20200903_WSRO_comments

Winston Salem Regional Office Comments
1. How will you maintain 679 stone fence outlets after any significant rainfall within a reasonable
time period (i.e. one week or next rainfall)? Have you considered adjusting the sediment basin
locations and/or LOD to reduce the drainage areas downstream of basins and diversions? These
outlets are considered stormwater outfalls and will need to be monitored per the NCG02
Stormwater Permit for mining. The amount of disturbed area draining through basins and traps
should be maximized.
Be advised stone outlets are not a sediment treatment device. Any reference to maximum
drainage area is not approvable nor applicable. They are not intended to function as a measure
in lieu of a sediment trap or basin and offer no filtration value for retention of sediment. Their
primary purpose is to prevent silt fence laydown for small retention areas that cannot otherwise
be directed toward a basin or trap. Redesign appears justified.
Silt fencing typically has a design life of six months. Will the silt fence and stone outlets be
replaced every six months for the life of this mine? Redesign is recommended.
2. Regarding general basin design, in addition to flow through three baffles, the percentage of
surface area in each cell should be roughly 25% of the total surface area of the basin. Please
refer to Sections 6.65 of the North Carolina Erosion and Sediment Control Planning and Design
Manual. Reconfiguration of the sediment basin and baffle placement may be necessary to
achieve approvable design.
The proposed rip rap transition areas are not measures included in the North Carolina Erosion
and Sediment Control Planning and Design Manual. Please provide details and specifications for
installation and maintenance of these measures.
4. Plan review includes evaluation of proposed measures to assure that they will be sufficient to
retain the sediment generated by the land -disturbing activity within the boundaries of the tract
during construction. This includes verifying that particular measures are appropriate for their
corresponding drainage areas, especially for perimeter measures which drain large areas, such
as basins. We require skimmer brands to be specified because approvable design of the basin
withdrawal rate is crucial for assuring compliance.
The supporting calculations provided to justify the skimmer selection and sizing are based on
the particular proposed skimmer. Verification that the skimmer has been sized appropriately is a
step in evaluating basin design. Additionally, each skimmer manufacturer has design criteria
specific to their brand. Therefore, in order to use an alternative skimmer, it will be the owner's
responsibility to submit an approvable modification to the plan with supporting calculations
verifying that substituting the proposed device will not result in off -site sedimentation.
Please clarify where the skimmer brand is specified on the plans. Please also specify the
skimmer arm length on the plans.