SW6221201_Stormwater Narrative_20221209

CSD
ENGINEERING
PROJECT NARRATIVE
With Project Calculations & Supporting Documents
Ample Storage — Erwin, NC
INTRODUCTION
032721
CA
10/2 8/2022.
Turtle Run, LLC is proposing to build an Ample Storage facility at 404 E. Jackson Boulevard in the Town of
Erwin, Harnett County, NC. The development will consist of buildings for indoor storage, office, and on -
site apartment for facility manager. Submittals for approval and permitting include detailed construction
drawings for site plan, grading, paving, drainage, and stormwater management for the site and minor
improvements within the public right-of-way for driveway access. Approval of the construction drawings
are under the jurisdiction of the Town of Erwin for site plan approval and zoning permit; N.C. Department
of Environmental Quality (NCDEQ), Division of Water Resources (DWR) for stormwater management
permit; and NCDEQ, Division of Land Resources (DLR), Land Quality Section for approval of the erosion
and sediment control plan. The proposed driveway access will be reviewed and permitted by the N.C.
Department of Transportation (NCDOT) District Office for Harnett County.
CjOM
Soil data was obtained from the NRCS Web Soil Survey for the project area of interest in Harnett County,
NC. The soil map from the web application is included with the project calculations and supporting
documents. The site mostly consists of loamy sand to sandy loam soils. The soils on the site are associated
with uplands and are not a flood prone soil. The soils for the Goldsboro and Marlboro soil series are
moderately well -drained soils; therefore, ponding on the surface of bare soil is not expected during
construction. Unified classification (used in channel analysis) is SM at the surface. The site grading requires
import of soils for the building and driveway foundations. A geotechnical engineering study for the borrow
soils is recommended for structural purposes; however, for the purposes of stormwater runoff and
erosion and sediment control, the properties of the in -situ soils were used. Regarding drainage, the soils
have moderate permeability and fall into Hydrologic Soil Group B. The soil survey states that the water
table is from 2 to 3 feet below the surface in the winter and spring; therefore, water may be encountered
during installation of storm drainage pipes and the wet pond.
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Project Narrative
Ample Storage — Erwin, NC
The soil composition was considered during design and analysis for hydrology, hydraulics, and erosion and
sediment control. The soil map for the area of interest and sections of the soil report that were used to
gather data used in the design and analysis have been included in the calculations and supporting
documents.
HYDROLOGY
The area of improvement consists of one (1) drainage area within the site boundary. The total site area is
6.18 acres; however, the hydrologic analysis was conducted for the drainage area to the proposed wet
pond (4.65 acres) which is all the new impervious area plus some pervious areas within the site
development. The remaining on -site may be disturbed during construction; however, this area will remain
pervious and not affect the pre- and post -development analysis. The hydrologic calculations for the
drainage area to the pond includes pre -development curve number (CN) for the existing conditions and
post -development CN for the proposed conditions. The drainage area analyzed was delineate using the
post -development grading which was designed to drain all on -site impervious areas to the proposed wet
pond. The existing area within the post -development drainage area drains to the lowest point on the site,
which is the outfall point for the proposed wet pond.
The pre- and post -development Drainage Area Maps show drainage area delineations, along with
unimproved areas that do not drain to the proposed wet pond. The impervious surfaces on the Ample
Storage site will be drained through a closed drainage system to the proposed wet pond.
NRCS Peak Runoff Method
The methods outlined in NRCS TR-55 were used to calculate peak runoff for land uses, time of
concentration, and peak runoff for the drainage area. The Curve Numbers used were calculated using a
composite curve number calculation. Precipitation data was obtained from NOAA's Precipitation
Frequency Data Server (web -based), Atlas 14 Point Precipitation Frequency Estimates. The latitude and
longitude were entered for the location and the 24-hour precipitation depths for the 1, 2, 10, 25, and 100-
year storm recurrence interval were recorded and used in subsequent hydrologic calculations. The
precipitation depth for the 1-year. 24-hour storm is 3.70 inches according to the NOAA estimates and this
was used in time of concentration calculations.
The computer software program Hydraflow Hydrographs was used to calculate peak runoff hydrographs
and used to determine the storage required for proposed detention ponds. A copy of the Hydraflow
Hydrograph reports for peak flow have been included in this submittal. The hydrologic calculations are
summarized as follows:
➢ Drainage Area:
➢ Pre -development CN
➢ Post -development CN
➢ Time of Concentration
➢ Pre -development runoff (cfs):
➢ Post -development runoff (cfs)
4.65 acres
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94
12 minutes
Q1=3.88; Q2=6.47; Q10=15.30; Q25=21.54; Q100=32.65
Q1=15.39; Q2=19.19; Q10=30.06; Q25=36.81; Q100=48.08
STORMWATER CONTROL MEASURE
A stormwater wet pond is proposed to capture runoff from the site and release it slowly; and the size
needed has been calculated and represented in the plans. Hydraflow Hydrographs was used to calculate
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Ample Storage — Erwin, NC
the peak flow hydrographs for multiple storm recurrence intervals and subsequently used to route the
inflow hydrographs and generate an outflow hydrograph for the temporary pool and outlet structure.
