In a natural setting, rainfall lands on the tree canopy, drips to the earth, and drains through vegetation into the soil. Plants take some of this water into their roots, and the excess slowly seeps underground, eventually returning to the surface as a spring or stream. Developed landscapes, with compacted soils, hardened exteriors, and impervious surfaces, disrupt this delicate natural flow of water.
Reducing Impacts to the Landscape
At the landscape level, a green infrastructure network includes park and trail systems, green spaces, and open vistas that improve our quality of life, create wildlife habitat, and provide for cleaner air and water. Permanent protection of these features can be accomplished by land preservation and the use of Low Impact Development (LID) techniques, which limit the impact of development by concentrating improvements to areas where they are most appropriate.
Limiting earth disturbance and hard, impervious surfaces to as small a footprint as possible reduces the need for gray infrastructure: pipes, curbs, inlets, and detention structures that disrupt the natural way water flows through our communities.
When the disruption of natural water flow is unavoidable, green stormwater infrastructure (GSI) can provide significant water quality protection. Using GSI features—extended detention, bioswales, rain gardens, green roofs—a designer puts plants between sky and soil, and soil between plants and streams, recreating and better mimicking the natural processes that allow water to soak into the earth.
A Place on the Spectrum
The design of every stormwater project sits on a spectrum of gray to green—somewhere between pipes and streams, asphalt and meadow. Finding the right place on that spectrum to meet project goals is a significant challenge. Though the choice often depends on the desired intensity of the intended land use, consider also these eight important questions:
- How will the site be used now—and in the future?
- Who will maintain the site?
- What is the cost of maintenance?
- What is the cost of construction, and can GSI strategies provide a savings?
- Where can exposure to regulation be limited (land use, wetlands, streams, steep slopes)?
- How can the project help meet other goals, making limited funds go further?
- What design is compatible with adjacent land uses?
- Most importantly: How can the use of LID and GSI best serve the owner?
Using LID techniques preserves natural flow paths wherever possible. But when LID isn’t possible, natural hydrology can be re-created using GSI. It’s all about finding the right balance between gray and green.