THE RESPONSE OF GRAVEL BEACHES IN THE PRESENCE OF CONTROL STRUCURES

T.T. Coates, N. Dodd

Abstract


Much of the research on beach control structure concentrates on sand beaches. Relatively little work has been done on gravel beaches though they are important features along temperate and sub-arctic coastlines, particularly in the UK. Until recently attempts to manage these beaches and to control erosion have relied on traditional approaches such as timber groynes, designed by past experience and engineering judgement. To improve this situation HR Wallingford have undertaken a programme of flume and wave basin physical model studies followed by the development of numerical models and design guidelines. The most recent phase of this programme has been an investigation of beach response to detached breakwaters. This work was undertaken at a scale of 1:50 in a mobile bed, random wave basin. The breakwaters varied in length, crest elevation and distance offshore. The sea conditions varied in wave period and water level. Measurements were taken of wave heights around the structure, beach planshape development and longshore transport rates. For each breakwater configuration and sea condition an efficiency value has been derived which relates the longshore transport rate in the presence of the structure to the potential open beach rate. These efficiency values can be used to tune breakwater dimensions to the actual long term drift rates of a specific beach, thereby providing a stable beach. This paper briefly reviews some of the previously published work, then discusses the major findings of the recent model tests and suggests methods for applying the results. The results are, at present, only applicable to a limited range of sea and beach conditions, but it is anticipated that the approach will be developed for general application. The findings are also related to a recently completed detached breakwaters and beach recharge scheme on the UK south coast.

Keywords


gravel beach; control structure

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