SW3230303_Stormwater Narrative_20230803

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UNIONCOUNTY
WAT E R •
Twelve Mile Creek WRF 9.0 MGD Expansion
Post - Construction Storrnwater
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9101 Southern Pine Boulevard, Suite 250
Charlotte,North Carolina 28273
July 2023

Post - Construction Stormwater
Twelve Mile Creek WRF 9.0 MGD Expansion
Waxhaw, North Carolina
Prepared by:
Hazen
Hazen and Sawyer
9101 Southern Pine Boulevard, Suite 250
Charlotte, North Carolina 28273
July 2023
H&S Project No. 30831-061 Twelve Mile Creek WRF
Post Construction Stormwater Plan

TABLE OF CONTENTS
Project Description 1
Site Description 1
Water Resources 2
Post-Construction Stormwater 2
Future Construction Sequence 3
Other Permitting 4
References 5
LIST OF FIGURES
Figure 1 General Vicinity Map
Figure 2 Soil Distribution
Figure 3 USGS 1:24,000 Topographic Map-Waxhaw Quadrangle
Figure 4 Detailed Stormwater Drainage Maps
Figure 5 Future BUA Figure
LIST OF APPENDICES
Appendix A Contract Drawings
Appendix B FEMA Flood Insurance Rate Maps (FIRMs)
Appendix C Supporting Calculations
Appendix D USGS StreamStats Delineation
Appendix E Supplement Forms
Appendix F Wetland Certification Statement
Appendix G Current Property Deed
Appendix H Geotechnical Report
H&S Project No. 30831-061 Twelve Mile Creek WRF
II Post Construction Stormwater Plan

Project Description
This Post-Construction Stormwater narrative has been prepared for the proposed construction of
the 9.0 million gallons per day(MGD) expansion of the Twelve Mile Creek Water Reclamation Facility
(WRF). Proposed improvements include installing new influent piping, constructing a new influent
pump station (IPS), odor control facility,chemical facility, secondary clarifier,truck parking canopy,
electrical upgrades,and fiber optic upgrades.
The total property area is 56.66 acres. There is 3.42 acres of surface water on the property which
yields a total project area of 53.24 acres. Total land disturbance for the project is approximately 12
acres. The overall built upon area (BUA) will increase from 7.23 acres to 7.98 acres, for a final
coverage of 15 percent.
Prior to the implementation of stormwater rules on July 1, 2007,there was a total of 5.63 acres BUA
on this site. Since then, BUA has been added to the site for a current total of 7.23 acres of BUA. This
permit application is seeking coverage for a total increase of 2.36 acres of BUA. This coverage area
includes this proposed project's BUA increase of 0.76 acres as well as the existing unpermitted BUA
of 1.63 acres.
Site Description
The Twelve Mile Creek WRF is a 7.5-MGD wastewater treatment facility located in Waxhaw, North
Carolina owned and operated by Union County,North Carolina.The WRF was originally built in 1997
with an initial permitted capacity of 2.5 MGD and has since undergone two plant expansion projects
to its current capacity. Current conditions at the site include approximately 7.23 acres of impervious
surface, 1.64 acres of non-impervious structures (aeration basins, thickening basins, and clarifiers,
1.6 acres of open water surface from Twelve Mile Creek and tributaries,and 46.18 acres open grassy
areas.
This property was originally deeded to Union County on October 7, 1981. This is the most current
property deed and is shown in Appendix G.
Soil types found at the WRF were identified based on information obtained from the United States
Department of Agriculture (USDA), Natural Resources Conservation Service (NRCS). The USDA Soil
Distribution Map of the property is shown in Figure 2. Topography of the Twelve Mile Creek WRF
consists of silty clay loam soils with mostly 8 to 15 percent slopes. Smaller portions of the property
have steeper(15 to 45 percent) and flatter(2 to 8 percent) slopes.There is no evidence of significant
erosion under existing site conditions.
There are two existing berms located at the main entrance separated by the main road. The existing
storm drainage on sites consists of 11 separate systems (A-K)with numerous pipes and catchments,
each with a controlled outlet.All of the systems were previously implemented to convey flow under
roadways. Nine (9) are located on the main site and two (2) at the influent pump station. Much of
the site utilizes curb and gutter. All stormwater outlets have rip rap aprons and discharge to
vegetative areas in a non-erosive manner to promote dispersed flow. Existing storm drainage is
illustrated in Figure 4.
H&S Project No. 30831-061 Twelve Mile Creek WRF
1 Post Construction Stormwater Plan

