Low Impact Development

In natural environments such as forests, rain is intercepted by trees and other vegetation as it falls to the ground. After passing through the forest canopy and bushes, rainwater is absorbed in the soil or is held in small, temporary ponds where it will eventually evaporate into the atmosphere. Except in the case of flash flooding, a relatively small portion of the rainwater falling in forests flows into surface streams.

However, significant runoff develops when trees and other vegetation are cleared, soil is scraped off or eroded, and natural depressions in the ground are graded to make a site uniform for new construction.

When buildings are erected and roads and parking lots are paved with nonabsorbent materials, rainfall has nowhere to go. Instead of soaking into the ground or evaporating from temporary ponds, rain falling on impervious surfaces quickly runs off. As it travels, water gathers force, debris, and pollutants. It flows across roads, sidewalks, and parking lots, picking up spilled oil, detergents, solvents, road salts, pesticides, and fertilizer and depositing them in small streams and rivers. The increased volume and velocity from the urban runoff resulting from thunderstorms or rapid snowmelt can cause stream banks to erode, carve new channels, and choke waterways with sediment.

Flooding can be a serious consequence of urban runoff. Sometimes property damage caused by urban runoff occurs near the point of origination. More frequently, flooding and water pollution resulting from runoff caused by urban development is delivered to communities further downstream.

EPA Promotes a Better Way

In December 2007, the Environmental Protection Agency (EPA) published Reducing Stormwater Costs through Low Impact Development (LID) Strategies and Practices. This publication explains how Low Impact Development (LID) mitigates the impacts of urban stormwater erosion, pollution, and flooding by mimicking the way rainfall is absorbed and runoff is slowed and filtered in a healthy natural setting.

“LID comprises a set of site design approaches and small-scale stormwater management practices that promote the use of natural systems for infiltration, evapotranspiration, and reuse of rainwater,” explains the report. “These practices can effectively remove nutrients, pathogens, and metals from stormwater, and they reduce the volume and intensity of stormwater flows. LID techniques manage water and water pollutants at the source and thereby prevent or reduce the impact of development on rivers, streams, lakes, coastal waters, and ground water.”

Managing Runoff, Naturally

Instead of removing stormwater as quickly as possible and managing it in large facilities at the bottom of drainage areas (as many older drainage systems were designed to do), LID stormwater management works by installing smaller landscape features onsite to mitigate the volume and rate of runoff while also removing pollutants. LID landscaping features rain gardens, grassed swales, cisterns, rain barrels, permeable pavements, and green roofs.

Although LID is typically used to maintain the predevelopment hydrology of a site, it can also be used to retrofit existing developments to reduce runoff volume, speed, and pollutants.

LID Landscape Features

LID employs a number of strategies mimicking the natural slowing and filtering of rainwater runoff in an undeveloped site. Rain gardens are shallow, low-lying places in residential or commercial areas that have been planted (often with native vegetation) to manage runoff from nearby impervious surfaces by slowing it before it enters the groundwater system, filtering out pollutants, and absorbing some portion of the total rainfall. Plants used in rain gardens allow water vapor to return to the atmosphere through evapotranspiration.

Grassed swales are shallow, open channels constructed along roads or parking lots and planted with flood-tolerant and erosion-resistant plants. Grassed swales are designed to slow runoff and sometimes incorporate small check-dams, which are low enough to permit stormwater to continue to flow at its peak but encourage temporary ponding when volumes decrease. Ponding filters out pollutants before groundwater is absorbed.

Cisterns and rain barrels are aboveground, temporary storage containers designed to slow and hold rainwater, usually from the roofs of buildings. Downspouts from eaves or gutters direct the flow of rainwater to the container. Later, retained water can be reused for irrigating gardens or lawns.

Permeable pavement is porous material used to cover low-traffic areas such as residential driveways, alleys, walkways and entryways, terraces, and stalls in parking areas. Porous forms of asphalt and concrete allow water to percolate into the soil beneath while filtering out pollutants. Paving blocks also can be used to produce a porous parking or walking surface.

Green roofs are protected with some form of waterproofing, partially or completely covered with soil, and planted with vegetation. As much as 75 percent of stormwater can be retained in the soil and vegetation on a green roof before it is slowly released back into the atmosphere through condensation and transpiration.

Case Studies Show LID Savings

The EPA report summarizes 17 case studies of developments in the United States and Canada where LID practices were used. In most cases, implementing well-chosen LID practices saved money—total capital savings ranged from 15 to 80 percent, with only a few exceptions—for developers, property owners, and communities, while protecting and restoring water quality. EPA anticipates that, as LID practices gain wider use, they will become even less expensive.

While the EPA report focuses on the cost reductions and savings achievable through the use of LID practices, it also describes the many amenities and associated economic benefits communities can experience when LID features are installed. "These include improved habitat, improved aesthetics, expanded recreational opportunities, increased property values due to the desirability of the lots and their proximity to open space, increased total number of units developed, increased marketing potential, and faster sales,” says the EPA report.

Mitigation Best Practice in Kane County, Illinois

The Mill Creek subdivision in the town of Geneva, Illinois, is a 1,500-acre, mixed-use community built as a conservation-site design development. It uses open swales to channel and treat stormwater, and the subdivision has a lower percentage of impervious surfaces than conventional developments.

When compared with conventional development, the conservation-site design techniques used at Mill Creek saved approximately $3,411 per lot. Nearly 70 percent of these savings resulted from reduced costs for stormwater management, and 28 percent of the savings were found in reduced costs for site preparation.

Mill Creek is one of 17 case studies of LID costs and benefits featured in the December 2007 report released by the Nonpoint Source Control Branch of the U.S. Environmental Protection Agency.

EPA Recommendations

According to the EPA, Reducing Stormwater Costs through Low Impact Development (LID) Strategies and Practices can serve as a primer for low-impact site design and supply background information about the benefits of LID. The report provides developers and planners interested in implementing or promoting LID projects in the community with a breakdown of site development costs for traditional and low impact scenarios, which can be useful when presenting new designs to stakeholder groups who are unfamiliar with LID costs and benefits. More information is available at the EPA Green Infrastructure website and EPA LID website.

Lynd Morris began working with the NFIP Bureau and Statistical Agent in 1983. She has been the primary writer and production coordinator for Watermark since 1998.