20080519 Ver 1_Stormwater Info_20080318

Stormwater Management Calculations -
Construction Drawing Submittal
For the proposed
Sunset Creek Subdivision
Wake County, North Carolina
Prepared for
Silver Developers, LLC
Prepared By
Stewart Engineering, Inc.
MAR ~ 12?08

STEWART
DENt~- WA1~tk~~iUALJ7T
WETLANDS AND STORPolWATER BRAPJCH
March 7, 2008

Stormwater Management Calculations
For
Sunset Creek Subdivision
Wake County, North Carolina
Prepared for:
Silver Developers, LLC
Prepared by:
Stewart Engineering, Inc.
421 Fayetteville Street, Suite 400
Raleigh, NC 27601
919.380.8750
SERI-
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PRELIMINARY -FOR REVIEW
PURPOSES ONLY
Stewart Project No. C7090
March 7, 2008

Stormwater Management Calculations
SUNSET CREEK SUBDIVISION
WAKE COUNTY, NORTH CAROLINA
EXECUTIVE SUMMARY
The following Stormwater calculations have been prepared to accompany the
Preliminary Subdivision Plan Approval for the Sunset Creek Subdivision. The
proposed subdivision is located on a 7.76 acre parcel at 4912 Yates Mill Pond Road in
Wake County. The existing parcel contains one single family home and existing
wooded building/shed. The majority of the site is currently wooded. A large portion
of the Stormwater from the site drains to the north into an intermittent stream that
traverses the north potion of the site. The existing stream will be buffered by a 50-
foot riparian buffer and additional 20' building setback.
The site is within the Swift Creek Water Supply Watershed which places the
maximum impervious area at 12%. The proposed percent impervious cover is
16.03% which necessitates the use of BMPs to capture and treat runoff from a
portion of the site. Per the enclosed Wake County Stormwater Management
Spreadsheet Calculations, the required storage for the subdivision is 7,632 cf. This
runoff will be captured in two bioretention areas located adjacent to the proposed
private roadway. Runoff from the proposed roadway will be directed into the
bioretention through the use of grass lined swales.
A Nitrogen buydown payment of $1,715.75 will also be required to mitigate the
Nitrogen export from the site.
The existing intermittent stream will be crossed in order to provide access to Lot 7 of
the subdivision. The drainage area to the stream is approximately 58.4 acres,
therefore a (2) 36" RCP culvert has been designed to ensure that HW/D does not
exceed 1.5 in the 25-year storm event.

Site Data

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Wake County
Stormwater Spreadsheets

-~.....,~ Stormwater Management Plan SID No.: C7039
w~~li,~ Date: 2/27/2008
COUNTY
kUHIH CA 0.UIJ IVA Designer: DMB
SUMMARY .INFORMATION
Site Data
Total Area: 7.759 acres
Zoning: R-40W
Number of Lots: 7
Percent Im envious: 16.27
Volume Mana ement Calculations
Target Curve Number: 63
Proposed Curve Number: 66
Total Volume Required: 7632 ft3
Total Volume Provided: 7653 ft3
Nitro en Calculations
TN Loading from Site: 3.86 Ibs/ac/yr
Buy-down (if in NRB): $1,715.75
Peak Flow Mana ement Calculations:
Pre-Development Peak Flow: 1.17 cfs
Post-Develo ment Peak Flow: 1.17 cfs
Wake County Stormwater Tool
Version 2.2 3/10/2008 12:10 PM

Stormwater Management Plan
COUNTY
k(>0.I fl (:A Rtllf kA
S/D No.: C7039I
Date: 2/27/2008
DMBI
I TARGET CURVE NUMBER CALCULATION I
Wake County Target Curve Numbers
Zonin District A B C D
R-80W and R-80 37 60 73 79
R-40W and R-40 41 62 75 80
R-30, R-20, R-15, R-5 43 63 76 81
Site Data:
Total Site Area: 7.759 acres "Excludes Area of Exisitng Lakes or Ponds on site
Existing Lake/Pond Area: acres
Area used for Overall Density Ca/cs: 7.759 acres
Zoning District: R-40W
Soil Composition:
Hvdrolo4ic Soil Group Site Area % Target CN
A 41
B 7.05 91 % 62
C 0.71 9% 75
D 80
100%
Target Curve Number: 63
Runoff (inches) = Q* = 0.43 inches
Wake County Stormwater Tool
Version 2.2 3/10/2008 12:10 PM

Stormwater Management Plan s/D No.:
~.``
W~E Date:
COUNTY
uoarn cnao~ ive Designer:
I POST-DEVELOPMENT CALCULATIONS
CURVE NUMBER:
Is Data from Preliminary Plan Still Valid?
Total Area of Lots:
Number of Lots:
Impervious Area per Lot:
Impervious on Lots:
Woods on Lots:
Impervious in R/W:
Percent Impervious for Site*:
'Using total site area plus existing lakes/ponds
Percent of Total Disconnected Impervious:
Yes
6.7147 acres
7
5066 sf
12.12
31.6163
45.1538
16.27
50 % (50% is Default. See Table 3.3)
HSG Land-use CN Area acres
B R/V1l w/Ditch Sections 88 0.975 12.57%
B Lots 64 6.784 87.43%

