SW1221002_Response To Comments_20230804

Gresham
Smith
August 2, 2023
Mr. Jim Farkas
NCDEQ
512 North Salisbury Street
Raleigh, NC 27699-1612
Subject: Asheville Regional Airport Terminal Modernization and Expansion
Asheville, NC
Gresham Smith Project Number: 43483.02
Dear Mr. Farkas:
The following are Gresham Smith responses to your comments from your letter dated May 22, 2023
regarding Asheville Regional Airport Terminal Modernization and Expansion.
1. Comment: Prior Comment 1 - "...Please modify the Application to indicate that this will be a
modification to SW1190702..." Please provide the following items associated with a
major modification to an existing stormwater permit:
a. Designer's Certification. This item is required per Section 11.1.b. of the Application,
Part 111.7 of the permit, and 15A NCAC 02H .1042(4)(a). The Designer's
Certification can be found at the end of the permit originally issued for this project.
b. Proof that the O&M Agreement has been recorded with the Register of Deeds. This
item is required per Part 11.12 of the permit and 15A NCAC 02H .1050(11).
c. Proof that the drainage easements have been recorded with the Register of Deeds, if
required (per Part 1.3 of the permit and 15A NCAC 02H .1042(2)(1) & .1042(4)(c)).
d. Please ensure that the project area used for this major modification includes the
entirety of the new BUA that was originally permitted in 2019 as it will still be
covered under this permit. Please also ensure that the SCMs that were approved under
the 2019 permit and will remain in place are included in the Supplement-EZ Form
222 Second Avenue South GreshamSmith.com
Suite 1400
(k,,,iii,e Ingenuity Nashville,TN 37201
615.770.8100

Jim Farkas
State Stormwater Engineer
DEMLR Post-Construction Stormwater Program
August 2,2023
and other submittal items. Please also note any previously permitted SCMs that will
be modified/removed in Section IV,1 of the Application. NOTE: New calculations
for currently permitted SCMs only need to be provided if the design of the SCM or
the drainage area to the SCM are being changed. NOTE: You may also utilize
previously approved SCMs that will remain to provide runoff volume reduction (you
will need to provide calculations for this though).
Response: a. The Designer's Certification for permit SW1190702 has been provided as part
of this resubmittal.
b. The O&M Agreement for permit SW1190702 has been provided as part of this
resubmittal. As part of a state-sanctioned independent airport authority, the
Greater Asheville Regional Airport Authority does not believe deed
restrictions shall be required for the completed project or future projects.
c. As part of a state-sanctioned independent airport authority, the Greater
Asheville Regional Airport Authority does not believe drainage easements for
permit SW1190702 shall be required for the completed project or future
projects.
d. The project boundary has been updated to include the areas covered under
permit SW1190702, with the new project boundary encompassing 43.29
acres. Additionally,the proposed water quality swales in the 2019 plans have
been accounted for in the updated design. Please see the stormwater
management report for information about the water quality swales. Swale 1
was removed in the current design, and its water quality treatment volume
was included in the current design. Swale 2 was relocated and analyzed for
the change in drainage area. Its water quality volume was also included in the
current design as a cautionary measure. Swales 3 and 4 remain undisturbed
in the current design and shall function as originally designed.
2. Comment: Prior Comment 2 - "As designed, this project does not meet the requirements of Runoff
Volume Matching (RVM) as defined in 15A NCAC 02H.1002(44)... In order to meet
RVM,you must demonstrate that the annual runoff volume for the entire site is not
increased by more than 10 percent from the predevelopment conditions."
Predevelopment conditions refer to the site conditions prior to the post-construction
stormwater rules going into effect in this area, not what currently exists on-site/what
is currently approved for the site. Please exclude the approximately 113,325 sf of new
BUA approved under this permit in 2019 from the existing BUA amount. Please see
earlier comment with regard to the project area to be used for this project. Please also
use the "Fletcher" rainfall station (P = 50.64") instead of the "Asheville" rainfall station
(P =36.99"). The Fletcher rainfall station is closer to and more representative of rainfall
patterns at the airport than the Asheville rainfall station. Please revise the calculations
and other submittal items as needed.
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Jim Farkas
State Stormwater Engineer
DEMLR Post-Construction Stormwater Program
August 2,2023
Response: The proposed site does not meet the requirements of Runoff Volume Matching, as
the ARV increase is 10.63% for the updated project boundary. Using the Runoff
Treatment requirements instead, the net increase of impervious area is 3.52 acres.
Having said that, the updated water quality pond provides enough water quality
volume for 4.95 acres of impervious area. The required water quality volume was
calculated using both the Discrete NRCS Curve Number Method and the Simple
Method,then utilizing the larger of the two outputs for sizing the pond volume.
