Many Low Impact Development (LID) stormwater manuals promote surface infiltration since it can maximize the potential to directly contribute to stormwater runoff reduction. Drivable grass (permeable pavers) provides an excellent opportunity for additional green space instead of using either non-porous or porous asphalt or concrete. Drivable grass can be implemented in both commercial and residential settings, but may not be an option depending on the intended land use, large footprint, or unsuitable subsurface conditions for infiltration.
An alternative approach to surface infiltration is to move stormwater infiltration underground. This strategy can maximize surface land use while simultaneously creating green space opportunities in urban settings. Still, subsurface infiltration is not immune to limitations since soils may be unsuitable for infiltration, a large footprint may be needed, the groundwater table is shallow, and/or bedrock may prohibit its implementation.
Rainwater harvesting manages stormwater runoff as a valuable asset and uses water that would otherwise have been lost. Harvested rainwater can be a source for non-potable water uses such as landscape irrigation, flushing toilets, decorative fountains, etc. However, some locales do not allow harvesting if it can deprive the groundwater table from recharge. Harvesting systems can require a large footprint or storage vessel that does not conform to the intended use of the site; or, there are other development issues or building codes that can limit its use.
Biofiltration spans the gap between the fundamental LID goal of runoff reduction and the need for water quality treatment. While biofiltration can provide a high level of treatment and manage a considerable volume of water on a per storm basis, the infiltration rates often will not allow for handling the majority of annual stormwater runoff. Biofiltration technologies, whether via land based designs or engineered solutions such as modular “tree boxes,” can also offer an effective level of water treatment while improving the viewscape of the property.
Water quality treatment systems can include the use manufactured treatment technologies such as hydrodynamic separation and media filtration. Both systems can be used either as standalone devices or in a wide variety of treatment train options to meet the fundamental goals of LID. For example, hydrodynamic separation can be a pretreatment technology in association with subsurface infiltration, detention or storage (harvesting) structures. Media filtration can also be used when a higher level of water quality treatment is needed. In addition, underground infiltration, detention and storage structures can be used following pretreatment by a “tree box” bio-filter system.