Variability in cohesive sediment settling fluxes: Observations under different estuarine tidal conditions

The mass settling flux, which is defined as the product of the concentration and the settling velocity, is of prime importance with respect to both stratified and well mixed estuarine conditions. The determination of these fluxes (for applied modelling purposes) in high energy tidal estuarine environments, is very problematic. This is because the muddy sediments which dominate in estuaries, flocculate producing a variety of sizes and settling velocities, and this flocculation process is not understood well enough to be fully described theoretically. By drawing on examples of floc spectra acquired in-situ using the INSSEV system, this study explains how mass settling fluxes in the near-bed region can vary by three or four orders of magnitude in meso- and macro-tidal estuaries throughout a single tidal cycle. A floc population representative of dilute suspension conditions on a neap tide, indicated only 35% of the floc mass was macroflocs (> 160 μm). However, the macrofloc settling velocity = 2.4 mm s− 1; three times faster than the microflocs, which meant the former fraction contributed 57% of the 205 mg m− 2s− 1 settling flux. Both highly concentrated (4–6 g l− 1) and very turbulent spring tide conditions (τ > 1.6 N m− 2) produced a bi-modal distribution in terms of the floc size and dry mass. With the former, 54% of the mass was contained within the 240–480 μm size fraction, with a further 25% of the dry floc mass in the flocs over 480 μm in diameter. These large flocs had settling velocities between 4–8 mm s− 1, which meant 99.5% of the settling flux (33.5 g m− 2s− 1) was accredited to the macroflocs. The high turbulence environment saw the dry floc mass distribution shift 60:40 in favour of the microflocs. The microfloc settling velocity was 1.45 mm s− 1, 0.35 mm s− 1 faster than the larger macrofloc fraction. In terms of the total mass settling flux, 0.9 g m− 2 s− 1, this translates into the microflocs contributing 70% during high turbulence. At slack water the flux only reached 12 mg m− 2 s− 1 and macrofloc growth was mainly attributed to differential settling. Continuous floc observations made over a complete tidal cycle revealed that the asymmetrical distribution of the tidal energy generated throughout the spring conditions in the Tamar estuary demonstrated a distinct control on the flocculation process. The less turbulent ebb produced 86% of the total tidal cycle mass settling flux, of which only 8% of the settling flux was outside the turbidity maximum. An attempt to simulate these large settling fluxes by using a constant settling rate of 0.5 mm s− 1, under-estimated the tidal cycle settling flux by 78%, with less than 15% of the total flux being estimated during the advection of the turbidity maximum on the ebb. In contrast, using a faster constant settling velocity parameter of 5 mm s− 1, (representative of the macrofloc fraction), resulted in a mass flux over-estimate of 116% for the tidal cycle duration.