Stormwater Calculations - Mathematics and Statistics for the Environment - page 809

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).

Assume for orifice area:

06 .

a =

06 . ()( 2322 70 .)(.)

For orifice diameter:

2 2

a = 0 047 . ft = π d 4

2

4 a 4 0 047 (. ft )

d = = = 0 245 . f t = 2.94 in.

π π

Use a 3-inch diameter water quality orifice.

Routing the water quality volume ( V wq ) of 0.76 ac-ft, occurring at elevation 88 ft, through
a 3-in. water quality orifice will allow the engineer to verify the drawdown line, as well
as the maximum elevation of 88 ft. This calculation will give the engineer the inflow-
storage-outflow relationship in order to verify the actual storage volume needed for the
extended detention of the water quality volume. The routing procedure takes into account
the discharge that occurs before maximum or brim storage of the water quality volume, as
opposed to the brim drawdown described in Method 2. The routing procedure is simply a
more accurate analysis of the storage volume used while water is flowing into and out of
the basin. Therefore, the actual volume of the basin used will be less than the volume as
defined by the regulation. This procedure will come in handy if the site to be developed is
tight and the area needed for the stormwater basin must be “squeezed” as much as possible.
The routing effect of water entering and discharging from the basin simultaneously will
also result in the actual drawdown time being less than the calculated 30 hours. Judgment
should be used to determine whether the orifice size should be reduced to achieve the
required 30 hours or if the actual time achieved will provide adequate pollutant removal.

Note: The designer will notice a significant reduction in the actual storage volume used when routing
the extended detention of the runoff from the 1-year frequency storm (channel erosion control).

Routing the water quality volume depends on the ability to work backwards from the
design runoff volume of 0.5 inches to find the rainfall amount. Using SCS methods, the
rainfall needed to generate 0.5 inches of runoff from an impervious surface (RCN = 98)
is 0.7 in. The SCS design storm is the Type II, 24-hour storm. Therefore, the water quality
storm using SCS methods is defined as the SCS Type II, 24-hour storm, with a rainfall
depth of 0.7 in. The rational method does not provide a design storm from a specified
rainfall depth. Its rainfall depth depends on the storm duration (watershed t c ) and the storm
return frequency. Since the water quality storm varies with runoff amount, not the design
storm return frequency, an input runoff hydrograph representing the water quality volume
cannot be generated using rational method parameters. Therefore, Method 1, routing of the
water quality volume, must use SCS methods.