Water Resources
The Twelve Mile Creek WRF is part of the Twelve Mile Creek watershed, which is a part of the
Catawba River Basin. The entire site ultimately discharges to Twelve Mile Creek. The
north/east/south sites drain directly to Twelve Mile Creek,while the west side drains to Twelve Mile
Creek Tributary 3, This tributary converges with Twelve Mile Creek at the southwest corner of the
property. USGS's StreamStats was used to delineate the drainage area at this conversion, and it was
determined to be 80.3 square miles (Appendix D).The Twelve Mile Creek WRF property boundary is
less than 0.001% of the total contributing drainage area. Due to the significant difference between
the total project area (53.24 acres) and the contributing drainage area, offsite drainage was not
considered in the BUA table (Drawing C6). BUA percentages were determined by using the property
line as the drainage area boundary (Drawing C5). This allows for the accurate comparison between
pre and post construction BUA.
A wetland investigation was performed at the site which found that no portion of the project's limits
of disturbance resides nor abuts wetlands. Refer to Note 1 on drawing C21 and the Wetland
Certification Statement in Appendix F for documentation.
Information provided by the Federal Emergency Management Agency (FEMA) was examined to
determine if designated floodplains are present at the Twelve Mile Creek WRF. Floodplains (Zones
AE and X) are present on the southern side of the property due to the neighboring Twelve Mile Creek.
Small portions of the work at the Influent Pump Station and Plant Outfall areas extend in to the 100-
year Floodplain and/or 100-ft Union County stream buffer. Please refer to FEMA Flood Insurance
Rate Map Panel 4474J,Map Number 3710447400J (October 16,2008)included in Appendix B of this
submittal.
Post-Construction Stormwater
Post-Construction Stormwater permitting at the site address is managed by the North Carolina
Department of Environmental and Natural Resources Division of Energy, Mineral and Land
Resources,under Rule 15A NCAC 2H .103(2).
According to Rule 15A NCAC 02H.1017 (4) of the North Carolina Administrative Code,a project shall
be considered a low-density project if it meets the low-density criteria set forth in Rule.1003(2)and
contains no more than 24 percent built-upon area (BUA). Based upon the built-upon area tabulation
in the application form,the project site meets this threshold with a total BUA of 15-percent.
The stormwater drainage system was evaluated using Stormwater Studio for drainage basins
affected by this project. This was done to determine whether the existing system and proposed
system changes could maintain sufficient capacity and conveyance in the pre and post construction
conditions. The storm systems that are affected by this project are Systems C, J, and K. The
Stormwater Studio model in Appendix C shows that the HGL lines in systems C, J, and K remained
below the grade line during the 10-year storm event.
Since Stormwater Studios calculates peak runoff based on the Rational Method,evaluating the runoff
generated during the 10-year storm event required calculating the variables in the Rational Equation.
H&S Project No. 30831-061 Twelve Mile Creek WRF
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The runoff coefficient,C,is dependent upon the type of soil/cover in the drainage area. Since the land
use and soil cover is not homogeneous throughout the drainage area,the runoff coefficient value was
determined by a weighted average calculation (Appendix C). The time of concentration, Tc, was
assumed to be 5 minutes for each catch basin analyzed.Once Tc was determined,the intensity of the
design storm was determined using NOAA Atlas 14,Volume 2,Version 3 (Appendix C).
Stormwater Studio was also used to determine the velocity of the stormwater to develop proper
erosion control measures for each outfall affected by the project. These velocities were used against
Manning's equation for full pipe flow out of each outfall.Whichever velocity was greater was used to
calculate the required rip rap sizing and apron configuration.The riprap aprons were sized using NC
