SW3210701_Stormwater Narrative_20210913

TI MCADAMS STORMWATER IMPACT ANALYSIS
GENERAL DESCRIPTION
Located in Cabarrus County northwest of the intersection of Morrison Road and Flowes Store Road,
within the Town of Midland, is the proposed Wicker Park subdivision development. The existing site
consist of heavily wooded areas as well as open grassed areas. Proposed development on this site will
consist of the construction of single-family homes, along with the associated roads and sidewalks, utility
improvements, and three stormwater control measures.
The proposed development is located within the Yadkin-PeeDee River Basin with stormwater runoff from
the proposed development draining into Caldwell Creek which flows to Reedy Creek. This development is
located within the Town of Midland and as a result is subject to both the local and state stormwater
requirements. The local stormwater requirements do not require stormwater detention for this site, but
the state does require stormwater quality treatment as outlined below:
Water Quality Requirements:
The proposed facilities have been designed in accordance with the Minimum Design Criteria (MDC)
standards set forth in the NCDEQ Stormwater Design Manual. The proposed facilities will provide the
required 85% TSS removal. Portions of this development have been designed as low -density and are
shown on the post -development drainage map included in this report. These areas use sheet flow and
vegetated conveyance to treat stormwater runoff before entering the stream buffers on this site.
CALCULATION METHODOLOGY
> Rainfall data for the analysis was taken from the most current data available from the National
Oceanic and Atmospheric (NOAA) Precipitation Frequency Data Server website.
> Using maps contained within the Cabarrus County Soil Survey, the on -site soils were determined
to be hydrologic soil groups (HSG) 'B' and 'C' soils. Since the method chosen to compute post -
development peak flow rates and runoff volumes is dependent upon the soil type, care was taken
when selecting the appropriate Soil Conservation Service Curve Number (SCS CN). A composite
SCS CN was computed for each post -development sub -basin. For example, Sub -basin #1 in the
post -development analysis consists of approximately 56% HSG 'B' and 44% HSG 'C' soils.
Therefore, for the wooded cover condition, the composite SCS CN is computed as follows
(assuming good conditions):
> Composite Wooded SCS CN = (56% x 55) + (44% x 70) = 62
> A composite SCS Curve Number was calculated for the post -development condition for each sub -
basin using SCS curve numbers and land cover conditions. Land cover conditions for the post -
development condition were taken from the proposed development plan.
> The time of concentration was assumed to be a minimum of 5 minutes for each drainage area to
each facility.
creating experiences through experience 3430 Toringdon Way, Suite 110, Charlotte, NC 28277 / 704. 527. 0800

TI MCADAMS
STORMWATER IMPACT ANALYSIS
> Pond Pack Version V8i was used in determining the post -development peak flow rates for the 1-,
10-, and 25-year storm events, as well as routing calculations for the proposed stormwater
control measure.
> The proposed stormwater control measure was designed in accordance with the Minimum
Design Criteria (MDC) standards set forth in the NCDEQ Stormwater Design Manual.
DISCUSSION OF RESULTS
Peak Runoff Control Requirements:
This site is exempt from peak runoff control requirements. But the required water quality control will
result in mitigating the increased stormwater runoff resulting from the proposed development.
Water Quality Requirements:
The proposed facilities have been designed in accordance with the Minimum Design Criteria (MDC)
standards set forth in the NCDEQ Stormwater Design Manual. The proposed facilities will provide the
required 85%TSS removal.
CONCLUSION
If the development on this tract is built as proposed within this report, then the requirements set forth by
both the Town of Midland and the State will be met. However, modifications to the proposed
development may require that this analysis be revised. Some modifications that would require this
analysis to be revised include:
> The proposed site impervious surface exceeds the amount accounted for in this report.
> The post -development watershed breaks change significantly from those used to prepare this
report.
The above modifications may result in the assumptions within this report becoming invalid. The
computations within this report will need to be revisited if any of the above conditions become apparent
as development of the proposed site moves forward.
creating experiences through experience 2 of 2