Projects & Programs
Outfall Design and Mixing Zone Study, Alaska
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 flushing 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 comparable to the field data.
The outfall relocation would eliminate the water quality issues associated with poor effluent flushing at the existing discharge location. Since the proposed outfall site ues higher available natural flushing and reduces the effluent to concentrations well below the applicable standards, the discharge would not be expected to cause water quality impairment at the new location. The outfall effluent temperature was approximately 10 degrees F greater than ambient water temperature. The outfall was designed for 100-times dilution, thereby reducing the outfall temperature change to less than 0.1 degrees F. The design ensured that the temperature increase due to the outfall was less tan the Alaska criterion of 0.25 degrees F. The outfall effluent temperature was approximately 10 degrees F greater than ambient water temperature. The outfall was designed for 100-times dilution, thereby reducing the outfall temperature change to less than 0.1 degrees F. This design ensured that the temperature increase due to the outfall was less than the Alaska criteria of 0.25 degrees F.
Project Highlights:
- Hydrodynamic model results were used to select an optimum site for the new outfall
- Transport models were used to design a new diffuser and conduct mixing zone analysis that showed all ADEC criteria were satisfied in order to obtain a Section 401 water quality certificate
- Drogue simulation and dye simulations were performed to ensure long-term accumulation of effluent would not occur and the required mixing zone would be in compliance with Alaska Department of Environmental Conservation requirements
- Hydrodynamic model results showed that existing outfall was located in a low-energy environment with poor flushing.