Before Rule Change:
This comparison shows a typical small lot single-family subdivision designed to meet the stormwater management rules effective prior to March 2021, with compact development, implementation of some grass swales, and a typical stormwater management wet pond.
There is little to no developable area at the upper end of the site due to preservation of existing wooded areas and the large amount of area needed for the wet pond. The size of the wet pond is driven by the design approach of using an “end-of-pipe” system. All drainage areas on site are captured via storm sewer or overland flow and conveyed to the wet pond which acts as a single point of management.
After Rule Change:
The theoretical design of the same subdivision applying the amended stormwater rules adds several decentralized green infrastructure BMPs and yields two additional building lots. This is achieved by intercepting and infiltrating a significant portion of the overall stormwater runoff within the developed portion of the site as opposed to conveying the majority of the runoff to the end-of-pipe wet pond.
Pervious driveway aprons treat and infiltrate both driveway and roadway runoff; multiple rain gardens treat and infiltrate yard, roof, and roadway runoff; a roof recharge pipe in the center of the subdivision infiltrates roof runoff. This reduces the peak flow and volume of runoff to the end-of-pipe wet pond, allowing the wet pond to be designed with a smaller footprint. Note that a naturalized pond edge has also been added to the wet pond to provide water quality benefits, discourage geese, and add aesthetic appeal. Although not illustrated, the wet pond would also be utilized as a source of irrigation water. Morever, as water quality is already addressed prior to discharge to the wet pond, the minimum required volume is reduced.