Observations of fine-sediment transport in a semi-enclosed sheltered natural harbour (Pagham Harbour, UK)

Abstract

Preliminary analysis of results from a series of studies of fine sediment transport in the macrotidal Pagham Harbour (U.K.) have revealed patterns of sediment transport which are related to tidal range and wind speed. This paper will present an analysis of some preliminary results from continuous monitors from two sites, which logged turbidity, salinity, water level and wind speed/direction at a frequency of 10 minutes or less. At one site in particular (the Ferry Pool site), detailed results of the variation in salinity and suspended sediment concentration with depth, and near-bed flow velocity, are presented. In general, shorter, faster flood-tide currents lead to concentrations of suspended sediment which are highest during the first 90 minutes of the flood tide. These decrease rapidly due to settling, during the relatively low velocities around the time of high slack water. The magnitude of the peak flood-tide suspended sediment concentrations appear to correlate more with tidal range, than with wind speed, pointing to a greater sediment mobility during spring tides. Further analysis of the variation in salinity and suspended sediment concentration with depth reveal that these high flood tide suspended solids concentrations are related to short-lived local peaks, both of near-bed velocity and of vertical salinity gradient. Rapid settling of sediment occurs following flow deceleration and vertical mixing of the water with respect to salinity. It is suggested that this mechanism is of great importance in describing the landward transport of fine sediment in harbours of this kind.