of reducing erosion at the coast. Storm-induced erosion is significantly reduced due to the presence of wave farms in the areas most at risk from this phenomenon. However, the effects of wave farms on the coast do not lend themselves to general statements, for they will depend on the wave farm design (WEC type, layout and farm-to-coast distance) and the characteristics of the area in question, as shown in this document for Perranporth and Xago. In summary, this synergy will improve the economic viability of wave farm projects through savings in conventional coastal defence measures, thereby fostering the development of this nascent renewable, reducing greenhouse gas emission and converging towards a more sustainable energy model. Thus, wave energy contributes to mitigating climate change by two means, one acting on the cause, the other on the effect: (i) by bringing down carbon emissions (cause) through its production of renewable energy, and (ii) by reducing coastal erosion (effect).