Observations on Floc Settling Velocities in the Tamar Estuary, United Kingdom

Abstract

Measurements in the mesotidal Tamar estuary (UK) reported previously indicate a dependence of the floc settling velocity on the shear rate and the suspended fine sediment concentration or volume fraction. Typical time-independent analytic formulas for the floc settling velocity tend to follow the mean trend but fail to provide an explanation for the characteristic data spread. Moreover, they assume floc diameter to be single-valued such as the mean or the median. Given this constraint, an examination of tide-induced trends in the Tamar settling velocities is attempted by simple time-dependent modeling of aggregation, i.e., the dynamics of floc growth and breakup. The effect of aggregation on the settling velocity and diameter is simulated over a representative one-half tidal cycle. Starting at low water (LW) slack at the onset of the sediment erosional phase during the first quarter tide, as the shear rate and volume fraction increase, floc growth is shown to increase the settling velocity that peaks at a shear rate in the range of 15-30 s-1. At higher shear rates, as floc breakup supersedes the effect of sediment concentration in promoting growth, the settling velocity gradually decreases until the shear rate reaches its maximum value on the order of 102 s-1 at the strength of flow. During the following depositional phase in the second quarter tide, as the shear rate decreases the settling velocity increases continually until high water (HW) slack when it achieves its overall maximum value as the shear rate approaches zero. Thus, the loci of settling velocity versus shear rate differ between the quarter tides and result in shear rate versus settling velocity hysteresis. Moreover, it is shown that in the Tamar, tidal variation prevents the settling velocity and diameter from always achieving the equilibrium assumed in analytic formulas. Thus, aggregation modeling serves as a useful guide for resolving temporal trends in floc properties.