SW3241101_Design Calculations_20241118

RESPONSE MEMORANDUM
Date: October 24t", 2024
To: Jacob Beeker
NCDEQ
[e-mail]
• • • design
From: James McGivern for Hy Nguyen
master planning . civil engineering
Proj: 19076- McCoy Farm urban design .landscape architecture
Re: Response to Staff Review September 24t", 2024 Comments
The following is an enumeration of responses to plan review comments:
Previous Comments.
1. Prior Comment:
Prior Comment-"Prior Comment 2.b.-"Section IV, 8-Please recalculate the
percent BUA for the project. Percent BUA for a project with no existing BUA is
calculated as the total amount of BUA proposed within the project area divided by
the project area(Section IV 7). For a project with existing BUA, the percent BUA is
calculated as the net increase in BUA divided by the difference between the project
area(Section IV, 7)and existing BUA. Please refer to 15A NCAC 02H.1003(1)for
more information."When calculating the percent BUA for the project area, the
surface water area is not included. For a project area of 61.97 ac and a total of 13.36
ac of BUA within the project area, the percent BUA for the site would be 21.6%
(13.36 ac/61.97 ac=21.6%). NOTE: The PCSO Summary(on the plans&in the
calculations)indicates that there is only 11.12 ac of BUA within this project. NOTE:
The BUA summary for the overall site(in the calculations)shows a total of 581,792 sf
of BUA whereas the Supplement shows a total of 581,962 sf of BUA. Please revise as
needed."
The Percent BUA listed in Section IV,8 of the Application is 19.3%.This does not
correspond to the actual Percent BUA calculated from the provided BUA
measurements.((aes,v7o+lzo,szz)-7a,aoo * 0.193)As a result,this comment remains
2,686,781-78,400
unaddressed.
DPR:Per the emails from Jacob Beeker:
Proposed BUA(Excluding Of fsite)—Existing BUA(Whether removed or remaining)
Total Project Area(Section IV,7)—Existing BUA(Whether removed or remaining)
Proposed BUA = 581,792 sf= 13.356 AC
#8 = (13.36AC- 1.80 AC)/(61.68 AC- 1.80 AC) = 0.193 = 19.3%.
2. Comment:
DPR DESIGN. • 901 Berryhill Road,Suite. 101,Charlotte,NC 28208 • PH 704.332.1204 • www.dpr.design 1

Prior Comment—"Prior Comment 2.c.—"Section Iv, 9& 10—SCM 18 does not
appear to be required in order to meet State post-construction permitting
requirements.Please revise this and the other submittal items to indicate this(either
remove the SCM or indicate that the information is provided for reference only."
Section IV,9 of the Application still indicates that there are 3 SCMs proposed for this
project and the lower table in Section Iv, 10 still indicates drainage area"18" NOTE:
The torte narrative indicates that there are"three(2)"SCMs in this project.
Please revise as needed."
Section IV,9 of the Application still references three drainage areas(SCMs).This
comment remains unaddressed.
DPR:Line 9 was updated from (3) SCMs to(2) SCMs. There are only two SCMs in this project.
3. Comment.
Prior Comment—"Prior Comment 4.—"O&M Agreement Form—See earlier
comment with regard to SCM 18."We require an original,signed and notarized hard
copy of this form(an altered photocopy of the original appears to have been
provided)."
IThe version of the O&M Agreement that was provided is outdated and therefore
incomplete.As a result,this comment remains unaddressed.The up-to-date
version of the O&M Agreement can be found at https://www.deq.nc.gov/energy-
mineral-and-land-resources/Wrmwatej/state-syy-supplemental-forms/o-m-ez-
20201 21 5-2-1-3/download.
DPR:Done.
New Comments:
1. Surface Water:
The Surface Water surface area is inconsistent between Section IV,6 of the
Application(given as 2.69 ac)and Line 4 of the Cover Page of the Supplement-EZ
Form(2.73 ac).
DPR: The Application, line#6, was updated to 2.73 AC. The Application and the Supp EZ both now have
the same values for the surface water area.
2. Project area:
The Project Area is inconsistent between Section IV,7 of the Application(given as
61.68 ac)and Line 2 of the Cover Page of the Supplement-EZ form (given as 59.88
ac).
DPR:Done. The typo on Supp-EZ was updated. Both the Application and Supp-EZ have the site area (-)
water surface area = 61.68 AC.
Application Section IV, #10:
• Proposed Impervious:per Jacob Beeker=just on-site BUA;count only Existing BUA that will Remain
Proposed BUA + Of fsite BUA — Existing BUA
• % Impervious Onsite Drainage Area +Of fsite Drainage Area — Existing BUA
• Drainage 1 = (464,270 + 31,700-0)/(1,096,504 +31,700- 0) = 0.4396 =44.0%
• Drainage 2= (117,522 + 3,000-0)/(271,428 + 3,000- 0)= 0.450 =43.9%
DPR DESIGN. • 901 Berryhill Road,Suite. 101,Charlotte,NC 28208 • PH 704.332.1204 • www.dpr.design 2