Totals: 7.759 100.00%
Post-Development CN = 66
Runoff (inches) = Q* = 0.54 inches
Warnings/Notes:
Proposed CN is greater than the Target CN therefore volume management will be required.
Percent woods on lots exceeds allowable. Please show the wooded area to be preserved on the construction drawing.
Wake County Stormwater Tool
Version 2.2 3/10/2008 12:10 PM

Stormwater Management Plan S/D No.: C7039
/~.,'.`
w1~E Date: 2/27/2008
COUNTY
~{,R,,, ~ ,R~,,,N,, Designer: DMB
I PRE-DEVELOPMENT CALCULATIONS
(CURVE NUMBER:
User Defined
User Defined
User Defined
User Defined
HSG Land-use CN Area acres
B Im ervious 98 0.1098 1.42%
B Woods, Good Cond. 55 5.0012 64.46%
B Pasture or Open Space, Good Cond. 61 2.648 34.13%
98
98
98
98
98
98
98
98
98

Totals: 7.759 100.00%
Pre-development CN = 58
Runoff (inches) = Q* = 0.27 inches
ME OF CONCENTRATION:
Sheet Flow:
Sheet Flow:
Shallow Flow,
Shallow Flow,
Channel Flow:
Channel Flow:
Length (ft) Slope (ft/ft) Surface Cover Tt (hrs)
100 0.027 Grass 0.20

335 0.134 Un aved 0.02

Flow
Length (ft) Slope (ft/ft) n-value Area (ftz) Wetted Perimeter (ft) Tt (hrs)
Pre-development T~ = 0.21 hrs (Minimum 5 minutes)
Wake County Stormwater Tool
Version 2.2 3/10/2008 12:10 PM

Stormwater Management Plan
COUNTY
I~t)Aill C:A 0.[)IINA
DMBI
I POST-DEVELOPMENT CALCULATIONS I
ME OF CONCENTRATION:
Sheet Flow.
Sheet Flow
Shallow Flow
Shallow Flow
Channel Flow:
Channel Flow:
Length (ft) Slope (ft/ft) Surface Cover Tt (hrs)
100 0.027 Grass 0.20

335 0.134 Unpaved 0.02

Flow
Length (ft) Slope (ftlft) n-value Area (ftz) Wetted Perimeter (ft) Tt (hrs)
Post-development T~ = 0.21 hrs (Minimum 5 minutes)
S/D No.: C7039
Date: 2/27/2008
Wake County Stormwater Tool
Version 2.2 3/10/2008 12:10 PM

Stormwater Management Plan S/D No.: C7039
W1~E Date: 2/27/2008
COUNTY
<,ar„ ~ ~a~>,,..~A Designer: DMB
PEAK FLOW CALCULATIONS
(CURVE NUMBER:
Pre-development CN = 58
Post-Development CNadj = 58
ME OF CONCENTRATION:
Pre-development T~ = 0.21 hrs
Post-development T~ = 0.21 hrs
PEAK FLOW:
Method: TR-55 Graphical Peak Discharge Method for Tvpe II Distribution
Qp = qup4r,Q*
Where:
Qp =Peak Discharge (cfs)
q„ =Unit peak discharge (csm/in) TR-55 Appendix F
Am =Drainage Area (miz)
Q* =runoff (inches)
Pre-development
qu = 352.97 csm/in
Am = 0.01 mil
Q* = 0.27 inches
Q~-year= 1.17 cfs
Post-development
qu = 352.97 csm/in
Am = 0.01 mil
Q* = 0.27 inches
Q1-year= 1.17 Cfs
Wake County Stormwater Tool
Version 2.2 3/10/2008 12:10 PM

~~ Stormwater Management Plan s/D No.: C7039
W~E Date: 2/27/2008
COUNTY
h~, a,„ ~:~k,,,,„„ Designer: DMB
PROPOSED STORAGE DEVICES
Enter only runoff volume below that will be infiltrated or drawn down over 2 to 5 days. Additional
volume provided in devices should not be entered in this worksheet.
Storage Volume
Name/Location T e of Device Provided ft3 Additional Comments
1 Bio A Bioretention 5028
2 Bio B Bioretention 2625
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Total Volume Provided: 7653 ft
Volume Required by Wake
County Ordinance: 3154 ft3
Minimum Volume Required to Meet Peak
Flows for Neuse Rules: 4478 ft3
Total Volume Required: 7632 ft
Post-Development Curve Number Adjustment based on Volume Provided
Source: "Modeling Infiltration Practices Using TR-20", October, 1983, Maryland Department of the Environment
200
CNadj = (P+2Q+2) - (5PQ+4Q2)1/2
Where:
P =Rainfall (inches) = 3 inches
Q =Runoff Depth for Proposed CN (Q*) -Volume Provided (inches)
Volume Provided (Inches) = 0.27 inches
Q* = 0.54 inches
Q = 0.27 inches
Post-Development CNadj = 58
Wake County Stormwater Tool
Version 2.2 3/10/2008 12:10 PM

