Floc properties in the turbidity maximum of a mesotidal estuary during neap and spring tidal conditions

Abstract

In order to improve the understanding of how fine cohesive matter behaves during different tidal conditions within an estuary turbidity maxima, high quality in-situ data is of a prime requirement. As part of the European Commission COSINUS project a series of measurements were carried out in the upper reaches of the Tamar estuary, to investigate how flocculated mud properties vary within the turbidity maximum zone. Floc populations were sampled at regular intervals using the optical INSSEV instrument. The floc data was supplemented by simultaneous time series of near-bed profiles of turbulent shear stress and suspended particulate matter. The proportion of macroflocs (> 160 μm) changed significantly across the turbidity maximum. Before the passage of the turbidity maximum the total floc mass tended to be equally divided between the macroflocs and microflocs on neaps, whilst on springs the macroflocs contributed 60–70%. At spring tides a peak near-bed SPM concentration of 6 g l − 1 was measured in the turbidity maximum; this was over an order of magnitude higher than observed at neaps. The turbulence measurements showed significant drag reduction at SPM concentration gradients above ∼ 3.6 kg m − 4 . Within the turbidity maximum the macrofloc proportion rose rapidly until they contributed at least 75% of the floc dry mass during neap conditions, and at spring tides this reached in excess of 90–96%. An abundance of fast settling macroflocs from spring tides, accounted for a time series average of 83% of the mass settling flux. Where as during neap tides, the macroflocs contributed 15% less to the settling flux rate. This was partly due to a time series averaged macrofloc settling velocity of 3.9 mm s − 1 from the spring tidal data; 2.1 mm s − 1 higher than for neap tide conditions. During the turbidity maximum passage at springs, macroflocs reached 2.2 mm in diameter; these flocs had settling velocities of up to 16.6 mm s − 1 , but effective densities were less than 50 kg m − 3 , which means they would be prone to break-up when settling to a region of high shear. At the opposite end of the scale, low turbidity and quiescent slack water conditions severely restricted floc production.