Wave farms and coastal protection on gravel dominated beaches

Abstract

The repercussions produced by the use of fossil fuels, e.g. climate change and sea level rise, have increased attention on the development of renewable energies. The European Commission has adopted the replacement of carbon fuels by renewable energy sources as one of the main objectives for the XXI century. Among these, wave energy is poised to become one of the most important due to the extensive worldwide resource and comparatively low environmental impacts. Although this renewable energy is yet in an early stage of its development and further research is required, the global resource, including the Mediterranean Sea, provides a promising future for wave energy which can enhance the presence of renewable energies in the global energy mix.

However, for wave energy to become a fully fledge carbon-free energy source, the repercussions for nearshore hydro- and morphodynamics must be fully understood prior to undertaking any wave farm installation. In fact, wave farms (i.e. arrays of wave energy converters) have been recently proposed to fulfil a dual function as renewable energy producers and coastal defence elements countering erosion. Although these impacts have been previously studied on sandy beaches, gravel dominated coasts have not been addressed so far. These coasts are common in high latitudes and steep hinterlands and they are growing due to the use of coarse sediments for nourishment works on sandy beaches. They are common in the UK, the Mediterranean coast, Canada, New Zealand and Denmark among other areas.

The research investigates the impacts of wave farms on gravel dominated beaches. To this end, changes produced by the presence of a wave farm on the shoreline evolution of a gravel dominated beach (Playa Granada, southern Spain) have been studied. This beach has suffered erosion problems since the discharge of the Guadalfeo river, the main source of sediments for the beach, was regulated in 2004 by the construction of Rules' Reservoir. With the construction of this river damn, 85% of the discharge was regulated which altered system's dynamics. Approximately 0.3 hm3 of sediments have been lost which have produced a maximum retreat of the shoreline of 87 m since the construction of the reservoir. The impact on the wave field of different wave farm scenarios and designs has been studied by means of a third-generation spectral wave propagation model (SWAN). Different parameters of the wave field at the breaking line have been extracted from the results of the wave propagation model. To this end, the fraction of breaking waves in each cell was obtained, and the breaking line was fixed in those cells with a 5% of waves breaking. These results were used to apply a longshore sediment transport (LST) formulation on the study site. Finally, LST rates computed were implemented in a one-line model to address changes on the shoreline position. This model, which is based on the continuity equation, relates the advance (or retreat) of the shoreline to the volume gradient, calculated as the sediment balance by means of LST rates. The selected WEC to form the modelled wave farms was WaveCat, an overtopping-type device, which has shown promising results in terms of coastal protection in recent research. This WEC is formed by two hulls, similar to those found in a catamaran, with a total diameter of D = 90 m.

Wave farm effects on the wave field and shoreline position were assessed by means of several ad hoc indicators, comparing significant wave height, LST rates, shoreline advance and dry beach surface between scenarios with wave farm and the natural (no-wave farm) baseline scenario. These indicators act as a benchmark of the performance of each wave farm scenario with respect the natural situation, allowing to study the impact of the wave farm on each parameter involved in the shoreline evolution. Finally, the dry beach surface difference is obtained as a representation of the overall impact on the whole stretch of beach. More precisely, the effects of the alongshore location, the layout and the inter-device spacing of a wave farm were studied.

Regarding the alongshore position of the wave farm, the results indicate that the wave farm location is a key parameter in terms of coastal protection; indeed, it was found that only three of the eight locations studied generated a weighted increase in dry beach surface with respect to the baseline. The rest of wave farm locations generated loss in dry beach area at the studied stretch of beach with respect to the baseline, thus producing negative effects in terms of coastal protection. The most advisable location for the wave farm increased the dry beach area in 25.58 m2 per storm whereas in the worst location 13.17 m2 are lost. These results show that the alongshore position of a wave farm needs to be carefully selected, as this parameter is able to turn the behaviour of a coast from accretionary to erosionary. In addition, different layouts consisting of a variation in the number of rows of wave energy converters composing the wave farm keeping constant their number are investigated. Results indicate that the best layout is that composed of two rows of WECs. In this case, the increase in dry beach surface with respect to the baseline scenario is 25.94 m2, whereas the farms with 3 and 4 rows yield a lower increase in beach surface. This shows that the extent of coast covered by the wave farm prevails over the number of rows as the impact of the shadow produced by the wave farm reaches a longer stretch of coast. Finally, four inter-device spacings were modelled. In this case, a lower spacing between devices increases local accretion in the lee of the wave farm but rises erosion elsewhere, whereas a greater spacing counter this erosion but accretion is also reduced. The best results were obtained for a spacing of 2D, where D = 90 m is the space between the two bows of the WaveCat.

As main findings of this thesis, the research carried out in this work shows that the most important parameter in order to achieve the expected results in terms of coastal protection performance of a dual wave farm is the alongshore position. The analysis of the alongshore position effects shows that the location of the wave farm is a key parameter, not only in terms of energy production but also in order to use the wave farm as a coastal management element. Once the alongshore position is fixed, layout and inter-device spacing may help to improve the obtained results. A wave farm with a greater number of rows does not perform better in terms of coastal protection, as the extension of the stretch of coast covered is more relevant. A similar conclusion can be drawn from the inter-device spacing impact study. Wave farms with a lower inter-device spacing produce higher local peaks of accretion in their lee. On the contrary, wave farms with greater spacing reduce this erosion but accretion is also weaker, affecting the dry beach area difference with respect to the baseline. In this case, the wave farms with an intermediate spacing achieve a balance between these two extremes and stand as the best alternatives.

The methodology followed in this work constitutes a useful decision-aid tool which can be applied to other gravel-dominated coasts worldwide. It considers the repercussions for nearshore hydrodynamics and shoreline morphology. The results obtained open up the possibility of using wave farms not only to produce carbon-free energy but also to mitigate coastal erosion problems on coasts across the globe.