SW1220801_Stormwater Report_20230214

Sparks Drive Commercial Building
Stormwater Narrative
176 Sparks Drive
Forest City, NC
July 9, 2022
Revised: January 24, 2023
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Kevin Pulis, PE
Senior Civil Engineer
Tarr Group, LLC

Sparks Drive Commercial Development — Forest City, NC
Stormwater Narrative
Proiect Description and Existing Conditions
The purpose of this project is to construct a commercial development with one building totaling 7,100
square feet with associated parking, utilities, and landscaping on a 1.37-acre parcel. The site is located at
176 Sparks Drive in the Town of Forest City.
The parcel is currently entirely vegetated and historical aerial images show that it is primarily grass in
good condition. The site slopes from west to east at approximately a 5 percent slope.
Stormwater Quantity
Town of Forest City Requirements
Per Town Ordinance 10.3.1, post -development runoff rates be the same as pre -development runoff
rates for the 1- and 10-year, 24-hour storms. A wet pond is being proposed to meet this requirement.
NCDEQ Requirements
Since this project exceeds 1 acre of disturbance the NCDEQ stormwater ordinance is in effect. Since a
wet pond is being proposed, State Code 15A NCAC 02H.1053(8) requires that "the wet pond shall
discharge runoff from the one-year, 24-hour storm in a manner that minimizes hydrologic impacts to the
receiving channel.
Pre -Development
As mentioned in the Project Description, the site currently contains a mixture of scrub trees and grasses
in fair condition. According to the NRCS Soil Map, 100% of this site consists of Pacolet Sandy Clay Loam
which has a hydraulic soil group of "B".
Based on a topographic survey there are three definable watersheds on this site which flow offsite at
different locations. Each watershed was modeled and assumed to combine downstream to give a total
pre -development site runoff rate.
A curve number (CN) of 61 was assigned to each watershed based on the existing land use of "grass —
good condition". These CN values were used in the stormwater modeling for pre -development runoff
rates.
A time of concentration (Tc) for each watershed was calculated based on its most hydraulically remote
portion. Given the topography the Tc values ranged from 5 minutes to 13.3 minutes.
Using the information above, Hydraflow Hydrographs software (2020 Version) was used to calculate a
pre -development peak runoff rate for the site. The table at the end of the Post -Development section
displays the results of the pre -development study. Please refer to the Appendix for detailed calculations
for the CN, Tc, and peak runoff rates for the 1-, 10-, and 100-year storms.

Post -Development
In the post -development condition, 36,409 SF of built upon area (BUA) will be created. This BUA can be
broken down as 27,108 SF of parking, 1,435 SF of sidewalk, 7,378 SF of roof, and 488 SF of dumpster
enclosure. The site will be graded to create 4 distinct watersheds. Storm drain and curb will be installed
to take the bulk of the site (48,600 SF) to a proposed wet pond. The remaining portion of the site
(11,077 SF) will bypass the wet pond. The portions of the site that bypass the wet pond will not cause
any adverse impact to downstream properties.
A weighted curve number (CN) for each watershed was calculated based on their land uses. Watersheds
1 and 2 consist of landscaped areas and have been assigned a CN of 61 based on the land use of "lawn —
good condition". Watershed 3 was assigned a composite CN of 88 based on the land uses of "lawn —
good condition", "impervious area", and "open water" (the proposed wet pond's permanent pool).
Watershed 4 was assigned a composite CN of 85 based on the land uses of "lawn — good condition" and
"impervious area". Each of these CN values were used in the stormwater modeling for post -
development runoff rates.
A time of concentration (Tc) of 5 minutes was assigned to each watershed based on the convention for
post -development hydrology studies.
Using the information above, Hydraflow Hydrographs software (2020 Version) was used to calculate a
post -development peak runoff rate for the site for the 1-, 10-, and 100-year, 24-hour storms. These
calculations showed an increase in peak flow rates in the post -development condition.
In order to meet the requirements of the Town of Forest City and NCDEQ, a wet pond with a peak rate
attenuation control structure will be created. This wet pond was designed to limit overall site peak
runoff rates for the 1- and 10-year, 24-hour storms. The pond was also analyzed for the 100-year, 24-
hour storm to ensure that it would not overtop and maintains a minimum 1 foot of freeboard.
Following is a summary of the results of the site's post -development versus pre -development peak
hydrology study when routed through the wet pond:
Wet Pond— Permanent Pool = 994.00, Emergency Spillway = 997.00, Top of Dam = 998.00
(Refer to Appendix D for a stage -storage table)
Storm Event
Pre -Development
Post -Development
Wet Pond
Max WSE
1-year, 24 hr
0.606 cfs
0.421 cfs
995.58'
10-year, 24 hr
3.251 cfs
2.682 cfs
996.61'
100-year, 24 hr
7.261 cfs
13.16 cfs
996.90'
Based on the results shown in the table above the site is confirmed to meet the stormwater quantity
requirements of the Town of Forest City and NCDEQ.

