Low Impact Development: Controlling Runoff at its Source
(Conservation Currents, Northern Virginia Soil and Water Conservation District)
As soon as a new development starts in our neighborhood, many of us become concerned. We ask, “What will happen to our precious stream?” We know that development will dump high-volume runoff into our stream and put it under stress.
How can we protect our streams from the negative impacts of increased runoff due to development? For many years, jurisdictions have used conventional stormwater management systems to manage the problems associated with runoff.
Conventional stormwater management systems are designed to remove runoff from the site as fast as possible. A system of pipes and open channels brings the runoff to a pond, usually located at the lower end of the site. Creating stormwater management ponds is a common practice in conventional stormwater management systems. However, these systems only decrease the rate at which runoff enters the stream. They do not change runoff volume or frequency. Therefore, our stream will still be under stress.
One other approach we can take is low-impact development or LID. The goal of LID is to maintain the condition of the watershed as it was before development, thereby preserving the integrity of the receiving waters. To achieve this goal, LID emphasizes “source control” of runoff. Runoff is controlled where it is formed rather than being conveyed to ponds.
In LID, runoff is managed in small, cost-effective landscaped features such as rain gardens, filter strips, vegetated buffers, grassed swales, and rain barrels located near the source of impact. With these landscape features, runoff that enters the unit either will penetrate the soil and recharge the groundwater or enter a pipe at the base of the unit to be gradually released to a stream. This approach allows a developed site to be designed as an integral part of the environment and maintains the site’s predevelopment watershed conditions. Furthermore, because LID helps site planners decrease runoff volume in the first place, it also decreases the dependence on stormwater management ponds needed to detain or retain the stormwater runoff.
LID also is advantageous to developers. LID practices such as reducing impervious surfaces, decreasing the use of storm drain piping and inlet structures, and eliminating or reducing the size of large stormwater management ponds can significantly reduce development costs. In some cases, developers can achieve a greater yield per lot and, therefore, a greater return on investments.
While low impact development practices are being utilized more and more frequently these days, there is still a long way to go. NVSWCD has published guides and periodically holds workshops on rain gardens and other LID practices to educate both the public and industry to share best practices and lessons learned in this approach. For more information, see: Rain Gardens, Green Roofs and Other LID Practices.