SW4210301_STORMWATER_CALCS_2991.2_REV_ONLY_20210412

AEV PARKING EXPANSION
J EFFERSON, NC
H YDROLOGIC AND H YDRAULIC A NALYSIS R EPORT

Prepared By: Prepared For: Project Site:
BREC, P.A.
Kenneth Osborne 150 Northwest Drive
1520 Meadowview Drive
Jefferson, NC 28640
Wilkesboro, NC 28697
NCBELS Firm #C-3448
[phone]
[e-mail]

12 April 2021
Revised Pages 10,11,12 and Last Page of Post-Construction First Flush

Max Prestwood III, PE
Project Engineer
BREC, P.A. AEV PARKING EXPANSION 1

2.7 3 ­£ &¨«³¤± $¤²¨¦­

The combined area of the sediment chamber and sand chamber is determined
from the relationship of the max water depth observed during the water quality
event, , and the adjusted water quality volume. The design value for
was chosen as . When coupled with the adjusted water quality volume,
, the resulting stage storage for the sand filter is
summarized below.

Table 12 - Sand Filter State-Storage

0.0 2,989.0 1,922.00 0 0 0
1.0 2,990.0 2,516.00 1,110 1,110 2,219
2.0 2,991.0 3,182.00 2,534 2,534 5,068
2.2 2,991.2 3,323.84 2,861 2,861 5,721

At , .
2.7ȁΐ #§ ¬¡¤±  ­£ &¨«³¤± 3¨¹¨­¦

The sediment chamber and filter chamber are equally sized. The sediment
chamber has an additional depression to assist with maintenance and
primary sediment deposition; however, this added volume is not considered in
calculations to meet minimum design criteria requirements. Given the filter
chamber area, the sand media area, , was selected as . This value
maximizes the available filter chamber area and preserves an easily installable
geometric shape for the sand media while preventing the sand media from being
installed adjacent to the toe of slope grading. Discharge through the sand media

Where,

2.7.2 5­£¤±£± ¨­ 3¨¹¨­¦

BREC, P.A. AEV PARKING EXPANSION
10

Drawdown time is calculated as:

Discharge rate is calculated as:

After applying a safety factor of 10,

The diameter of a single pipe is calculated as:

Where,

Per Table 1 of Section A-5 of the NCDEQ Stormwater Design Manual, use (2) 6
inch pipes.

2.7.3 Anti-¥«® ³ ³¨®­ # «¢´« ³¨®­²

The outlet structure is a precast concrete box with side and
bottom wall thicknesses. Including concrete and reinforcing steel, the structure
has an empty mass of .

Assuming the total height of the box is exposed to buoyant forces, the volume of
water displaced by the box is calculated from the outside dimensions.

BREC, P.A. AEV PARKING EXPANSION
11

The buoyant force is defined from the hydraulic forces acting on the box.

No additional mass needed mass of structure is greater than the buoyant force.

ΒȁΏ 2¤²´«³²

The first flush storm is routed through the sand filter and reaches a max
storage depth of which does not exceed the design . The
water quality volume is discharged over a period of

Figure 1 - System Storage of First Flush

BREC, P.A. AEV PARKING EXPANSION
12

Storage Node : SAND-FILTER (continued)
Outflow Weirs
SNElementWeirFlapCrestCrestLengthWeir TotalDischarge
IDTypeGateElevationOffsetHeightCoefficient
(ft)(ft)(ft)(ft)
1EM-SPILLWAYTrapezoidalNo2993.504.5010.001.003.33
Outflow Orifices
SNElementOrificeOrificeFlapCircularRectangularRectangularOrificeOrifice
IDTypeShapeGateOrificeOrificeOrificeInvertCoefficient
DiameterHeightWidthElevation
(in)(in)(in)(ft)
1PRIMARY-OUTLETSideRectangularNo18.0014.002991.200.63
2SECONDARY-OUTLETBottomRectangularNo36.0036.002993.000.63
Output Summary Results
Peak Inflow (cfs) ......................................................................................3.55
Peak Lateral Inflow (cfs) ..........................................................................3.55
Peak Outflow (cfs) ...................................................................................0.08
Peak Exfiltration Flow Rate (cfm) ............................................................0.00
Max HGL Elevation Attained (ft) ..............................................................2990.90
Max HGL Depth Attained (ft) ...................................................................1.9
Average HGL Elevation Attained (ft) .......................................................2989.47
Average HGL Depth Attained (ft) ............................................................0.47
Time of Max HGL Occurrence (days hh:mm) .........................................0 05:42
Total Exfiltration Volume (1000-ft³) .........................................................0.000
Total Flooded Volume (ac-in) ..................................................................0
Total Time Flooded (min) ........................................................................0
Total Retention Time (sec) ......................................................................0.00