Circulation Dynamics and Dilution in Shallow Estuaries Case Study (1): Dilution Factor Calculations

Abstract

The ambient conditions of receiving waters such as currents and density stratification influence the dilution of treated wastewater discharged by WWTPs. Hendrix Creek, a semi-enclosed tidal tributary of Jamaica Bay that serves as receiving water of the 26th Ward WRRF, is not well studied. A near-field dilution model was developed and applied to the plant outfall to simulate plume transport and dilution in the initial mixing zone. The long-term ambient field conditions were characterized, and the dilution factor of a tracer was measured to calibrate the model. This was phase one of an integrated dilution and mixing study aimed at understanding the circulation dynamics which contribute to dilution in the creek. Field measurements during the dye release period also aided in the calibration and development of a 3D hydrodynamic model used to accomplish far-field dilution and mixing in phase two of this study. The near-field model results have good agreement with dye survey data and acoustic doppler current profiler (ADCP) measurements in the creek. Under critical conditions, the model predicts a dilution factor of less than 2 in the initial mixing zone, indicating poor dilution. Saline water from Jamaica Bay which served to dilute the treated wastewater appears to be dominated by the high plant discharge resulting in limited dilution in the near-field region of the creek.