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 a nd turbidity behaviour on intertidal mudflats bounding a 400 m wide cross-s ection in the central reaches of a small, macrotidal, ria estuary (the Tavy Estuary, Southwest England, UK). Measurements were made during low freshwa ter 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 sedi ment, with very small fractions of silt and clay. Two near-bed instrument p ackages 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-section's main channel. Longitudinal and vertical surveys of temperat ure, salinity and turbidity were made throughout the estuary in order to ai d interpretation. Maximum bed shear stresses were flood dominant in the mai n channel and strongly ebb dominant on the upper mudflats. Currents over th e intertidal mudflats had much slower peak speeds than those in the main ch annel. The data presented here indicate that vertically mixed, relatively h igh-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 ex ception, there was little evidence of any subsequent strong sediment resusp ension during the salinity-stratified ebb (at least when depths > 0.25 m). These results are consistent with the long-term depositional environment as sociated with rapidly rising sea levels of the early Halocene, and with the present, relatively slower rate of sea-level rise, (C) 2000 Elsevier Scien ce Ltd. All rights reserved.