Stormwater Calculations - Mathematics and Statistics for the Environment - page 812

3. Determine the average discharge (Q avg ) resulting from the 24-hour drawdown requirement.
Calculate the average discharge by dividing the required volume, in cubic feet, by the
required time, in seconds, to find the average discharge:

3

43 56, ft
(24hr)(3600 sec/hr)

Q avg = = 0.5 cfs

4. Determine the required orifice diameter by rearranging the orifice equation, Equation
25.12, to solve for the orifice area (ft 2 ) and then diameter (ft). Insert the values for Q avg and
h avg into the rearranged orifice equation to solve for the orifice area and then solve for the
orifice diameter:

QCa h = 2

Q
C h

a =

2

where
Q = Discharge (cfs).
C = Dimensionless coefficient = 0.6.
a = Area of the orifice (ft 2 ).
g = Gravitational acceleration = 32.2 ft/sec 2 .
h = Head (ft).

For orifice area:

05 .

0.052 ft 2

a = =

06 . ()( 2322 40 .)(.)

For orifice diameter:

2

4 a 4( 0.052 ft )

d = = = 0 257 . ft = 3.09 in.

π π

Use a 3.0-in.-diameter channel erosion extended detention orifice.

Method 1 results in a 3.7-in.-diameter orifice and a routed water surface elevation of 88.69 ft.
Additional storms may be “stacked” just above this volume if additional controls are desired.

REFERENCES AND RECOMMENDED READING
King, H.W. and Brater, E.F. (1976). Handbook of Hydraulics , 6th ed. McGraw-Hill, New York.
Linsley, R.K., Kohler, M.A., and Paulhus, J.L. (1982). Hydrology for Engineers , 3rd ed. McGraw-Hill, New
York.
Morris, H. M. and Wiggert, J.M. (1972). Applied Hydraulics in Engineering . John Wiley & Sons, New York.
Schueler, T. (1987). Controlling Urban Runoff: A Practical Manual for Planning and Designing. Metropolitan
Washington Council of Governments, Washington, DC.
USDOT. (1984). Hydrology , Hydraulic Engineering Circular No. 19. U.S. Department of Transportation,
Washington, DC.