COASTAL IMPACTS IN THE LEE OF A WAVE ENERGY SITE: WAVES, BEACH MORPHOLOGY AND WATER-USERS (WAVE HUB, CORNWALL, UK)

摘要

The Wave Hub facility in Cornwall (South West UK) is a marine renewables testsite, predominantly designed for the purpose of trialling wave energy convertersprior to commercialisation. Beach water-users such as bathers and surfers are ofeconomic importance to tourism in Cornwall, and during theWave Hub consultationthere were concerns among stakeholders that wave energy extraction would reducethe height and quality of coastal waves for sur ng, as well as a ecting sedimenttransport and beach morphology. This thesis investigates the interaction betweenwave conditions, beach morphology, and beach water-users, and proposes how awave climate altered by wave energy extraction is likely to alter these interactions. Amultidisciplinary research approach is adopted, involving the collection of qualitativeand quantitative social data, the collection of over 5 years of physical wave andbeach morphology data, and predictive modelling of the e ects of an attenuatedwave climate.Quantitative, structured interview data from 403 water-users, collected at twobeaches (Perranporth and Porthtowan) in the lee of Wave Hub, indicate that thepopulation of water-users in the area is predominantly made up of surfers (53%), butbodyboarding and swimming/bathing are also popular activities (29% and 11%, respectively).In-depth semi-structured interviews reveal that water-user perceptionsof wave energy extraction and its potential coastal impacts are constructed usingintuitive risk perceptions, rather than technical understanding. These risk perceptionsare constructed through a weighing of their perception of wave energy devices('technology') and their perception of the coastal environment ('nature'). To investigatehow waves are perceived, nearshore wave buoy measurements collected in 14 mwater depth and transformed to breaking height, are compared to concurrent visualobservations of mean breaker height and period. On average water-users underestimatedsigni cant wave height and period by 48% and 17%, respectively. Accountingfor variations in wave perception, the wave preferences of di erent water-user groupsare determined. Water-users are found to share a common preference towards wavevperiods of 9 - 20 s, but di erent water-user groups are found to have di erent rangesof preferred wave height, which is found to govern whether wave energy extractionwill decrease or increase the occurrence of preferred waves.Previous research indicates that three-dimensional (3D) beach morphology withcrescentic bar and rip features is the primary controller of surf-zone hazard, andalso strongly in uences the quality of sur ng waves at the coast. A dataset of 5.5years of quasi-weekly bar measurements, and quasi-monthly intertidal surveys fromPerranporth beach is used to quantify seasonal to inter-annual changes in threedimensionality.Integrated, cumulative uctuations in wave steepness, wave power,and relative tide range that occur over seasonal time scales are shown to be wellcorrelated to seasonal uctuations in beach three-dimensionality. 3D morphology iswell related to a disequilibrium term that predicts increases or decreases in threedimensionalityby examining the di erence between instantaneous wave conditionsand a temporally varying equilibrium condition, based on a weighted average ofantecedent waves. This indicates that periods of wave regime change between erosivewinter conditions with high steepness waves and accretive summer conditions withlow steepness waves are related to the growth of 3D features, and vice versa, whileextended periods with similar wave conditions drive the beach towards equilibrium.Using a range of realistic and extreme coastal wave height attenuation scenariosdetermined from previous Wave Hub modelling studies, it is predicted that none ofthe scenarios will have a universally positive or negative e ect on the occurrenceof wave conditions preferred by water-users. When used to predict beach threedimensionalityat Perranporth beach, the attenuated wave climates are found toreduce the variability in three-dimensionality. Even an extreme and unrealistic levelof wave energy extraction (100% energy capture) was shown to have an insigni cante ect on the occurrence of preferred waves, and only under an extraction scenariowhere the impact was not varied with wave frequency did this level of attenuationhave a signi cant e ect on the predicted beach three-dimensionality. The inshorewave attenuation from Wave Hub is therefore likely to have an insigni cant e ect onwave conditions and beach morphology of relevance to beach water-users. A numberof observations and recommendations are discussed for the development of a soundand robust methodological approach, which can be used to investigate the e ects ofwave energy extraction on beach water-users at future wave farm sites.