Publication Abstract

Suspended Sediment Concentrations in the Tamar Estuary

G.R. Tattersall, A.J. Elliott and N.M. Lynn

Suspended sediment concentrations, obtained from the lower Tamar estuary in SW England using optical back scatter sensors, showed a background concentration of 0.02-0.04 kgm ³ throughout most of the spring-neap cycle. On spring tides the concentration increased to 0.25-0.40 kgm³ either side of low water; however, the concentration maxima did not correspond to the time of maximum tidal flow which suggests the importance of sediment advection. The observations have been simulated using two-dimensional depth-averaged models of tidal currents and suspended sediment concentrations. The hydrodynamic model was validated against current meter and bed-mounted ADCP data; tidal current speeds were reproduced to within 5% and good agreement was obtained when simulating drogue trajectories. Harmonic constants generated by the tidal model were then used to estimate the advective terms and the bed shear stress in a 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 µm and 75 µm fractions) was resuspended in the upper model region and advected down estuary on the ebb tide in agreement with the observed concentration maxima. 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. At slack water some sediment settled out of the water column but was resuspended on the following phase of the tide. 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.

Reference:

G.R. Tattersall, A.J. Elliott and N.M. Lynn, 2003. Suspended Sediment Concentrations in the Tamar Estuary. Estuarine Coastal and Shelf Science, 57(4): 679-688.