Beach and oceanographic processes surrounding Jersey, Channel Island

Abstract

The Gulf-Normano Breton lies within the Western English Channel but can be considered distinct in terms of both physical and biochemical oceanography. Datasets covering the physical dynamics of the Gulf, particularly those concerning the stability of the coastline of Jersey, the largest of the Channel Islands within the Gulf, are identified and described. Overall, the coastline of Jersey, is stable in the short-to medium-term. Changes in beach profiles and volumes are discussed. Between 1992-1999, losses and gains of beach material have been balanced, either spatially or temporally over a period of 3-6 years in the major embayments (St Ouen's Bay, St Brelade's Bay, the south east coast and Grouville Bay). Underlying these fluctuations, there is a slow loss of material from the upper beach (possibly, to the lower beach) however, these changes are not statistically significant. In contrast, the beach profile volumes within St Aubin's Bay show a highly significant trend of accumulation. St Ouen's Bay forms the focus of this study. Re-analysis of geophysical data (from May 1998) showed that the sea floor area is dominated by exposed bedrock: namely metamorphic Jersey Shale and granite. The most recent survey (April 2001) has confirmed that sand (and, hence, bedforms) in the embayment are confined to; (a) the intertidal (perched) beach area, (b) within the Great Bank and, (c) in shallow sand lenses and channels contained within the rocky sea floor. Great Bank itself is a type 3a Banner (or Headland) Bank resting on the offshore abrasion platform. Whilst the Bank has many bedform types superimposed (ranging from ripples to large dunes 3m in height), there has been no major change in the nature of the sea floor, between May 1998 and April 2001 and the bank is stable in the medium term. Moreover, the Bank is assumed to be controlled by the present hydrodynamic regime; the ADCP confirms that the sea floor interacts with the tidal currents, to produce local eddy structures. There is only limited interaction between the offshore and onshore sand areas. However, onshore / offshore transport may occur within the nearshore sand lenses. Evidence for this are provided by bedforms (side- scan sonar, April 2001) the on/offshore tidal currents (ADCP) and deposition within the beach cores. The beach of St Ouen's Bay can be classified as ultra-dissipative; as such it is expected to be stable. Conversely, beach erosion has been documented since 1759. Although there is insufficient evidence to assess the long term sediment transport paths, the stratigraphy of the intertidal beach area has been examined on the basis of the collection and analysis of sediment peels. These sections show that sediment transport on the beach was dominated by tidal patterns and cross-shore sediment transport. The beach exhibits considerable short term variations in morphology but, unless the underlying peatbeds are eroded, the beach levels are assumed to recover naturally within a few years. is hypothesised that 'St Ouen's Bay contains a stable volume of sand which is transported around the intertidal area by a series of standing edge waves (although evidence for this mechanism is required). The morphology of the bay appears to be in equilibrium with the present climatic conditions as there is a recycling of material over the medium term (3-5 year period/. However, it is uncertain if the bay will be able to maintain its equilibrium, under conditions of rising sea levels and changes in climatic conditions (such as storm intensity and frequency). Whilst the depth-integrated tidal patterns have been studied, there is little knowledge of the current velocities at the sea floor and there is substantial scope for improving the understanding of both the tidal and wave climate within the embayments of Jersey. In addition, the rates of sea levels change around the Island could be better understood, by establishing the historical datum levels. To determine the significance of the trends in beach volume over the longer term, it is necessary to establish the underlying baseline and to extend the profile lines to the 'depth of closure'.

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