Low Impact Development

July 2004 » Feature Articles
Low Impact Development (LID) is a relatively new practice that attempts to unite site planning, land development, and stormwater management with ecosystem protection.
Philip Jones, E.I.T.

Integrated management practices,such as paving blocks that allow runoff to infiltrate into the underlying subbase, can reduce the amount of land dedicated solely to stormwater management.

Low Impact Development (LID) is a relatively new practice that attempts to unite site planning, land development, and stormwater management with ecosystem protection. It was first developed in the 1990s in response to the economic and environmental impacts of conventional stormwater management techniques. Put briefly, LID is a comprehensive development and design technique that strives to preserve predevelopment hydrology and water quality through a series of small-scale, distributed structural and non-structural controls.

Several design manuals currently exist for planners, engineers, and landscape architects who wish to implement LID techniques. In addition, several states now include LID practices in their stormwater management handbooks.

The benefits of LID are quantifiable and have been demonstrated in projects around North America and Europe. LID "hotspots" in the United States include the Washington, D.C., Philadelphia, Portland, Ore., and Seattle metropolitan areas, where it is being implemented in a variety of public and private projects. Numerous local jurisdictions around the country also have expressed a continuing interest in LID training and demonstration projects. Several branches of the armed services have recognized LID as an important planning and design tool for their facilities. "The Navy brass has been supportive of LID," said Krista Grigg, storm water program manager for Naval District Washington. An LID Unified Facilities Criteria document prepared by the Low Impact Development Center will be available soon from the Department of Defense. The Low Impact Development Center is a nonprofit organization dedicated to research, development, and training for water resource and natural resource protection issues.

LID projects commonly are implemented because they reduce lifecycle costs for stormwater infrastructure and shift maintenance burdens away from local governments. Additionally, they provide superior control of non-point source pollution, and hydrologic control of small, frequently-occurring storms and their effects on downstream ecology. Other benefits include National Pollution Discharge Elimination System (NPDES) Phase II Final Rule compliance, combined sewer overflow (CSO) mitigation, community development, and watershed pollutant load management.

The following is a brief review of some basic hydrologic concepts, which will provide a background for discussing LID and its differences from conventional stormwater management.

Hydrology review
Three basic processes control the fate of precipitation that falls on a site: surface runoff, evapotranspiration, and infiltration. Surface runoff flows toward receiving waters through natural and engineered conveyances. Evapotranspiration (ET) is the loss of water through evaporation (either from the ground or from the tree canopy) and through uptake into plants. Infiltration refers to the downward movement of water into the soil. Undeveloped land naturally and efficiently "manages" stormwater through the interplay of all three processes, and as little as 10 percent of annual precipitation may become surface runoff. In the built environment, however, conventional stormwater management often overlooks the potential utility of infiltration and ET, concentrating instead on controlling relatively large surface runoff volumes.



This bioretention strip at the University of Maryland, College Park, buffers the adjacent stream (on left) and provides a visual transition between the road and the existing green space.

Precipitation that does become surface runoff may be stored through retention or detention. Detained runoff is stored on site temporarily, often in a centralized facility such as a pond, and is released at a controlled rate. Typically, detention does little to reduce the volume of runoff. Retention captures runoff permanently and eliminates the runoff volume through infiltration and ET. Retention and detention practices may be centralized, distributed, or both.

Conventional stormwater management
Conventional stormwater management consists primarily of the conveyance and detention of surface runoff to control peak discharge rates and to improve water quality for relatively infrequent design storms, such as the two- and 10-year storms. A centralized detention (or occasionally, retention) pond stores the entire volume of site runoff and may be modified to enhance pollutant removal.


A bioretention cell was installed at a Northwestern High School parking lot in Hyattsville,Md. Plant uptake reduces runoff volume and pollutant concentrations.

One concern with the "pipe-and-pond" approach is poor control of small, frequent storms. Because the minimum design storm is often the one- or two-year storm, the peak rate, runoff volume, and pollutant loading of small, frequent storms is not attenuated, even though they account for the majority of the annual precipitation volume. For example, 70 percent of the annual precipitation volume in Washington, D.C., comes from storms of 1 inch or less.

Most urban pollutants are conveyed in the first 0.5 inches to 1 inch of runoff. Because conventional management strategies are intended for larger storms, they do little to address non-point source pollution from this "first flush" and may not assist with NPDES Phase II compliance. Additionally, failing to adequately manage small storms contributes to stream channel erosion.

Conveying large volumes of runoff to a centralized location requires the maintenance of a variety of infrastructure. As stormwater infrastructure ages, inspection and maintenance costs become an increasing financial burden on municipalities and homeowner associations. Because of the centralized nature of stormwater infrastructure, poor maintenance may compromise the hydrologic performance of the entire system, as well as public safety.

