SW4231002_Response To Comments_20240514

CRUMPLER
Consulting Services, PLLC
DEMLR Post-Construction Stormwater Program
Brianna Holland
512 N. Salisbury Street, Office 625Y
Raleigh, NC 27604
Re: Stormwater Permit No. SW4231002
Dollar Tree—Welcome, NC
Response to Comments
1. Design Prior Comment 4.b.—"The temporary pool elevation is determined by the elevation of
the lowest bypass... Revise the draw-down calculations when this volume is revised." See
comments with regard to the drawdown calculations and stage-storage tables below. Please
revise as needed.
The temporary pool has been revised as requested and shown on the storage table as Storage
Table 3, Design Volume Storage Table.These values were pulled directed from Autocadd
Volume Calculations.
2. Prior Comment 4.d.—"Per Wet Pond MDC 6:The main pool shall be equipped with a
vegetative shelf around its perimeter." Per the plan view of the wet pond,the vegetated shelf
is shown between elevations 862 ft and 863 ft whereas the other submittal items indicate that
the vegetated shelf is located between elevations 861 ft and 862 ft. Please revise as needed
for consistency.
The detail has been updated to be consistent with the plans and specifications.
3. Prior Comment 4.e.—"Per Wet Pond MDC 11, 50 plants per 200 square feet of shelf area is
required.The Landscaping requirements note on the plan says 460 plants will be
provided..." Similar to the original comment, there is still a note on the plans indicating that
there will be 460 plants provided, however the planting schedule and notes on the plan view
of the pond indicate that only 325 plants are being provided (see attached). Please revise as
needed for consistency.
The plantings have been updated to reflect the square footage of the shelf based on the
difference in surface are of 862 and 863 of the shelf on the storage table.
4. Prior Comment 5.b.—"Revise the narrative to match the rest of the package.The narrative
currently references a dry detention basin, which is not a primary SCM." This comment was not
addressed. Please revise as needed.
The narrative has been revised and attached with the submission.
5. Consistency: Prior Comment 5.d.—"The emergency spillway elevation is listed as 397.75 in the
emergency spillway detail, which appears to be an error."This comment was not addressed.
Please revise as needed.
The spillway detail has been updated.
6. Prior Comment 6.c.—"Wet Pond Page, Line 35—For a wet pond,the design volume is the
amount of storage that is provided in the wet pond above the permanent pool surface and
below the elevation of the lowest bypass weir/orifice in the outlet structure (temporary pool
elevation) and must be at least as large as the minimum required treatment volume. Please
revise accordingly." See below Supplement-EZ comments.
2308 Ridge Road I Raleigh, NC 27612 I p: 919-413-1704 I e:[e-mail]

CRUMPLER
Consulting Services, PLLC
The design volume has been updated and shown to be in compliance based on the
calculations of the simple method and the design volume of the pool between 863 and 864 as
shown on table 3 of the storage calculations.
7. Prior Comment 6.d.—"Wet Pond Page, Line 40—The drawdown time is the amount of time to
drawdown the design volume provided in Line 35."See below Supplement-EZ comments.
The drawdown time has been updated and note using the volume of the pool between 863
and 864 as shown on table 3 of the storage calculations.
8. Prior Comment 7.—"Please show the required vegetated setback from surface waters in
the main set of plans.This item is required per 15A NCAC 02H .1042(2)(g)(iii). Per 15A
NCAC 02H .1017(10),the vegetated setback width from surface waters is 30 ft. Revise the
Supplement-EZ, Cover Page, Line 7 accordingly." See below Supplement-EZ comments.
The EZ form has been updated to be N/A.
9. Please correct the following issues with the stage-storage tables:
a. a. Forebay—The provided stage-storage table does not appear to calculate the
incremental storage volumes correctly. For example,the cross-sectional area at
elevation 860' is shown as 272 sf, the cross-sectional area at elevation 861' is shown
as 502 sf, and the volume stored between these two elevations is shown as 797 cf(the
actual volume stored between these two elevations would be closer to 380 cf). Please
revise the table as needed.
The forbay storage table has been updated as requested.
b. Temporary Pool—Similar to the forebay table, this table does not appear to calculate
the incremental storage volumes correctly. For example,the cross-sectional area at
elevation 862' is shown as 3,578 sf, the cross-sectional area at elevation 863' is
shown as 5,250 sf, and the volume stored between these two elevations is shown as
2,690 cf(the actual volume stored between these two elevations would be closer to
4,400 cf). Please revise the table as needed.
The temporary pool has been updated as needed based on the 863 being the top of
the shelf and bottom of the temporary pool.These values were pulled directed from
Autocadd Volume Calculations.
10. As designed, the wet pond does not have a sufficient average depth, as required per Wet
Pond MDC 2. Using the provided information (Equation 2 used, main pool surface area =
2,790 sf, & main pool volume = 7,755 cf) results in an average depth of 2.78 ft (7,755 cf/
2,790 sf= 2.78 ft< 3.0 ft). Please revise as needed.
Based on the revised calculations of the main pool,the pond exceeds the 3.0' minimum as
7,755cf/2,790sf=3.01'.
11. As designed,the drawdown orifice is too small to draw down the temporary pool within the
2-5 day window required per Wet Pond MDC 7. Using the provided information (DV=
10,153 cf(interpolated based on the bypass invert of 864.0' and the provided surface areas
and elevations from the stage-storage table), Cd =0.6, Diameter= 0.5", Ho=0.66 ft) results
in a drawdown time of 22.17 days (A= pi * ((Diameter/2) * (1ft/12in))^2 = 3.1415 * (0.5in/2
* 1ft/12in)^2 =0.00136 sf, Q=Cd * A * (2g * Ho)^0.5 =0.6 * 0.00136 sf* (64.4 ft/s^2 *
0.66 ft)^0.5 = 0.0053 cfs,T= DV/Q= 10,153 cf/0.0053 cfs/86,400 sec/day= 22.17
days). NOTE:Your provided calculations use the minimum required treatment volume
instead of the design volume and there appears to be an issue with the orifice sizing
calculation (a circle with diameter of 0.5" has an area of 0.00136 sf). Please revise as needed.
2308 Ridge Road I Raleigh, NC 27612 I p: 919-413-1704 I e:[e-mail]

