Wind, Water Level, and Fluid Mud Thresholds in Lake Apopka, Florida

A study was undertaken at Lake Apopka in Florida to assess the minimum water depth required to contain a wind-induced episodic rise of fluid mud. In a year-long investigation, measurements were made at the mean water depth of 1.3 m to record the variation of suspended sediment concentration due to bed erosion and settling of the flocculated matter. The height of rise is defined as the elevation above the bed at which the mud floc volume fraction is at the threshold between the so-called flocculation settling and hindered settling regimes. The rise, which is considered significant when fluid mud occupies the 0.2 m high benthic boundary layer (BBL), occurs when the threshold wind exceeds about 9 m s−1 corresponding to a 4% cumulative probability of occurrence. Predictive modeling suggests that in 2 m water depth the required wind would be about 14 m s−1 with a low probability of 2%. Moreover, a transition occurs from wave-dominant resuspension at low depths to current-dominance in deeper water, which likely influences BBL dynamics with potential effects on the benthic biota. Provided a higher than present depth can be sustained in the large lake, the deduced relationship between fluid mud rise, wind speed, and water depth makes it feasible to select the depth at which the frequency of fluid mud occupying the BBL remains acceptably low. The developed protocol is general enough to be applicable to other similar shallow lakes where fluid mud rise must be contained.