SW3240204_Response To Comments_20240604

azeri Hazen and Sawyer
9101 Southern Pine Blvd,Suite 250
Charlotte,NC 28273•704.357.3150
June 4,2024
Brianna Holland
512 N. Salisbury Street, Office 625Y
Raleigh,NC 27604
Re: Grassy Branch WRF Expansion
Post Construction Stormwater Application—SW324024
Dear Ms. Holland,
This response letter addresses the comments received on April 12, 2024. These responses were previously
emailed on May 14,2024.We will upload and submit the requested revised hard copies upon further
clarification of our responses.
Review Comment 1: Application Section IV, 8-Accounting for the surface water area results in a project
density that is greater than is allowed for a low density permit. (22,715 SF total impervious area = 0.52
acres, 0.52 acres/1.40 acres = 37.2% impervious). Utilizing the rules below can reduce the percentage of
impervious area,while meeting the requirement to omit the surface water area from the project area. (This
rule can be applied because it appears that the existing BUA was installed prior to the rules going into effect
in the area.). 15A NCAC 02H .1003 (1) (b) A project with existing development has the option of
calculating project density as the difference of total built-upon area minus existing built upon area divided
by the difference of total project area minus existing built-upon area; (c)(i)Total project area shall exclude
the following: areas below the Normal High Water Line(NHWL).
13,245 SF of existing BUA=0.30 acres of existing BUA.
0.52 acres—0.30 acres
1.40 acres—0.30 acres —2096
• Can you clarify the issue with how the 4/4 resubmittal documents calculated the%impervious
area?
• As denoted on Sheet C6 (submitted as Plansheet — BUA), the Project % Impervious was
calculated using the described method(c)in your comment letter.
• There seems to be a comment being made regarding the conversion of the existing BUA values
from square footage to acres not being accurate. I'd like to acknowledge rounding up of the
acreage values were used to allow for the summation of the individual drainage areas to equal
that of the total drainage area—this is true for the Existing BUA line item. This has happened
due to the small drainage areas seen on our project site when converting from square footage
to acres.
• Based on the previous two comments,we believe no updates are necessary.
Review Comment 2: The calculations reflect the North American Green — P300 polypropylene mat —
TRM. Different TRM types have varying maximum velocity allowances. While the brand doesn't need to
hazenandsawyer.com

Hazen Brianna Holland
June 4,2024
be included in the plans,the specifications listed must be detailed enough to ensure the chosen mat allows
for a maximum allowable velocity equal to or exceeding the value calculated.
• We addressed this comment by updating the minimum shear stress value to 2.0 lb/ft2 in
paragraph 2.05F3 of the Erosion and Sediment Control specification to account for this, and
match what is described in the calculations and shown on the Drawings.
Review Comment 3: Per 15A NCAC 02H.1002(50) "Ten-year storm intensity"means the maximum rate
of rainfall of a duration equivalent to the time of concentration expected, on the average, once in 10 years.
Ten-year storm intensities are estimated by the National Oceanic and Atmospheric Administration(NOAA)
Precipitation Frequency Data Server (PFDS). The NOAA Precipitation Frequency Data Server table was
provided, but the time of concentration could not be verified based on the information provided. The
calculations stated that the 10-year storm utilized was based on StreamStats, but conformance with this
definition must also be demonstrated. If the flow for the 10-year storm needs to be revised, please revise
the swale and rip rap calculations accordingly.
• Based on your comment,we confirmed the sizing of the swale using the Rational Method and
calculating a time of concentration.
• The updated Low Density sheet has been uploaded. The Q, Vactual, and Additional
Information sections were updated.
• We have confirmed that the original rip rap design is sufficient.
Review Comment 4: Verify the flow from the culvert. The provided Culvert Report reversed the slope of
the culvert from what is indicated on the plans and could result in a different discharge condition. Per 15A
NCAC 02H .1003 (5), stormwater outlets shall be designed so that they do not cause erosion downslope of
the discharge point during the peak flow from the 10-year storm event.
• See the updated Culvert Sizing calculation based on the updated flow calculated mentioned in
the Comment 3 section.
Thank you very much for your consideration of the enclosed resubmittal. Please do not hesitate to reach
out to me with any questions by email([e-mail])or telephone[phone]).
Respectfully submitted,
Zachary Kemak,PE
Enclosure
cc:Thomas Mann(Union County Water)
Fred Braun (Union County Water)
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