SW6231002_Response To Comments_20240805

A0 O WithersRavenelOur People.Your Success.
July 2, 2024
Jim Farkas
Greenfield Serenity, LLC
1612 Mail Service Center
Raleigh, NC 27699-1612
RE: Serenity Smith Betts - Harnett Co
Stormwater Plan Approval
Jim Farkas Project No.SW6231002
WR Project No.02190647
To whom it may concern,
We are writing this letter in response to comments dated June 13,2024. Each of the comments are repeated
below,followed by our response.
DEPARTMENT NAME
Subheading
Page Reference
1. Please clearly indicate the extents of the on-site surface waters in the main set of plans (per 15A
NCAC 02H .1042(2)(g)(iii)).Surface waters are surface water features shown on a USGS topographic
map. Per the provided USGS topographic map, there appears to be a surface water located along
the eastern border of the property, along the western border of the property, along & near the
northern border of the property,and near the southern border of the property(see attached image).
Some of these, like Hector Creek, are shown, but others are not. Please ensure the following:
a. These surface waters are clearly shown on the plans (per 15A NCAC 02H .1042 (2) (g)
(iii)).
b. Vegetated setbacks from these surface waters are clearly shown on these plans(per 15A
NCAC 02H .1042(2)(g)(iii)). NOTE: Vegetated setback requirements for water supply
watersheds are different than those required for HQWs, the more restrictive, applicable
of which must be met(see 15A NCAC 02B .0624(11)).
c. The surface area of these surface waters is indicated in Section IV, 6 of the Application
and the Supplement-EZ Form (Cover Page, Line 4).
d. The surface area of these surface waters is excluded from the project area in Section IV,
7 of the Application and the Supplement-EZ Form (Cover Page, Line 2 & Drainage Area
Page, Entire Site Column, Line 5). NOTE: This also applies to the low-density drainage
area.
e. The Percent BUA for the project area is recalculated in Section IV, 8 of the Application
and the Supplement-EZ Form (Drainage Area Page, Entire Site Column, Line 18).
NOTE:This also applies to the low-density drainage area.
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NOTE:If you believe that the surface waters shown on the USGS topographic map do not accurately
reflect the extents of the on-site surface waters, you can reach out to the Fayetteville Regional
Office to schedule an on-site determination (per 15A NCAC 02H .1017(10)).
Current comment:
If you believe that the surface waters shown on the USGS topographic map do not accurately reflect
the extents of the on-site surface waters,you can reach out to the Fayetteville Regional Office to
schedule an on-site determination (per 15A NCAC 02H .1017(10)). Surface water determinations
must be performed on-site by a qualified individual within the Division. Revise the package as
needed.Clearly identify and delineate the top of banks of the stream in the main set of plans(Section
VI, 8h). If it is not possible to delineate the banks of the surface water (as would be the case for a
narrow stream), please show the centerline of the surface water and provide a callout listing the
surface area of that surface water that is within the project boundary. Note that the surface area
of the surface water is the area between the top of the banks of the streams and normal pool of
impounded structures. Note:the minimum vegetated setback must be 30 ft and must be measured
from the top of bank and not the centerline. 15A NCAC O2H.1017(10), 15A NCAC O2H.1003(4)(a),
15A NCAC 02H.1042(2)(g)(iii). Ensure the surface water area is revised across the package
accordingly. Ensure the previous comments were addressed.
A. Ensure the vegetated setbacks remain vegetated. Riprap cannot be located within the
vegetated setback.
