Sediment transport in the Bristol Channel: A review

Abstract

Investigations into the physical oceanographic characteristics of the Bristol Channel and Severn Estuary are reviewed; these include the general pattern of water movement and the influence of high freshwater discharges on the salinity distribution. Various approaches to the problem of defining sediment transport paths in the macrotidal estuarine system, for fine grained and coarse grained material, are summarised. Transport patterns are examined, for example, through: the distribution of surficial sediments and their associated bedforms; seabed drifter studies; the analysis of tidal current and wave data, separately and in combination, because of their influence on bedload transport paths; satellite imagery, within the visible part of the spectrum; and on the basis of output from depth-averaged numerical models. Comparison between the interpretations shows that there are discrepancies and contradictions in the derived sediment transport paths. In particular, it is difficult to explain the up-channel movement of fine grained material against a background of inferred offshore movement, shown by the orientation of sandwaves and the direction of movement of the surface waters. An early model to overcome this contradiction incorporated ‘two-way differential transport’ throughout the water column. Localised contradictions in the transport paths for sand and ‘fine sediment’ occur in the inner part of the Bristol Channel. The most recent conjectural models for sediment movement incorporate an ebb- dominated zone along the central axis of the Channel, together with flood-dominated coastal zones. This lateral variation in transport paths, across the Channel, constrasts with the earlier proposition for vertical differentiation; it is suggested by seabed drifter recovery patterns and confirmed by the output from one of the numerical models. Transport towards the flood-dominated zones at the boundaries is enhanced under the superimposed influence of waves, which is demonstrated through the use of sediment transport formulae and sand tracer experiments. Although the annual contribution of the rivers to the overall sediment budget is shown to be small, the time of retention of waters and sediment within the system could result in fluviatile sediment inputs being retained for tens of years. In an attempt to extrapolate observations of transport processes into the ancient record, a preliminary facies model for the macrotidal Bristol Channel is presented. The model shows the availability of sand increasing in an onshore direction; progressing in this direction, the facies pass from scoured bedrock and lag deposits, through cross bedded megaripple foresets and the topset planar bedding of intertidal flats, to Holocene fill and supratidal deposits.