Publication Summary

The City of Shelton commissioned a study of their wastewater discharge via an outfall that straddles two (sanitary) lines, which define a shellfish closure zone in Oakland Bay and Hammersley Inlet. Ecology created a three-dimensional computer model, Hammersley Oakland Bay Oceanographic (HOBO), which was validated against a dye release experiment funded by FDA. The model shows that extending the diffuser horizontally across Hammersley Inlet can be very effective in controlling dilution at the sanitary line, although releasing effluent too far north toward Munson Point can cause problems on the east end of the Oakland Bay sanitary line on flood tides. Holding back effluent at slack tide is another very effective method for controlling initial dilution at the sanitary lines. Finally, the Hammersley Inlet line could be moved closer to the discharge.

The City of Shelton is growing in population and is considering the possibility of increasing their wastewater discharge via an outfall that straddles two (sanitary) lines, which define a shellfish closure zone in Oakland Bay and Hammersley Inlet.

The Washington State Department of Ecology was approached to develop a model to help the Washington State Department of Health evaluate conditions at these sanitary lines, in response to Shelton′s needs for possible wastewater treatment plant expansion.

We developed the Hammersley Oakland Bay Oceanographic (HOBO) model based on the Environmental Fluid Dynamics Code (EFDC), a three-dimensional hydrodynamic computer primitive equation model (Hamrick, 1992, 1996). HOBO is driven by real data acquired at its boundaries. The air-sea boundary conditions were generally provided from the nearby airport, Sanderson Field. The open sea boundary conditions were recorded near Libby Point as a part of the overall study. We used the HOBO model to simulate various discharge scenarios and to determine consequences at the two sanitary lines during periods specified by the Washington State Department of Health.

Ecology also participated in a dye study to determine the far-field dilution factor at the sanitary lines at a specific time, April 2003, and to validate the model.

Model results show that extending the diffuser horizontally across Hammersley Inlet can be very effective in controlling the far-field dilution at the sanitary line, although releasing effluent further north toward Munson and away from Eagle Point can cause problems on the east end of the Oakland Bay sanitary line. Holding back effluent at slack tide is another very effective method for controlling initial dilution at the sanitary lines. Controlling the vertical plume trapping depth is not very effective as a control method since there is a substantial amount of vertical mixing in this estuary, and the plume gets mixed anyway by the time it gets to either sanitary line.