SW6221201_Response To Comments_20230404

(S) � CSD
ENGINEERING
April 4, 2023
N.C. Dept. of Environmental Quality
Division of Energy, Mineral, & Land Resources — Stormwater Program
512 N. Salisbury Street, Office 640E
Raleigh, NC 27604
Re: Proposed Ample Storage Facility in Erwin, NC
Subject: Stormwater Management Permit Application Re -Submittal
Dear Mr. Farkas,
CSD Engineering has completed comments received from your Request for Additional Information dated March 6,
2023. The following is the written response and summary of revisions made to address comments:
1. West Pond Design
➢ The top of sediment storage in the main pool is 185.0 and top of sediment storage in forebay is
187.5; labels have been added. The volume of the main pool and forebay were adjusted to not
include the sediment storage area.
➢ The forebay entrance is at the 36" pipe shown. The exit of the forebay is intended to be to the east
of entrance to get maximum flow path to outlet; with forebay sloping up to shallower elevation at
exit. The forebay outlet has been drawn and contours adjusted accordingly with labels for forebay
exit on plan and profile shown on sheet C4.2.
➢ With the minor revisions to contour areas to account for a depressed forebay outlet, the new main
pool perimeter is 560.1 feet. The calculation has been revised to show perimeter, max. depth, and
width of submerged shelf as input, resulting in a shelf volume of 1,680 c£ The average depth
calculations were adjusted to the revised shelf volume calculation and updated. Average depth is 4.0'
and corresponding calculations for water quality, etc. were revised accordingly and results are still
within the MDC.
➢ The drawdown calculation inputs used were from older min. design standards. The driving head was
revised to 1 /3 of the starting head and an orifice coefficient of 0.6 was used.
➢ The volume used was for the water quality pool volume (apparently an older criterion approved in
the past). The volume to the top of riser was used (21,695 ct) and the size of orifice was revised to
2.5 inches; resulting in a drawdown time of 2.3 days. The pond outlet structure detail on sheet C4.2
was revised accordingly.
2. O&M Agreement Form:
➢ The permanent pool elevation for forebay and main pool have been revised to 192.0.
➢ The temporary pool elevation for the forebay and main pool have been revised to 193.7.
Note: The sediment cleanout elevations were revised to 187.5 (forebay) and 185.0 (main pool)
according to the comments above and the revised volume calculations.
3. Supplement-EZ Form:
➢ Cover Page:
i. Lines 7-10: Vegetated setback along side property lines is 20 feet (added to form). This
area will remain a vegetated buffer zone and this was noted on form (Line 8).
➢ Drainage Area Page:
i. Site Column: the entire site area (6.18 ac) was added to the site column and the entire
BUA is within this site area. The existing BUA is very small and does not help meet
permitting requirements (all new BUA is being treated).
■3805 Cherry Street ■ Wilmington, NC 28403 ■ 115 E. Third Street (P.O. Box 1250) ■ Wendell, NC 27591 ■ ■ [phone] o ■ [phone] f

ii. The total BUA on non -subdivided lots has been entered on line 10.
iii. Drainage Area 1 Column, Line 20: The volume to the top of the riser (per previous
comment) has been added to this line.
b. Wet Pond Page:
i. Line 8: Revised.
ii. Line 25: changed to top of riser elevation (193.7).
iii. Line 26: Revised.
iv. Line 27: Revised according to revised calculations and response above.
v. Line 28: Revised according to revised calculations and response above.
vi. Line 31: Corrected. Note: Line 30 was revised after minor revisions to contours at forebay
outlet and interface with main pool.
vii. Line 32: Revised.
viii. Line 34: Updated (192.0-187.4 = 4.5 feet = 54 inches).
ix. Line 35: Revised.
x. Line 40: Revised per comments and response above. Note: the orifice size was revised to
2.5 inches on Line 37.
xi. Line 42: Revised.
xii. Lines 43 and 44: The depth of forebay to excavated bottom at entrance (at 36" pipe) is
60". The depth of forebay at exit is12 inches (rip rap lined notch is forebay berm). Not
sure what the comment is here. Please clarify as the correct numbers appear to be on the
right line.
4. Provide PDFs: Only one (1) sheet of the plan set was revised. Sheet C4.2 — Wet Pond Plan, Profile, & Details
was revised as follows:
➢ Sediment storage volume elevation drawn and labeled.
➢ Top of forebay berm drawn and labeled.
➢ Forebay exit location drawn and labeled.
➢ Drawdown orifice size label revised to 2.5" on outlet structure details.
The following items have been submitted under this resubmittal cover letter:
➢ Two (2) hard copies of the revised plan sheet C4.2 as described above and dated 3/20/2023
➢ One (1) hard copy of the revised documents and calculations:
• Updated/revised O&M Form (Wet Pond page was revised; original cover provided with resubmittal).
• Updated/revised Supplement-EZ Form
• Updated Narrative & Calculations; full set with the following updates:
i. Main Pool and Forebay Volume updated/revised tables and graphs
ii. Updated/Revised Forebay/Main Pool Volume Check and Average Depth Calculation
iii. Updated/revised Pond Sizing
iv. Updated/revised Water Quality Volume Check
v. Updated/revised Drawdown
If items received are not as listed above, and/or you need additional information, please contact me immediately at
[phone] (m).
Best regards,
John F. Oglesby, PE
Sr. Project Manager
■3805 Cherry Street ■ Wilmington, NC 28403 ■ 115 E. Third Street (P.O. Box 1250) ■ Wendell, NC 27591 ■ ■ [phone] o ■ [phone] f