A Dye, Current Meter, and Modeling Study in South Puget Sound — a Highly Refluxed Semi-Enclosed Estuary

The municipality of Shelton, WA discharges effluent from its municipal wastewater treatment plant (WWTP) into the partially enclosed estuary of Oakland Bay, which is connected to the rest of South Puget Sound by Hammersley Inlet. The Washington State Department of Health maintains shellfish closure boundaries (sanitary lines) landward and seaward of the WWTP discharge along which the concentration of effluent must not exceed strict standards. A three-dimensional hydrodynamic model of the estuary was made based on the open source Environmental Fluid Dynamics Code (EFDC). The model was forced with a time series of salinity, temperature, tidal height data collected at its open seaward boundary together with stream flows and meteorological data from a nearby airport. This EFDC application was calibrated against available field data from tide gauges, Conductivity, Temperature, with Depth probe (CTD) casts made in the interior, as well as NOAA tidal current predictions and was confirmed with a dye release study emanating from the WWTP. Despite a vigorous tidal exchange, net outflow is weak, primarily due to tidal refluxing.