Observations of currents, salinity, turbidity and intertidal mudflat characteristics and properties in the Tavy Estuary, UK

Results are presented from a study of the velocity, salinity, temperature and turbidity behaviour on intertidal mudflats bounding a 400 m wide cross-section in the central reaches of a small, macrotidal, ria estuary (the Tavy Estuary, Southwest England, UK). Measurements were made during low freshwater inflow, summer spring tides. The mudflats comprised a muddy mixture of predominantly silt and clay, with bulk densities typically in the range 1.2–1.4 g ml−1. Bed sediment in the main channel, and on the upper shores of both banks, comprised a mixture of predominantly coarse, non-cohesive sediment, with very small fractions of silt and clay. Two near-bed instrument packages were continuously deployed during the observational period, one on each of the intertidal mudflats located on opposite sides of the estuarine main channel. The instruments recorded water level, velocity, temperature, salinity and turbidity at 0.25 m above the bed. Tidal-cycle measurements of these variables were additionally made throughout the water column in the cross-sectionʹs main channel. Longitudinal and vertical surveys of temperature, salinity and turbidity were made throughout the estuary in order to aid interpretation. Maximum bed shear stresses were flood dominant in the main channel and strongly ebb dominant on the upper mudflats. Currents over the intertidal mudflats had much slower peak speeds than those in the main channel. The data presented here indicate that vertically mixed, relatively high-salinity, high-turbidity waters flooded onto the upper mudflats during spring tides, and that the suspended particulate matter largely settled to the bed there, both during the flood and over high-water slack. With one exception, there was little evidence of any subsequent strong sediment resuspension during the salinity-stratified ebb (at least when depths >0.25 m). These results are consistent with the long-term depositional environment associated with rapidly rising sea levels of the early Holocene, and with the present, relatively slower rate of sea-level rise.