Conventional stormwater management offers a way to "treat" surface runoff but does little to prevent it from being generated. In many ways, surface runoff is not the problem, but rather a symptom of the site design. Site design governs fundamental hydrologic characteristics, such as the curve number (CN) and time of concentration (Tc), and therefore determines the volume, peak discharge rate, frequency, and duration of surface runoff. In other words, site design directly influences the amount and type—and therefore the cost—of required stormwater management infrastructure before the first storm drain profile is ever drawn.

The LID alternative
Beginning with site design, LID directly addresses many of the hydrologic shortcomings of conventional stormwater management. LID site design minimizes the proportion of precipitation that is converted to surface runoff and maximizes the amount that is lost to ET and infiltration, meaning it preserves the initial abstraction. It can be considered, therefore, a preventative approach to managing surface runoff. Site design becomes a variable that can be manipulated to decrease stormwater management construction, environmental, and maintenance costs, rather than remaining a fixed process in which hydrology is not considered until it is time to calculate the resulting CN and Tc. By minimizing the generation of surface runoff, LID site design helps to reduce non-point source pollution because less runoff is available to convey pollutants.

To treat the surface runoff that still is generated, LID offers an effective alternative to centralized detention. Integrated Management Practices (IMPs) are small-scale devices distributed evenly over a site that reduce the peak rate and volume of runoff and improve water quality. They are a principal component of LID design. For example, rather than enter a storm drain, runoff is captured and retained by IMPs such as bioretention cells or vegetated swales (learn more about IMPs on page 29). The runoff volume is lost to infiltration and ET, processes that also improve water quality through natural filtration. Underdrains and overflow pipes are typically provided to ensure adequate conveyance of runoff from large storms.


This parking lot at the Washington, D.C., Navy Yard was reconstructed in 2001 and two permeable paving strips were installed.The parking lot was graded to drain to the strips.

As mentioned above, most storm events generate less than 1 inch of precipitation, and these cumulatively account for the majority of the annual precipitation volume. Together, LID site design and IMPs excel in significantly reducing the peak rate, volume, and non-point source pollutant loading of surface runoff from these small, frequently occurring storms. By doing so, LID also reduces the annual frequency and duration of surface runoff. IMPs can be sized to capture and infiltrate the entire volume of runoff from such storms. Depending upon site constraints, LID may be used successfully as a stand-alone management strategy for up to the two-year storm.

Of course, LID techniques can be augmented by conventional infrastructure if control of larger storms is required or if an existing site is being redeveloped. In this case, the volume and peak reductions achieved by LID site design and IMPs potentially can lower infrastructure requirements.

Site design guidelines
Conventional site design promotes efficient drainage of surface runoff, but lowers the Tc and increases the CN in the process. LID site design has a different goal: to use non-structural techniques to preserve the pre-development hydrology of the site and receiving waters without impeding the site’s intended function. The benefits of LID site design can be quantified by the resulting Tc and CN. Common LID site design strategies are outlined below.

  • Siting—Constrain the development envelope by clustering roads and buildings, and by minimizing the area that is cleared, graded, and compacted. Where disturbance occurs, restore soil permeability by aerating and amending the soil and revegetating. Avoid placing stormwater facilities in streams and wetlands.
  • Conveyance — The post-development Tc must match the pre-development Tc. Maintain the existing topography as much as possible to preserve flow paths and to prevent runoff from concentrating along a single path. Avoid curbs, gutters, and storm drains in favor of vegetated swales. With swales, use circuitous routes, increase Manning’s n surface roughness (for example, by planting taller grasses or mowing less frequently), and add check dams to increase travel time. Maximize the length of sheet flow through vegetated areas, and use level spreaders to convert concentrated flow to sheet flow and divert it to open space. Preserve natural depressions to promote ET and infiltration.
  • Soils—Preserve Hydrologic Soil Group (HSG) A and B soils as open space or use them for IMP construction to utilize fully the soils’ infiltration potential. Construct impervious surfaces on HSG C and D soils.
  • Vegetation—Preserve natural areas, especially riparian buffers, whenever possible. Remove individual trees only when necessary. Pay special attention to preserving native vegetation. Clustering development will minimize the disturbance to vegetated areas. Revegetate cleared and graded areas.
  • Impervious area — Precipitation that falls on impervious surfaces quickly becomes surface runoff. This relatively large runoff volume accelerates pollutant transport and stream erosion. Impervious areas also prevent infiltration and contribute to the urban heat island effect. Use permeable surfaces instead of impervious surfaces whenever possible. Minimize the required impervious area; for example, reduce street widths, share driveways, and modify roadway configurations. Disconnect impervious areas so that runoff from roof drains, streets, and parking lots flows into IMPs or stable open space instead of storm drains, gutters, or other impervious surfaces. This way, flow from several impervious areas does not accumulate.
  • Pollution prevention—More an aspect of operations and maintenance than site design, pollution prevention techniques identify and isolate potential point sources of pollution, reducing or eliminating their impact on stormwater quality.

