Dynamics of Turbidity in the Tweed Estuary

Abstract

Results are presented of turbidity measurements made in the Tweed Estuary during 14–29 September 1993. The observations covered a spring-neap period of very strong and relatively weak tides, which included strong wave activity at the mouth of the estuary, the aftermath of a strong freshwater inflow event and a minor inflow event which coincided with neap tides. Turbidity levels between the mouth and the limit of saline intrusion during this period were observed to lie in the range 2–30ppm. Temperature–salinity relationships, based on rapid sampling throughout the estuary, often indicated conservative mixing between riverine and coastal waters. Turbidity–salinity relationships throughout the estuary were approximately linear for most of the lower salinity range (<30) although the relationships varied throughout a tidal cycle. At the highest salinities, in the lower 2·5km of estuary, significantly enhanced turbidities occurred during strong, onshore swell-wave conditions. Near-mouth turbidities were very low (<5ppm) at high-water (HW) when the swell-wave height was small (<0·3m). There was no correlation between near-mouth turbidity at HW and tidal range, whereas the correlation between near-mouth turbidity and swell-wave height explained over 90% of the variance in near-mouth turbidity. The temporal trends in freshwater turbidity and freshwater inflow during the fieldwork period were fairly similar. A statistically significant relationship existed between these variables when freshwater turbidity was correlated against inflow 30h earlier. A ‘model’ of turbidity, based on 5 days of inflows and used to hindcast turbidity from 14 days of inflows between 16–29 September, explained over 80% of the variance in freshwater turbidity. Therefore, inflow exerted a significant control on the fluvial turbidity. It appears that very fine-grained particles were responsible for the observed turbidity in the central and upper reaches of the Tweed. During the flood, in the presence of strong wave activity in the coastal zone, larger sediment was rapidly winnowed in the lower reaches, close to the mouth, whereas fine silt particles remained in suspension and reached the limit of saline intrusion.