SW3201203_Stormwater Report_20210412

Encore at Streamside
Stormwater Impact Analysis and Final Design of Stormwater Management Facilities

STORMWATER IMPACT ANALYSIS
GENERAL DESCRIPTION
Located in Union County east of the intersection ofS Providence Road and Cuthbertson Road is the
proposed Encore at Streamside single-family subdivision development.The existing site consists of
heavily wooded areas as well as openagricultural fields. Proposed development on this site will consist of
the construction ofsingle-family homesand amenity center along with theassociatedroadsand
sidewalks, utility improvements, and three stormwater controlmeasures.
The proposed development is located within the CatawbaRiverbasin with stormwater runoff from the
proposed developmentdraining intoEast Fork Twelvemile Creekas well as a small portion of the site
draining to West Fork Twelvemile Creek. Ultimately all runoff combines into Twelvemile Creek at the
western property boundary. This development is located within Union Countyand as a result the
proposed stormwater management on this site shall meetboth Union County’s Development Standards
as well asthe stormwater managementperformance standards for development set forth byNCDEQ’s
Phase II Post-Construction SW requirements. These are asfollows:
Union County DevelopmentStandards:
60.170-C Stormwater Management
3.Post-development stormwaterdischargerates at the propertyboundary of anapproved
cluster development may not exceed the pre-development rate for the 2-and 25-year storm events. This
regulationdoes not apply if discharging directly into a FEMA-regulated floodplain.
This project is exempt from thisrequirementaseachfacility dischargesdirectly into a FEMA-regulated
floodplain. As a result stormwaterdetention is not required. Stormwater quality is required and is per the
state performance standards.
In order to meet the water quality requirements,threewetponds are proposed. The existing pond on-
site will serve as an amenity andisnot proposed to be used for either stormwater quantity or quality
control. This report serves as he stormwaterimpact analysis for the proposeddevelopmentand includes
design calculations for these facilities.
CALCULATION METHODOLOGY
>Rainfall data forthe analysis was taken fromNOAA Atlas 14.Please reference the precipitation
information section within this report for additional information.
>Using maps contained within the Union county SoilSurvey, the on-site soils were determined to
be hydrologicsoilgroups (HSG)‘B’and‘C’soils. Sincethemethod chosen to compute post-
developmentpeak flow rates and runoff volumes is dependent upon the soil type, care was taken
when selecting the appropriateSoil Conservation ServiceCurve Number (SCS CN). A composite
SCS CN was computed for eachpost-development sub-basin. For example,Sub-basin #3in the
post-developmentanalysis consists of approximately75% HSG‘B’, and25% HSG‘C’soils.
Therefore, for the woodedcovercondition, the composite SCS CN is computed as follows
(assuming good conditions):
>Composite Wooded SCS CN=(75%x55)+ (25% x70) =59
creatingexperiencesthroughexperience3430Toringdon Way, Suite 110, Charlotte, NC 28277 /704.527. 0800

STORMWATER IMPACT ANALYSIS
>A composite SCS Curve Number was calculated forthepost-developmentcondition for each sub-
basin using SCS curve numbers and landcoverconditions.Landcoverconditions for the post-
development condition were taken from the proposed development plan.
>The time of concentration was calculated using SCS TR-55 (Segmental Approach, 1986). The Tc
flow path was dividedinto three segments: overland flow,concentratedflow, and channel flow.
Thetravel time was then computed for each segment, from which the overall time of
concentration was determine by taking the sum of each segmental time.For each sub-basin
draining to astormwater control measure a minimum time of concentration of 5 minuteswas
assumed.
>PondPack Version V8i was usedin determined thepost-development peak flow rates for the1-,
2-, 10-,25-, and 100-yearstorm events, aswell as routing calculations for the proposed
stormwater control measure.
>Theproposed stormwater control measuresweredesigned in accordance with the Minimum
Design Criteria (MDC) standards set forth in the NCDEQ Stormwater Design Manual.
DISCUSSION OFRESULTS
Peak RunoffControl Requirements:
Peak runoff control is not required for this project. Per theUnion County development standards quantity
control is not required ifdischargingdirectly into a FEMA-regulated floodplain which such is the case for
this development.The stormwater control measures have been designed to safely pass thehigherstorm
events.
Water Quality Requirements:
Theproposed facilitieshas beendesignedin accordance with theMinimum DesignCriteria (MDC)
standards set forthin the NCDEQ Stormwater Design Manual.The proposed facilitieswill provide the
required 85% TSSremoval.
CONCLUSION
If thedevelopment on this tract is built as proposed within thisreport, then the requirements set forth in
Union County’s Development Standardswill be met. However, modifications to the proposed
development may require that thisanalysis be revised. Somemodifications that would require this
analysis to be revised include:
>The proposed site impervious surface exceeds the amount accountedfor in this report.
>The post-development watershed breaks change significantly from those usedto prepare this
report.
The above modifications may result in the assumptions within this report becoming invalid. The
computations within this report will needto be revisited if any of the aboveconditions become apparent
as development of the proposed site moves forward.
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SUMMARY OF RESULTS
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MISCELLANEOUS SITE INFORMATION
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NC Center for Geographic Information & Analysis, Esri, HERE, Garmin, ©
OpenStreetMap contributors
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SITE AERIAL MAP
01,0002,0004,000
PROJECT #: DWH-19010
Feet
1 inch = 2,000 feet
UNION COUNTY, NORTH CAROLINA

Copyright:© 2013 National Geographic Society, i-cubed
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USGS TOPO MAP
01,0002,0004,000
PROJECT #: DWH-19010
Feet
1 inch = 2,000 feet UNION COUNTY, NORTH CAROLINA

WATERSHED SOILS INFORMATION
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PRECIPITATION DATA
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POST-DEVELOPMENT HYDROLOGIC
CALCULATIONS
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DESIGN OFSTORMWATER CONTROL
MEASURE A
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DESIGN OF STORMWATER CONTROL
MEASURE B
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DESIGN OF STORMWATER CONTROL
MEASURE C
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