Data used in site runoff calculations was used to estimate the size needed to meet water quality and water
quantity treatment requirements as to improve the post -development stormwater management of the
overall drainage area. Once the area needed for the pond was estimated, design for the actual pond
grading was entered to determine the actual storage.
Various outlet structure and spillway combinations were tried until an optimal solution was reached. A
concrete riser with sloped top and trash rack is proposed and adequately stores the 1-year, 24-hour. 2-
year, 24-hour storm , and 10-year 24-hour storm volumes. The secondary spillway will be a 15-feet weir
in the wet pond dam. A copy of the Hydraflow calculation reports for the 1, 2, 10, 25 and 100-year storms
are included in the calculations and supporting documentation. A schematic showing the estimated water
surface elevations for the pond and the orifice sizing calculation for release of the overall storage volume
have also been included. A summary of the routing calculations follows the next section of the narrative.
Minimum Design Criteria
The proposed wet pond was designed for water quality treatment and water quantity storage. The main
pool surface area and volume was computed using the SA/DA and average depth method. Through
iterations of main pool, forebay, and vegetated shelf configurations, the average depth of the pond was
calculated to meet MDC (3 to 8 feet below permanent pool elevation). The average depth of the pond
was used to determine the minimum surface area of the permanent pool to remove 85% TSS; from Table
1 in Chapter C-3 of the NCDEQ Stormwater Design Manual. The actual surface area at the permanent pool
elevation is nearly twice what is needed for 85% TSS removal. The "Simple Method" was used to
determine the minimum storage needed for the temporary water quality pool (volume of the 1"
precipitation or the "first flush"). The final pond design adequately provides storage above the permanent
pool for water quality treatment (see summary and supporting calculations).
For the main pool depth, Equation 3 in Chapter C-3 of the Stormwater BMP Manual was used because the
vegetated shelf was excluded from the average depth calculation. Through several iterations, the forebay
volume was designed to be within 15 to 20 percent of the main pool volume. The proposed grading
contours on the plans show the forebay entrance is deeper than the exit. The berm that separates the
forebay from the main pool will be submerged and flows will be non -erosive. The forebay and main pool
will be excavated to meet sediment storage requirements.
The wet pond inlet and outlets were configurated to maximize the flow path. The pond will outlet along
the length of the pond giving a wide area to facilitate settling. The pond has been routed for an outlet
structure that will adequately convey storm events and provide adequate detention to protect
downstream property. The drawdown of wet pond will be gradual within 2 to 3 days. The sloped top of
the riser shall be covered by a prefabricated metal grate and help prevent debris from blocking top of
riser.
The wet pond will include a 12-foot-wide vegetated shelf at a 6:1 slope; 1 foot elevation above the
permanent pool to one foot elevation below the permanent pool. A landscape plan is included in the
construction drawings for planting of the vegetated shelf and mesic area around the pond. The plant
schedule calls for Pickerelweed, Arrow Arum, and Bull Tongue within the submerged vegetated shelf; and,
a variety of sedges within the wet mesic part of the vegetated shelf. Juncus will be planted along the
water's edge to protect the more fragile submergent plants from potential rouge mowing by maintenance
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Ample Storage — Erwin, NC
crews. The embankments may be planted with love grass; however, a non -clumping turf grass will likely
be used.
