Sediment transport processes : with particular reference to Hayling Island

Abstract

The post-replenishment behaviour of an artificially renourished shingle beach (using offshore dredged gravel) at Hayling Island is examined in association with the short- and long-term changes occurring within the adjacent coastal zone. Details from an extensive range of data from a variety of sources (including: bathymetric, sedimentological, meteorological, hydrodynamic, mathematical model and described. and investigated information) profile beach are Artificial beach replenishment has become a popular method of sea defence, however, there is relatively little information with regards to its subsequent behaviour. It has been found that fine-grained material, present within the replenishment material, leads to the development of a vertical beach face, and a beach with low permeabilty (a feature observed also at other artificially replenished beaches). This reduces effectively the protective capacity of an artificially replenished beach, when compared with a natural shingle beach. Higher than predicted losses of shingle along the replenishment area are attributed to this unforeseen feature. Along the natural (unreplenished) western section of the Hayling shoreline, a westerly longshore transport of beach material can be attributed to low-energy, 'background', wave conditions. Evidence from wave refraction model results indicate that the offshore bathymetry of the outer East Solent, and the numerous nearshore banks, control strictly the nearshore distribution of wave energy and wave patterns. Nearshore variations in the level of the sea bed are found to be confined mostly to the extensive sand banks (situated at the harbour entrances), with only very localised and nearshore areas of gravel sea bed subject to movement. The evidence indicates that only small volumes of sedimentary material are being transported onshore, and that any onshore movement is restricted to areas only close inshore. The major part of Hayling Bay is considered to be relatively stable over the time-scales examined. The study indicates that sediment movement offshore in this region is limited, however, further information is required to determine the conditions required for movement of 'natural' sea bed gravel deposits.

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