Quantifying mud settling velocity as a function of turbulence and salinity in a deltaic estuary

Abstract

Mud settling velocity in coastal regions is controlled by flocculation, which in turn strongly depends on turbulence, chemistry, and biology of the water-sediment mixture. As a result, mud settling velocity can be poorly constrained, and vary in space and time by orders of magnitude. Here we quantified mud settling velocity in Barataria Basin, a deltaic estuary in Louisiana (USA), using three independent methods: eddy covariance (one station for 200 days), floc cameras (4 stations at one time), and Rouse profile inversion (14 stations, replicated 10–30 times each). Eddy covariance indicates that settling velocity increases with turbulence, at least within the range experienced at the site (shear rate G up to 10 Hz). Settling velocity increases with salinity (in the 0 to 6 psμ range) for moderate turbulence levels (5 < G < 10 Hz), but it is nearly independent of salinity for low levels of turbulence (G < 5 Hz). Consistent with this finding, floc camera measurements – taken at low turbulence levels – indicate similar floc sizes for salinities from 0.4 to 20 psu. Settling velocity estimated from a Rouse profile inversion also lacks a dependence on salinity, likely because they were taken at low turbulence levels. This study is novel in that it utilizes three methodologies to independently predict the mud settling velocity, with quantified settling velocity values ranging 0.1–1 mm/s, and with most values between 0.2 and 0.5 mm/s. Overall these measurements confirm that mud is flocculated in both the saline and freshwater zones of Barataria Basin, and that turbulence is the largest factor controlling mud settling velocity. Nonetheless, salinity can increase mud settling velocity up to a factor of two. These results could inform the management of sediment imported into estuaries from freshwater sources, such as through natural drainages, crevasse splays, and engineered river diversions.