LID and urban retrofits
Urban retrofit projects present a series of unique problems and opportunities. Many IMPs can be installed with comparatively little disruption to the existing infrastructure while providing significant water quality and quantity improvements. At the same time, the goals and methods of an LID project often are significantly different in urban settings than in greenfields. The goal of replicating the predevelopment hydrology may not apply because the old hydrologic regime may be impossible to ascertain. In addition, true retention may not be possible for all storms because the opportunity for infiltration may be limited by compacted or polluted soils and by proximity to building foundations. In such cases, infiltration IMPs, such as bioretention cells, can be constructed with impermeable liners and underdrains and will still provide many of the same benefits.

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A sign marks a combined sewer overflow gate on the Anacostia River
in Washington, D.C., under the 11th Street Bridge
.

In urban areas, LID is especially useful for water quality improvements and mitigation of CSOs. Both goals often can be served by one retrofit project. As little as 0.2 inches of rainfall can trigger a CSO event lasting several hours, as well as convey a variety of nonpoint source pollutants from impervious surfaces into storm drains. Increasingly, LID is being recognized as an important tool for reducing the magnitude, duration, and number of CSOs.

For example, Portland, Ore., and Washington, D.C., are using LID techniques as part of a series of CSO-reduction measures. Portland’s Downspout Disconnection Program is a voluntary program that so far has disconnected downspouts in more than 42,000 households, eliminating an estimated 942 million gallons of roof runoff from the combined sewer system. "The voluntary aspect has been very successful. Quite a lot of the homes that can disconnect, do," said Bob Batz, a canvasser for the city who assesses 25 homes a day. Homeowners are eligible for a $53 refund per approved downspout.

In Washington, D.C., the D.C. Water and Sewer Authority commissioned a study to examine the feasibility of LID retrofits at its facilities. The use of LID for CSO control is also an emerging research topic. The Low Impact Development Center is leading a research team that is preparing a report for the Water Environment Research Foundation on the use of decentralized controls for CSO mitigation.

Green roofs provide another way to reduce urban stormwater runoff at the source while involving property owners. "Rooftops are the one major impervious surface we can do something about in the city," observed Dawn Gifford, executive director of D.C. Greenworks, a nonprofit environmental design and community training organization in Washington, D.C. In 2003, Gifford designed and oversaw the installation of one of the first residential green roof projects in the Washington, D.C., area. A 450-square-foot roof patio was fitted with a 3-inch green roof at a cost of about $5,000. Homeowner Nina Garfield has been pleased with the results. "Every time I come up here it’s a little more grown out," Garfield said.


A green roof was installed on Nina Garfield’s 450-square-foot
roof patio in Takoma Park, Md.Total cost was $5,000, excluding arbor,
waterproofing, and timber parapet.

Gifford values LID as a way to invest in communities and to improve water quality, and appreciates its scalability to a wide variety of projects. "The beauty of LID is that it’s decentralized and accessible," she said. "You can be an engineer or you can be Joe Homeowner and there’s something you can do."

Each small-scale, distributed control measure enhances stormwater management, and moves the area’s hydrology and water quality closer to its pre-development state. 

Philip Jones, E.I.T., is a civil engineer at the Low Impact Development Center in Beltsville, Md. He can be reached by e-mail at psjones@lowimpactdevelopment.org or by phone at 301-982-5559. The Low Impact Development Center (www.lowimpactdevelopment. org) is a non-profit 501(c)(3) organization dedicated to research, development, and training for water resource and natural resource protection issues.

Integrating Stormwater Management Practices into the Landscape
Integrated management practices (IMPs) are used as distributed, onsite retention or detention devices.An important point about IMPs is that they integrate stormwater management into the landscape, meaning that nearly all IMPs combine aesthetic, hydrologic, and infrastructure benefits. Because they are integrated into the landscape, IMPs help to reduce the amount of land dedicated solely to stormwater management. Infiltration IMPs facilitate pollutant removal and groundwater recharge, and help maintain stream baseflow.

