The following is an excerpt from Florida's new Stormwater Quality Applicant Handbook.
Section 12.0 discuss how Florida will treat green roofs and cisterns with respect to stormwater. For a discussion of concerns read subsequent posts.
12.0 Green Roof/Cistern Design Criteria
A greenroof/cistern stormwater treatment system is a vegetated roof followed by storage in a cistern for the filtrate which is reused. A greenroof/cistern system is a retention BMP and its effectiveness is directly related to the annual volume of roof runoff that is captured, retained, and reused. The filtrate from the greenroof is collected in a cistern or, if the greenroof is part of a BMP Treatment Train, the filtrate may be discharged to a downstream BMP such as a wet detention pond. A cistern is sized for a specific amount of filtrate and receives no other runoff water. Other pond storage must also provide capacity to detain a specified quantity of filtrate. The detained water is used to irrigate the roof. Irrigation must be provided to maintain the plants. A back up source of water for irrigation is necessary. Excess filtrate and excess runoff can be discharged to other stormwater treatment systems, infiltrated into the ground, or used for irrigation or other nonpotable purposes. The greenroof/cistern system functions to attenuate, evaporate, and lower the volume of discharge and pollutant load coming from the roof surface. Greenroof systems have been shown to assist in stormwater management by attenuating hydrographs, neutralizing acid rain, reducing volume of discharge, and reducing the annual mass of pollutants discharged. Concentrations of pollutants discharged from a greenroof with pollution control media have been shown to be approximately the same as would be anticipated from a conventional roof. Thus, the concentration and mass must be managed. If no pollution control media are used, greenroof concentrations are greater than those from conventional roofs. In addition, with fertilization of the plants, increased nutrients are expected and storage for the filtrate is required.