Urban/Suburban Nonpoint Source Best Management Practices
There are many best management practices (BMPs) that are particularly well suiting to urban environments and addressing sources of nonpoint source pollution. Most urban BMPs are classified under Low Impact Development (LID). LID is a design tool that mimics the natural hydrology of a site by installing landscaping that allows storm water to infiltrate into ground water. LID is popular in urban areas where impervious surfaces are prevalent and opportunities to infiltrate storm water are not always apparent. While some of the more common types of BMPs are discussed here, IDEM Section 319 grantees should also consult IDEM’s Urban Guidance for information on grant-eligible BMPs. The grantee guidance also details how pollutant load reductions are calculated for several popular urban BMPs.
Curb cuts are spaces cut into parking lot curbs to allow storm water to flow onto a pervious surface. Considering the large amount of parking lots in urban and suburban areas, curb cuts are a good option for reducing storm water run-off. They can be especially valuable if combined with the parking islands that are in many larger lots.
Also known as rooftop gardens, green roofs occur when plants and shrubs are planted on top of buildings. Green roofs lower the temperature of the building, filter pollution and reduce the amount of run-off from rain. They can also reduce the heat island effect in cities. A waterproof, root-safe membrane, drainage system and lightweight plants protect the roof from damage.
Porous pavement is a term that applies to any surfacing material that allows storm water to move through it. Porous pavement reduces rainwater run-off and excess flow through local streams. There are many different types of porous pavement, but they all have the same general principles: a layer of porous material, such as like sand, is overlaid by a porous matrix or plantable grid designed to bear the weight of vehicles and the extremes of weather.
There are several types of permeable surfaces available, depending on the needs of the area. In some cases, paving stones are set slightly apart, allowing water to flow through the crushed rock or plant material used between. In areas where traffic would not support gaps in paving material, concrete and pavement that allows for water movement has been developed.
A rain barrel is a container, usually 40-60 gallons, which collects rainwater that falls on a roof. Downspouts direct the water into the barrel during rain and a hose attached to the bottom of the barrel can be used to water lawns and gardens after the rain. Some rain barrels are commercially available at hardware and home improvement stores, but they are easy to make at home, using online instructions.
A rain garden is a planted depression that helps rainwater run-off from impervious urban areas, such as roofs, driveways, walkways and compacted lawn areas, absorb into the ground. This reduces rain run-off by diverting rainfall away from storm drains. Native plant species with long root systems are recommended for rain gardens, as they don’t require watering.
Storm Water Pond Modification
Storm water ponds, or retention ponds, are common in urban and suburban areas. While these ponds allow quick and efficient movement of storm water off of roads, parking lots and other impervious surfaces, they don’t help to improve water quality. Storm water washes nonpoint source pollution into the ponds and in some cases can release water so fast that local water bodies flood and erode.
Storm water ponds can be modified to improve how they work. Altering a pond’s riser can change how it releases water. Additionally, wetland plants can be planted in strips along the banks of storm water ponds to maximize nonpoint source pollution filtration and absorb some storm water.
A swale is very similar to a rain garden. Both are depressions where storm water is allowed to infiltrate deep into the ground. Swales are usually larger than rain gardens, able to treat greater amounts of storm water and linear in shape. A swale’s benefit to water quality can be increased with soil amendments and planting native vegetation.