New Understanding and Predicting Storm Impacts on Gravel beaches

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Gravel barriers and beaches consists of sediments coarser than 2 mm and are very common in England and Wales. They extend along more than 1,000 km of its coastline and protect low-lying back-barrier regions from flooding, and coastal cliffs from undercutting during storm events. Their importance to society is widely acknowledged and coastal engineering structures (e.g., seawalls and groins) and management techniques (recharge, recycling and reshaping) are extensively used, at significant cost, to maintain and enhance their protective ability. Unfortunately, as highlighted in a recent report commissioned by DEFRA, regular breaching and extensive storm damage has occurred at many gravel barrier sites in the UK, and this is likely to increase in the future as a result of sea-level rise and enhanced storminess due to climate change. The DEFRA report concludes that limited scientific guidance is currently available to provide beach managers with operational management tools to predict the response of gravel barrier and beaches to storms. Specifically, we are currently unable to predict under what conditions a gravel barrier will withstand a certain storm event, or whether the barrier will be overwashed, or even breached. Similarly, we have no means of evaluating the effect of certain management interventions (seawall construction, beach nourishment, profile reshaping) on gravel barrier stability.The principal aim of the proposed research is therefore to obtain new understanding of storm impacts on gravel beaches and barriers, and to develop a predictive tool that is capable of modelling these impacts. Rather than developing a new model from first principles, our approach is to use an existing model that has been applied successfully to sandy beaches and modify the model for use on gravel beaches using field data. The model used as a starting point is the XBeach model, which has been specifically been developed to predict hurricane impacts on sandy barriers.The proposed research is best summarised by the following closely linked objectives. (1) A 4-week field experiment will be held on a gravel beach to measure swash processes, sediment transport and beach response under a range of wave conditions, including a storm. (2) These field data will be used to help modify, parameterise and calibrate the existing XBeach model so it can be used for gravel beaches. (3) An additional field data set on storm response will be collected on nine UK gravel barrier systems, representing a range of morphological, sedimentological, wave and water level conditions. (4) These field data will be used to validate and verify the XBeach model developed under (2). (5) A tool will be developed for end-users, based on the model formulated under (4), for predicting berm formation, overtopping, overwashing and breaching of gravel beaches and barriers. The 3-year project is led by Professor Gerd Masselink and involves two co-investigators, one post-doctoral research fellow, one project student, two Visiting Researchers and two Project Partners. The research team is ideally placed to conduct the proposed research, because over the past 5 years they have acquired: (1) knowledge and understanding of gravel beach dynamics under calm and energetic conditions: (2) experimental capability and instrumentation for measuring gravel beach morphodynamics under storm conditions; and (3) expertise with XBeach to customise the model for use on gravel beaches. Involvement of two Project Partners (one from industry and one from government) will ensure that the results from this research will be appropriately disseminated and used for practical applications.