Physical properties and processes in the Bristol Channel and Severn Estuary

Abstract

This article firstly describes the physical properties of the Bristol Channel and Severn Estuary in the context of its adjacent seas. These properties include tidal ranges and tidal phases at mean spring tides, storm surge characteristics, wind-driven currents and water levels, and frontal evolution. Simulated data on peak current speeds within the Channel at mean spring and mean neap tides are then presented, together with simulated data on annual mean wind speeds and wave heights. Analyses of observed data are described in the second part of the article. The analyses cover elements of three topics: the influences of tides, topography, runoff, salinity, wind and atmospheric pressure on residual currents and mean water levels; the identification of mechanisms involved in residual water, salt and suspended particulate matter (SPM) transport; and the identification of an exceptionally strong estuarine turbidity maximum (ETM) in the upper reaches of the Severn. Gravitational circulation was evident and strong spring-neap variations in near-bed, mid-depth and upper water column residual currents were identified that demonstrated the importance of non-linear tidal generation due to advection of momentum. Near-bed and mid-depth wind-driven residuals generally were oppositely directed to the wind in the central and inner Channel. A combination of data analyses, analytical modelling of vertical current structure and depth-averaged hydrodynamic modelling was used to interpret features of the data. Analyses of the mechanisms driving residual water, salt and SPM transport at a station in the lower Severn during neap tides showed that vertical shear mechanisms were relatively unimportant to salt and SPM transport. The residual advection and tidal pumping transport mechanisms were quantified.