This publication is intended to provide a summary of common terms and definitions used in urban stormwater management. These terms and definitions are used throughout the Urban Stormwater Best Management Practices (BMP) fact sheet series 1-15, Virginia Cooperative Extension (VCE) publications 426-120 through 426-134. A companion glossary is included with each fact sheet; this document provides a compilation of the terms used in the series.

Rooftop disconnection (RD) is one of the simplest means of reducing stormwater from residential lots. RD takes roof runoff that has been collected in gutters and piped directly to streets, storm drains, and streams and redirects it away from impervious surfaces to landscaped areas (figure 1). Rooftop disconnection is a very sustainable best management practice (BMP) because it controls pollutants in runoff near their source. Redirected runoff from downspouts is infiltrated, filtered, treated, or reused prior to draining into a stormwater conveyance system.

Sheet flow to open space (SOS) is a group of best management practices (BMPs) designed to disperse concentrated runoff to sheet flow into filter strips or a riparian buffer. An SOS reduces runoff volume and associated sediment and nutrients that are carried with it (see figure 1). It is used as a stormwater treatment practice in both urban and rural areas. This practice is often used after another treatment practice to disperse or eliminate runoff. In a few cases, an SOS can be used as a pretreatment to remove small amounts of sediment via a vegetated filter strip — prior to a bioretention device, for example.

Grass channels (GCs) are wide, gently sloping, open channels with grass sides used as a stormwater conveyance system (see figure 1). Grass channels are similar to ditches; however, their side slopes are much more gradual. GCs provide treatment via filtering through vegetation. When compared with traditional curb and gutter, or inlets and pipes, which remove no pollutants, GCs may provide a modest amount of runoff reduction and pollutant removal. The extent of this reduction depends on the underlying soil characteristics, slope, and flow velocity. At higher velocities, stormwater is only conveyed and is not treated. Unlike dry swales, (VCE publication 426-129), GCs do not include a soil media and/or specific storage volume.

Soil restoration (SR) is the technique of enhancing compacted soils to improve their porosity and nutrient retention. It includes biological (worms) and mechanical aeration, mechanical loosening (tilling), planting dense vegetation, and applying soil amendments. Soil amendments involve the spreading and mixing of mature compost into disturbed and compacted urban soils.

A vegetated roof (VR) is a best management practice (BMP) that reduces stormwater runoff and pollution. Vegetation and media create a permeable system on a previously impervious surface. The VR intercepts rainfall and filters runoff while reducing the volume and velocity. Vegetated roofs consist of a waterproofing barrier, drainage system, and engineered growing media. There are two types of VRs: intensive and extensive. Intensive vegetated roofs are deeper and heavier, while extensive vegetated roofs are shallower, lighter, and more common. The type of VR determines the amount of maintenance necessary to maintain the vegetation.

Rainwater harvesting (RWH), also known as rainwater harvesting systems or cisterns, are devices that intercept, divert, store, and release collected roof runoff from rainfall for later use as an alternative water supply. RWH can also be designed to provide runoff reduction benefits. Therefore, it is classified as a best management practice (BMP) for treatment of urban stormwater. Because of its dual purpose and benefit, RWH is often classified as a sustainable urban BMP.

Permeable pavement (PP) is a modified form of asphalt or concrete with a top layer that is pervious to water due to voids intentionally created during mixing. PPs include pervious concrete, porous asphalt, and interlocking concrete pavers. These materials are used as stormwater treatment practices in urban areas. They are used in place of traditionally impervious surfaces to allow infiltration and storage, thus reducing runoff.

Infiltration practices provide temporary surface and/or subsurface storage, allowing infiltration of runoff into soils. In practice, an excavated trench is usually filled with gravel or stone media, where runoff is stored in pore spaces or voids between the stones. These systems can reduce significant quantities of stormwater by enhancing infiltration, as well as provide filtering and adsorption of pollutants within the stone media and soils. Infiltration practices are part of a group of stormwater treatment practices, also known as best management practices (BMPs)

A bioretention cell, or rain garden, is a best management practice (BMP) designed to treat stormwater runoff from roofs, driveways, walkways, or lawns. They are a shallow, landscaped depression that receives and treats polluted stormwater with the goal of discharging water of a quality and quantity similar to that of a forested watershed (figure 1).

A dry swale (DS) is a shallow, gently sloping channel with broad, vegetated, side slopes. Water flow is slowed by a series of check dams (see figure 1). A DS provides temporary storage, filtration, and infiltration of stormwater runoff. Dry swales function similarly to bioretention, and are comparable to wet swales; however, unlike a wet swale, a DS should remain dry during periods of no rainfall. A DS is an engineered best management practice (BMP) that is designed to reduce pollution through runoff reduction and pollutant removal and is part of a site’s stormwater treatment practice.

A wet swale (WS) is an engineered, best management practice (BMP) arranged in a straight line that is designed to reduce stormwater pollution. A WS consists of a shallow, gently sloping channel with broad, vegetated, side slopes and slow flows. Wet swales typically stay wet because the bottom of the swale is below the water table. This is done to encourage the growth of wetland vegetation, providing water quality treatment similar to a natural wetland. This stormwater treatment practice also functions as part of the stormwater conveyance system. Wet swales have a relatively low capital cost; however, maintenance can be is intensive and expensive when compared to other BMPs.

A stormwater filtering practice (FP) treats stormwater runoff by passing it through an engineered filter media consisting of either sand, gravel, organic matter, and/ or a proprietary manufactured product, collecting it in an underdrain, and then discharging the effluent to a stormwater conveyance system. FPs are stormwater treatment practices that are often obtained from the marketplace due to unique proprietary technologies.

Constructed wetlands are a series of ponds with varying depths that treat stormwater using wetland processes. In terms of biological activity, wetlands are extremely productive; and thus constructed wetlands can provide significant water quality treatment to urban runoff. This fact sheet describes these benefits, and provides guidance on their design and limitations.

Wet ponds are ponds designed to retain water through storage. They provide treatment through settling and biological uptake. They can also attenuate peak flows and provide flood and streambank protection. This fact sheet describes wet ponds and their benefits and limitations.

Extended detention ponds (EDs) are dry detention ponds that provide 12 to 24 hours of runoff storage during peak runoff events (see figure 1). Releases from the ED ponds are controlled by an outlet structure. During a storm event, as the discharge restriction is reached, water backs up into the ED pond. The pool slows flow velocities and enables particulate pollutants to settle. Peak flows are also reduced. ED ponds have the lowest overall pollutant- removal rate of any stormwater treatment option, so they are often combined with other upstream, lowimpact development (LID) practices to better maximize pollutant-removal rates. Due to their placement at the exit point of the watershed, ED is often the last opportunity to treat stormwater before it is discharged to a stream. Because of its low treatment performance, an ED should be viewed as the treatment option of last resort.

An overview of rain barrels, one of the most common residential stormwater management practices.

An overview of permeable paving, one of the most common residential stormwater management practices.

An overview of grass swales, one of the most common residential stormwater management practices.

An overview of rain gardens, one of the most common residential stormwater management practices.

An overview of buffers, one of the most common residential stormwater management practices.

Overview of rooftop redirect, one of the most common residential stormwater management practices.