DYNAMICS OF TURBIDITY IN THE TWEED ESTUARY

Results are presented of turbidity measurements made in the Tweed Estu ary during 14-29 September 1993. The observations covered a spring-nea p period of very strong and relatively weak tides, which included stro ng wave activity at the mouth of the estuary, the aftermath of a stron g freshwater inflow event and a minor inflow event which coincided wit h neap tides. Turbidity levels between the mouth and the limit of sali ne intrusion during this period were observed to lie in the range 2-30 ppm. Temperature-salinity relationships, based on rapid sampling thro ughout the estuary, often indicated conservative mixing between riveri ne and coastal waters. Turbidity-salinity relationships throughout the estuary were approximately linear for most of the lower salinity rang e (<30) although the relationships varied throughout a tidal cycle. At the highest salinities, in the lower 2.5 lan of estuary, significantl y enhanced turbidities occurred during strong, onshore swell-wave cond itions. Near-mouth turbidities were very low (<5 ppm) at high-water (H W) when the swell-wave height was small (<0.3 m). There was no correla tion between near-mouth turbidity at HW and tidal range, whereas the c orrelation between near-mouth turbidity and swell-wave height explaine d over 90% of the variance in near-mouth turbidity. The temporal trend s in freshwater turbidity and freshwater inflow during the fieldwork p eriod were fairly similar. A statistically significant relationship ex isted between these variables when freshwater turbidity was correlated against inflow 30 h earlier. A 'model' of turbidity, based on 5 days of inflows and used to hindcast turbidity from 14 days of inflows betw een 16-29 September, explained over 80% of the variance in freshwater turbidity. Therefore, inflow exerted a significant control on the fluv ial turbidity. It appears that very fine-grained particles were respon sible for the observed turbidity in the central and upper reaches of t he Tweed. During the flood, in the presence of strong wave activity in the coastal zone, larger sediment was rapidly winnowed in the lower r eaches, close to the mouth, whereas fine particles remained in suspens ion and reached the limit of saline intrusion. (C) 1997 Academic Press Limited.