Volume Control Regulations
A number of SMPCs have implemented some kind of volume control regulations, as can be seen in Table 1. An example of a volume control regulation is one that requires that a specified volume of water be kept and infiltrated on site. Infiltration is the process by which water is absorbed into soil. The water is then taken up by plants or percolates through the soil and becomes groundwater. Infiltration is a natural mechanism by which water travels which improves water quality and recharges aquifers. As discussed earlier, impervious surfaces limit the amount of infiltration that can take place. Current technologies may limit a site from being completely free of impervious surface, but BMPs can be utilized to maximize infiltration.
The first inch of runoff is commonly referred to as the first flush due to the high amount of pollutants it typically contains. Using a traditional runoff calculation analysis, one inch of runoff from a paved surface corresponds to approximately 1.21 inches of rainfall. This analysis is based on the amount of rainfall and the land cover.
In the Draft Cook County Watershed Management Ordinance, properties which develop over 5,000 square feet of impervious area are required to capture and retain an inch of runoff. This can be achieved through the use of volume control practices. Examples of these practices may be the green technologies described earlier or through traditional infiltration practices such as trenches or retention ponds. The ordinance does allow for a fee in lieu of program for sites where it may not practical to implement volume control practices.
Stronger volume control ordinances like the one proposed for Cook County could have a dramatic affect on the quality and quantity of stormwater runoff. One inch of runoff over a 5,000 square foot parking lot relates to approximately 3,116 gallons.