Stormwater Quality
Town of Forest City Requirements
The Town of Forest City doesn't have any water quality requirements.
NCDEQ Requirements
Since this project exceeds 1 acre of disturbance the NCDEQ stormwater ordinance is in effect. Since this
project is considered high density and is located in an urbanizing area, State Code 15A NCAC
02H.1017(5) requires water quality treatment for the one -inch storm to achieve an 85%TSS annual
removal.
SCM Utilized
Under the most current version of the NCDEQ Stormwater Design Manual, all "Primary" SCMs are
considered to provide at least 85% TSS annual removal if they're designed in accordance to the NCDEQ
minimum design criteria. For this project a wet pond was utilized. This wet pond is designed to treat the
first inch of runoff (85%TSS removal) per the MDC in 15A NCAC 2H.1053 and SCM credit document C-3
of the NCDEQ Stormwater Design Manual.
To meet the MDC for wet ponds, there are two main design elements that must be considered — water
quality treatment volume and the design of the main pool. The following information describes those
requirements and how they are met.
Water Quality (First Inch) Treatment
To meet the MDC each wet pond has to be designed to capture and treat the first inch of runoff from
the impervious areas that drains to it. For this project though there is a portion of the site that is
downhill of the pond and therefore cannot drain to it. The pond was oversized to account for the lack of
water quality treatment of that area. Below is a breakdown of the impervious areas and the calculated
water quality volume:
• Direct area draining to wet pond = 48,600 square feet (1.116 acres)
• Impervious area to wet pond = 32,560 square feet
• Impervious area that bypasses the wet pond = 3,877 square feet
• Impervious area percentage not including bypass impervious = 67%
• Impervious area percentage including the bypass imperious = 75%
• Design Volume (including bypass) = 3630 * 1 inch * (0.05 + 0.9 * 0.75) * 1.116 = 2,936 cubic feet
For this design an overflow weir will be installed at elevation 995.25 which provides a water quality
treatment volume of 3,188 cubic feet.
Additionally, the water quality volume must be drawn down over a period of 2 to 5 days. Per the results
of the hydrology study the following draw down times are achieved:
• Beginning of drawdown at 24.80 hours
• Complete drawdown at 78.83 hours
• Total drawdown time of 54.03 hours = 2.25 days

Main Pool Design
To meet the MDC the main pool shall be designed with an average depth between 3 and 8 feet and a
surface area to drainage area ratio (SA/DA) that varies based on the average impervious area of the
watershed draining to it. Below is a breakdown of those values and how they were met:
Average Depth
• Equation 3 of the MDC is being used since the vegetated shelf is submerged.
• Volume of permanent pool = 2,562 cubic feet (Refer to Appendix D for a stage -storage table)
• Volume of submerged portion of vegetated shelf = 330 cubic feet
• Net volume of main pool = 2,232 cubic feet
• Area at bottom of submerged shelf = 733 square feet
• Average depth = 3.05 feet
SA/DA Ratio
• Drainage area to wet pond = 48,600 square feet
• Impervious area of drainage area (including bypassed impervious area) = 75%
• Required SA/DA ratio per Table 1 of the MDC = 2.72
• Surface area of permanent pool = 1,642 square feet
• Provided SA/DA ratio = 3.38
Forebay Design
To meet the MDC the forebay shall be designed with a volume that is between 15 and 20 percent of the
volume of the main pool. The entrance elevation of the forebay shall be deeper than the exit elevation
and the total depth shall be a minimum of 36" with the bottom 6" reserved for sediment storage. Below
is a breakdown of those values and how they were met:
• Entrance elevation = 990.0
• Exit elevation = 991.0
• Sediment storage elevation = 991.5
• Permanent pool elevation = 994.0
• Volume of main pool = 2,562 cubic feet (Refer to Appendix D for a stage -storage table)
• Volume of forebay = 496 cubic feet (Refer to Appendix D for a stage -storage table)
• Forebay = 19.4% of main pool
Operations and Maintenance Requirements
Permanent maintenance of the wet pond is critical to ensure they function as designed. The wet pond
will require inspections and maintenance on a regular basis. The engineering drawings contain a specific
operations and maintenance schedule. NCDEQ also provides guidance and requirements on operations
and maintenance of the wet pond. The owner of record shall refer to NCDEQ documents throughout the
life of the property as the guidance is constantly being updated and improved.