Preliminary findings of a study of the upper reaches of the Tamar Estuary, UK, throughout a complete tidal cycle: Part II: In-situ floc spectra observations

Abstract

A series of field experiments funded by the Natural Environmental Research Council were conducted in the upper reaches of the Tamar estuary (UK), which placed the measurements within the tidal trajectory of the turbidity maximum. The aim of the study was to examine how the distribution of floc characteristics evolved with respect to changes in the turbulent shear stress, suspended concentration and biological constituents, throughout a complete tidal cycle. The main objective of the experiment was to measure simultaneous floc properties (which included: floc size, shape, settling velocity, effective density, porosity and floc dry mass) using the optical INSSEV instrument and associated hydrodynamic components, in-situ , throughout a complete tidal cycle. This paper reports the preliminary findings of the measurements made on the 15 th April 2003, during a spring tide. During the ebb a concentrated benthic suspension layer formed in close proximity to the bed producing a peak concentration of 4.2 g/1 and a maximum shear stress of about 1.5 N/m 2 . The more dynamic flood produced a shear stress which exceeded the peak ebb stress by 0.15 N/m 2. This in turn meant that the suspended matter was more evenly mixed throughout the entire water column on the flood. Local salinity values ranged from 14 at high water, down to completely flesh at low water. A total of 24 INSSEV floc samples were collected on the ebb flow and a further 34 floc populations were obtained on the flood. A combination of a shear stress of 0.38 N/m 2 and a concentration of 4.2 g/l, produced the optimum flocculation conditions which was signified by a bi-modal floc distribution. The microflocs represented 25% of the population, but only 9% of the dry floc mass. These microflocs were generally dense, slow settling aggregates, with typical effective density values of 80–1550 kg/m 3 and settling velocities ranging from 0.03–1.1 mm/s. The macroflocs, which constituted the second mode, contained 91% of the floc mass, one third of which were flocs over 400 μm in spherical-equivalent diameter. These macroflocs had individual settling velocities ranging between 2.2 to 7 mm/s. This translated into the macrofloc fraction constituting 98% of the total mass settling flux.