Stormwater Calculations - Mathematics and Statistics for the Environment - page 807

Various local stormwater management regulations allow for the control of downstream chan-
nel erosion by detaining a specified volume of runoff for a period of time. Specifically, 24-hour
extended detention of the runoff from the 1-year frequency storm is proposed as alternate criteria
to the 2-year peak rate reduction. The channel erosion control volume ( V ce ) is calculated by first
determining the depth of runoff (in inches) based on the fraction of rainfall to runoff (runoff curve
number) and then multiplying the runoff depth by the drainage area to be controlled.
25.7.9.1 Retention Basins—Water Quality Volume
The permanent pool feature of a retention basin allows for settling of particulate pollutants, such
as sediment and other pollutants that adsorb to these particulates. Therefore, it is essential that the
volume of the pool be both large enough and properly configured to prevent short-circuiting . (Short-
circuiting results when runoff enters the pool and exits without sufficient time for the settling process
to occur.) The permanent pool, or “dead” storage volume, of a retention facility is a function of the
water quality volume. For example, a permanent pool sized to contain four times the water quality
volume provides greater pollutant removal capacity than a permanent pool sized to contain two times
the water quality volume. Our example analyzes a 25-acre watershed. The water quality volume and
permanent pool volume calculations for a retention basin serving this watershed are calculated next.
25.7.9.1.1 Procedure
1. Calculate the water quality volume ( V wq ) for the given watershed. Assume the commercial/
industrial development disturbs 11.9 acres, with 9.28 acres (404,236 ft 2 ) of impervious
cover after development.
V wq = 404,236 ft 2 × 1/2 in./12 in./ft = 16,843 ft 3
16,843 ft 3 /43,560 ft 2 /ac = 0.38 ac-ft
2. Size the permanent pool based on the desired pollutant removal efficiency or the drainage
area impervious cover . The pool volume will be sized based upon the desired pollutant
removal efficiency. The permanent pool must be sized for 4 × V wq for a pollutant removal
efficiency of 65%.
Permanent pool volume = V wq × 4.0 = 0.38 ac-ft × 4.0 = 1.52 ac-ft
25.7.9.2 Extended Detention Basins—Water Quality Volume and Orifice Design
A water quality extended detention basin treats the water quality volume by detaining it and releas-
ing it over a specified amount of time. In theory, extended detention of the water quality volume
will allow the particulate pollutants to settle out of the first flush or runoff, functioning similarly
to a permanent poll. Various local stormwater management regulations pertaining to water qual-
ity specify a 30-hour drawdown time for the water quality volume. This is a brim drawdown time,
beginning at the time of peak storage of the water quality volume. Brim-drawdown time means the
time required for the entire calculated volume to drain out of the basin. This assumes that the brim
volume is present in the basin prior to any discharge. In reality, however, water is flowing out of the
basin prior to the full or brim volume being reached. Therefore, the extended detention orifice can
be sized using either of the following methods:
1. Use the maximum hydraulic head associated with the water quality volume ( V wq ) and
calculate the orifice size needed to achieve the required drawdown time, and route the
water quality volume through the basin to verify the actual storage volume used and the
drawdown time.
2. Approximate the orifice size using the average hydraulic head associated with the water
quality volume ( V wq ) and the required drawdown time.