Effects of Salinity and Turbulent Shear Variations on Floc Size in the Lower Mississippi River

Abstract:

The Mississippi River delivers an estimated 300 million tons of sediment each year to the Gulf of Mexico. A major fraction of this sediment load is cohesive in nature, thereby resulting in aggregates known as flocs. Flocs change shape in response to driving factors such as variations in chemical, biological, and hydrodynamic characteristics of water, making forecasting their movement difficult. To examine the existence and behavior of flocs in the Mississippi River in southern Louisiana, we conducted mixing tank experiments on water samples collected from multiple points along the Mississippi River and its distributaries in June of 2020. The main goal was to investigate the effect of salinity and turbulent shear rate on floc size and settling velocity. Observations showed the presence of flocs throughout the freshwater reaches of the lower Mississippi and its distributaries. Laboratory results showed that adding salt to water samples from the freshwater reaches mixing in the tanks had little to no effect on floc size, suggesting that flocculation is driven by biological processes or low amounts of ions already present in the water. During the experiments in which turbulent shear rate was varied, flocs were found to respond to changes in shear by growing in size when shear was reduced and decreasing in size with increases in shear. Some amount of memory or slow reaction time was observed in the floc sizes when shear was varied through increasing and decreasing cycles.