(a) Using the following equations: CPWQv = C * P * A/12
Where: CPWQv = Channel protection and water quality volume in acre-feet
C = Runoff coefficient appropriate for storm less than 1 inch (See Table 2)
P = 0.75 inch precipitation depth
A = Area divining into the BMP in acres.
TABLE 2: RUNOFF COEFFICIENTS BASED ON TYPE OF LAND
USE FOR CPWQv CALCULATION
Land Use | Runoff Coefficient |
Industrial & Commercial | 0.8 |
High Density Residential (>8 dwellings/acre) | 0.5 |
Medium Density Residential (4 to 8 dwellings/acre) | 0.4 |
Low Density Residential (<4 dwellings/acre) | 0.3 |
Open Space and Recreational Areas | 0.2 |
Where the land use will be mixed, the runoff coefficient should be calculated using a weighted average. For example, if 60% of the contributing drainage area to the storm water treatment structure is low density residential, 30% is high density residential, and 10% in open space, the runoff coefficient is calculated as follows: (0.6)(0.3) + (0.3)(0.5) + (0.1)(0.2) = 0.35
(b) An additional volume equal to 20% of the CPWQv shall be incorporated into the BMP for sediment storage and/or reduced infiltration capacity during construction.
(c) BMP's shall be designed such that the drain time is long enough to provide treatment, but short enough to provide storage available for successive rain events as described in Table 3 below.
(d) Commercial, institutional and industrial sites that are currently developed are only required to provide 20% of the calculated CPWQv upon re-development, alternatively the re-development site may provide a 20% net reduction in impervious area.
TABLE 3: TARGET DRAW DOWN (DRAIN) TIMES FOR STRUCTURAL
POST-CONSTRUCTION TREATMENT CONTROL PRACTICES
Best Management Practice (BMP) | Drain Time of CPWQv |
Infiltration | 24 - 48 Hours |
Vegetated Swale or Filter Strip | 24 Hours |
Extended Detention Basins (Dry Basins) | 48 Hours |
Retention Basins (Wet Basins)* | 24 Hours |
Constructed Wetlands (above permanent pool) | 24 Hours |
Media Filtration, Bioretention | 40 Hours |
(Ord. 15-O-2820, passed 5-19-2015)