Low Impact Development (LID) is a term used to describe a land planning and engineering design approach to managing stormwater. LID emphasizes the conservation and use of a site’s natural features to protect water quality and to maintain the hydrologic integrity of the site. This approach entails the use of engineered small-scale hydrologic controls distributed throughout the site to replicate the pre-development hydrologic regime of the site by infiltrating, filtering, storing, evaporating, and detaining runoff close to its source. This distributed nature of the strategy makes it a sort of “decentralized concept of stormwater management”. LID practices have also been termed “green infrastructure”. It is closely allied with development practices that emphasize minimizing disturbance and/or maximizing the natural features of a site, such as “conservation development”.
LID practices are selected for an individual site in consideration of the site's land use, hydrology, soil type, climate and rainfall patterns. There are many variations on these LID practices. Many are practical for retrofit or site renovation projects, as well as for new construction. Frequently-used practices include:
- Bioretention cells, also known as rain gardens
- Rainwater catchment and sequestration in cisterns or rain barrels
- Green roofs, essentially another strategy for rainwater catchment and sequestration
- Permeable paving, also called porous pavement
- Grassed swales, also known as bioswales
One of LID’s primary goals is to maintain not just runoff quality but also runoff volume at the predevelopment level, by infiltrating rainfall to groundwater, evaporating rainwater back to the atmosphere after a storm, and/or by routing sequestered rainwater to beneficial uses – including the maintenance of stream baseflow – rather than exporting runoff as a waste product down storm sewers or drainage channels. The result is a landscape functionally equivalent to predevelopment hydrologic conditions, thus maintaining the hydrologic integrity of the watershed.
Excerpted and adapted from a proposal I submitted to plan and design the water resources infrastructure for a development near Austin, Texas, this paper reviews the LID stormwater management strategy that I advocate for greenfield development in this area. This approach favors retention on the site of the increased volume of runoff produced by development, with an aim to beneficially utilize this water rather than wasting it off-site, at the same time blunting the water quality impacts imparted by development. The distributed nature of the management devices also minimizes the vulnerability of the stormwater management practices, producing a robust management strategy.
Bioretention Strategy for a “Big Box” Store Parking Lot
An analysis of how a bioretention-based stormwater management strategy could have been implemented for a “big box” store parking lot in the same area allotted to the parking lot and the large-scale end-of-pipe devices that were employed – devices that simply collect, treat and drain the significantly increased runoff volume – while delivering additional benefits beyond simply addressing water quality objectives.
