20200294 Ver 2_1 - narrative_20220621

110- WithersRavenet
Our People. Your Success.
STORMWATER MANAGEMENT PLAN
Carpenter Square
Cary, NC
Prepared For:
Pulte Group
1225 Crescent Green Drive
Cary, NC 27518
919.816.1155
Prepared By:
WithersRavenel
115 MacKenan Drive
Cary, NC 27511
919.469.3340
License No. C-0832
January 2020
WR No. 02180863
PRELIMINARY
Wesley R. Perry, PE CFM
Tadd Alexander, El

Carpenter Square
Stormwater Management Plan
Cary, North Carolina
INTRODUCTION
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The proposed site development is located within a 25.36-acre property situated between Morrisville
Parkway, Louis Stephens Dr, and Carpenter Upchurch Road in Cary, North Carolina
Existing Conditions
Carpenter Square is a ±25.36-acre tract of land located within the Town of Cary planning jurisdiction on
Morrisville Parkway.
Currently zoned RMF-CU, this property features a stream and wetland draining to the southeast. Mature
hardwoods and pines follow the stream creating natural buffers. There is a drainage divide at approximately
halfway through the southern field, splitting the property into two existing Points of Analysis (POAs). The
property boundary follows the peaks and saddles of the existing topography.
The existing soil groups are Creedmore-Green and Chewacla that were assumed hydrologic soils group D
when saturated. Much of the existing condition is row crops.
Proposed Development
The proposed development comprises of multi -family buildings, totaling 98 lots. Proposed impervious area
associated with the site will increase to approximately ±32.3% of the site from ±1.9% impervious area in
the existing condition (namely, scattered roads and a farm). The impacts of increased development will be
mitigated with the installation of three (3) Stormwater Control Measures (SCMs).
The NRCS Curve Number and Pond Sizing Calculations are based on the percent impervious associated with
the Townhouse lot types. Based on architectural planning, it was assumed that ±1800 square feet of
impervious would be developed on each lot. Thus, the calculation for percent impervious is as follows:
% impervious on Lots =
Estimated Impervious area on Townhome Lot
Total proposed lot acreage
Carpenter Square
Stormwater Management Plan
WR No. 02180863
January 2020

Carpenter Square
Stormwater Management Plan
Cary, North Carolina
Z
98 Lots * 1800 Lot
307516.3 f tz
% impervious on Lots = 57%
STORMWATER MANAGEMENT REQUIREMENTS
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The development shall comply with the requirements outlined by the Town of Cary. The project straddles
both the Northeast Creek catchment (Jordan Lake Watershed) and the Upper Crabtree Creek catchment
(Neuse River Basin). The site must therefore meet state and local requirements relating to town wide
ordinances and policies for stormwater compliance for this watershed.
The following requirements apply to the subject site:
Post -development peak stormwater runoff from the 1-year storm leaving the site shall be no greater
than pre -development conditions.
Post -development peak stormwater runoff from the 2-year, 5-year and 10-year storm leaving the
site shall be equal to or less than a 10% increase from pre -development conditions. If post
development calculated discharge is greater than a 10% increase then a downstream impact analysis
shall be supplied.
Post -development nitrogen loading rates shall not exceed 3.6 Ibs/acre/year. Nitrogen reduction
requirements may be met, in part, through offsite management measures or the purchase of nutrient
credits; however, the total nitrogen loading rate cannot exceed 10.0 Ibs/acre/year for
nonresidential or multifamily residential developments. Onsite SCMs must be used to reduce the
nitrogen loading to the 10.0 Ibs/acre/year limit.
All engineered SCMs shall be designed to provide a minimum of 85% TSS removal and sized to
capture runoff from the first 1 inch of rainfall from the corresponding drainage area.
Water quality and quantity control for the site is accomplished via two (2) wet ponds (SCM#1 & SCM#2)
and one wetland (SCM#3) designed per the North Carolina Department of Environmental Quality
Stormwater Design Manual's Minimum Design Criterial (MDCs).
METHODOLOGY
Hydrology
The SCS Method, in conjunction with NOAA Atlas 14 rainfall data for the subject site's location was used to
develop runoff hydrographs for the Type II, 24-hour duration, 1-year,10-year, 25-year, and 100-year storm
events. The SCS method requires three basic parameters: drainage areas, curve number (CN), and time of
concentration (Tc).
Existing drainage areas were delineated based on topographic field survey data and Wake County aerial
topographic data (LiDAR 2-ft contours). Proposed drainage areas also incorporated proposed site plan
grading and storm drain networks.
Curve numbers were based on soil type and land use. Soil types were delineated from the Soil Survey of
Wake County, North Carolina. Land uses for existing conditions were based on survey information and 2015
aerial photography. Land uses for proposed conditions also incorporated proposed site plans.
Times of concentration for pre- and post -development drainage areas were developed using the TR-55
method. This method models the flow path in segments; each segment is considered as either sheet flow,
shallow concentrated flow, or channel flow. A minimum time of concentration of 5 minutes was used for
the proposed piped drainage systems.
Drainage area maps during existing and proposed site development are included in the Maps section of this
report. A Summary of Hydrology Inputs Per Sub -Drainage Area for both existing and proposed conditions
Carpenter Square WR No. 02180863
Stormwater Management Plan January 2020