3. Supplement-EZ Comments
a. Cover Page
i. Project Area and Surface Water Area are inconsistent with what is
shown on the Application.
DPR:Done. These were addressed under New Comments#1 & #2 above.
b. Drainage Areas Page
i. Line 8 of the Entire Site Column does not reflect the Total BUA in
Project(495,970+ 120,522*503,392).
DPR:Per the emails from Jacob Beeker:
Proposed BUA(Excluding Of fsite)—Existing BUA(Whether removed or remaining)
Total Project Area(Section IV,7)—Existing BUA(Whether removed or remaining)
Proposed BUA = 581,792 sf= 13.356 AC
#8 = (13.36 AC- 1.80 AC)/(61.68 AC- 1.80 AC) = 0.193 = 19.3%.
o Drainage Areas items clarified by Jacob Beeker:
• Line 5, Total Drainage Area:
• Entire Site =Project area (It does not equal the sum of any column) =Application,
Section IV, #7= 61.68 AC =2,686,781 sf.
• Each SCM=sum of individual column#6+ #7
• Line 6, On-site Drainage Area:
• Entire Site =Application, Section IV, #7= 61.68 AC =2,686,781 sf.
• Each SCM=Actual measured on-site drainage area respective to each SCM.
• SCM 1 = 1,096,504 sf.
• SCM 2=271,428 sf.
• Line 7, Offsite Drainage Area:
• Entire Site = N/A =Equals Zero! "Off-site is not on-site"-
• Each SCM=Actual measured offsite drainage area respective to each SCM.
• SCM 1 =31,700 sf.
• SCM 2=3,000 sf.
• Line 8, Total BUA in Project:
• Entire Site =All On-site BUA within the project area, whether or not it drains to an
SCM. =581,792 sf
• Each SCM=All on-site and offsite BUA draining to each SCM.
• SCM 1 =495,970 sf.
• SCM 2= 120,522 sf.
• Line 9, New BUA on subdivided lots:
• Entire Site = Total Amount of BUA on subdivided lots. =430,000 sf
• Each SCM=BUA on subdivided lots directed to each SCM.
• SCM 1 =343,140 sf.
• SCM 2=86,860 sf.
• Line 10, New BUA not on subdivided lots:
• Entire Site = Sum of line 12, entire site column = 151,792 sf.
• Each SCM=Sum of line 12, each individual column.
• SCM 1 = 121,130 sf.
• SCM 2=30,662 sf.
• Line 11: Off-site BUA:
• Entire Site =N/A =zero.
• Each SCM=Amount of offsite BUA accounted for in each SCM's sizing.
• SCM 1 =31,700 sf.
• SCM 2=3,000 sf.
DPR DESIGN. • 901 Berryhill Road,Suite. 101,Charlotte,NC 28208 • PH 704.332.1204 • www.dpr.design 3

• Line 12:Breakdown of new BUA (all inside the project)
• Entire site =Accounting of all new BUA not on subdivided lots.
• Each SCM=Accounting of BUA not on subdivided lots draining to each SCM.
Breakdown of BUA Supplement EZ form
NOT on subdivided
Lots Entire Site SCM 1 SCM 2
Parking 0 sf 0 sf 0 sf
Sidewalk 31.000 sf 24,738 sf 6.262 sf
Roof 0 sf 0 sf 0 sf
Roadway 84.600 sf 67.511 sf 17.089 sf
Future 25.000 sf 19.950 sf 5.050 sf
Other 11,192 sf 8.931 sf 2.261 sf
Total only for checking 151,792 sf 121 130 sf 30 662 sf
• Line 18:Breakdown of new BUA (all inside the project)
• Entire Site = Section IV, #8 = 19.3%
• SCM columns:include off-site DA area as 100% BUA =Application:Section IV, 10,
"%Impervious Area"
• SCM 1 =44%.
• SCM2=439%
ii. Lines 20 and 21 are not completed for either SCM.
DPR:As clarified by Jacob Beeker:
• Line 20:Design Volume of SCM
• Entire Site =N/A =zero.
o Each SCM= The minimum required treatment volume for each SCM=Sand Filter
Section, line 2.
• SCM 1 =32,083 cf, this is the WQ volume with 25%discount applied.
• SCM 2= 7,804 cf; this is the WQ volume with 25%discount applied.
• Line 21, Calculation Method for design volume. Entire Site =N/A. Each SCM=Simple
Method(This is the method in the excel sheet used to calculate WQ volume from 1"
event.)
• Line 23: Vol Sediment Chamber= volume that can be stored in the sediment chamber
area from the bottom (Line 25)and the invert of the first bypass device(Line 35). The
top of the rip-rap berm will serve as a boundary.
o SCM 1:31.775 cf, calculated in Excel using elevations 596.00 to 597.30.
o SCM 2:6,050 cf, calculated in Excel using elevatins 612.00 to 613.00.
• Line 25:Bottom elevation of the sediment chamber;due to slope, this is higher than the
sand elevation.
o SCM 1:596.00
o SCM 2:612.00
•
c. Sand Filter Page
i. Line 26, SCM 2-This value is not consistent with what is shown in the
provided plans.
Line 26:Area of the Sediment Chamber. NCDEQ:is "the surface area within the bottom
elevations of the berms and banks which make up the Sediment chamber."
o SCM 1:8,050 sf., measured.
o SCM 2:2,650 sf., measured.
• Line 27:Physical Depth of the sediment chamber, measured from the bottom (Line
25) to the invert of the first by-pass device(line(35) which is normally the small
orifice.
o SCM 1:597.30(-) 596.00 = 1.3'= ±16"
o SCM 2:613.00(-) 612.00 = 1.0'= 12"
DPR DESIGN. • 901 Berryhill Road,Suite. 101,Charlotte,NC 28208 • PH 704.332.1204 • www.dpr.design 4