DEQ's design spreadsheet"Riprap Outlet Protection" (Appendix C).
Main Plant Drainage
The main plant refers to the northernmost section of the site where most of the plant resides.
Drainage in this area was delineated by drainage system, the extents of which were found by using
ArcGIS topographic data.This portion of the site uses a combination of curb and gutter in areas with
heavy truck traffic and soft shoulders where roadways are not subjected to heavy truck turning and
movement. There are a total of nine (9) existing disconnected stormwater systems that have been
constructed to convey stormwater under roadways.On this side of the plant this project will include
additional BUA in system C, as well as a modification of system C. Therefore, only system C was
analyzed for post construction stormwater on the main portion of the plant.The modification can be
seen in drawing C42.The modification includes a proposed catch basin near the new chemical facility,
which will only be used as a floor drain under a covered canopy. Additionally, there is a new yard
inlet(structure C3C)to reduce flow by an electrical box situated near an existing yard inlet(structure
C3).There are also roof drain pipes tied into the system (structures C4&C5) from the truck parking
canopy.The system was rerouted to avoid process piping and the canopy.Appendix C shows that the
system still maintains a 10-year level of service.
Influent Pump Station Drainage
The southernmost portion of the site is disconnected from the main plant area by a roadway running
along the crest of a hill.This part of the site is subjected to substantial truck traffic which is why the
pavement around the Influent Pump Station is lined with curb and gutter. Majority of the structures
in this area sit on a hill with storm drainage systems J and K conveying runoff to the lower and flatter
area surrounding it.Appendix C shows that even though there will be an increase in runoff coefficient,
the systems J and K still maintain a 10-year level of service.
Future Construction Sequence
See Figure 5 for planned future facilities. Planned facilities will be installed in at least one more plant
expansion project: 12.0 MGD Expansion. The exact scope of this expansion project, and any future
phased expansion,will be determined at a later date.The timing of the expansion will be dictated by
population growth and demand within the collection system.
H&S Project No. 30831-061 Twelve Mile Creek WRF
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Other Permitting
In addition to the NC DEQ Post-Construction Stormwater permit,the following other permits will be
obtained:
• Army Corps of Engineers 401/404 Permit (submitted January 12, 2023)
o 401 Permit(WQC005722) received March 6, 2023
o 404 Permit(SAW-2023-00133) received April 6, 2023
• Union County Floodplain Development Permit(submitted April 4, 2023)
o Union County Permit (FP23002) received July 20, 2023
• NC DEQ Erosion and Sediment Control Permit
o Initially submitted January 2023 (withdrawn) -UNION-2023-051
■ To be resubmitted with Post Construction Stormwater
o Have been in contact with NC DEQ Mooresville regional office
• Union County Commercial Site Plan Permit(submitted April 4, 2023)
o To be resubmitted with Post Construction Stormwater
• Union County Building Permit
o To be submitted upon Union County Commercial Site Plan approval
H&S Project No. 30831-061 Twelve Mile Creek WRF
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References
Federal Emergency Management Agency(FEMA). (www.fema.gov/nfip)
Natural Resources Conservation Service.United States Department of Agriculture(NRCS USDA).Web
Soil Survey. (http://websoilsurvey.nrcs.usda.gov/)
North Carolina Department of Environment and Natural Resources.Division of Land Resources.Land
Quality Section (NCDENR). Erosion and Sedimentation Control Planning and Design Manual. May
2013,revision.
North Carolina Floodplain Mapping Information System (NCFMIS).
(http://www.ncfloodmaps.com/firm indexes/) (Accessed December 2022).
H&S Project No. 30831-061 Twelve Mile Creek WRF
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