SW3230606_Response To Comments_20240208

j!j THOMAS & HUTTON
1020 EUCLID AVENUE
CHARLOTTE, NC 28203 I 980.201.5505
WWW.T H O MASAN D H UTTO N.CO M
January 23, 2024
Brianna Holland Re: Finger Mill
NCDEQ High Density Wet Pond
512 N. Salisbury Street, Office 640E Lincoln County, NC
Raleigh, NC 27699-1612 J-30017.0000
To whom it may concern:
This letter serves as acknowledgement that we have revised the previously submitted package
to address all comments below as follows:
Previous Comments:
- Original Comment 1: "The Drainage Area 1 column provided in the Application
Section IV, 10 and the Pond 01 column on the Drainage Areas page of the
Supplement EZ are intended to describe the same drainage area."
- Current Comment: Application Section IV, 10 and the Pond 01 column on the drainage
areas page of the EZ are still inconsistent. For clarification, I have attached a snip of the
relevant portions of the application and the Supplement-EZ which should show the
same BUA and drainage information below outlined in red.
0 Response: The stormwater management permit application form section IV, 10
and Supplement-EZ form drainage areas page have been revised for
consistency. The following items should be consistent across plans, Application,
Calculations and supplement-EZ:
■ Total Drainage Area (on-site+off-site): 680,843 SF; 15.63 ac
• On-site: 13.37 ac; (582,397 sf)
• Off-site: 2.26 ac; (98,446 sf)
• On-site impervious: 8.25 ac (359,776 sf)
• Off-site impervious: 1.62 ac (70,567 sf)
• Total impervious (on-site+off-site): 9.88 ac (430,343sf)
- Original Comment la: "Per the Application Section IV, 10, the total drainage area
directed to the SCM is 608,872 SF (13.97 ac). The calculations list the drainage area as
13.37 acres. Please revise for consistency."
- Current Comment: The Application Section IV, 10 lists the total drainage area directed
to the SCM as 680,843 SF (15.673 ac). Per Sheet C3.0, the total drainage area appears
to be the SCM as 680,843 SF (15.63 ac). Per sheet C3.0, the total drainage area
appears to be currently list the project/drainage area as 13.37 ac. The required water
quality volume needs to account for the total drainage area directed to the SCM. This
will need to be updated on the Supplement-EZ, Wet Pond Page, Line 2.
0 Response: The 2.26-acre off-site area is included in the 15.63-acres.The

construction plan sheet C3.0 and the required water quality volume have been
revised to account for the total drainage area of 15.63 acres.
- Original Comment lb: "The BUA information should match between the Application
and Supplement-EZ and currently does not. The total BUA of new BUA in the
Application Section IV, 10 is stated as 359,776 SF, but is listed as 479,160 SF on the
Supplement-EZ. The impervious area listed in the calculations is 8.00 acres which does
not match the Supplement-EZ or Application. The percent impervious is listed as 59%in
the Application, but 42%in the Supplement-EZ Pond 01."
- Current Comment: The total BUA in the Application Section IV, l0 is stated as 430,343
SF, but is listed as 479,160 SF on the Supplement-EZ. Sheet C 1.0 notes the impervious
area as 8.3 acres. The wet pond sizing calculations list an impervious area of 9.90 acres.
The BUA needs to be consistent across the package. Verify the total BUA in the project.
The Supplement-EZ, Drainage Area Information Table, Line 5 lists the total BUA in the
project as 479,160 SF. This does not appear to correspond to the BUA that is listed within
the remainder of the Drainage Area Information Table. The values that were provided
in the Supplement-EZ as draining to the SCM are listed below and totaled as the last
line. Please revise as needed.
o Response: The 430,434 sf(15.63-acres) is correct. The supplement-EZ form and
sheet C1.0 have been revised to account for the total drainage area of 15.63-
acres and total impervious area of 9.88 acres.
- Original Comment 2a: "Application Section IV, 8: The total BUA provided in the table is
359,776 SF (8.26 ac), the total project area provided in Section IV, 7 is 24.55 acres.
Project %impervious = (Total Impervious/Total Project) X 100 =8.26 ac/24.55 ac = 33.6%
<55%."
- Current Comment: The total BUA provided in the table is 430,343 SF (9.88 ac), the total
project area provided in Section IV, 7 is 24.55 acres.
Project%impervious = (Total Impervious/Total Project) x 100 = 9.88ac/24.55ac = 40.2%>32.6%.
o Response: The stormwater management permit application form section IV, 8
has been revised to show the 40.2%.
- Original Comment 4c: "Vegetated shelf: Per MDC 11, please clarify on the plans that
the minimum requirement of 50 plants per 200 square feet of shelf area will be met."
- Current Comment: For clarification, please provide the area (in square feet) of the
vegetated shelf (often this is just a note on the Plant Schedule Table, it does not need
to be broken up by plant type.) The quantity of plants provided is 1413, so it just needs
to be shown that the vegetated shelf has an area of less than or equal to 5,652 SF.
o Response: Plan sheets L1.0-L1.3 plant schedule tables have been revised to
show a column that includes total area of plantings and total vegetative shelf.
2nd Technical Review Comments:
1. Per 15A NCAC 02H. 1050 (4), the inlets and outlets of the SCM need to be protected
from erosion resulting from stormwater discharges during peak flow from the 10-year