• Line 28:Line 28:Ponding depth:this is when the sand filter has a permanent pool.
This depth would be the difference from the elevation of the permanent pool to the
first by-pass invert. This does not apply to SCM 1 or 2.
ii. Line 30—These values are not consistent with what is shown in the
provided plans or the provided calculations. Please note that the
volume stored in the sand chamber is only calculated above the top of
the sand layer.
• Line 30 Volume of the Sand Chamber= the volume per Jacob Beeker= volume
"directly above"the sand media up to the 1st bypass device =area sand media *(1st
orifice elevation(-)sand media elevation).
o SCM 1:8,050 sf*(597.30(-)595.00) = 18,515 cf.
o SCM 2:2,650 sf*(613.00(-) 611.50) =3,975 cf.
o Check to ensure that the Sand Chamber volume is at least 50% of the
required WQ design volume, and ensure the Sand Chamber volume is less
than the Sediment Chamber volume—Done!
iii. Line 31 —These values are not consistent with what is shown in the
provided plans or the provided calculations.
• Line 31:Bottom of sand chamber= "the excavated bottom of the sand chamber; this
is used to verify the separation from the SHWTE":
o SCM 1:592.50.
o SCM 2:609.00.
• Line 32:Area of the sand chamber=area of just the sand media.
o SCM 1:8,050 sf.
o SCM 2:2,650 sf.
• Line 33:Physical depth =first by-pass invert elevation (-)elevation of bottom of
underdrain gravel.
o SCM 1:597.30(-) 592.50 =4.8'= ±58"
o SCM 2:613.00(-) 609.00 =4'=48"
• Line 34:Ponding depth of the Sand Chamber above the sand media =elevation of
1st by-pass device(-)elevation of sand media.
o SCM 1:597.30(-) 595.00 =2.3'=27.6"= ±28"
o SCM 2:613.00(-) 611.50 = 1.5'= 18"
• Line 35:Elevation of 15t bypass device. Note: these elevations are specifically called
out on the SCM sheets in the CDs.
o SCM 1:597.30
o SCM 2:613.00
DPR DESIGN. • 901 Berryhill Road,Suite. 101,Charlotte,NC 28208 • PH 704.332.1204 • www.dpr.design 5

iv. Line 36—Only SCM 2 appears to utilize an orifice as bypass.
• Line 36: Both SCMs use orifices in their outlet structures. Each Orifice is now clearly
labeled for each SCM.
• Line 39: Depth above the underdrain pipe(inches) =NCDEQ "Line 39 refers to the
depth of media above the underdrain pipe. This is measured from the top of the
underdrain pipe to the sand surface."
o SCM 1:595.00 sand media elevation (-)594.00(bottom of sand elev.) + 0.33'
(depth of stone above the underdrain pipe) = 1.3'= ±16".
o SCM 2:611.50 sand media elevation (-) 610.50(bottom of sand elev.) + 0.33'
(depth of stone above the underdrain pipe) = 1.3'_ ±16".
4. Per the Supplement-EZ form, both sand chambers will hold more volume than their
respective sediment chambers. Per Sand Filter MDC 2, storage volume in each
chamber shall be equivalent.While the sediment chamber is allowed to be larger
than the sand chamber to provide peak flow mitigation,the sand chamber is not
allowed to be larger than the sediment chamber.
• DPR: The volume values were updated for each SCM. Both SCMs'Sediment
Chamber volumes are larger than the Sand Chamber volumes.
DPR DESIGN. • 901 Berryhill Road,Suite. 101,Charlotte,NC 28208 • PH 704.332.1204 • www.dpr.design 6