Observations of velocity, salinity and suspended sediment concentrations in a semi-enclosed tidal lagoon (Pagham Harbour, UK)

Recent concerns about sea level rise and the loss of areas of natural habitat have focussed attention on the problems associated with siltation in tidal lagoons. In the UK, Pagham Harbour is a small (4 km2) tidal system that is part of a series of tidal lagoons characterised by a narrow entrance and multiple fresh water input points. While some progress has previously been made in measuring long-term siltation rates in these tidal lagoons, there is a need for a greater understanding of the mechanisms by which fine sediment is transported by tidal action, fresh water flows and wind-induced wave action. Three field deployments are described at 3 different sites in the lagoon for three spring-tide tidal cycles, and results presented of varying water depth, velocity and vertical profiles of salinity and suspended sediment concentration. From these data, some preliminary conclusions have been made about the likely mechanisms that control fine sediment transport near the tidal limits of these harbours, including the influence of strong vertical salinity gradients, modified at one site by the influence of intermittent pumped discharge of fresh water into the lagoon. These tend to enhance flood-tide sediment transport by increasing near bed velocity. It is further suggested that the absence of significant observed sediment transport at a point in mid lagoon, away from the tidal limit, points to a cycling of fine sediment at and around the tisal limits. No new material is imported, and no material is lost, from the pool of sediment at these tidal limits, under these relatively quiescent conditions. It is posited that any changes to the overall sediment budget of the lagoon linked with siltation must occur at other times of year, possibly under more stormy conditions.