Sediment circulation in mixed gravel and shingle bayhead beaches on the South East Dorset coast.

Abstract

This thesis investigates and attempts to clarify the morphological characteristics, processes and sedimentology of five local mixed gravel and shingle beaches. A conceptual framework of process response has been adopted which considers energy inputs, sediment transport and coastal morphology, under conditions of limited sediment supply and a "closed cell" situation. Onshore field investigations using conventional methods of surface sediment sampling, together with beach profiling, platform and cliff measurements provided quantitative data with respect to contemporary sediment inputs, sediment characteristics and transport. Extensive and intensive measurements of beach sections described spatial and temporal morphological and volumetric change and revealed a neutral sediment budget at the scale of 12-14 months. The beaches are in equilibrium with prevailing and dominant south westerly wave regimes; prolonged periods of south easterly wave climates influence the foci of wave energy and cause significant littoral drift and exposure of the chalk platform. The offshore data used were derived from available sources and supplemented by fieldwork by the author. Investigations in the nearshore zone by side-scan sonar and echosounder revealed the morphological and sedimentological nature of the seafloor along the northern shore of Weymouth Bay. Sediment distribution and bedforms suggested preferred sediment transport paths. Within each bay sediment sampling by grab and/or divers elucidated the nature of each sediment cell. The distinct differences of textural composition and the presence of natural offshore barriers to sediment movement highlighted the sedimentological/morphological containment of each bayhead unit. Theoretical considerations and field data have helped to gain a better understanding of the relationship between cliff, beach, platform and nearshore processes and illustrate that selected embayments along the northern shore of Weymouth Bay are morphologically contained sediment cells sharing the same hydrodynamic system.