Two-dimensional depth-averaged tidal and sediment transport models have been developed to simulate tidal currents and suspended sediment concentrations in the lower Tamar estuary in SW England. Tidal resuspension is the main process responsible for the removal of material from the bed in the Tamar estuary. A depth-averaged hydrodynamic model was used to generate harmonic constants for elevation and velocity. Tidal prediction was then used to simul ate the velocity and to estimate the bed shear stress in the sediment transport model. The tidal model has been validated against tide gauge, current meter and bed-mounted ADCP data. High and low water tidal heights have been simulated with an accuracy of 0.15m while tidal current speeds were reproduced to within 0.1ms^-1. Good agreement was obtained in simulations of the movement of drogues that were re leased in the estuary. Numerical modelling of tidal processes is relatively well established but sediment transport, particularly in estuaries, has received less attention due to the complex nature with which cohesive sediment is eroded from the bed.
Suspended sediment concentrations were obtained from the lower estuary using
optical back scatter sensors. The data showed a background concentration of 0 .015-0.030 kgm^-3 which persisted throughout most of the spring-neap cycle. At spring tides the concentration increased to 0.25-0.30 kgm^-3 at low water; however, there was no comparable increase at the times of maximum tidal flow. This suggests that the advection of sediment from farther up the estuary was more important than local resuspension at the instrument site. The sediment model was used to simulate erosion, transport and deposition because sediment movement is dominated by the suspended load in the Tamar. The observed background concentration was reproduced by including a 2µm grain size to represent the low settling velocity wash load. During spring tides the fine grained bed sediment, of 25µm characteristic grain diameter, was resuspended in the upper model region and advected down estuary on the ebb tide in
agreement with the observed low water concentration maxima.