Critical Flow for Water Management in a Shallow Tidal River Based on Estuarine Residence Time

To support the development of protective water resources management strategies, a 3D hydrodynamic model was applied to the Little Manatee River (LMR) to evaluate the effects of reducing river flow and drought on the Estuarine Residence Time (ERT). ERT is an important indicator for estuarine environmental quality. The Little Manatee River is a small tidal river estuary with a yearly mean gaged freshwater inflow of 4.8 m3/s. The hydrodynamic model was calibrated and verified by using two continuous data sets for a six month period. Model simulations were conducted for 17 river inflow scenarios. Among the flow scenarios, 13 scenarios were within a flow range from 0.26 m3/s to 10 m3/s total freshwater inflow. A regression equation (R2 = 0.98) fitted by a power-law function was derived from analysis of the hydrodynamic modeling results to correlate model predicted ERT to total river inflow, though ERT can be predicted from gaged freshwater inflow as well. The study indicates that the estuarine residence time reaches 53.3 days under an extreme drought condition of 0.26 m3/s total inflow. When river inflow falls below the critical flow (4 m3/s or less), further flow reductions can cause the substantial increases of ERT by a factor of 2 to 10 times. This suggests that the management of flow reductions is particularly critical when total river flows are 4 m3/s or less if adverse impacts to the water quality and ecological characteristics of the Little Manatee River are to be avoided.