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# Cosumnes power plant (01-afc-19) data response, set 1A

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#### Attachment W&SR-133

 project: Cosumnes Power Plant job no.: 13561 discipline: Civil subject: Drainage Volumes and Areas calculation no.: 13561-C-1 originator: L. Gasparetti date: 7/26/01 file no.: checker: date: Sheet 1 of 5 Sheets

 Purpose of Calculation Determine the detention pond storage volume required to contain the total runoff from a 10-year, 24-hour storm. Summary of Results and Conclusions Starts in Sheet No. _ Sources of Design Criteria Sources of Formula and References: FR-1 Rainfall Depth Duration Frequency for Eagles Nest (Department of Water Resources) FR-2 Plot Plan, Drawing No. G1, Rev. by PB Power Record of Issues No. Description By Date Checked Date Approved Date Date Filmed Preliminary Calculation x Committed Preliminary Design Calculation Superseded Calculation Final Calculation
 project: Cosumnes Power Plant job no.: 13561 discipline: Civil subject: Drainage Volumes and Areas calculation no.: 13561-C-1 originator: L. Gasparetti date: 7/26/01 file no.: checker: date: Sheet 2 of 5 Sheets

1. PURPOSE

See cover sheet.

1. METHODOLOGY

1. The Rational Method determines the peak runoff:

Q = CiA, where

Q = peak runoff (cfs)

i = rainfall intensity (in/hr) (based on time of concentration)

A = contributing area (acres)

1. 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.

1. DESIGN CRITERIA

1. Use 10-yr frequency, 24-hour storm.

2. 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

1. Rainfall intensity for 10-year, 24-hr storm, i:

Rainfall depth = 2.60 in/24-hr (FR-1)

Therefore, i = 0.11 in/hr

1. Storage volume required is the difference between the runoff from the developed site minus the runoff from the undeveloped site.

1. SUMMARY

Required volume for detention pond is 100,000 cubic feet.

1. AREAS

1. Total site:

AT = 25.2 ac

1. Paved areas, AP:

Ap = 13.5 ac

1. Crushed rock areas, ACR:

Switchyard: ACR = 6.2 ac

1. Landscaped areas, AL:

Entrance: AL = 1.1 ac

1. Compacted earth, AE

Cooling towers: AE1 = 2.8 ac

O/W Separator: AE2 = 0.7 ac

Gas Metering Station: AE3 = 0.9 ac

AE = AE1-3 = 4.6 ac

1. RUNOFF COEFFICIENTS

For developed site, CDEV:

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

1. 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

1. VOLUME OF DETENTION POND, VPOND:

VPOND = VDEV - VUDEV = 167,616 – 71,712 = 95,904 ft3, say 100,000 ft3

#### EBMUD American River Diversion Analysis

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