Seasonal Variability of Fine-sediment Concentrations in the Turbidity Maximum Region of the Tamar Estuary

Abstract Measurements describing the intratidal, spring-neap and seasonal variations of fine, cohesive, suspended particulate matter (SPM) concentrations at two sites (CK and HQ) within the upper reaches of the Tamar Estuary, U.K. are presented. The data were obtained using two, near-bed instrument packages. Correlations between daily-averaged SPM concentrations and runoff and tidal range during the separate deployments often showed a significant dependence of SPM on these variables. Where statistically significant, increasing tidal range led to enhanced SPM levels because of resuspension of bed sediments. There was an asymmetry between the daily-averaged SPM concentrations on the rising and falling 15 day spring-neap cycle. On the rising cycle, the SPM levels were significantly lower than on the falling cycle for the same tidal range. This asymmetry was a feature of all deployments. The daily-averaged SPM during 3·8 m tidal-range rising spring tides at stations CK and HQ was approximately 0·3 kg m -3 in July and November 1988 and then fell to 0·01 kg m -3 during December 1988. This reduction in SPM was a manifestation of the winter period of bed-sediment depletion. Sediment was scoured from the upper estuary by strong ebb currents, which were enhanced by high and increasing runoff. SPM levels remained low during March 1989 (0·08 kg m -3 ) at the end of the high runoff, winter period. The daily-averaged SPM at HQ during April was much higher (0·77 kg m -3 ) than that further up-estuary at CK (0·11 kg m -3 ), which was comparable with the March levels. This indicates that the bed-sediment supply had moved up-estuary to the vicinity of HQ, so that local resuspension was very important there at that time, but had not reached the vicinity of CK. Fine sediment continued to move into the upper reaches during the spring to summer period. By May 1989 the daily-averaged SPM concentrations at CK were much higher than those further down-estuary at HQ(0·98 as opposed to 0·22 kg m -3 ). For the June and (both) July 1989 deployments, SPM levels at CK exceeded those at HQ, although at both stations the levels were lower in July. As the limited amount of fine sediment moved further up-estuary, through CK and closer to the head, the amount of bed-source sediment available for local erosion at CK and HQ was reduced.