PREDICTING BREACHING OF SHINGLE BARRIER BEACHES-RECENT ADVANCES TO AID BEACH MANAGEMENT

Shingle barriers and ridges provide a natural line of coastal protection and flood defence for many sites around the coast of England. Examples include: Chesil Beach, Porlock, Hurst Spit, Medmery and Cley. Long term barrier evolution is linked with sea level rise, longshore transport rate and sediment supply, local geomorphology, incident hydraulic conditions, and the barrier geometry. Changes in any combination of these controlling variables may result in either building or degrading of the barrier; the latter may result in periodic overtopping, overwashing and landward migration. The evolutionary process is a significant management issue, particularly when an increasing frequency of barrier overwashing results in migration towards coastal developments or important habitats. Intervention in the natural evolutionary process is commonplace, yet there is little practical management guidelines available. Management strategies vary considerably; some barriers are artificially managed by beach recharge, or reforming of the crest following storms, whilst others have been allowed to develop naturally. Regular breaching and extensive storm damage has occurred at many sites, but limited scientific guidance is currently available to provide beach managers with operational management tools, to predict the response of these beaches to storm conditions. Some sites are currently the subject of high profile strategic management reviews, or have already incurred high capital or maintenance expenditure costs; significant decisions must currently be made with a limited understanding of the way that these beaches perform under storm attack. This paper examines the relationship between the geomorphological barrier response, hydraulic conditions, and the pre-storm barrier geometry at a temporal scale based on typical storm events. A wide range of geometric configuations have been examined. Key variables which affect the profile response of shingle barriers to storm activity are identified and their influence is quantified. An empirical framework, based upon an extensive series of physical model and field studies has been analysed to develop a dimensionless predictive threshold model. The empirical framework has been tested and partially validated within the development of coastal strategy plans and in operational beach management, at several sites.