Low Impact Development
This manual explains how to use Low Impact Development (LID) to preserve or mimic the natural water cycle and protect water quality — generally by reducing runoff. Reducing runoff also reduces the amount of pollution delivered to downstream waterways. What’s more, it reduces stream bank erosion — so more soil along stream banks stays put, instead of polluting the waterway. LID works best and costs least when a site is examined as a whole. That means considering stormwater and natural resources from the beginning and incorporating Best Management Practices (BMPs), rather than adding one or two large stormwater facilities as an afterthought at the end of the planning process.
What is Low Impact Development?
Also known as Green Infrastructure, Green Development Practices, or Alternative Storm Water Management Systems, LID is a pattern of land development that preserves natural resources and promotes managing stormwater where it falls. LID relies on carefully selected techniques for reducing, receiving and cleaning stormwater runoff to protect and improve water availability and quality.
The Puget Sound Partnership defines LID as “a stormwater and land use management strategy that strives to mimic pre-disturbance hydrologic processes of infiltration, filtration, storage, evaporation and transpiration by emphasizing conservation, use of on-site natural features, site planning and distributed stormwater management practices that are integrated into a project design.”
What is a BMP?
The EPA defines a best management practice (BMP) as "a device, practice, or method for removing, reducing, retarding, or preventing targeted stormwater runoff constituents, pollutants and contaminants from reaching receiving waters.”
Despite widespread use of “best,” these practices may not be effective unless properly applied:
- Use several BMPs together, not just one.
- Select BMPs appropriate for the site’s opportunities and constraints and for the project’s objectives.
- Apply BMPs during the appropriate phase(s) of the project. Some BMPs are important during more than one project phase.
Why use Low Impact Development?
- It can cost less to build and maintain.
- It reduces water pollution, which supports community health and welfare with better drinking-water quality, reduced treatment costs and cleaner waterways for recreation and commerce.
- It reduces local flooding during storms, except at sites with a history of flooding.
- It’s the most accepted path to meeting a community’s water quality goals as defined by an MS4 (National Pollutant Discharge Elimination System [NPDES] Municipal Separate Storm Sewer System) permit, or the Total Maximum Daily Load (TMDL) program.
- It contributes to a attractive, healthy economically vibrant community.
In 2009 the EPA wrote, “Until recently, stormwater programs established to address water quality objectives have been designed to control traditional pollutants that are commonly associated with municipal and industrial discharges, e.g., nutrients, sediment and metals. Increases in runoff volume and peak discharge rates have been regulated through state and local flood control programs. Although these programs have merit, knowledge accumulated during the past 20 years has led stormwater experts to the conclusion that conventional approaches to control runoff are not fully adequate to protect the nation’s water resources.”