20200579 Ver 1_2023-10-02 WhirligigStation_As-BuiltReport_20240528

AS-BUILT NARRATIVE > CWI-19000
2905 Meridian Parkway, Durham, NC 27713 / 919. 361. 5000 creating experiences through experience
WHIRLIGIG STATION
As-Built Sand Filter Nutrient Credit Calculations

GENERAL DESCRIPTION
The proposed design for the Whirligig Station – Parking Lots Improvements underground storage and sand filter SCM
was previously approved by the City of Wilson. The approved Stormwater Master Plan (SMP) can be found as an
attachment to this report. The project is located on a lot between Tarboro Street SW and Goldsboro Street SW in the
downtown area of Wilson, North Carolina and is owned by the City of Wilson. The project is within the Neuse River
Basin. The pre-project condition of the site consisted of a parking lot area with limited stormwater drainage piping. The
proposed design included increased stormwater routing of onsite and offsite surface runoff to the SCM which is
capturing approximately 8.61 acres of total and impervious area. The associated amount of impervious does not change
from the pre-development to the post-development condition.

CALCULATION METHODOLOGY
These calculations provide the determination of nutrient credits, specifically nitrogen, generated by the sand filter once
constructed. The sand filter design computations per the current NC DEQ MDCs include a correction to the ‘hmax’,
ponding depth value, from the previously provided calculations in the original Stormwater Design Calculations (See
Appendix). The sand filter design calculations have been updated to show As-Built values as shown in the As-Built survey
of the SCM also included as an attachment in this report. The original sand filter nutrient calculations were completed
using the North Carolina Stormwater Nitrogen and Phosphorus Tool (SNAP Tool). These have been updated using the
Neuse Method 2 Nutrient Offset calculations. The original calculations were referenced for area inputs. The Method 2
calculations are included in this report. As demonstrated in the As-Built survey, the sand filter was built per the design
and therefore, no revision to the proposed nutrient removal calculations is required.

DISCUSSION OF RESULTS
NUTRIENT CREDITS GENERATED
The onsite underground storage and sand filter SCM is capturing approximately 8.61 acres of surface runoff. The sand
filter is designed per MDC standards. Nutrient treatment was not required by the development in the original design,
however, as the sand filter is providing nitrogen treatment in the downtown area, the credits generated have been
calculated to be utilized in the associated MBI. The total generated nitrogen credits were calculated to be 1,916.7 lbs.

