Outfall Sitting and Effluent Flushing Analysis in Tongass Narrows, Alaska, Using a 2-D Hydrodynamic and Transport Model, 1996 Environmental Conference Proceedings

An outfall’s performance is based on its ability to provide sufficient mixing to reduce the effluent concentrations to acceptable levels at the regulatory mixing zone boundary. Ketchikan Pulp Company’s existing outfall discharges at the surface of Ward Cove, near Ketchikan, Alaska. Although Ward Cove experiences large tidal variation in water surface elevation, the velocities observed are low, consisting mainly of vertical movement of the water surface with little lateral movement. This results in very low dilution of the discharged effluent, and has caused water quality compliance problems. In response to growing concern over water quality issues in Ward Cove, Ketchikan Pulp Company decided to site a new outfall at a location outside of Ward Cove in the adjacent Tongass Narrows, to provide optimum dispersion and flush-ing of the effluent. The outfall design process included two-dimensional hydrodynamic and transport modeling analysis of Tongass Narrows and Ward Cove using RMA2 and RMA4 models. The simulated current and tidal levels were compa-rable to the field data. The results from the calibrated hydro-dynamic model were used to determine the mixing zone boundary at the point where water quality standards would be met.