Putting the LID on Stormwater Runoff
The following post is provided by guest blogger Thomas Powers, P.E., LEED AP, CFM, CPESC, A Project Manager with Wight & Company in Chicago Illinois and former colleague of The Inlet’s Carolyn Howard.
How many gallons of rain do you think falls each year on just one acre of land in Norfolk, Va.? Ten thousand? One hundred thousand?
Would you believe more than one million? Unfortunately, most of that water isn’t absorbed by the land and instead becomes stormwater runoff, carrying debris, chemicals, sediment or other pollutants. The U.S. Environmental Protection Agency (EPA) ranks urban runoff and storm-sewer discharges as the fourth most prevalent source of impairment of our lakes, streams and rivers.
The current best practices in stormwater management is called low impact development (LID), which refers to a comprehensive land planning and engineering design approach that emphasizes conservation and the use of on-site natural features to protect water quality. According to the nonprofit Low Impact Development Center, its goal is to “maintain and enhance the predevelopment hydrologic regime of urban and developing watersheds while allowing for development or infrastructure rehabilitation.”
LID strategies typically focus on reducing the quantity of stormwater runoff while improving its quality. This often requires creating a multistep “treatment train” at its source and as the runoff flows through the site and into retention areas or downstream conveyance systems. At Wight’s headquarters, for example, stormwater becomes progressively cleaner as it moves from parking lot bioswales through filtration trenches, across a restored prairie and into a naturalized detention basin.
Whenever possible, stormwater systems should follow nature’s lead and manage rain where it falls. Such solutions usually involve the use of many distributed, small-scale systems that infiltrate, filter, store, evaporate and detain runoff. In contrast to traditional stormwater practices, LID focuses on treating smaller and more frequent storm events through various cost effective features in upland areas of the watershed.
The amount of rainfall expected at the site will be a critical factor in LID strategy and design. Typically, the goal is to be able to treat 90 or 95 percent of historical storms. Another common design objective is to treat a certain depth of stormwater runoff. For example, the goal might be to treat one inch of runoff for new sites and a half-inch for redeveloped sites.
One of the most effective stormwater runoff strategies is “disconnection,” which decouples roof downspouts, roadways and other impervious areas from stormwater conveyance systems. This allows runoff to be collected and managed on-site or dispersed into the landscape. A LID best practice is to daylight roof downspouts at grade, preferably to a rain garden or bioretention area. Pavements should also sheet drain into bioswales or rain gardens.
Stormwater management best practices can be integrated into all types of buildings and sites. More information about LID can be found in this recent article in Building Design + Construction.