What is Low Impact Development?
Low Impact Development is a way to protect the environment and reduce construction costs at the same time. Conventional design and construction methods generally use expensive systems of curbs, gutters, pipes, and ponds to collect and treat runoff. In contrast, the Low Impact Development approach uses a more decentralized approach; the idea is to reduce the amount of runoff and treat it closer to the source using smaller, less expensive techniques. Basic design strategies seek to reduce the extent of rooftops and paved areas, use infiltration techniques such as bioretention areas and grass swales, and design the site to protect natural features that improve water quality. Some important techniques are listed below. Click on the links to access a fact sheet about each technique.
- Careful site design
- Narrow roadways
- Smaller parking areas
- Bioretention areas (also known as Rain Gardens)
- Vegetated swales
- Grassed filter strips
- Infiltration trenches and dry wells
- Rain barrels and cisterns
- Green roofs
Does it take up a lot of land?
In many cases, Low Impact Development techniques can reduce the amount of land devoted to stormwater management, because smaller detention ponds are necessary. Green roofs take up no extra space at all, and techniques such as bioretention areas, grass filter strips, and swales can also help to fulfill site landscaping/open space requirements. Narrower roadways and smaller parking lots actually conserve land.
What about standing water where mosquitoes can breed?
All Low Impact Development techniques are designed to prevent standing water and pest problems. Bioretention areas should drain within 72 hours after a rainstorm, and should be designed with an overflow outlet to prevent flooding. On sites with tight soils, bioretention and infiltration areas can be designed with perforated underdrains to allow rapid drainage. Cisterns and rain barrels should all be fitted with screens to prevent mosquito breeding.
Won’t narrower roadways impede fire trucks and ambulances?
Narrow streets in low-traffic residential areas are generally not a problem for emergency access, especially since on-street parking is rare (most new homes have off-street parking.) Conventional street widths are based on a scenario in which two fire trucks going in opposite directions need to pass each other at full speed with cars parked on both sides of the street. Alternative roadway cross-sections can provide sufficient room for passage of emergency vehicles, with the recognition that oncoming traffic may need to pull into a parking lane. According to the Center for Watershed Protection, the U.S. Fire Administration and the Massachusetts State Fire Marshal approves of street widths as narrow as 18 feet.
Won’t flooding be a problem without curbs and gutters?
Properly designed Low Impact Development techniques will convey water away from buildings and paved areas as quickly as conventional stormwater systems. Because LID emphasizes infiltration and reduction of impervious surfaces, there will actually be less runoff and a lower potential for flooding downstream. With a decentralized approach, one component (such as one bioretention cell) might fail without compromising the integrity of the entire system.
Does Low Impact Development cost more than a conventional approach?
Some Low Impact Development techniques can cost more than conventional approaches, but overall LID is cost-competitive because it can reduce the size of stormwater pipes and downstream ponds, reduce the amount spent on paving, enhance site aesthetics and value. The life-cycle cost of green roofs is lower than conventional roofs due to a much longer life span and considerable savings on heating and cooling costs.
Does Low Impact Development involve expensive and complicated maintenance?
All stormwater management structures require some sort of ongoing maintenance, and LID techniques are no different. However, many of the maintenance activities associated with LID strategies are quite simple and can be conducted by property owners or landscaping crews, with proper direction. Examples include mulching bioretention areas, reseeding or revegetating swales and rain gardens, picking up trash, removing accumulated sediment and dead vegetation, and monitoring performance of the system. Rarely, more complex maintenance is necessary and will involve professionals and heavy equipment; excavation and rehabilitation of bioretention areas or infiltration trenches is one such activity.
Do these techniques work in cold climates?
In Massachusetts, all stormwater management systems need to be designed to account for freezing conditions, snow plowing, and runoff from springtime snow melt. LID techniques should be selected accordingly, with design modifications (such as those outlined in the publication Stormwater BMP Design Supplement for Cold Climates) to improve performance during the winter months. Many techniques such as bioretention areas, filter strips, and swales can be used as snow storage areas during the winter months, and low-impact site and roadway design can reduce the volume of snow that needs to be plowed. Adequate setbacks and design standards will reduce the risk of frost heave.