3. Comment: Prior Comment 3 - "The provided design volume of this bioretention cell... is
significantly undersized for this drainage area (a 100%sized SCM for this drainage area
would have a design volume of about 11,000 cf). Please revise. SCMs less than 50%of
their full (100%) size are too small to receive credit per the SCM Credit Document."The
design volume of a bioretention cell is based on the area draining to the SCM, not the
amount of runoff volume required to be treated for a project. For example, if a project
were only required to treat 500 cf of stormwater runoff to meet treatment
requirements, but the SCM has a proposed drainage area of 5.6 ac with 80%
impervious,the SCM would need to provide a storage volume of 15,653 cf to be 100%
sized (Rv = 0.77, Rd =1.0" runoff, DV = Rd * Rv * DA = 1.0"* 0.77* 5.6 ac = 15,653 cf).
While you are only required to treat 500 cf of runoff,the SCM must be sized based on
the actual drainage area to it (per 15A NCAC 02H .1050(1)) so only providing an SCM
with 500 cf of storage volume would result in an undersized SCM, since it is
undersized for its drainage area, even if it would be adequately sized to meet the
treatment requirements for the site.
One way to address this would be to reduce the drainage area to the SCM so that the
runoff generated from the drainage area to the SCM is closer to the required treatment
volume for the project (500 cf in this example). Please revise as needed.
Response: Per the meeting 20 July 2023, the bioretention pond was removed to prevent any
wildlife habitat and to not have ponding water on the airport per FAA and NCDEQ
regulations.Addition hardship is due to the lack of space within the airport to provide
a primary and secondary SCM for water quality and quantity. In this scenario,the dry
detention pond will be utilized as the primary SCM for water quality and water
quantity as well.The pond capacity/volume has been updated to capture the volume
of the drainage area discharging to it, in lieu of the design volume required for the
site, as this drainage area volume is larger.
4. Comment: Prior Comment 4.a. - "General MDC 4 - Please provide riprap sizing/design
calculations showing that they are stable in accordance with this MDC." Please provide
the calculations, or elaborate on where these riprap sizing configurations came from,
showing that these riprap configurations will be stable per the MDC.
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Jim Farkas
State Stormwater Engineer
DEMLR Post-Construction Stormwater Program
August 2,2023
Response: Minimum riprap calculations have been provided in the stormwater management
report according to the NCDEQ Erosion and Sediment Control Planning and Design
Manual. The headwalls onsite now drain to a riprap forebay, which provides riprap
protection greater that the minimum dimensions calculated.
5. Comment: Prior Comment 4.b. - "Bioretention MDC 1... - It is unclear where the provided SHWT
elevation (2102.30) came from. Per the soils report, the existing ground surface
elevation in the vicinity of the Bioretention Cell/Dry Pond is approximately at elevation
2119'and the borings (Ksat-1& Ksat-2) were only drilled to 36"& 34"below existing
grade respectively. Please clarify where the provided value came from."The response to
this item indicates that the area where the proposed SCM will be located in was filled
from about elevation 2095'to about 2119'. It is still unclear where 2102.3' came from.
Please revise/clarify as needed.
Response: The 2102.3' was an assumption of depth based on the geotechnical borings not
hitting bedrock.The correct elevation is, at a maximum, 2095' due to the conditions
prior to the 2019 project.
6. Comment: Prior Comment 7.b.i.2 - "Line 18 - Please include this item." Please include the
percent BUA for the drainage area to the SCM.
Response: As requested, the percent BUA for the drainage area to the SCM is provided in the
submittal documents.
7. Comment: NOTE: Changes to the Application and Supplement-EZ Form will need to be made in
order to address the earlier comments. Since it is not known how the earlier comments
will be addressed at this time, specifics about what needs to be changed in these
forms are not provided. Most likely,the project area information in Section IV, 4-9, the
BUA accounting shown in Section IV, 10 & Drainage Areas Page of the Supplement-EZ
Form, and SCM design on the Bioretention Page of the Supplement-EZ Form will need
to be revised. Please revise as needed.
Response: As requested, the Application and Supplement-EZ Form have been revised
accordingly.
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Jim Farkas
State Stormwater Engineer
DEMLR Post-Construction Stormwater Program
August 2,2023
8. Comment: Provide PDFs of all revisions, 2 hardcopies of revised plan sheets, 1 hardcopy of other
documents, and a response to comments letter briefly describing how the comments
have been addressed.
a. PDFs must be uploaded using the form at:
https://edocs.deq.nc.gov/Forms/SW-Supplemental-Upload
b. Hard copies must be mailed or delivered to the following address:
For Fedex/UPS:
Jim Farkas
512 N. Salisbury Street, Office 640M
Raleigh, NC 27604
ii. For USPS:
Jim Farkas
1612 mail Service Center
Raleigh, NC 27699-1612
iii. Hand Delivery:
Please reach out to me prior to hand delivering a submission to make
sure that I (or someone else in my group) will be able to receive the
submission. Do not leave the package in the foyer with the security
guard.
NOTE: Hard copies should not be sent to a Regional Office. Doing so will delay
the review process and the submission package may be lost while being sent from
the Regional Office to me in the Central Office.
Response: The requested items have been submitted with this resubmittal package.
If you have any questions, please do not hesitate to call me at 615-770-8155.
Sincerely,
Nathan Hudgens, P.E.
Gresham Smith
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