Carpenter Square
Stormwater Management Plan
Cary, North Carolina
■■ WithersRavenel
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is included in the Hydrology Analysi Section of this report along with a soil report generated through the
USDA NRCS Wet Soil Survey, and NOAA Atlas 14 precipitation data for the subject site.
Bentley System's PondPack v8i software (PondPack) was utilized to generate cumulative rainfall curves for
the 1-Yr, 10-Yr, 25-Yr, and 100-year storm events using 24-hour NOAAAtlas 14 rainfall data for the subject
site in conjunction with the above data to develop runoff hydrographs using the SCS unit hydrograph
method. Peak flows obtained from runoff hydrographs were routed through a hydraulic model of the site.
Hydraulics
The hydraulic design requires the development of stage -storage and stage -discharge functions for the
proposed SCMs. The rainfall/runoff hydrographs, stage -storage, and stage -discharge functions have been
compiled to create a computer routing simulation model using PondPack. This PondPack model was then
used to assess the peak water surface elevations in the proposed wet ponds for the design rainfall events.
Computer simulated reservoir routing of the 1-year, 2-year, 5-year, 10-year, 25-year, and 100-year design
storms utilized stage -storage and stage -discharge functions. A non -linear regression relation for surface area
versus elevation was derived from the proposed geometry of each SCM. This relation estimates the
incremental volume of the basin to the stage or elevation of the basin. Stage -discharge functions were
developed to size the proposed outlet structures for the SCMs. The results of the hydraulic analysis can be
found in the Hydraulic Analysis Results section of this report.
The proposed culvert crossing under Road 01 was sized for the 100-year 24-hour design storm using HY-8.
See supplemental culvert memo for the hydraulics analysis of this crossing and a condition of the existing
culvert under Louis Stephens Dr.
NITROGEN EXPORT
The nitrogen export rate was calculated using Method 2 from land use and impervious area data entered
into the hydrologic model. Appropriate nutrient reduction credits were applied to areas draining through the
onsite wet ponds (30% TN removal) and wetland (44% TN removal). The Nutrient Control Plan worksheet
summarizing the nitrogen loading rate calculations is included in the Nutrient Export Calculations section
of this report.
1*61► GAIP114II&I
Water Quality
Based upon the routing study, the proposed stormwater management plan has sufficient measures to
provide the necessary removal of nutrients and TSS as required by Town and State policies. The below
nitrogen loading rate calculations have adjusted values based on existing impervious areas on site (Cary
apportioning method). Detailed calculations are supplied in Nutrient Export Calculations section of this
report.
Nitrogen loading rates calculations for the subject site resulted in:
Total TN Export (excluding existing impervious and SCM treatment): 180.21 Ibs/yr
SCM #1 TN Loading Rate Reduction: 18.06 Ibs/yr
SCM #2 TN Loading Rate Reduction: 25.91 Ibs/yr
SCM #3 TN Loading Rate Reduction: 8.51 Ibs/yr
Total Project TN Load Rate: 127.72 Ibs/yr
Per the above nitrogen loading rate calculations, the proposed SCMs reduce post -development nitrogen
loading rates to 5.25 Ibs/ac/yr. An additional 1204 Ibs nitrogen will be mitigated through the purchase of
nutrient credits to meet the nitrogen loading rate requirements of 3.6 Ibs/ac/yr.
Carpenter Square WR No. 02180863
Stormwater Management Plan January 2020

