Stormwater Calculations - Mathematics and Statistics for the Environment - page 811

the basin is to control additional storms, such as 2-year or 10-year storms, the additional storage
volume would be “stacked” just above the water quality volume. The invert for the 2-year control,
for example, would be set at 88.1 ft.

25.7.9.3 Extended Detention Basins—Channel Erosion
Control Volume and Orifice Design
Extended detention of a specified volume of stormwater runoff can also be incorporated into a
basin design to protect downstream channels from erosion. Virginia's Stormwater Management
Regulations, for example, recommend 24-hour extended detention of the runoff from the 1-year
frequency storm as an alternative to the 2-year peak rate reduction. The discussion presented here
is for the design of a channel erosion control extended detention orifice. The design of a channel
erosion control extended detention orifice is similar to the design of the water quality orifice in that
two methods can be employed:

1. Use the maximum hydraulic head associated with the specified channel erosion control
(V ce ) storage volume and calculate the orifice size needed to achieve the required draw-
down time, and route the 1-year storm through the basin to verify the storage volume and
the drawdown time, or

2. Approximate the orifice size using the average hydraulic head associated with the channel
erosion control volume ( V ce ) and drawdown time.

The routing procedure takes into account the discharge that occurs before maximum or brim
storage of the channel erosion control volume ( V ce ). The routing procedure simply provides a more
accurate accounting of the storage volume used while water is flowing into and out of the basin,
and results in less storage volume associated with the maximum hydraulic head. The actual storage
volume needed for extended detention of the runoff generated by the 1-year frequency storm will
be approximately 60 percent of the calculated volume ( V ce ) of runoff for curve numbers between 75
and 95 and time of concentration between 0.1 and 1 hour. The following procedure illustrates the
design of the extended detention orifice for channel erosion control.

25.7.9.3.1 Procedure

1. Calculate the channel erosion control volume ( V ce ). Determine the rainfall amount (inches)
of the 1-year frequency storm for the local area where the project is located. With the
rainfall amount and the runoff curve number (RCN), determine the corresponding runoff
depth using the runoff equation (see Chapter 4 in TR-55 ) or rainfall-runoff depth charts.

Given:
1-year rainfall = 2.7 in., RCN = 75
1-year frequency depth of runoff = 0.8 in.

V ce = 25 ac × 0.8 in. × 1 in./12 ft = 1.66 ac-ft.

To account for the routing effect, reduce the channel erosion control volume:

V ce = 0.6 × 1.66 ac-ft = 1.0 ac-ft = 43,560 ft 3

2. Determine the average hydraulic head ( h ave ) corresponding to the required channel erosion
control volume:

h avg = (89 - 81) ÷ 2 = 4.0 ft