Multiphase Layering and Mobility of Suspended Fine Sediment in Lake Apopka, Florida

Fine-grained sediment in Florida’s eutrophic lakes displays a characteristically multiphase and layered structure including fluid mud that accounts for most of the nutrient-rich suspended matter potentially contributing to water quality degradation. The viscometric properties of fluid mud layer are particularly important for calculating the sediment load and rate of accumulation. Following a description of fine-sediment layering, a method is outlined to determine the yield stress and viscosity of the characteristically viscoplastic fluid mud. Based upon previous analytic work, these two quantities are deduced from the flow curves for sediment samples (with a mean organic content of about 63%) collected by coring at four sites in Lake Apopka. The analysis indicates an increase in the yield stress and decrease in the relative viscosity with increasing floc volume fraction. Inherent in these trends is the influence of organic content that increases with the floc volume fraction. Comparison with flow curves for sediment of higher density, greater cohesion and lower organic content from a bayou in Louisiana reveals three orders of magnitude higher yield stresses and somewhat lower viscosities relative to Apopka. High yield stresses in the bayou are associated with a dense bed subject to tidal current, which possibly prevents the retention of weak sediment at the bottom. Calculation of the sediment load and annual rate of accumulation due to a fluid mud undercurrent is illustrated for viscous flow over a mildly sloping bottom. For a realistic assessment of the accumulation it will be essential to take into account the role of the episodic wind field and the turbulent flow driving the suspended matter in the lake.