CRUMPLER
Consulting Services, PLLC
The drawdown time has been updated and note using the volume of the pool between 863
and 864 as shown on table 3 of the storage calculations.The draw down time using a 1" hole
is 4.35 days within the 2-5 window.
12. Please correct the following issues with the Supplement-EZ Form:
a. Cover Page:
i. Line 7—Please list the vegetated setback width (as indicated in 15A NCAC 02H
.1017(10)). It is noted that there are no surface waters located within the project
area, we just ask that his information be included on the cover page of this form
(you may change Line 8 to "N/a"to reflect that there are no vegetated setbacks
on this site).
The line item has been updated to N/A.
b. Drainage Areas Page:
i. Entire Site Column, Line 18—The provided value does not appear to be correct.
This value should be calculated in the same manner (and using the same values)
as Section IV, 8 of the Application. NOTE: For both this item and the Application,
you can exclude existing BUA(even if it is being removed) in the project density
calculation [Density= Net Increase in BUA/(Total Project Area— Existing BUA)].
The Density calculation has been updated to 43%for the site.
ii. DA 1, Line 20—Please show the design volume (volume stored between the
permanent pool surface and the invert of the lowest bypass elevation), not the
minimum required treatment volume.
The design volume has been updated based on the storage table.
c. Wet Pond Page:
i. Revise as needed per other comments (some items are listed below, but
depending on the nature of the revisions, other items may need to be revised).
Noted.
ii. Line 25—This item should refer to the invert of the lowest bypass device (top or
riser structure per plans).
The invert of the lowest bypass at top of riser has been added.
iii. Line 28— Revise as needed.
Avg.depth has been revised per calculations.
iv. Line 32—Revise as needed.
Volume of forebay has been added per the storage tables.
v. Line 35—Revise as needed.This value should correspond to the volume stored
between the permanent pool and the invert of the lowest bypass elevation.
The temporary pool has been updated as needed based on the 863 being the
top of the shelf and bottom of the temporary pool.These values were pulled
directed from Autocadd Volume Calculations value has been revised per the
storage tables.
vi. Lines 37 &40—Revise as needed.
The orifice sizing and drawdown time has been updated per drawdown
calculation.
vii. Lines 43 &44—The depth of the forebay is measured from the permanent pool
surface to the excavated bottom of the forebay near either the entrance or exit.
2308 Ridge Road I Raleigh, NC 27612 I p: 919-413-1704 I e:[e-mail]

CRUMPLER
Consulting Services, PLLC
Per the plans,the elevation of the permanent pool is at elevation 862.0 ft,the
elevation of the forebay entrance is at elevation 859.0 ft (36 in) and the
elevation of the forebay exit is at elevation 859.5 ft (30 in). Revise as needed.
The depth of the forebay has been updated per the detail based on the design.
13. 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.
The plans have been uploaded to the website and hard copies provided to the office.
2308 Ridge Road I Raleigh, NC 27612 I p: 919-413-1704 I e:[e-mail]