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Low Impact Development

Low-impact development (LID) is an approach to stormwater management that was introduced to the U.S. in the 1980s in Maryland to help Prince Goerge’s County to address the growing economic and environmental limitations of conventional stormwater management (Coffman 2001). The concept of LID is to design stormwater management systems to recreate the natural hydrologic cycle. The general approach is to manage stormwater using a distributed set of source controls. To recreate the natural hydrologic cycle a key element of the source controls is retention and infiltration using microscale structures.

LID, in general, is a concept applicable to flood control, stormwater quality management, combined-sewer overflows, urban water shortage management, and urban microclimate/air quality control. It is applicable to newly urbanizing areas, retrofits, and redevelopment/revitalization. The value of LID practices from the perspective of a regulator, permitting agency, developer, and designer has been demonstrated through several case studies (Coffman 2001). Common benefits of LID projects include reduced lifecycle costs, multifunctionality, additional environmental and social benefits, reduced off-site costs, and functional use of open space. The common concern that LID approaches will be more expensive because of additional time for design and construction has been shown to be a non-issue once designers gain experience and a market develops in a region. The use of permeable pavement is an example here. The cost of using permeable pavers or porous concrete when first introduced was higher than traditional concrete or asphalt pavement. But as designers, regulators, and contractors gained experience working with the technologies and more technologies and materials were introduced by a wider array of vendors the costs reduced and now in some places of the country do not represent a significant cost addition to an LID project.

The concept of LID is focused on the smaller storm events, not the 10-year, 50-year, or 100-yr return period events. The control of the more frequent events is the means to restore the natural hydrologic cycle and prevent downstream receiving water impacts. LID is often thought to be contrary to flood control needs because of its focus on the smaller, more frequent rainfall events rather than the rare, high intensity, long duration flood causing events.

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