For the final pond grading design, outlet details, and landscape plan, see the construction drawings. The
water quality and water quantity calculations, hydrograph reports, routing, drawdown, and anti -floatation
calculations are included in the project calculations and supporting documents. A summary of the final
wet pond design and calculations is provided below:
➢ Permanent Pool Water Elevation:
➢ Forebay Volume:
➢ Main Pool Volume:
➢ Vegetated Shelf Volume:
➢ Surface Area at Bottom of Shelf:
➢ % Forebay Volume:
➢ Average Depth:
➢ Drainage Area:
➢ Impervious Area:
➢ % Impervious:
➢ SA/DA (Table 1; interpolated)
➢ Surface Area Required (85% TSS):
➢ Actual Surface Area @ 192.0 feet:
➢ Water Quality Volume to be controlled:
➢ Depth of runoff added to permanent pool
➢ Temporary Water Quality Pool Elevation:
➢ Actual Storage at 193.36 feet:
Pond Routing Summary:
➢ Top of Dam:
➢ Emergency Spillway weir elevation and length
➢ Riser weir elevation and length:
➢ Culvert size/type and invert elevation:
➢ 1-Year Stage, Storage, and Outflow:
➢ 2-Year Stage, Storage, and Outflow:
➢ 10-Year Stage, Storage, and Outflow:
➢ 25-Year Stage, Storage, and Outflow:
➢ 100-Year Stage, Storage, and Outflow:
➢ Water Quality Volume to Drawdown:
➢ Drawdown Orifice Size:
➢ Drawdown Time:
HYDRAULIC DESIGN
192 feet
3,959 cubic feet (cf)
22,545 cf
6,390 cf (below Permanent Pool Elevation)
5,070 square feet (sf)
17.6
3.19 feet (Main Pool — Veg. shelf / SA at shelf)
4.65 Acres
3.92 Acres
84.3
2.95
5,975 sf
10,035 sf
13,651 cf
1.36 feet
193.36 feet
16,833 cf
197.00 feet
195.40 feet; 15.0 feet (Cipoletti)
193.70 feet; 5.0 feet (one side of box)
18 inches; R.C. Pipe; 191.60 feet
193.92 feet; 24,908 cf; 1.87 cfs
194.18 feet: 28,878 cf; 5.60 cfs
194.94 feet: 41,077 cf; 12.70 cfs
195.38 feet: 48,963 cf; 14.40 cfs
195.82 feet; 56,887 cf; 28.14 cfs
16,833 cf
1.5 inches (hole drilled in 4-inch PVC cap)
2.4 days
The closed storm drainage system were designed and analyzed using Hydraflow Storm Sewers software,
using the methods outlined by Dr. Rooney Malcolm in "Elements of Stormwater Design" for closed system
design (Rational Formula used for estimating runoff to inlets). The worst case scenario for water surface
elevation in the wet pond was used for the starting elevation in the hydraulic grade line calculations. Some
of the drainage system will be wet during normal conditions; therefore, the pipes were sized to
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Project Narrative
Ample Storage — Erwin, NC
accommodate the storm runoff under "wet" conditions. The 100-year storm was analyzed for potential
site flooding and the 100-year hydraulic grade line does not exceed the finish floor elevations of the
building. The grading plan also shows areas where ponding water may runoff site prior to flooding
buildings.
The roadside channel along St. Matthews Road was analyzed for stability using a custom design
spreadsheet workbook that was developed using the normal depth method outlined by Dr. Rooney
Malcolm in "Elements of Stormwater Design" and methods outlined in FHWA HEC-15 "Design of Roadway
Channels with Flexible Linings" (3rd Ed.). The spreadsheet inputs require discharge, slope, channel
geometry, soil data, and permissible shear stress values. For the input given, the normal depth of flow is
determined through iteration of values until the solution is met. The shear stress is calculated, and
appropriate liners are determined for stability. Normal depth for the channel downstream of driveway
culvert was used in the sizing of the driveway pipe. The channel analysis calculations have been included
in this submittal.
EROSION AND SEDIMENT CONTROL
The erosion and sediment control plan includes measures to keep runoff from disturbed areas from
leaving the site, minimize initial erosion, and to capture sediment from disturbed areas. The plan is divided
into two phases: demolition/clear and grub (1) and intermediate/final (2). The plans show a constructed
berm at the low end of the site to be built to finished slope and grade. The berm will act as a temporary
diversion to drain disturbed areas to the sediment basin. The construction drawings show detailed
sections for the berm/diversion. The sediment basin will be constructed in the footprint of the proposed
wet pond. The contractor has the option of building the pond riser and grading the pond first. The site will
be graded through stages so that runoff will be intercepted by the diversion. A basin dimension worksheet
summarizes the basin design, and the sediment basin will be dewatered from surface with a skimmer. The
wet pond riser detail shows how a skimmer can be attached to the permanent riser. Temporary inlet
protection will be utilized to keep sediment out of the drainage system boxes and pipes. The site will be
stabilized throughout construction with erosion control matting, gravel subgrade, pavement, concrete,
and permanent vegetation as construction progresses.
UTILITIES
The site will be served by an existing water service. A septic permit has been obtained to repair/relocate
the existing septic system. Construction of a new septic system is included in the limits of disturbance;
however, there will be limited disturbance in this area beside Building A. Minimal disturbance is
anticipated for modifications of the water service from the meter to the buildings.
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