RESPONSE
We included a stream buffer identification pdf performed by Inver Environmental Consultants
in our package and updated our Existing Conditions CD sheet with respective buffers and labels
to reflect that analysis. We show the top of banks on our plans and calculated the respective
Surface Waters area on our Land Use and Curve Number calculations sheet. That number was
subtracted from the total project area.See below:
Total Project Area= 155.11 ac
Total Onsite Surface Waters area= 1.50 ac
OFFISH WOODS B 60 -.. 796,603 1829
0NSETE•PHASES 8 6 V WOODS 8 60 126.066 2.89
OFFtST1 WOODS D T9 403.734 9.27
0NSTE•Pt4ASES8&V �.�.�,.� D !+ 140.618 3.23
Total Omlte Impervious Area: 143.430 3.29
Tool lot Impervious Area: 139.109 3.19
_ total Phase 8&9 Area: _1.232.134 28.30
Total Omite Area: 1.232.834 28.30
Total Sa0Dralnate Basin Area: 3,302.959 71.83
Total Impervious Area 165JM2 3.81
Cnedadve Curve Number;CN): 62
1iMi9rtM0ainlIM_
Taal Omit.Impervious Area 2,342.563 53.78
Total lot impervious Area Btpasslre SCMi: 210.225 4.83
Total Phase 8&9 Area: 6,756.606 155.11
Total Omit.Area: 7415.535 17443
1 Tr.tal Omit*Surface Waters: 65A00 1.50
r Total lespervlous Area: 2,3 9.024 54.61
Total Watershed Area: 11.352.266 260.61
PROJECT INFORMATION
1 Project Name Serenity-Phases 8&9
2 Project Area(ac) 155.11
3 Coastal Wetland Area(ac) 0
4 Surface Water Area(ac) 1.5
5 Is this project High or Low Density? Both High and Low Density
6 Does this project use an off-site SCM? No
Serenity Smith Betts - Harnett Co Comment Response Letter WR Project No.02190647
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A. In the past, we have been advised to extend the riprap out to the top of bank to ensure
integrity of the outlet channel by preventing preferential flow through the buffered areas
and potential undercutting when the flow leaves the riprap dissipator. We believe 15A
NCAC 02H.1003(4)(f)justifies this practice.
2. Previous comment: Please correct the following issues with the design of the wet ponds:
a. Please ensure that the sediment storage zone is not accounted for in the volume of the main
pool or the volume of the forebay for the wet ponds. For example, for Wet Pond 6, the
sediment storage zone for the main pool is shown between elevations 321.0' & 322.0' so
incremental and cumulative volumes should start at elevation 322.0', not at elevation 321.0'
(this issue also appears in the forebays and for each wet pond SCM). Please revise as needed.
Current comment:
The comments response document indicated that this issue was resolved, however,the issue is still
present within the calculations. Using SCM #6 as an example, the top of sediment storage per the
plans is 328.0"; therefore storage should not be included between the 327.0" and 328.0" contours
in the stage storage calculations.
RESPONSE
Our volume calculations do not include sediment storage.Please see red boxes below from snips
of SCM#6 as an example,the sediment storage volume is subtracted from the total volume at
Normal Pool on both Main Pool&Forebay basin sizing talcs:
SCM#6 Pond Volume Information
Incremental Accumulated
Countour Contour Contour Contour
Pond Area Elevation Area Volume Volume
(ft) (sf) (cf) (cf)
8,931 MEM=®(-Normal Pool
t r MEZ 44 ‹—Bottom of Littoral Shelf
Main Pool =1/11f f MEE= 6,907
MgEEM
330.00 11,960 17,066
328.00 MEE:M=1MM mom(-Sediment Storage Volume
MEEECM=EC=ME:M1 (—Pond Bottom
MEDEMMEMM=IETEM MN=(-Normal Pool
METIMMEEMMIal= 5.353 (—Bottom of Littoral Shelf
Forebay t t ! 1,614 3,316
330.00 1.414 1.110 1,702
•t. MEME ) (—Sediment Storage Volume
328.00 MOMS 1 (-Forebay Bottom
OPTION 2: Average depth when the shelf Is being excludedm fro average depth akulation
Day-
L
-Vp.,,a.paar-0.5 x Omax owrsanJ x Ppe.a.-p.a X Wsvemnemsa.el
Aaor_seer J
Where: D,,.=Average Depth(ft)
V,,,,,„a=Volume of Permanent Pool(c0
D,,,o,2,,,,,.-Depth of water at the deep side of the shelf to permanent pool(ft)
P a,,,.,a•Perimeter of the main pool at the bottom of the shelf(ft)
W„e,,,e,a,v,,,,•Width of the shelf at permanent pool(ft)
Ay„,.,,,•Area of Wet Pond at the Bottom of the Shelf(sf)
b I V.,.,,..- 30,626 cf (Total Volume-Sediment Storage Volume)I
P,•,,,,,,,- 420 ft
W.,m,m,a.a•r• 3.00 ft
A.,.a•.= 7,890 sf
Dw.= 3.84 ft
Use Average Depth• 3.50 ft
SCM#6 Forebay Sizing Information
Per NCDEQ'Stormwater Design Manual•Minimum Design Criteria:
The forebay volume shall be 15-20%of the main pool.
