20070295 Ver 6_0.01 Narrative_20191220

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Holding Village — North Lake
Storm woter Impact Analysis
GENERAL DESCRIPTION
NARRATIVE > RAP-19000
The proposed North Lake at Holding Village development is located in Wake Forest, North Carolina on S.
Franklin Street. The proposed development will consist of 276 single-family lots (including 68 townhomes)
along with utility and stormwater management improvements associated with the development. The
proposed development is located within the Neuse River basin with stormwater runoff draining to Smith
Creek. The overall Holding Village project is governed by the original Master Plan approval from May 15,
2007. As a result of this vested right the town ordinances, rules, and regulations applicable to Holding
Village are those that applied when the project was originally approved on May 15, 2007 which consisted
of detention to pre -development conditions in the 2- and 10-year storm events. Due to proposed impacts
to onsite jurisdictional features a 401 Water Quality Certification from NCDEQ is required. As a result the
portions of the project that do not drain to the existing Holding Lake are designed to meet the NPDES
Phase II post construction stormwater management controls to provide for no degradation of the
downstream receiving surface waters.
The NPDES Phase II post construction stormwater management controls includes properly managing
stormwater runoff rate, volume, and pollutants as necessary to protect the environment, property,
health, safety, and welfare of the Town's citizens. More specifically stormwater control measures shall be
installed that control and treat the difference in stormwater runoff volume leaving the project site
between the pre- and post -development conditions for, at a minimum, the 1-year, 24-hour storm. Runoff
volume drawdown time shall be a minimum of 24 hours, but not more than 120 hours. All structural
stormwater treatment systems used to meet the requirements of the program shall also be designed to
have a minimum of 85% average annual removal for Total Suspended Solids (TSS). To meet these
requirements the stormwater treatment devices that will be used include three wet ponds designed as
Primary SCMs using the state Minimum Design Criteria (MDCs) to remove total suspended solids (TSS) and
provide slow release (i.e. 2 to 5 days) of the water quality volume. Additional benefits of these facilities
includes detention of the peak flowrates at the site discharge analysis points in the 1-year, 2-year, and 10-
year storm events.
There is a portion of this project that drains directly to the existing Holding Lake which is a buffered
jurisdictional feature. The portion of the project that drains directly to the existing Holding Lake must
demonstrate no degradation of downstream surface waters which includes addressing any increases in
peak flowrates and discharging the stormwater non-erosively. To meet these requirements level
spreaders to provide diffuse flow of the stormwater runoff during the water quality event are proposed
along with riprap bypass channels that are designed to discharge the larger storm events non-erosively
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NARRATIVE > RAP-19000
directly into the existing Holding Lake. The existing Holding Lake facility was previously retrofitted as part
of a separate project and calculations have been provided demonstrating stormwater detention of the 1-
year, 2-year, 10-year, and 100-year storm events is provided in the existing Holding Lake. By providing
diffuse flow into the existing Lake and stormwater detention of the peak flowrates within the existing
Lake to the extent practical it is expected no degradation of downstream surface waters will occur.
CALCULATION METHODOLOGY
• Rainfall data for this area in the Wake Forest, NC region is from NOAA Atlas 14. This data contains a depth -
duration -frequency (DDF) table describing rainfall depth versus time for varying return periods in the area.
These rainfall depths are input into the meteorological model within Pond Pack for peak flow rate calculations.
Please reference the precipitation information section within this report for additional information.
• Using WebSoil Survey, the on -site soils were determined to range from hydrologic soil group (HSG) 'A' soils to
HSG 'D' soils. Since the method chosen to compute pre- and 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).
• An approximate proportion of each soil group was determined for the entire site using NRCS Soil Survey Maps.
Once an approximate proportion was determined, a composite SCS CN was computed for each cover condition.
• A composite SCS Curve Number was calculated for both the pre- and post -development condition for each
subbasin using SCS curve numbers and land cover conditions. Land cover conditions for the pre -development
condition were taken from Wake Forest GIS. Land cover conditions for the post -development condition were
taken from the proposed development plan.
• The time of concentration was calculated using SCS TR-55 (Segmental Approach, 1986). The Tc flow path was
divided into three segments: overland flow, concentrated flow, and channel flow. The travel time was then
computed for each segment, from which the overall time of concentration was determined by taking the sum
of each segmental time.
• PondPack Version V8i was used in determining the pre- & post -development peak flow rates for the 1-, 2-, 10-
and 100 year storm events, as well as routing calculations for the proposed stormwater management facilities.
• Existing topographic information used in this analysis is from an aerial survey performed by Avioimage and
McAdams.
DISCUSSION OF RESULTS
PEAK RUNOFF CONTROL REQUIREMENTS
As shown in the Summary of Results in Section 1 of this report, the proposed Stormwater Control Measures provide
the necessary on -site peak runoff control such that there are no predicted increases in the 1-, 2-, and 10-year storm
events at the points of analysis leaving the site. Additionally, flows decrease at every point where diffused flow enters
an existing stream in the 1, 2, and 10-year storm events.
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CULVERT CALCULATIONS
There are three proposed culverts on this site. Calculations can be found in section 8 of this report.
CONCLUSION
If the development on this tract is built as proposed within this report, then the requirements set forth in the Town of
Wake Forest regulations will be met without additional stormwater management facilities. However, modifications to
the proposed development may require that this analysis be revised. Some modifications that would require this
analysis to be revised include:
1. The proposed site impervious surface exceeds the amount accounted for in this report.
2. 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.
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