The costs for installing IMPs depend on a variety of factors and cannot be generalized easily. In some cases, initial construction costs are comparable to conventional infrastructure, while in others, LID construction costs are higher. Lifecycle costs, however, may be lower for LID for several reasons. First, IMP maintenance often is relatively simple and can be performed by property owners rather than local governments or homeowner associations. Second, some stormwater management strategies may employ "hybrid" designs involving a combination of LID site design, IMPs, and conventional infrastructure. In hybrid designs, peak flow rate and volume reductions achieved by decentralized controls reduce the size and scope of conventional conveyance and storage structures, thereby decreasing their construction and long-term maintenance costs. In addition, mitigation measures, such as those mandated by the Clean Water Act, may be reduced or eliminated if LID techniques are used to avoid stormwater impacts to receiving waters. Several common IMPs are explained below.

  • Bioretention cells — Also known as rain gardens, bioretention cells are vegetated depressions that store and infiltrate runoff. Uptake into well-suited plants (those classified as self-sustaining, drought and flood tolerant, and hardy) reduces runoff volume and pollutant concentrations. The soil media is engineered to maximize infiltration and pollutant removal.
  • Green roofs—The soil and plants in green roofs detain, absorb, and filter precipitation, reducing the annual volume of roof runoff. Extensive green roofs contain several inches of soil and hardy, self-sustaining plants, and may be added to many existing roofs. Intensive green roofs are full-fledged rooftop gardens.
  • Permeable pavements—Permeable asphalt, concrete, and paving blocks allow runoff to infiltrate into the underlying subbase and soil. Underdrains may be provided if drainage problems are anticipated.
  • Tree box filters — Located below grade, tree box filters provide "bioretention in a box" along road curbs, integrating the water quality benefits of bioretention into ultra-urban settings. Located upstream of existing curb inlets, underdrains ensure adequate drainage.
  • Vegetated swales — Shallow vegetated swales simultaneously serve as conveyance, infiltration, and storage devices. They may be designed as wet or dry features, depending upon the degree of runoff retention.
  • Filter strips—Located adjacent to a runoff source, vegetated filter strips intercept and slow runoff, facilitating removal of a variety of suspended pollutants.They are effective pretreatment devices for other IMPs.
  • Dry wells/infiltration trenches — Dry wells are aggregate-filled pits that promote infiltration into the surrounding soil. Infiltration trenches are similar in function, but alternately may be filled with sand or bioretention soil.
  • Rain barrels or cisterns—These devices retain rainwater indefinitely for uses such as landscaping and potable water applications. Rain barrels often are easy to add to an existing downspout.
  • Soil amending—This non-structural practice can increase soil permeability dramatically. Amending with compost or lime reduces runoff volume and pollutant transport, and improves plant viability.


Illustration By: D.C. Greenworks

 
           Photo by Emily Ayers

How effective is LID?
LID has become a hot research topic in the stormwater management community in recent years. Currently, municipalities and universities are conducting numerous monitoring studies. Following are partial results from studies on four LID practices:

  • At the University of Washington, Derek Booth and Benjamin Brattebo conducted a six-year study of four permeable pavements and a conventional asphalt surface used for weekday parking. The permeable surfaces infiltrated "virtually all precipitation" and surface runoff was "insignificant." The quality of infiltrated water from the permeable pavements was compared to surface runoff from the asphalt. Copper concentrations in the infiltrated runoff were 83 percent to 89 percent lower than in the surface runoff, zinc concentration reductions were 39 percent to 69 percent, and motor oil concentrations were reduced to below detection limits.
  • David Beattie of Pennsylvania State University reports that an extensive green roof with a 4-inch soil layer retained 0.36 inches of rainfall from a total of 1.1 inches that fell during a 30-hour period; a conventional roof retained only 0.13 inches.The peak discharge rate from the green roof was 0.011 inches per 5 minutes, compared with 0.041 inches per 5 minutes from the conventional roof.
  • A study of compost amendments by Tom Glanville and Tom Richard at Iowa State University compared the performance of three types of compost blankets to topsoil and compacted subsoil. In a 30-minute rainfall with an intensity of 4 inches per hour, the mean runoff depth from the unvegetated compost test plots ranged from 0.01 mm to 0.13 mm, compared to 26.2 mm for compacted subsoil and 15.5 mm for topsoil.This volume reduction, combined with the compost blankets’ erosion resistance, resulted in substantial decreases in the mass of soluble and adsorbed metals and nutrients in the runoff.
  • Allen Davis at the University of Maryland investigated metal removal in two existing bioretention cells. He found that when 9.7 inches of synthetic runoff was applied to 7- x 7-foot portions of the cells for six hours, copper, lead, and zinc removal was more than 95 percent in one cell and 43 percent, 70 percent, and 64 percent, respectively, in the other. He concluded that "metal accumulations in bioretention should not be a significant issue for many years."

 A list of LID reference materials is available at
http://www.lowimpactdevelopment.org/cenews.


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