Influence of tidal range and river discharge on suspended particulate matter fluxes in the Forth estuary (Scotland)

Abstract Moored current meters and transmissometers have been used to study the transport of suspended particulate matter (SPM) in the lower Forth estuary. Deployment periods of upwards of 11 weeks enabled both semi-diurnal and neap/spring aspects of the tidal regime to be examined. Additional surveys have been made to examine particle size distributions and the extent of stratification within the semi-diurnal tidal cycle. On the semi-diurnal scale, SPM concentrations are closely related to current velocities. In near bottom waters, the current velocities required to make resuspension and deposition evident are 0·60 and 0·30 m s −1 respectively. Within the semi-diurnal tidal cycle, these velocities are exceeded for longer periods when the tidal range is large, thus spring cycles are the dominant influence in controlling the transport of SPM within the estuary. Close to the sea-bed, residual sediment transport was landwards for all six periods of spring tides observed in detail. Nearer to the surface however, transport was seawards for five of the six periods. The near-bed landward transport during spring tides has a strong dependence on the mean tidal range, an increase from 4 to 4·8 m resulted in an order of magnitude increase in sediment transport. The seaward transport is relatively independent of mean tidal range and for tidal ranges of 4 m is of similar magnitude to landward transport. Individual large spring tides exert a disproportionately large influence on sediment transport. Single tides within a period of springs can be responsible for up to 17% of the total observed transport. It was anticipated that fluvial discharge would be a significant factor in lower estuary sediment transport as it is in the dynamics of upper estuary turbidity maxima. However, no firm direct evidence was found for either increased SPM concentrations due to downstream flushing or to the suppression of sediment transport due to high fluvial discharge enhancement of stratification. This lack of influence is attributed to fluvial-induced stratification being most significant during neap tides when sediment transport in the lower estuary is minimal.