storm. The flows used in the outlet protection calculations correspond to the flows
provided in the 1-year flow calculations.
a. Response: Peak flows shown in appendix H for FES #1 and #38, are 41.0 cfs and
21.5 cfs, respectively, which corresponds to the calculated 10-year peak flow
found in appendix G. Pipe capacity and energy grade line calculations in
appendix G were based on the 10-year storm. 1-year flows were not
calculated for the purposes of pipe sizing. Peak flow for FES#41 is 39.73 cfs and
corresponds to the peak flow associated with the modeled discharge from the
pond from the 50-year storm.
2. Per Wet Pond MDC 1, the volume of the main pools and the forebays should not
include the sediment storage zone since this portion of the pond is intended to be filled
up with sediment and can therefore not count towards provided storage volume.
Please revise the stage tables, associated volumes, and calculations (such as average
depth). Revise the Supplement-EZ accordingly.
a. Response: Revising the bottom of storage in this way resulted in reducing the
average depth to below 3.0'; therefore,the bottom of the main pool and
forebays has been changed from 805.00 to 804.50. Sheet C6.1 has been revised
accordingly, along with the supplement EZ. The stage storage calculations
remain the same, as the sediment storage zone has now been lowered by 6
inches.
3. Per the plans the width of the submerged vegetated shelf is 3 ft as the main pool is at
an elevation of 808.5, and the top and bottom of the vegetated shelf are 809 and 808
respectively. Please revise the Supplement-EZ, Wet Pond Page, Line 31 and the
average depth calculation as required per Wet Pond MDC 2.
a. Response: Supplement-EZ line 32 updated to 3.0 feet and average depth
calculation updated accordingly.
4. The calculations list the total forebay volume as 9,965.25 CF, but the Supplement-EZ,
Wet Pond Page, Line 32 lists the total volume of the forebay as 13,996 CF. The values
provided in the Supplement.
a. Response: Supplement EZ has been revised to show a forebay volume of
9965.25 cf to match calculations.
5. Supplement-EZ, Wet Pond Page, Line 34-The cleanout depth of the forebay is the
distance from the permanent pool surface elevation to the top of the sediment
storage zone. Please revise.
a. Response: Due to the increased depth of forebays discussed in response
number 2, this depth will remain 42 inches. (Permanent Pool = 808.5, new
sediment storage elevations = 805.00)
6. Forebay 1 and Forebay 2 appear to have different depths per the plan view. Ensure
that the cross-section view reflects both forebays and sediment cleanout depths, entry
and exit depths are clearly shown per Wet Pond MDC 5.
a. Response: Plans were missing 805 contours; this has been addressed. Revised
grading sheets have been provided.

7. Clarify the drawdown calculations. Per Wet Pond MDC 7, the drawdown volume is the
volume between the temporary pool (elevation 810) and the permanent pool
(elevation 808.50) and must be greater than the design volume.The 2-5-day
drawdown requirement does not apply to any peak attention volume that the pond is
designed to handle, but the hydraulics of any peak flow attenuation capacities should
be considered when calculating the two to five-day drawdown rate.
Clarification received via email 2/2/2024: Utilizing a stage storage table, provide the
volume of the main pool at the temporary pool elevation (810) and the permanent
pool elevation (808.50). The volume between these elevations is the drawdown
volume. Use this volume in the drawdown calculations.
a. Response: A line has been added to the Water Quality Calculations in the
upper right corner, corresponding to "Provided water quality volume"; i.e. the
volume of the temporary pool at elevation 810. Drawdown time was then re-
calculated based on this new volume and the height of head above the orifice
(1.5'). The new drawdown time is now 81.72 hours/3.41 days. This new value is
also revised in the EZ form, line 40 for consistency.
8. A scanned copy of O&M Agreement was provided. An original, signed & notarized
hard copy form is required per 15A NCAC 02H. 1042(2)(j) and per Sections V & VI, 3 of
the Application.
a. Response: See attached original, signed & notarized hard copy O&M form.
9. 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.deg.nc.gov/Forms/SW-
supplemental-Upload
b. Hard copies must be mailed or delivered to the following address:
i. For Fed Ex/UPS:
Brianna Holland
512 N. Salisbury Street, Office 640E
Raleigh, NC 27604
ii. For USPS
Brianna Holland
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.
a. Response: See attached revised documents.

Sincerely,
THOMAS & HUTTON
Jason K. Seaman, PE
Project Manager