Low Impact Development
By Larry S. Coffman

Low Impact Development (LID) represents the most advanced stormwater management technology and has evolved from the lessons learned over the past 30 years here in the United States and around the world. Simply put, LID is a new approach using decentralized integrated source control practices making more cost effective and efficient use of a site to maintain the watershed hydrology and water quality.

LID is a comprehensive source control technology. Prince George's County, Maryland first pioneered LID in 1997 to help address the growing economic and environmental limitations of conventional stormwater management practices. As LID was developed by a local government, it is sensitive to addressing local government's unique environmental and regulatory needs in the most economical manner possible by reducing costs associated with stormwater infrastructure design, construction, maintenance and enforcement.

LID is simple and effective. Instead of the large investments in complex and costly centralized conveyance and treatment infrastructure, LID allows for the integration of treatment and management measures into urban site features. LID encourages the multifunctional cost-effective use of the urban green space, buildings, landscaping, parking lots, roadways, sidewalks, and various other techniques to detain, filter, treat and reduce runoff. LID is completely different from conventional management strategies.

Conventional practices use highly efficient drainage systems to get water off a site as quickly as possible to a centralized treatment device (i.e., stormwater pond). LID uses many decentralized micro-scale at the source control techniques to manage runoff. This involves strategic placement of distributed lot-level controls that can be "customized" to more closely mimic a watershed's hydrology and water quality regime. One of the primary goals of LID design for new development (greenfields) is to reduce runoff volume through infiltration, recharge, evaporation and finding beneficial uses for rainwater rather than disposing of it as a waste product into storm sewers. The result is a landscape that is functionally equivalent to predevelopment hydrologic conditions that generates less surface runoff, less pollution, less erosion and damage to lakes, streams, and coastal waters.

LID is economical. It costs less than conventional stormwater management systems to construct and maintain, in part, because of fewer pipes, few conveyance structures and less impervious surface. But the benefits do not stop there. Space once dedicated to stormwater ponds can now be used for other purposes. The greater use of on lot multipurpose landscaping / vegetation also offers human "quality of life" opportunities by greening neighborhoods thus contributing to livability, value, sense of place, and aesthetics. Other benefits include enhanced property values and re-development potential, greater marketability, improved wildlife habitat, thermal pollution reduction, energy savings, smog reduction, enhanced wetlands protection, and decreased flooding. LID is a multi-dimensional approach with multiple benefits.

LID is flexible. It offers a wide variety of structural and nonstructural techniques to provide for both runoff quality and quantity benefits. LID works in highly urbanized constrained areas and environmentally sensitive areas for urban infill or retrofit projects. In a combined sewer system, LID can reduce both the number and the volume of sewer overflows. Opportunities to apply LID principles and practices are infinite as any feature of the urban landscape can be modified to control runoff (e.g., buildings, roads, walkways, yards, open space) or reduce the introduction of pollution.

LID tools include five basic techniques:

• 1) encourages conservation of natural features (wetlands, streams, woodlands and buffers);
• 2) promotes impact minimization techniques (reduce clearing and grading, limit lot disturbance, impervious surface reduction and disconnection);
• 3) provides for strategic runoff timing (slowing water down to allow infiltration and evaporation );
• 4) uses an array of integrated management practices (rain gardens, rooftop storage, rain water use, parking lot storage, and amended soils); and
• 5) advocates pollution prevention measures to reduce the introduction of pollutants to the environment.

The more LID techniques used the closer one can get to restoring the natural hydrologic and water quality regime of a watershed. LID now gives us the tools to design the built environment to remain a functioning part of the ecosystem instead of apart from it. The effectiveness of LID technologies is only limited by the knowledge and skills of the site engineers / designers. There is no one technique for LID. Its power lies in the cumulative benefits of all its techniques.

Copies of the Prince George's LID design manual, the national LID guidance manual and information on bioretention can be obtained by calling Prince George's County's Department of Environmental Resources at (301) 883-5834. For more information see www.lowimpactdevelopment.org. [ opens in a new window ]

Larry Coffman is the Associate Director, Prince George's County, Maryland Department of Environmental Resources. His work on Low Impact Development for ecologically based and environmentally sensitive site designs won the county the First Place National Excellence Award for Municipal Stormwater Programs from the US Environmental Protection Agency in 1998.

Rain Gardens Reduce Runoff
By Gweneth Newman

When it rains, we witness water flowing from our roofs, onto lawns, down streets and into storm drains. What we don't see are the incredible amounts of pollution this rainwater picks up along the way - things like motor oil, brake fluid, pesticides, fertilizers and animal waste. Because most properties aren't designed to absorb and utilize stormwater, all these pollutants sit around collecting and, when it rains, wash into the drains, into our streams, and in the case of my backyard, into the Boston Charles River.

When the EPA and MIT sponsored a national contest looking for fresh ideas that - costing less than $10,000 to implement - would stop the free flow of bacteria and pollutants into the river, my classmate Katherine Alberg Anderson and I took the opportunity to share some of our ideas about how to retain this resource on site, beginning in the front yard of a typical Cambridge residence.

Our project focused on several objectives. We viewed the house as representing its own miniature watershed. We wanted to provide a prototypical design whose wide palette of stormwater treatment options could allow homeowners to select some - or all - of the recommendations. It was important to us that construction of the project was both doable and durable. Homeowners could build it themselves and feel comfortable having their families use it, let the kids run around and have the impact on the landscaping be minimal. Our intent was to incorporate both functional and aesthetic design in order to bring stormwater out of the drain and visualize the benefits of retaining it on site.

The final product of our design harvested stormwater from the roof and groundsurface and converted it into uses that were visual, ephemeral and sustaining in a multitude of ways. Rainwater gardens were placed along all four sides of the house, collecting water generated by downspouts and surface runoff. Water routed to these landscaped depressions would be naturally filtered by plants and soils, removing excess nutrients and pollutants. Rock filters and dry streambeds were included to assist in guiding and filtering water into these gardens, as well as to stimulate visual interest of water tumbling and moving.

A rain barrel was included in the design to collect runoff generated by the roof. Attached to an existing gutter with minimal modification, the water collected in rain barrels can be saved as a reserve with which to irrigate the landscape. To quantify: for a roof roughly 625 square feet in area, a storm generating a quarter inch of rainfall would produce 98 gallons of water.

Porous pavers were suggested as an alternative to the typical asphalt driveway. These pavers are unique in that they consist of strong structural materials regularly interspersed with voids that are filled with sandy loam or grassed turf, allowing for rainwater retention and infiltration.

It was important to us that the design reinforced our notion of getting water out of the gutters and really celebrate the qualities of its sound, fluidity and significant visual attributes. The result of these efforts took form in what we call the rain-chain pergola walk. A decorative alternative to downspouts, rain chains originated in Japan, where they have been around for hundreds of years to provide a beautiful and functional watercourse for rain.

Too often we are ready to shuttle away what storms bring us, viewing water as a problem rather than as an opportunity. Rather than installing expensive pump-powered fountains and water features into our properties, why not capitalize on what we get naturally? We need to increase people's awareness of what an important resource rainwater is by interventions at the local scale, beginning in our own front yards. And while the impact of one house improving the condition of regional streams and rivers might seem hard to fathom, just imagine what one entire block or one neighborhood might be able to achieve. It all adds up. And all it takes to start are ourselves and some creative ideas for a gutter.

Gweneth Newman and Katherine Alberg-Anderson are studying Landscape Architecture at Harvard' s Graduate School of Design. Together they won the EPA/MIT first prize for utilizing rainwater.

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