Suspended sediment concentrations in the Tamar estuary

Abstract

Suspended sediment concentrations, obtained from the lower Tamar estuary in South West England using optical back scatter sensors, showed a depth-averaged background concentration of 0.02 kg m −3 throughout most of the spring–neap cycle. On spring tides the depth-averaged concentration increased to 0.25–0.40 kg m −3 either side of low water; however, the concentration maxima did not correspond to the time of maximum tidal flow, which suggests the influence of sediment advection. The observations were simulated using two-dimensional depth-averaged models of tidal currents and suspended sediment concentrations. Harmonic constants generated by the tidal model were used to estimate the advective terms and the bed shear stress in the sediment transport model. The sediment model included three size fractions which represented the low settling velocity wash load (2 μm), the cohesive (25 μm) and the non-cohesive (75 μm) suspended loads. During spring tides the simulated fine bed sediment (25 and 75 μm fractions) was resuspended in the upper model region and advected down estuary on the ebb tide. The sediment transport model reproduced the observed low water concentration increases to within a factor of two to five. The quantity of eroded sediment increased from medium tides to spring tides, as maximum bed shear stress increased, and formed a mobile pool of suspended sediment. After spring tides less sediment was remobilised on successive phases of the tide and accumulation occurred. The finer silty material (25 μm) was deposited in the shallower upper model region whereas the sand sized particles (75 μm) accumulated in the deeper parts of the estuary, which was in general agreement with published bed composition data. At neap tides, in accordance with the observations, the simulations showed no evidence of sediment resuspension.