CONCLUSION
The proposed stormwater control measures (underground sand filter) at Whirligig Station were built per the design and
will function as intended in compliance with current NC DEQ MDC’s for an underground sand filter.
WHIRLIGIG STATION
CWI-19000
MODIFIED SAND FILTER DESIGN R. STUBBS, PE
3/21/2023
Determination of Water Quality Volume (WQV)
Note: The following design calculations are based upon requirements from the NCDEQ Minimum Design Critieria.
where,
WQV= water quality volume (CF)
RV = 0.05+0.009(I) where I is percent impervious cover
A = area in acres
P = rainfall (in inches)
Input data:
Total area, A =8.61 acres
Impervious area =8.61 acres
Percent impervious cover, I =100.0 %
Rainfall, P =1.0 inches
Calculated values:
RV =0.95
WQV =0.68 acre-ft
=29693 cf.
Sizing is based upon the discounted water quality volume, WQV dis (75% of the total WQV).
WQVdis =22270 cf.
Size Sand Filter and Sedimentation Chamber
Determine the required Sand Filter Surface Area
The minimum required sand filter surface area is the discounted WQ volume divided by the ponding depth:
Ponding Depth (D) =2.50 ft
ASF =8,908 sf. (minimum sand filter surface area required)
The minimum sand chamber area must accommodate 50% of the treatment volume.
Design Summary
ASF ≥8,908 sf (minimum total sand and sedimentation chamber surface area)
ASF ≥9,230 sf (total sand and sedimentation chamber surface area provided)
ASand ≥4,454 sf (minimum sand chamber area required)
ASand ≥6,602 sf (minimum sand chamber area provided)
ASed ≥2,628 sf (minimum sedimentation chamber area provided)
D =2.50 ft (maximum WQV dis ponding depth in facility)
A SF = WQV dis / D
WQV = (P)(R V )(A)/12 x 43,560
WHIRLIGIG STATION
CWI-19000
MODIFIED SAND FILTER DESIGN R. STUBBS, PE
3/21/2023
Determination of Underdrain Sizing
The following design calculations are based upon requirements from the 2009 NC BMP Design Manual.
Compute drawdown through filter bed
where,
A = provided surface area of filter bed (sf)
k = coefficient of permeability of filter media (ft/day)
hF = height of water above filter bed (feet)
dF = filter bed depth (feet)
Input data:Design As-Built
A =6602 6602 sf
k =3.50 3.50 ft/day
hF = 2.50 2.50 ft
dF = 2.00 1.92 ft
Calculated values:Design As-Built
Q =51993 53196 cf/day
=0.602 0.616 cfs
Compute minimum drawdown discharge
WQV =22270 cf
t = 1.667 days
Minimum drawdown, Q = 0.155 cfs
Compute underdrain pipe capacity
Number of rows, n = 8
Diameter of underdrains, d = 6 inches
Area, A =0.1963 sf
Wetted perimeter, P =1.5708 ft
Slope, S = 0.0050 ft/ft
Capacity of one pipe = 0.395 cfs
Total capacity = 3.161 cfs
Assume 50% clogging =1.580 cfs DESIGN ACCEPTABLE
Compute underdrain perforation capacity
3/8-inch perforations spaced 3 inches on center, along 4 longitudinal rows spaced 90 o apart
Number of rows, n = 8
Diameter of underdrains, d = 6 inches
Length of underdrains, L = 40 ft
Number of perforations =5120 holes
Capacity of one hole = 0.008 cfs
Total capacity = 42.280 cfs
Assume 50% clogging =21.140 cfs DESIGN ACCEPTABLE
Q = A(k)(h F +d F )/(d F )
WHIRLIGIG STATION
CWI-19000 NITROGEN EXPORT CALCULATIONS
H. DEW, PE
4/15/2020
TN-Loading Input Data
Open Wooded Impervious Pond Total
To SWMF 0.00 0.00 8.61 0.00 8.61
Totals =0.00 0.00 8.61 0.00 8.61
TN-Loading Output Data
To SWMF 8.61 182.54 35%118.65 Sand Filter
Totals =8.61 182.54 118.65
Total TN-Load After Treatment =118.65 lbs/yr
=13.78 lbs/ac/yr
Estimated TN Credits Generated
Total Nitrogen Analysis Area =8.61 acres
TN-Load Before Treatment =182.54 lbs/yr
21.20 lbs/ac/yr
Total TN-Load Generated =63.89 lbs/yr
7.42 lbs/ac/yr
# of Years requiring offset =30 yrs
Total TN-Export Generated =1,916.7 lbs
Sub-basin ID Nitrogen Analysis Area [acres]
Sub-basin ID Nitrogen Analysis Area
[acres]
TN-Load Before
Treatment [lbs/yr]% Removal TN-Load After
Treatment [lbs/yr]BMP Type
[PROJECT NAME]
[PROJECT NUMBER]
POST-DEVELOPMENT
NITROGEN EXPORT CALCULATIONS
To SWMF
[DESIGNER/ENGINEER]
4/15/2020
METHOD 2:
STEP 1:Determine the area for each type of land use and enter in Column (2).
STEP 2:Total the areas for each type of land use and enter at the bottom of Column (2).
STEP 3:Multiply the areas in Column (2) by the TN export coefficients in Column (3) and enter in Column (4).
STEP 4:Total the TN exports for each type of land use and enter at the bottom of Column (4).
STEP 5:Determine the export coefficient for the site by dividing the total TN export from uses at the bottom of
Column (4) by the total area at the bottom of Column (2).
(2)(3)(4)
Area TN export coeff.TN export from use
[acres](lbs/ac/yr)(lbs/yr)
8.61 ---182.54
Total TN Export =21.2 lbs/ac/yr
% impervious =100.0%
areas, etc.)
Permanently protected managed
Permanently protected undisturbed
open space (forest, unmown meadow)
1.2
lots, driveways, roofs, paved storage
Type of Land Cover
Quantifying TN Export from Residentail / Industrial / Commercial Developments when Footprints of all
Impervious Surfaces are shown.
(1)
0.6
TOTAL
182.54
0.00
0.000.00
0.00
8.61 21.2
open space (grass, landscaping, etc.)
Impervious surfaces (roads, parking