Structural BMPs
There are several types of structures designated as BMPs in modern stormwater management. They each support one or more of the system’s management goals.
Porous Pavement
Porous surfaces, such as interlocking tiles and bricks, allow precipitation to pass through otherwise hard surfaces and infiltrate the soil directly where it falls. This prevents stormwater from traveling over extensive surfaces and picking up a myriad of pollutants, as well as providing effective erosion control.
Infiltration Basins
Infiltration basins are vegetated depressions in the ground which are designed to store surface runoff and allow it to gradually infiltrate the soil. These basins remain dry except in periods of heavy rainfall. Infiltration basins are effective in reducing runoff within a drainage area and are simple and cost-effective to construct. Ultimately, the water in an infiltration basin flows through the soil and recharges groundwater.
Wet Ponds
As the name suggests, wet ponds always contain water, but when new stormwater runoff flows in, the existing (and relatively clean) pond water is gradually released into local waterways. This detains arriving stormwater in the pond, where suspended pollutants can slowly settle to the bottom. If the detention time is adequate, the former stormwater - now pond water - will carry fewer pollutants. Wet ponds are effective at reducing silt and sand loads, as well as certain other pollutants, but they don’t provide much capacity for flood prevention.
Extended Detention Ponds
Extended Detention Ponds are designed to accept and store stormwater instead of allowing it to be diverted into a local waterway without any treatment. Like wet ponds, these are intended to hold water in place until pollutants settle to the bottom, at which point the water is released slowly into the river. Extended detention ponds are generally dry between storm events, maximizing the storage capacity and their ability to minimize flooding throughout the system.
Water Quality Inlets (WQI)
Specialized stormwater inlets known as WQI are designed to trap and remove a variety of pollutants commonly associated with stormwater. Large sediments, suspended solids, the pollutants they carry, and free oil (as opposed to emulsified or dissolved oil) from parking lots can be removed using proprietary devices or specifically engineered structures. WQIs can be effective in reducing the pollutant load in stormwater but are generally used only for pretreatment.
Organic and Sand Filters
Rather than live, vegetation-based treatment, organic and sand filters are designed to capture and retain pollutants in stormwater using mechanical means. These tools are generally used in smaller drainage areas and the systems may include separate chambers for capturing large debris, followed by a filtering medium such as sand, leaf compost, or peat. Since these filtering structures are not as attractive as vegetated areas, for example, they’re more frequently used in commercial or industrial areas where visual appeal is not as high a priority. Media filters such as peat and leaf compost can include an underdrain, which makes them effective for handling PSHs (potential stormwater hotspots). These are locations characterized by the possibility of generating high volumes of polluted runoff or highly concentrated runoff. In these cases, an underdrain is necessary to divert stormwater for further treatment, since simple organic filtration is likely to be inadequate.
Curb and Gutter Elimination
Road features like curbs and gutters are used to collect and quickly move stormwater runoff to a nearby drain, where it travels underground and is often discharged into local waterways without having had the opportunity for soil infiltration or pollutant removal. When curbs and gutters are removed, runoff can be directed to areas where pervious surfaces or vegetated areas permit infiltration and pollutant removal. Swales and rain gardens are commonly used to direct and absorb runoff, in the place of curbs.
Vegetative BMPs
BMPs that take advantage of the natural filtering activity of plants provide a wide variety of options for minimizing, reusing and recycling stormwater. Grassy swales and ditches that run alongside roads and highways, or attractive rain gardens that line residential streets and parking lots are inexpensive to install and increase the appeal of any urban area. These BMPs help reduce downstream runoff by promoting soil absorption and providing storage for partially filtered stormwater to be reused.
Natural Areas
Existing or restored natural areas offer many benefits for successful stormwater management and are an important part of an overall movement toward sustainable, green infrastructure. In urban spaces, areas planted with native species are credited with mitigating the effect of urban heat islands, improving air quality, and supporting a healthy ecosystem by providing habitat and food sources for local wildlife.
Green Roofs
Also known as rooftop gardens, living roofs, and eco-roofs, green roofs are specially designed roofs intended to support vegetation. There are different types of green roofs, but when the choice is made, energy savings is usually the primary goal. Still, green roofs are very effective at reducing stormwater runoff and can provide additional savings in reduced stormwater fees. Even stormwater that is not directly absorbed by the plants still benefits from vegetative filtration and can be collected for reuses appropriate for graywater resources.
Bioretention Areas
Stormwater tools designed for bioretention include rain gardens, infiltration basins, and even grass-covered ditches. These areas allow stormwater to gather and infiltrate the soil over time. In contrast to concrete or stone lined ditches, the vegetation in a bioretention area naturally filters out sediment, sand, and many pollutants, leaving higher quality water to pass into the soil.
Swales
Swales, at their most basic form, are vegetated drainage paths and channels designed to transport water. They’re most effective when employed to handle low volume runoff from relatively small drainage areas. In comparison to underground storm sewers or open concrete channels, they reduce the speed of stormwater runoff, encourage infiltration, and filter out pollutants.
Constructed Wetlands
Also known as stormwater wetlands, constructive wetlands fall under both structural and vegetative BMPs and are a high-value, relatively inexpensive stormwater control. Natural wetlands such as marshes, swamps and bogs are areas where water typically covers the ground at least part of the year and are full of specialized plants that have the ability to remove pollutants through physical, chemical, and biological processes. Constructed wetlands are designed to closely mimic this activity. Essentially, they’re shallow depressions filled with vegetation, built with a level bottom that permits the flow of water to be controlled. Depending on their design, constructed wetlands can be an effective flood control tool, as well as providing high levels of pollutant removal. In fact, constructed wetlands rank among the most effective stormwater controls that can be incorporated into already developed areas. Despite not being as biologically diverse as natural wetlands, they also provide valuable wildlife habitats and food sources.
Riparian Buffers
A riparian buffer is an area adjacent to surface water features such as streams, lakes and wetlands that support a variety of plant life, ranging from trees to shrubs and grasses, to perennial and semi aquatic plants. These plants provide many benefits, including consuming and filtering out excessive nutrients as stormwater flows across the ground toward surface waters. Plants that occupy the riparian zone remove pollutants and sediment from incoming water, very much like vegetated swales. In addition, riparian zones support plants with prolific roots that help anchor the soil and protect stream banks from erosion, especially during storm events. Riparian buffers can absorb water and then release it slowly, which mitigates river surges during stormwater events.
