SW1221002_Response To Comments_20230223

Gresham
Smith
February 20, 2023
Jim Farkas
NCDEQ
512 North Salisbury St. 11612 Mail Service Center
Raleigh, NC 27699-1612
Subject: Asheville Regional Airport Terminal Modernization and Expansion - SW1221002
Asheville, NC
Gresham Smith Project Number: 43483.02
Dear Mr. Farkas:
The following are Gresham Smith responses to your comments from your letter dated January 23, 2023
regarding Asheville Regional Airport Terminal Modernization and Expansion - SW1221002.
Comment: Parts of the project area (notably the area where the SCMs are proposed to go) are
already covered under Permit Number SW1190702. Please modify the Application to
indicate that this will be a modification to SW1190702 as an area cannot be covered
under two different permits.
Response: The application has been updated to be a modification to permit SW1190702.
2. Comment: As designed, this project does not meet the requirements of Runoff Volume Matching
(RVM) as defined in 15A NCAC 02H .1002(44). The provided RVM calculations show
the difference in runoff volume from 1.0" of rainfall within the drainage area to the
bioretention SCM. 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. Annual runoff volume can be calculated using the
following equation:
222 Second Avenue South GreshamSmith.com
Suite 1400
Genuine Ingenuity Nashville, TN 37201
615.770.8100

Jim Farkas
February 20, 2023
ARV = Pj * Rv * A * (P/12) - Vrry
Where:
- ARV = Annual Runoff Volume (in cubic feet)
- Pj = Fraction of Rainfall Events that Produce Runoff (assumed to be 0.9)
- Rv = Runoff Coefficient (calculated in the same manner as in the Simple Method)
- A = Area of Interest, usually the project area or part of the project area if there is a
watershed boundary/drainage split on the project area (in square feet)
- P = Average Annual Rainfall Amount (in inches) [About 37 inches for Asheville]
- Vrry = SCM Runoff Reduction Volume, if applicable (Based on the percent sizing of
the SCM and the fate information as shown in the SCM Credit Document:
https://deq.nc.gov/media/l1819/download)
Example: Assume a project has an area of 14.59 ac,11.45 ac of pre -development BUA,
a net increase in 1.35 ac of BUA (for a total of 12.80 ac of BUA in post -development
conditions), and an annual rainfall amount of 37.0 in.
The ARV for pre -development conditions would be calculated as ARVpre = Pj * Rv * A*
(P/12) = 0.9 * [0.05 + 0.9 * (11.45 ac / 14.59 ac)] * (14.59 ac * 43560) * (37.0 in / 12)
= 1,333,000 cf.
The ARV for post -development conditions would be calculated as ARVpost = Pj * Rv *
A * (P/12) - Vrry = 0.9 * [0.05 + 0.9 * (12.80 ac / 14.59 ac)] * (14.59 ac * 43560) * (37.0
in / 12) - Vrry = 1,480,000 cf - Vrrv.
Since the increase in ARV from pre- to post- cannot exceed 10%, we can determine the
minimum required Vrrv. Vrrvmin = 1,480,000 cf - (110% * 1,333,000 cf) = 13,700 cf.
This volume of stormwater must be treated via ET&I (refer to the SCM Credit
Document) annually by the SCM in order for RVM to be met for this example. NOTE:
Vrry is based on the percent sizing of the SCM based on the drainage area to the SCM.
Using the drainage area information provided in the calculations for the bioretention
cell(DA = 169,219 sf, BUA = 141,167 sf, & HSG = A) a 100% sized SCM for this drainage
area would have a design volume of about 11,000 cf and would provide about 318,047
cf of Vrrv.
Response: The site area has been redefined as the limits of disturbance on the landside of the
airport property and a small portion of area on the airside to equal 6.89 acres. The
ARV has been calculated for pre -developed and post -developed conditions and has
a 9.8% increase, which is under the 10% maximum. Therefore, the runoff volume
match method has been used to determine the required SCM sizing for the 6.89
acres. The calculated Vrry is 21 cf, which is provided in the updated bioretention
pond.
Page 12

