SW6080602_Stormwater Narrative_20240730

Stormwater Impact Statement
Purpose
The purpose of this report is to assess stormwater impacts associated with proposed
construction of a parking lot and building expansion in Robeson County. The site has
existing stormwater permits associated with the site. This project will be a modification
to existing permit SW6080602. The property falls within the NCDEQ Fayetteville
Regional Office's jurisdiction but stormwater is reviewed by the Raleigh Regional
Office. This analysis addresses requirements prescribed in the State's regulations related
to stormwater runoff control.
Existing Site Conditions and Drainage Characteristics
The subject property is located at 2120 NC-71 in Maxton,NC. The site is developed with
industrial buildings and supporting infrastructure. Runoff leaves the property in three
locations, at the north west property line through a 24" culvert pipe, to the east via an
existing drainage canal, and to the south west property line to the Lumber River.
Impervious Surface Area
The east side of the project will add 118,373 SF of new impervious on site. These areas
are associated with the parking lot expansion. All new impervious areas will drain a
proposed wet pond for treatment.
The west side of the project will add 100,574 SF of new impervious on site. These areas
are associated with the building addition and truck dock. The building addition will be in
existing drainage area B and drain to drainage area A to reorganize drainage areas from
existing stormwater permit SW6080602.
Applicability of Local Stormwater Regulations
The relevant performance requirements for this site are:
High density projects SCMs shall be designed, constructed, and maintained so that the
project achieves either "runoff treatment" or "runoff volume match" per 15A NCAC
02H .1003.
Stormwater outlets shall be designed so that they do not cause erosion downslope of the
discharge point during the peak flow from the 10-year storm event as shown by
engineering calculations.
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TSS Removal
Runoff from proposed site development will be managed by two on-site wet ponds.
These ponds have been designed per the MDC stormwater design criteria and will be
used to satisfy water quality requirement.
1-Year Volume Management
Stormwater runoff will be managed utilizing the on site wet ponds. The ponds will catch
a portion of the drainage area limiting the outgoing runoff from the increased impervious.
The analysis of the stormwater device was performed using Hydraflow Hydrographs with
Autodesk Civil 3D. This software is accepted by industry standards. The point of
interest (POI)was modeled in the pre and post conditions for the 1-, 2-, and 10-year
storm events.
Peak Runoff Rate
From a stormwater runoff mitigation perspective, the wet ponds will slow down runoff
rates, which inherently provides reductions in runoff quantity and peak runoff rate.
Allowing for the effective drainage area mitigation, the Qi value will not significantly
change between the Baseline Date and Post-Development conditions.
West Side Drainage Basin Summary
Storm Events (cfs)
Site Condition 1 Year 2 Year 10 Year
Pre Development 4.76 7.35 15.57
Post Development 0.06 0.07 3.14
East Side Drainage Basin Summary
Storm Events (cfs)
Site Condition 1 Year 2 Year 10 Year
Pre Development 4.34 6.94 15.34
Post Development 0.05 0.20 6.03
Conclusion:
Based on the information contained herein, proposed site improvements for the subject
project will meet State regulations and policies applicable to management of stormwater
runoff quantity and quality from the subject property.
End
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