SW5240603_Stormwater Narrative_20240911

Dewberry°
Stormwater Narrative
Eaddy Building Addition
September 11, 2024
Project Description: The Eaddy building is located on PIN 0785121112814 within the City of Raleigh's
jurisdiction zoned mix use. The Eaddy Building Addition will include the addition of 4,761 square feet and
will include a walkway for a total built upon area of 5,895 square feet. This addition will be located on the
northeast side of the existing Eaddy Building adjacent to the loading dock and mechanical area. The
existing site contains three (3) Connex boxes and a trailer located on the north side of the asphalt paved
service area. An existing fire hydrant, light pole, underground water, underground electrical and
underground gas service is also currently located in this area and will need to be relocated to allow for the
new construction. The total limits of disturbance is 0.78 acres. The Connex boxes, trailer and the
mentioned utilities that will be relocated are included within the limits of disturbance.
Existing Site Conditions: The Eaddy Building is located within the 999.5-acre parcel at 4300 Reedy
Creek Road. The LOD drains to Richland Creek, located to the north of the property. Richland Creek
drains into the Neuse River. The USGS map of the entire parcel is provided for reference.
Soil Characteristics: The soil within the LOD predominantly includes Pacolet sandy loam, the USGS
Web soil survey report of the LOD is included for reference. Also, the historical soil map is included to
show the soils for the entire parcel.
Erosion and Sediment Control Requirements: The land disturbance for this project is 0.78 acres.
Since the disturbance is less than 1 acre, and ESC permit is not required
Permanent Stabilization: No area shall remain unstabilized for more than 14 days, see the temporary
seeding detail. Also see the permanent seeding detail for permanent stabilization measures.
Stormwater Runoff Considerations/Management: The existing site is within the City of Raleigh's
jurisdiction; however, NCDEQ will review the stormwater management for the site since it is owned by the
State of North Carolina.
This project meets the criteria to be considered a low-density project. The total property area is 999.5
acres with a total of 22.06 acres of surface water. Therefore, the total project area is 977.44 acres (999.5
ac-22.06ac = 977.44). The total existing impervious area is 73.42 acres. In the proposed condition there
will be an added 0.14 acres of impervious area, resulting in a total of 73.56 acres of impervious area.
Only 7.53% of the project area is impervious, classifying it as a low-density project. The project is
designed to maximize dispersed flow through vegetated areas and minimize channelization flow.
Vegetated conveyance is used to transport stormwater from the developed area into riprap lined area to
disperse flow and ensure the velocity will not cause erosion downslope. It is released at the edge of an
existing vegetated setback and will be allowed to flow through the setback as dispersed flow.
The SNAP Tool was used to calculate the nitrogen and phosphorus runoff from the project area. This
parcel is more than one square mile; therefore, the project area was split into two to be able to utilize the
SNAP Tool. This method was suggested to us by NCDEQ, see attached correspondence. The "North
Section" is the project area north of Reedy Creek Road, and the "South Section" is the project area south
of Reedy Creek Road. The total export target and total load leaving the site that was calculated via the
SNAP tool of each section was combined to get the totals for the whole project area. See summary table
below.
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Stormwater Narrative
Eaddy Building Addition
Project No.50157867
September 11,2024
SNAP Tool -Total Project Area Combined
Total Load Total Export Total Load <Target Export
Leaving (lb/yr) Target (lb/yr)
Nitrogen 1,078.65 3,335.26 Yes
Phosphorus 464.90 926,462.05 Yes
Local stormwater control requirements are not required for this site based on the following City of Raleigh
UDO Section 9.2.e.2.b "Stormwater Runoff Controls- Exemptions". To confirm the UDO Requirements, a
point of discharge at the nearest receiving water course downstream of the site was used to evaluate the
peak stormwater runoff. The peak stormwater runoff between pre-development and post-development
conditions for the 2-,10-, 25-and 100-year storms is less than 10% at the point of discharge. See table
below for peak flow rates.
Peak Outflow(cfs)
2-yr 10-yr 25-yr 100-yr
Pre-Development 40.20 74.17 95.75 130.47
Post-Development 41.97 76.48 98.19 132.99
Percent increase 4.4% 3.11% 2.55% 1.93%
Calculations: See attached.
Dewberr
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