20191303 Ver 1_Permeable Pavement Calcs_20190930

Bradford Circle Water Quality Depth Calculations
❑wq = P(1+R)
n
where: D. q = Depth of aggregate needed to treat the water quality storm (inches)
P - Rainfall depth for the water quality storm (inches)
R = A,,fAp. ratio of the additional SUA to permeable pavement area
N = Percent voids, unitless decimal (from ASTM C29)
Given Depth of Aggregate
P= 1 in When:
Aa 1778 sf R = Aa/Ap = 0.38
Ap 4665 sf
N 0.38 Dwq = 3.63 inches
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Draw Down Calculations
Bradford Circle Permeable Pavement
Water Quality Volume (Ft')
501.6
Total WQv Required
Top Elevation of Temp Pool (Ft)
337.15
Overflow Elevation
Orifice Invert Elevation (Ft)
334.97
Reservoir 3 Elev. Used
Maximum Head (Ft)
2.16
At Centroid of Orifice
Driving Head Ho (Ft)
1.08
1/2 Maximum Head
Orifice Diameter (in)
0.50
Orifice Diameter (Ft)
0.04
Orifice Area (Ft2)
0.001
Orifice Coefficient
0.59
Orifice Discharge Q (Ft3/s)
0.01
Days to Complete Drawdown
0.87
Between 2 and 5 Days
Q = CDA 2gHo
Stage Storage Calculations
Elev. (Ft)
Area (sf)
Volume (cf)
336.82
4000
1320
337.15
4000
Note -
Reservoir Elevations Vary
WQv
1320
Voids
0.38
Total WQv Provided (cf)
501.6
Where: Q = Discharge (cfs)
CD = Coefficient of discharge (dimensionless) - see Table 3-8
A = Cross -sectional area of flow at the orifice entrance (sq ft)
g = Acceleration of gravity (32.2 ft/ sec2)
Ho = Driving head (ft), measured from the centroid of the orifice area to
the water surface - Note: Usually use Ho/3 to compute
drawdown through an orifice to reflect the fact that head is
decreasing as the drawdown occurs.
Figure 3-3
Schematic section through an orifice
=zR2
Table 3-8
Values of Coefficient of Discharge, CD (Malcom, 1989)
Entrance Condition CD
Typical default value 0.60
Square -edged entrance 0.59
Concrete pipe, grooved end 0.65
Corrugated metal pipe, mitred to slope 0.52
Corrugated metal pipe, projecting from fill 1.00
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