Water Quality Volume Equation: WQv = P A Rv (water quality volume equation is dimensionally consistent)
Rv = a + b i + c i2 + d i3
Water Quality Volume
Land development changes the hydrologic properties of watersheds and generally increases the sediment load transported by runoff compared to pre-development conditions.
The additional suspended sediment produced after land development can harm receiving waters.
Water quality volume is the stormwater runoff storage volume required to capture suspended sediment before it is transported to receiving waters.
Precipitation Depth P for Water Quality Volume
While it is cost prohibitive to implement water quality storage facilities to capture all sediment in runoff from all storms,
research shows that capturing 80% of sediment annually can be accomplished by capturing runoff from the 90th percentile storm (WEF, 2012, p. 68).
In the context of water quality volume computation, a 90th percentile storm has a higher precipitation depth than an 80th percentile storm for example.
The use of "percentile" nomenclature is different from storm magnitude probability (e.g. a 100-year storm has a probability of occurrence of 1% per year).
Since the annual load of sediment runoff (e.g. pounds per year) is more a function of the number of storms than the magnitude,
using storm percentile, rather than magnitude probability, has been shown to be better for sizing facilities for water quality (WEF, 2012, p. 68).
In the United States for the water quality volume equation, the state of Ohio uses P=0.9 inch (Ohio EPA, 2018).
U.S. EPA (2016, pp. 3-7) published a summary of state water quality criteria.
The list shows most states use P between 0.75 inch and 1.5 inch.
Volumetric Runoff Coefficient Rv
Different states in the U.S.A. specify different coefficients for the volumetric runoff coefficient Rv equation.
Further, the coefficients for the state's particular Rv equation may require site imperviousness i to be in decimal or in percent.
The LMNO Engineering water quality volume calculator allows you to select whether the coefficients use i in decimal or percent.
Our water quality volume calculator has two built-in equations for Rv.
The Driscoll equation uses the coefficients a and b in the Rv equation.
The Urbonas equation has values for all of the coefficients a, b, c, and d. (WEF, 2012).
Your locale may require different coefficients for the Rv equation, so you can enter them into the calculator
by selecting "Enter coefficients" from the drop-down menu.
Example water quality volume calculation:
If P = 0.9 inch, A = 20 acre, i = 40%, and the Driscoll equation is used for Rv (using i in decimal), then:
Rv = a + b i + c i2 + d i3 = 0.05 + (0.9)(0.4) + 0 + 0 = 0.41
WQv = P A Rv = (0.9 inch)(20 acre)(0.41) = 7.38 acre-inch or 0.615 acre-ft or other units selected in calculator.
In the water quality volume calculator, P, A, and i must be entered as greater than or equal to zero.
Depending on the values for the a, b, c, and d coefficients, Rv and/or WQv could be computed as negative which would be unrealistic.
Units in water quality volume calculator: cm=centimeter, ft=foot, m=meter, mm=millimeter.
References
Ohio Environmental Protection Agency Division of Surface Water. (2018, Oct.).
Post-Construction Storm Water Questions and Answers: Water Quality Volume.
NPDES Construction General Permit #OHC000005.
Retrieved from
https://fairfieldswcd.org/wp-content/uploads/2024/06/47d1a5e4-8653-4419-8a9e-88d7b84a1ac5.pdf
U.S. Environmental Protection Agency. (2016, July).
Summary of State Post Construction Stormwater Standards.
Retrieved from
https://www.epa.gov/sites/production/files/2016-08/documents/swstdsummary_7-13-16_508.pdf
Water Environment Federation and American Society of Civil Engineers. (2012).
Design of Urban Stormwater Controls. WEF Manual of Practice No. 23,
ASCE/EWRI Manuals and Reports on Engineering Practice No. 87.
Prepared by the Design of Urban Stormwater Controls Task Force of the Water Environment Federation
and the American Society of Civil Engineers/Environmental and Water Resources Institute. WEF Press.
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