Maln Pool Volume• 30,626 cf (Total Volume-Sediment Storage Volume)
Forebay Volume= 6.059
IForebay Volume•19.8%of Main Pool I
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A0 O WithersRavenelOur People.Your Success.
b. Previous comment: Please ensure that the forebay volume is within 15%-20%of the volume
of the main pool (per Wet Pond MDC 5a). For example, for Wet Pond 6, the provided
calculations show the forebay volume being 20.8% of the main pool volume which is not
acceptable since this is not within 15%-20% of the main pool volume. (this issue also
appears in other wet ponds). NOTE: Since the main pool and forebay volumes need to be
recalculated, it may change which SCMs this comment applies to. Please revise as needed.
Current comment:
This issue appears in SCM #9,and may apply to other ponds when the volumes are recalculated per
the previous comment.
RESPONSE
All forebays were revised to be between 15-20%as reflected in our calculations package
c. Previous comment: Drawdown calculations - Please ensure that the design volume of the
wet pond (volume that can be stored between the permanent pool and lowest bypass
invert) is used in these calculations(per Wet Pond MDC 7).
Current comment:
This issue remains unresolved. Using SCM#6 as an example,the temporary pool elevation is 335.0',
the permanent pool elevation is 332.5'. Per the stage storage calculations, the volume between
these elevations is 35,894 CF. Per Part B of the manual, when computing drawdown through an
orifice it is recommended to use Ho/3 to reflect the fact that head is decreasing as drawdown
occurs.
RESPONSE
We revised our drawdown calculations to ensure that the WQ volume drains within the
required 2-5 days considering just the required WQ volume AND the volume at the lowest
bypass invert.We use Ho/3 in our drawdown calculations,please see the snip below from SCM
#6 as an example.
Serenity Smith Betts - Harnett Co Comment Response Letter WR Project No. 02190647
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ORIFICE CALCULATOR
Per NCDEQ"Starmwater Design Manual-Minimum Design Criteria:
The design volume shall draw down to the permenant pool level in 2-5 days.
Q=C4A 2gh
1"WATER QUALITY STORM VOLUME
AT WATER QUALITY POOL ELEVATION H O
Variables Constants
WQ Volume: 0.513 acre- 22346 of g= 32.2 ft/s
Head/Driving Head: ECM (EEO Ca- 0.6
Draw down time: 52 hen 187920 sec
Orifice Area• 0.034 sq.ft. 4.897 sq.in. H o/3
Orifice Diameter= 2.50 In
USE 2.5 INCH DIAMETER ORIFICE , Ho
AT OVERFLOW ORIFICE ELEVATION
Variables
WQ Volume: 0.824 acre-. :94 H o/3
Head/Driving Head: Elm 0.83 ft
Draw down time: • • •ri=vs sec
Orifice Area= 0.034 sq.ft. 4.890 sq.in.
Orifice Diameter= 2.50 in
I USE 2.5 INCH DIAMETER ORIFICE I
SCM#6 Normal Pool Information
Avg Depth- 3.50 ft Normal Pool Elevation-
SA/DA ratio- 1-71 From Table I Main Pool SA Provided= ey41 Sq.rt.
DA x SA+DA ratio 0.205 acres
Minimum pond surface area(SA)—
100
SA• 8.831 sq.ft.
