project: Cosumnes Power Plant
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job no.: 13561
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discipline: Civil
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subject: Drainage Volumes and Areas
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calculation no.: 13561-C-1
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originator: L. Gasparetti
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date: 7/26/01
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file no.:
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checker:
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date:
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Sheet 2 of 5 Sheets
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PURPOSE
See cover sheet.
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METHODOLOGY
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The Rational Method determines the peak runoff:
Q = CiA, where
Q = peak runoff (cfs)
C = runoff coefficient
i = rainfall intensity (in/hr) (based on time of concentration)
A = contributing area (acres)
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Modify the rational method by using the rainfall depth for the 24-hour event to determine “i” rather than a shorter time period based on the time of concentration. This will yield a volume which is more realistic than using a higher intensity and assuming the storm maintains that intensity over a 24 hour period.
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DESIGN CRITERIA
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Use 10-yr frequency, 24-hour storm.
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Runoff coefficient, C:
C = 0.30
for crushed rock surfacing, landscaped,
and undeveloped areas
C = 0.75 for compacted earth
C = 0.90 for paving and roofs
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Rainfall intensity for 10-year, 24-hr storm, i:
Rainfall depth = 2.60 in/24-hr (FR-1)
Therefore, i = 0.11 in/hr
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Storage volume required is the difference between the runoff from the developed site minus the runoff from the undeveloped site.
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SUMMARY
Required volume for detention pond is 100,000 cubic feet.
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AREAS
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Total site:
A
T = 25.2 ac
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Paved areas, AP:
A
p = 13.5 ac
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Crushed rock areas, ACR:
Switchyard: A
CR = 6.2 ac
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Landscaped areas, AL:
Entrance: A
L = 1.1 ac
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Compacted earth, AE
Cooling towers: A
E1 = 2.8 ac
O/W Separator: AE2 = 0.7 ac
Gas Metering Station: AE3 = 0.9 ac
AE = AE1-3 = 4.6 ac
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RUNOFF COEFFICIENTS
For
developed site, C
DEV:
AT, CDEV = X
AP, C = 0.90
ACR, C = 0.30
AL, C = 0.30
AE, C = 0.75
CDEV = [0.9(13.5) + 0.3(6.2+1.1) + 0.75(4.4)]/25.2 = 0.70
For undeveloped site, CUDEV: AT = 25.2 ac, C = 0.30
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VOLUME OF RUNOFF
From developed site, Q
DEV:
Q DEV = CiA = 0.70(0.11)(25.2) = 1.94 cfs
For 24-hour duration:
VDEV = (1.94 ft3/sec)(3600 sec/hr)(24 hr) = 167,616 ft3
From undeveloped site, QUDEV:
QUDEV = CiA = 0.3(0.11)(25.2) = 0.83 cfs
For 24-hour duration:
VUDEV = (0.83 ft3/sec)(3600 sec/hr)(24 hr) = 71,712 ft3
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VOLUME OF DETENTION POND, VPOND:
V
POND = V
DEV - V
UDEV = 167,616 – 71,712 = 95,904 ft
3, say 100,000 ft
3