Particle size distribution in an estuarine turbidity maximum region

Abstract

Observations are presented of primary, deflocculated particle size and suspended solids concentration in the turbidity maximum region at Burringham, on the Trent estuary, UK, during two tidal cycles, in July 1996 and July 1997. Pumped samples were obtained at intervals throughout both tidal cycles, for subsequent analysis for suspended solids concentration and particle size distribution. Both deployments took place for spring tide conditions, but the antecedent fresh water flow conditions were much higher in July 1997 than in July 1996. As a result the turbidity maximum region was located further downstream in July 1997 than in July 1996. A generally higher flood-tide median particle size (D 50 = 50 μm) in July 1997 compared with July 1996 (D 50 =10 μm) points to the possibility of sediment size sorting as a mechanism for the maintenance of larger particles at the landward end of the turbidity maximum. The difference in particle size regime also helps to explain the higher flood-tide suspended solids concentrations (8-10 g/l as opposed to 5-7 g/l), together with higher particle-induced density gradients during the ebb tide, in July 1997, for a similar hydraulic regime. One important consequence of the presence of larger particles is the nature of the relationship between suspended solids concentrations obtained by gravimetric analysis of pumped samples (g/l) and turbidity, measured by optical transmissometer. At high slack water, preferential settling by these larger particles below the level of the lens of the turbidity sensor may lead to anomalously high readings caused by the presence of high concentrations and flocs of smaller particles. Since optical turbidity sensors are often used to estimate trends in fine-sediment transport in estuaries, this preferential settling has important consequences for calibration of these sensors in highly turbid estuarine environments. For the data presented, a simple straight-line relationship can be demonstrated for all times during the tidal cycle except for slack water periods, when a different calibration should be applied.