Green infrastructure/low impact development (LID) is an environmentally-sensitive approach to managing development and stormwater runoff. Development techniques and best management practices that mimic predevelopment forested conditions to reduce the volume of stormwater runoff and introduce surface water back into the soils and ground water. It can protect aquatic resources, water quality, and the natural hydrology of a watershed as development takes place.
Prior to development, most rainfall was slowed down by tree needles and leaves in Sammamish’s forests. The water then spread out over the forest floor, where it is absorbed into the ground, taken up by the roots of trees and other plants, or evaporated.
However, when forests and natural open spaces are cleared, and buildings, roads, parking areas and lawns dominate the landscape, rainfall becomes stormwater runoff, carrying pollutants to nearby waters.
Much less stormwater infiltrates and is taken up by plants, less stormwater evaporates back to the atmosphere, and much more stormwater becomes surface runoff.
Green infrastructure/low impact development elements work to mimic the pre-development processes and allow the natural movement of water through a site.
When combined with other key elements of a comprehensive stormwater program, effective land-use planning under the Growth Management Act and watershed or basin planning, green infrastructure/LID can help communities more efficiently and effectively manage stormwater, and protect their water resources.
• Green infrastructure/LID can help better protect the environment. green infrastructure/LID techniques remove pollutants from stormwater, reduce the overall volume of stormwater, manage high storm flows, and —or replenish—streams and wetlands.
• Green infrastructure/LID can help reduce flooding and protect property. Reducing impervious surfaces, increasing vegetation and dispersing and infiltrating stormwater results in less runoff. This reduces the likelihood of flooding from big storms.
• Green infrastructure/LID helps protect human health by more effectively removing pollutants from stormwater. Untreated stormwater can be unsafe for people to drink or swim in.
• Green infrastructure/LID protects drinking water supplies by ensuring that rainfall infiltrates where it can recharge aquifers, rather than being treated as a waste and discharged to marine waters.
• Green infrastructure/LID is good for the economy. Green infrastructure/LID can help protect shellfish growing businesses, water quality and marine sediment quality. This ensures that our resources remain clean and Puget Sound remains a great place to operate a business and attract employees. Taxpayers don’t have to pay for expensive cleanup efforts for polluted waters and sediments. And because green infrastructure/LID projects in many cases are less expensive to build, it means that developers and builders can often save money on overall development costs by using green infrastructure/LID.
• Green infrastructure/LID can increase the appearance and aesthetics of communities. Green infrastructure/LID projects leave more trees and plants and have less impervious surfaces, which makes for greener developments and communities.
• Rain gardens and bioretention areas can be used to collect runoff from hard surfaces. Pollutants are removed by the plants and a large portion of the runoff is infiltrated.
• Disconnecting downspouts reduces the amount of storm runoff into the public surface water system. Runoff can be routed to a grass or gravel area and infiltrated. This replenishes groundwater and helps reduce the increase flow to small creeks during rain events. Be careful not to route runoff directly onto a neighbor’s property, or in a place that could cause drainage problems.
• Pervious pavement can be used for walkways, parking areas, driveways, and patios. The pervious pavement reduces the amount of storm runoff by allowing rain to infiltrate through the surface and into the ground. Examples are pervious concrete, pervious asphalt, permeable pavers, and grass pavers.
• Rain barrels and cisterns capture roof runoff to be used later for irrigation. This reduces the increase in summer water usage.
• Amending soils with compost will increase infiltration and absorption. Nutrients in the composted soils work to break down and remove pollutants from the runoff.
Chapter 21A.85 – Low Impact Development
Department of Ecology Low Impact Development (LID) Resources
Low Impact Development Technical Guidance Manual for Puget Sound
Homeowner’s LID BMP Maintenance Brochure
Rain Garden Handbook for Western Washington Homeowners
Rain Garden Summary