Jim Farkas
February 20, 2023
3. Comment: The provided design volume of this bioretention cell (618 cf) 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.
Response: The site area has been redefined, and the design volume of the bioretention cell has
been updated. Please see Appendix C of the Stormwater Management Report for
design calculations.
4. Comment: The following items were noted with regard to the design of the SCMs:
a. General MDC 4 - Please provide riprap sizing /design calculations showing
that they are stable in accordance with this MDC.
b. Bioretention MDC 1 & Dry Pond 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.
Response: a. Riprap sizing dimensions are provided in detail 08 on sheet CL905.P2.
b. The area of the bioretention pond and detention pond was recently filled from
an elevation of 2095 to 2119. The SHWT was assumed to be greater than the
existing elevation due to saturation in the added soil and assumed existing
ponding at the low point of 2095.
5. Comment: The following items were noted with regard to the design of the Bioretention Cell:
a. Bioretention MDC 4 - Please provide a detail of the outlet structure so that the
IWS depth may be verified (The detail should include the invert of the IWS).
b. Bioretention MDC 10 - Please ensure that the maximum 5-year tree and shrub
canopy coverage of the bioretention cell does not exceed 50% coverage of the
bioretention cell.
c. The percent composition of the bioretention media shown in the Supplement-EZ
Form (Lines 33-35 on the Bioretention Cell Page) do not add up to 100%.
Response: a. A bioretention outlet structure has been added to the plans. See detail 04 on
sheet CL905.P2.
b. The landscape plan has been updated to keep 5-year tree and shrub canopy
coverage below 50%. See sheet L200.
c. The bioretention media composition has been updated to add up to 100%.
6. Comment: The following items were noted with regard to the design of the Dry Pond:
a. Sizing/design calculations for the Dry Pond were not provided for review.
b. Dry Pond MDC 5 - Pretreatment is required.
Page 13

Jim Farkas
February 20, 2023
c. Dry Pond MDC 6 - This item could not be verified based on the provided
information.
Response: The proposed dry pond is not designed to function for water quality treatment. The
detention pond has been included in the design to protect the existing ditch
downstream in all post -developed storm events up to the 100-year storm.
7. Comment: Please correct the following items in the Supplement-EZ Form:
a. Cover Page:
i. Line 2 - The value here (14.59 ac) does not correspond to the project
area value shown in Section IV, 7 of the Application (12.54 ac). Please
revise either or as needed.
ii. Line 7 - It is noted that there are no surface waters located on -site,
however, please list the width of the required vegetated setback form
surface waters (per 15A NCAC 02H. 1003 (4)(&.1017(10)). You may
leave Lines 8-10 as -is since there are no surface waters located on -site.
b. Drainage Areas Page:
i. General:
1. Line 10- Please include this item. This item is the sum of the items
of Line 12.
2. Line 18 — Please include this item.
3. 3. Lines 19-21 - See earlier comment.
ii. Entire Site Column:
1. Lines 5-7 - The "drainage area" for the entire site is equal to the
project area as shown in Section IV, 7 of the Application. Typically,
there are no off -site areas for the entire site column since the entire
site column includes the entire project area.
2. Line 12 - The items of Line 12 in this column should not be less
than the items of Line 12 in the other columns. For example, the
entire site parking area is shown as 17,97 sf and bioretention cell
DA parking rea is shown as 33,950 sf (there can't be more on -site
paring BUA draining to an SCM than there is on -site parking BUA).
3. Line 18-This item should correspond to Section IV, 8 of the
Application.
4. Lines 19-21 - These items are not required fo this column.
5. NOTE: The entire site column is an accounting of the entire site and
includes all on -site BUA, whether or not it drains to an SCM.
c. Bioretention Cell & Dry Pond Pages
i. See earlier comments.
Page 14

Jim Farkas
February 20, 2023
Response: a. Cover Page: The project area has been updated to 6.89 acres, and the
vegetated setback has been updated to 30'.
b. Drainage Areas Page: Line items have been updated accordingly.
c. Bioretention Cell and Dry Pond Pages: The bioretention page has been
updated accordingly. Because the dry pond does not function for water
quality treatment, it has been removed.
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:
i. 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 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 click here and insert your phone number
Sincerely,
251- �-- 4
Nathan Hudgens, P.E.
Page 15