NC0001422_Other_20200616

March 9, 2020
To: Sergei Chernikov
From: David Hill
Subject: CORMIX model review for Duke L.V. Sutton NC0001422
Geosyntec Consultants of NC has submitted a CORMIX model on behalf Duke Energy Progress, LLC for
the discharge of Sutton Lake water to the Cape Fear River (classified C, Swamp, PNA) at 0.35 and 3.5
MGD.
The CORMIX model was used in conjunction with a previously calibrated Environmental Fluid Dynamics
Code (EFDC) model to determine tidal inputs to the model. The outfall configuration was updated with
the latest changes to the outfall following the separation of the Cooling Pond wastewater discharge to
Sutton Lake. The model was completed in two parts. The first, the embayment stage, is a roughly
uniform channel to the main river. The plume characteristics at the end of the embayment model run
are used as inputs to the river hydrodynamics portion of the model. For reasons discussed below, mixing
zone considerations were reserved to the embayment channel.
The model results show a buoyant plume undergoing near -field mixing for approximately three meters
before contacting the surface of the water. Near -field mixing occurs along the surface contact for
another half meter before moving into passive ambient diffusion. The plume continues with passive
diffusion and contacts the nearest bank at 14 meters eventually ending approximately 8 meters wide
and 0.25 meters deep from the surface. EPA guidance for sizing acute mixings zones (section 4.3.3 of the
Technical Support Document for Water Quality -based Toxics Control, 1991) were reviewed with the
criteria of 50 times the discharge length scale (square root of the port cross -sectional area) being the
most restrictive of the three options, in this case 13.5 m with a dilution of 10.5. The Division believes
that 13.5 m is too large of an acute mixing zone considering the receiving water classification of Primary
Nursery Area by the NC Division of Marine Fisheries, the length of the passive buoyant plume, and that
the dilution allowed at this distance too great to protect downstream saltwater standards in accordance
with 15A NCAC 2B .0203. To minimize the mixing zone and the pollutant concentrations in the ambient
spreading plume, the mixing zone will be set where the plume contacts the water surface approximately
3 m from the outfall. The dilution at this point is 4.5.
Dilution Chronic/Acute- 4.5:1
IWC%- 22%
Diffuser -Existing outfall structure.
Regulatory Mixing Zone- three meters downstream of outfall.