W1~E
COUNTY
t~C)R 7l1 <'A RC )l IfJA
Stormwater Management Plan
S/D No.: C7039
Date: 2/27/2008
Designer: DMB
NITROGEN CALCULATIONS
TN export TN export
coefficient from use
T e of Land Cover Area (acres) (Ibs/ac/yr) Ibsl r
Permanently protected undisturbed open
s ace forest, unmown meadow 2.14 0.60 1.29
Permanently protected managed open
s ace rass, landsca in ,etc. 4.35 1.20 5.22
Impervious surfaces (roads, parking lots,
drivewa s, roofs, etc. 1.26 21.20 26.77
TOTAL 7.76 33.28
Nitrogen Loading w/out BMP Reductions: 4.29 Ibs/ac/yr
Neuse Buy-down w/out BMP Reductions: $4,553.33
Nitrogen Loading with BMP Reductions
See Wake County Stormwater Design Manual Chapter 4 for Details
IDrainaae Area 1:

T e of Land Cover
Area (acres) TN coeff.
(Ibs/ac/yr) TN export
Ibs/ r
Permanentl rotected o ens ace 0.60
Mang ed o ens ace 0.28 1.20 0.34
Im ervious surfaces 0.27 21.20 5.72
TOTAL 0.55 6.06
Nitrogen Removal Efficiency of BMP(s)
Nitrogen Loading for DA 1:
IDrainaae Area 2:
35
7.16 Ibs/ac/yr

T e of Land Cover
Area (acres) TN coeff.
(Ibs/ac/yr) TN export
Ibs/ r
Permanentl rotected o ens ace 0.60
Mang ed o ens ace 0.11 1.20 0.13
Im ervious surfaces 0.16 21.20 3.39
TOTAL 0.27 3.52
Nitrogen Removal Efficiency of BMP(s) 35
Nitrogen Loading for DA 2: 8.48 Ibs/ac/yr
rainage Area 3:

T e of Land Cover
Area (acres) TN coeff.
(Ibs/ac/yr) TN export
Ibs/ r
Permanentl rotected o ens ace 0.60
Mang ed o ens ace 1.20
Im envious surfaces 21.20
TOTAL
Nitrogen Removal Efficiency of BMP(s)
Nitrogen Loading for DA 3: Ibs/ac/yr
Final Nitrogen Loading from Site: 3.86 tbs/ac/yr
Buy-down (Neuse Only): $1,715.75
Version 2.2 3/10/2008 12:10 PM

Bioretention Calculations

STEWART
Bioretention Area Sizing - BIO A
(for 85% TSS removal)
Project: DeFreitas Subdivision
Number: C7039
Date: 30-Nov-07
Draina a Basin
Total Drainage Area = 24158 sf
Total Impervious Area = 11850 sf
Percent Impervious = 49%
Rainfall Depth = 1 in
421 Fayetteville Street, Suite 400, Raleigh, NC 27601 Tel [phone] Fax [phone] www.stewart-
eng.com

STEWART
Bioretention Area Sizing - BIO B
(for 85% TSS removal)
Project: DeFreitas Subdivision
Number: C7039
Date: 30-Nov-07
Draina a Basin
Total Drainage Area = 11623 sf
Total Impervious Area = 6790 sf
Percent Impervious = 58%
Rainfall Depth = 1 in
421 Fayetteville Street, Suite 400, Raleigh, NC 27601 Tel [phone] Fax [phone] www.stewart-
eng.com

Riprap Outlet
Protection Calculations

RAJ
STEWART
RIPRAP OUTLET PROTECTION WORKSHEET
Project Sunset Creek Subdivision Date 7-Mar-08
Project No, C7039 Designer DMB
Outlet ID
Outlet flowrate 4.36 cfs
Pipe diameter 15 inches
Outlet pipe slope 1.02 percent
Full flow velocity i.6 ft/sec
Figure 8.06.b.1
1 2 3 4 5 6 7 8 9 10
Pipe diameter (ft)
Zone from graph above = 1
Outlet pipe diameter 15 in.
Outlet flowrate 4.4 cfs
Outlet velocity 3.6 ft/sec
Material = Class A
Length = 5.0 ft.
Width = 3.8 ft.
Stone diameter = in.
Thickness = 9 in.
Zone Material Diameter Thickness Len th Width
1 Class A
~ ~ 3 9 ~ 4 x D(o) 3 x D(o)
2
3 Class B
Class I 6
'~ 13 22
22 i 6 x D(o) ~
~ 8 x D(o) 3 x D(o)
3 x D(o)
4 _
Class I ~ 13 22 i 8 x D(o) 1 3 x D(o)
5 Class II ~ 23 27 10 x D(o) 3 x D(o)
6 Class II 23 ~ 27 10 x D(o) 3 x D(o)

7 --
Special study
re uired
Calculations based on NY DOT method -Pages 8.06.05 through 8.06.06 in NC Erosion Control Manual
421 Fayetteville Street, Suite 400, Raleigh, NC 27601 Tel [phone] Fax [phone] www.stewart-eng.com

Culvert Calculations

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PROJECT PAGE OF
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