0.203 acres
SCM#6 Water Qualit Information
1-Inch Runoff Volume Calculation(Water Quality Volume) Provided Water Quality Volume
Using'Simple Method"Runoff Volume Cal(ulalmns Water Quallly Pool Elev= 334.08 ft
A.dr,tnbvd br 5,Annb;p7an Overflow Etev= 335.00 ft
RI,=0.05 t 0.9 Cl Where.Rv-Runoff coefficient(ia/ia) Storage Volume Provided= 35,894 cu.ft.
I=Percent impervious 0.824 acre-ft
Rv• 0.52 in/in
Total runoff volume from 1•inch precipitation:
['unoff Volume(S)=Design Rainfall x Re x Drainage Area
S S. 22.346 cu.R.
0.513 acre-ft
d. Previous comment: Wet Pond 7 - As designed,this wet pond appears to short-circuit(per
Wet Pond MDC 4). Please either move the outlet location to the corner of the wet pond
furthest away from the inlets or revise the pond configuration (move inlets/outlet, regrade
the pond,add berms/baffles, etc...)as to avoid short-circuiting through the pond.
Current comment:
The response to comments document referenced recommending gabion baskets, but these could
not be located within the plans. Please direct the reviewer to this item.Thank you.
RESPONSE
Callout added to Plan View and Profile View
3. Current comment:
The inlets of SCMs shall be designed to protect the SCM from erosion resulting from stormwater
discharges. 15A NCAC 02H .1050 (4). Provide calculations demonstrating all inlets and outlets to
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the SCMs are protected from erosion. Ensure the class and sizing information provided on the plans
matches the calculations.
RESPONSE
Callouts have been added for SCM inlet riprap protection sizing. Please see page 138 of the
'Storm Drainage Calculations'report added to the submittal package for calculations.
4. Previous comment: Please provide the per lot BUA allocation table to the reviewer as an Excel
spreadsheet so that the values may be verified(it is also needed to draft up the permit as the permit
includes the BUA allocation table).
Current Comment: Please provide as part of the deed restriction documentation as this will need to
be recorded on a per lot basis. The table format has been pasted below. List the lots by number,
instead of by"type".
RESPONSE
This has been added to the submittal package as an Excel spreadsheet.
5. Current Comment: Please correct the following issues with the O&M Agreement Form:
a. Revise the O&M Agreement Form to be consistent with the rest of the package. For example:
SCM 6 lists the permanent pool elevation as 327.5', but the rest of the package shows the elevation
of the permanent pool to be 332.50'.
RESPONSE
Corrected
6. Previous comment: Please correct the following issues with the Supplement-EZ Form:
a. Cover Page:
i. Lines 2 &4 - See earlier comment with regard to surface waters.
ii.This form will require a new signature.
b. Wet Pond Page:
i. Line 40 - See earlier comment with regard to the drawdown calculations. Revise as
needed.
c. Low-Density Page:
i. Previous Comment: You will need to fill in the swale section of the low-density page of
the form for any vegetated conveyances located within the low-density area. There
appear to be swales within the low-density area.
d. Filterra Page:
i. Lines 25-26: Please provide
RESPONSE
a.Surface water area is included on this page
b.Please see earlier response explaining our drawdown calculations
c.There are no swales within the low-density drainage area. The low-density areas
include only the back of lots,which discharge runoff via sheet flow.
d.These lines have been filled in
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7. Current comment:
Per 15A NCAC 02H .1003(5),stormwater outlets shall be designed so that they do not cause erosion
downslope of the discharge point during the peak flow from the 10-year storm event as shown by
engineering calculations. Provide calculations for each location that stormwater outlets from the
site(including low density areas).
RESPONSE
There are no piped or vegetated conveyances in the low density areas. Riprap calculations for
SCM discharges can be found in the SWMP.
8. Current comment:
Please include the following labels on the main set of plans: the depth of storage above the filter
media surface, surface area/dimensions of the Filterra Bioretention system, specification of media
(so that infiltration rate can be confirmed), and planting information. This item is required per 15A
NCAC 02H .1042(2)(h)(i).
RESPONSE
Standard specs and notes have been added to the plan sheet detailing the above information.
Sincerely,
WithersRavenel
440.4404. -
Brandon Miller, PE
Project Manager
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