Beach morphological response to surface and subsurface flows: measurement and modelling

The purpose of the visit is to partipate in a field experiment in Australia with the Coastal Engineering Research Group at the University of Queensland in order to collect a high-quality data set which can be used to validate numerical models which simulate short-term beach profile evolution and long-term coastal change. In particular, the research will focus on validation of the swash and groundwater components of the models in order to improve understanding of the relationships between swash motions, beach groundwater flows and beach morphological change. The swash zone is the part of the beach alternately covered and exposed by wave uprush and backwash, where final wave energy dissipation occurs. Although this region above the still water level is perhaps the most critical in terms of shoreline management, it is the area of the nearshore environment about which least is known and is a difficult part of the beach to measure and model. The swash zone is highly dynamic and influences the morphological response of beaches over both short (i.e. storm) durations and longer time-scales. Erosion and accretion of the beach profile, and the resulting movement of the position of the shoreline, are a direct result of sediment transport processes occurring in the swash zone. The interaction of surface and subsurface flow regimes in the swash zone affects the morphology of the intertidal beach by controlling the potential for onshore sediment transport and deposition above the still water level, or transport offshore to the inner surf zone, and swash zone processes provide an important control on beach recovery in response to storms. However, the complex fluid and sediment interactions in the swash zone are not well understood or modelled. At present, most numerical models of shoreline change and beach morphological evolution either do not include sediment transport processes in the swash zone or simplify the representation of swash and groundwater processes to the extent that the predictions are not realistic. This failure to model the swash zone correctly means that beach profile evolution and sediment transport at the shoreline will not be adequately represented. In particular, models of beach profile evolution are not generally successful in predicting beach accretion. Since an accretionary event is defined by the deposition of sediment above mean sea level, the lack of detailed knowledge of swash and beach groundwater dynamics is an important factor in the inability of profile models to simulate accretionary events accurately. The limitations of swash zone modelling represents a particular problem in long-term coastal modelling, where coastal change is simulated over the timescale of years or longer. Swash processes are even less likely to be represented adequately in such models. When process details are not well understood, uncertainty is amplified as the number of variables and dimensions increase and as small-scale processes are integrated up to larger scales in both space and time. As a result, predictions of the long term response of the coastline to changes in environmental conditions must be viewed with caution. However, time spans of decades and lengths of coastline in the order of tens of kilometres are the scales at which coastal managers must take decisions, and there is a need to improve the representation of swash processes in such models.

Beaches are the primary defence against wave attack along much of the world's coastline. With flooding and coastal erosion predicted to increase significantly as sea level rises, understanding and prediction of beach response is critical to coastal managers. In particular, process-based predictions of beach dynamics and groundwater movement are central to the design of coastal protection schemes and coastal aquifer management. Continued improvement in the numerical modelling of beach profile evolution is important in the further development of 'soft' solutions to coastal protection. In addition, long-term prediction of sediment transport and morphological behaviour in the coastal zone in response to human intervention or to changing environmental conditions is an increasingly important issue. The research project will help to improve understanding, and improve predictions, of both short-term and long-term beach morphological evolution. Improvement of the short-term beach profile model (Li et al. 2002) will provide better predictions of beach crest height (and therefore the potential for wave overtopping), shoreline position, and beach profile changes. Further validation of the long-term model (Villarroel-Lamb 2007) will improve simulation of the evolution of coastal morphology, including the effects of storms, cyclone conditions and sea level rise, on the long-term behaviour of beaches. Better predictive capacity will benefit UK coastal managers and government through reduced expenditure on coastal protection works. In particular, the outcome of the research will benefit Defra and the Environment Agency, who are responsible for managing the coastline and reducing coastal erosion and flooding due to beach overtopping. The research will also be of use to coastal engineering firms and local authorities, who must devise strategies to deal with beach response to storm events, and to coastal groups which are involved in developing Shoreline Management Plans for beach management over the next century. The research will also be of benefit to Australian coastal managers. Coastal environments and resources are of great significance for Australia economically and socially, as over 80% of the population lives in the coastal zone, which is under increasing pressure in terms of resource management, ecological balance and infrastructure demands. The Australian coast is a significant natural resource for one of the country's largest industries - tourism - and is under increasing pressure for development. The Gold Coast, where the field site is located, is a particularly important area for coastal tourism in Australia. Researchers from Geoscience Australia, the national agency with the primary responsibility for research in coastal zone processes, will participate in the field experiment, which is also supported by the Gold Coast Council. Coastal urbanisation in the Caribbean region is increasing due to both population growth and growth in the tourism industry, and tools are needed for improved management of Caribbean beaches. By assessing and validating the swash sediment transport module used in the long-term model against additional field data, the model will improve its predictive capability of long-term morphology on Caribbean coastlines and will provide guidance for coastal managers in the region. All senior members of the team have close contacts with coastal managers in Defra and the Environment Agency and Australian governmental and research organisations such as the Queensland Department of Environment and Resource Management, the NSW Department of Environment and Climate Change, Geoscience Australia, and the Gold Coast City Council, and will pass reports and data directly to them. Information will also be provided directly to beach users during the field experiment. The COZONE network will also be used to publicise the project and disseminate the research outcomes amongst UK researchers and practitioners.