Carpenter Square ■■ WithersRavenel
Stormwater Management Plan r. Our People. Your Success.
Cary, North Carolina
Nitrogen Buydown Calculation (with rounding)
N Buydown = Loading rate standard defecit * 30 years * (site acres — existing imp. acres)
_ (5.25 — 3.6) lbs * 30 yrs * (24.33 ac) = 1204 lbs N
ac —yr
In addition, the subject site has been designed so that runoff resulting from all impervious surfaces, as
reasonably practical, is captured by the proposed SCMs. The proposed SCMs have been designed per the
North Carolina Department of Environmental Quality Stormwoter Design Monuol's to meet 85% TSS removal
requirements. Calculations used to size the SCMs and their associated outlet structures can be found in the
SCM Sizing Calculation! section of this report.
Peak Flows
Post -development peak flows associated with the 1-year, 24-hour design storm will be attenuated so as not
to exceed pre -development conditions. Post -development peak flows associated with the 10-year 24-hour
design storm are less than 10% greater than the pre -development 10-year, 24-hour design storm.
Furthermore, the proposed SCMs will be designed to safely pass the 100-year storm event while maintaining
a freeboard of >1.0 feet.
The following illustrates the pre and post -development peak flows for a 1-Year, 10-Year, 25-Year, and 100-
Year, 24-Hour storm events as calculated using the Bentley Systems PondPack v8i software at each POA:
rug► 41 Teak Flow Analysii.
POA #2 Peak Flow Analysis
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Stormwater Management Plan January 2020

Carpenter Square
Stormwater Management Plan
Cary, North Carolina
DESIGN SUMMARY
SCM #1 - Wet Detention Pond
Drainage Area = 5.26 acres
Percent Impervious Area = 51%
Top of Dam:
4' X 4' Riser Structure:
Overflow Weir:
Overflow Orifice:
Temporary Pool:
Normal Pool:
Drawdown Orifice:
Outlet Barrel:
SCM #1 Performance Summary
Elev. 390.0'
Crest Elev. 388.5'
1.0' at Elev. 388.25'
12" (W) x 18" (H) at Elev. 385.75
12,763 CIF at Elev. 385.75'
9,021 SF at Elev. 384.5'
1.50" Diameter at Elev. 384.5'
24" RCP at Elev. 381.0'
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1-Year, 24 Hr
Water Surface
386.25
386.65
FreeboardPeak
3.75
3.35
Peak Outflow
2.15
3.84
2-Year, 24 Hr
5-Year, 24 Hr
387.23
2.77
6.26
10-Year, 24 Hr
387.65
2.35
7.86
25-Year, 24 Hr
1.81
9.49
100-Year, 24 Hr
388.78
1.22
19.03
SCM #2 - Wet Detention Pond
Drainage Area = 7.84 acres
Percent Impervious Area = 49%
Top of Dam:
5' X 5' Riser Structure:
Weir:
Temporary Pool:
Normal Pool:
Drawdown Orifice:
Outlet Barrel:
SCM #2 Performance Summary
Elev. 370'
Crest Elev. 368.50'
3.0 LF at Elev. 366.25'
15,028 CIF at Elev. 366.25'
7,232 SF at Elev. 364.5'
1.5" Diameter at Elev. 364.5'
36" RCP at Elev. 360.0'
1-Year, 24 Hr
Water Surface
367.09
FreeboardPeak
2.91
Peak Outflow
7.01
2-Year, 24 Hr
367.53
2.47
13.09
5-Year, 24 Hr
368.06
1.94
22
10-Year, 24 Hr
368.39
1.61
28.23
25-Year, 24 Hr
1 368.7
1.3
1 39.46
100-Year, 24 Hr
1 368.99
1.01
1 58.47
Carpenter Square WR No. 02180863
Stormwater Management Plan January 2020

Carpenter Square
Stormwater Management Plan
Cary, North Carolina
SCM #3 - Constructed Wetland
Drainage Area = 2.41 acres
Percent Impervious Area = 44%
Top of Dam:
4' X 4' Riser Structure:
Overflow Weir:
Temporary Pool:
1 Normal Pool:
/ Drawdown Orifice:
/ Outlet Barrel:
SCM #3 Performance Summary
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Elev. 355'
Crest Elev. 353.5'
2x 2.0' and 1x 1.0' Weir at Elev. 388.25'
5,314 CF at Elev. 353.25`
3,211 SF at Elev. 352.0'
1.0" Diameter at Elev. 352.0`
24" RCP at Elev. 350.5`
1-Year, 24 Hr
353.59
1.41
3.92
2-Year, 24 Hr
353.69
1.31
7.22
5-Year, 24 Hr
353.78
1.22
10.62
10-Year, 24 Hr
353.3
1.7
12.91
25-Year, 24 Hr
353.89
1.11
15.75
100-Year, 24 Hr
353.98
1.02
20.35
Carpenter Square WR No. 02180863
Stormwater Management Plan January 2020