Green Streets
A Green Street includes a rain garden and vegetated area along a street that reduces runoff and filters out pollutants to protect our local streams, Potomac River and the Chesapeake Bay. Green Streets help address environmental and regulatory stormwater needs.
In 2011, the County assessed all County watersheds and ranked spaces on County land where green street projects could potentially be installed. Although this study was an important first step in identifying potential green street locations, it did not include detailed project feasibility analysis. Upon closer examination, many of the ranked locations are found not to be suitable for green streets.
How Green Streets Work
The bowl-shaped, vegetated areas in a green street are rain gardens. They provide a temporary place for water to collect, be filtered by the soil and plants, and soak into the ground or be released into the stormwater system. An underground pipe, or underdrain, is incorporated into their design, connecting the rain garden with the stormwater system.
The pipe ensures that the rain garden will drain in less than 48 hours to prevent mosquito breeding. The underdrain allows water in the rain garden to exit to the storm sewer system if underlying soils drain slowly.
The rain garden is maintained by the County monthly during the growing season, including visits to weed, prune, clean out sediment and litter, replace plants when necessary, and to make sure underdrains and overflows function correctly. We welcome help from neighbors to keep the facilities free of litter and leaves.
This diagram shows a cross section of a typical Green Street rain garden in the street median. The rain garden has a small ponding area for runoff, with several feet of soil and gravel underneath. There is an underdrain at the bottom to allow water to flow to the storm sewer system after it is filtered.
Plant Palettes
Native plants are selected that can tolerate both wet and dry conditions. We select native plants that grow no more than 2.5 feet above the curb.
Project Process
Green Street projects are prioritized for construction based on several factors, such as the ability to be combined with another County project, capacity for water quality treatment, impervious area treated, and community support. Green street projects are often implemented in partnership with other County programs such as the Arlington Neighborhoods Program, the Stormwater Management Program and the Complete Streets Program.
The green street program is currently funded for design and construction of approximately two green streets per year.
Projects In Design, Planning, Construction
Completed Projects
Neighborhood Impact
How Green Streets Are Designed to Avoid Water Problems Nearby
Green Streets are designed to avoid local water problems in homes or basements. They are located at a lower elevation are far away enough from homes that basement seepage does not result.
The design of the rain gardens with the overflow and underdrain also ensures that the rain garden will not create local drainage problems.
Safety Features of Green Street Design
Green Street rain gardens include a curb and/or gentle slopes for safety.
The rain gardens are designed to contain no more than 6-12 inches of ponded water, which helps keep area residents and kids safe. Additional water will overflow to a stormwater inlet.
The special soil installed in the rain garden ensures that the water will quickly infiltrate into the soil. The rain garden will only hold water for a few hours following a storm.
Green Streets and Pollutants
The three most important water pollutants in the Chesapeake Bay watershed are nitrogen, phosphorus and sediment. Keeping these three out of our local streams and waterways is a major target of our Chesapeake Bay goals. While sediments and nutrients are a problem for waterways, plants love them. That's a key reason Green Streets are so effective.
There are also other pollutants from cars, tires and roads that rain gardens can help capture and filter. These pollutants bind to the soils or are taken up by the plants. Studies from Portland, Oregon have analyzed soils from green street gardens and haven’t found any evidence of pollutants accumulating or posing a health risk. If necessary, the rain garden soils can be replaced to ensure proper filtration.
Green Streets and Parking
If a green street rain garden is located in the median, parking spaces won’t be removed. If the rain garden is installed at the curb, it can displace parking spaces and in some cases may eliminate parking across the street.
We work to co-locate green streets with community-based neighborhood projects and traffic calming projects that benefit neighborhoods. In our project process, we also look at area parking conditions and where possible, try to maintain on-street parking.