2-year storm
A storm that has a 50% chance of occurring in any single year (PA Stormwater BMP Manual)

100-year storm
A storm that has a 1% chance of occurring in any single year (PA Stormwater BMP Manual)

Aquatic Bench
A platform of soil in a pond that extends inward from the shoreline and has a maximum depth of eighteen inches below the normal pool water surface elevation; benches can be used to promote the growth of aquatic plants (

A layer of permeable rock that holds water

A large area of lower elevation than the surrounding areas

Benthic Invertebrates
Insects, mollusks, crustaceans, worms and other organisms without a backbone that live in, on or near the bottom of lakes, streams and oceans

Best Management Practices (BMPs)
A method, activity, maintenance procedure, or other management practice for reducing the amount of pollution entering a body of water

Bioretention Basins
Treating stormwater by capturing it in a mulch-lined basin or bed planted with grasses and shrubs; captured runoff pools on the surface, which allows filtering and settling of suspended solids and sediment at the mulch layer, prior to entering the plant/soil/microbe complex media of the root system for infiltration and pollutant removal. Properly designed bioretention techniques mimic natural ecosystems through species diversity, density and distribution of vegetation, and the use of native species, resulting in a system that is resistant to insects, disease, pollution, and climatic stresses. See also rain garden. (PA Stormwater Manual Sec. 6.4.5)

Capture and Reuse System
Runoff storage technique designed to collect runoff, hold it for an amount of time, and then reuse it, for irrigation or other nonpotable uses, such as for flushing toilets (Stormwater BMP Manual Sec. 6.5.2)

Check Dam
A 6 to 12 inch tall dam made of natural wood, concrete, or stone that is used to slow or “check” the speed of runoff to promote settling of pollutants and infiltration in a swale (Stormwater BMP Manual Sec. 6.4.8)

Combined Sewer System
A sewer system that has separate channels for carrying wastewater and stormwater; large quantities of runoff can cause the system to overflow, which results in raw sewage being discharged to local waterways (Wikipedia)

A line drawn on a map connecting points of equal elevation

Detention Basin
A traditional stormwater practice of capturing and holding runoff in a basin; runoff is held in the basin to reduce the peak flow of runoff entering a stream during a storm; an outlet in the basin is sized to allow runoff to drain out of the basin over time

Digital Elevation Model (DEM)
A representation of the topography of the earth in digital format, that is, by coordinates and numerical descriptions of altitude

Dry Pond
Also called detention basins or extended detention ponds; temporary runoff storage basins formed from a natural depression in the ground or by excavating soil; basins are constructed with an outlet designed to release a specific amount of water, which reduces the peak amount of water that is released from the basin; prior to its release, water is held or “detained” in the basin, which allows sediment particles and pollutants to settle out and remain in the basin (PA Stormwater BMP Manual  Sec. 6.6.3)

Dry Well
Also called a seepage pit, is a subsurface storage facility that temporarily stores and infiltrates stormwater runoff from the roofs of structures (PA Stormwater BMP Manual Sec. 6.4.6)

The release of water from vegetation into the atmosphere

Fecal Coliform
Microscopic, single-celled organisms found in the wastes of warm-blooded animals

First Flush
The initial wave of runoff, which also contains the highest concentration of pollutants (PA Stormwater BMP Manual Sec. 2)

The relatively level area of land bordering a stream channel and inundated during moderate to severe floods

French Drain
A trench that is covered with gravel or rock for the purpose of collecting runoff and directing it away from a building’s foundation; French drains may be designed with a perforated pipe running along the base of the trench to help with venting water from above (Wikipedia)

Gabion Wall
A retaining wall made from filling steel baskets with rock that are placed along steep-sloped areas to prevent erosion; the voids between rocks allow for drainage (Wikipedia)

Geographic Information Systems (GIS)
A computer system for capturing, storing, checking, integrating, manipulating, analyzing and displaying data related to positions on the earth’s surface

Geotextile Fabric
Non-woven cloth-like material used to form a barrier to prevent soil from clogging BMP storage reservoirs and void spaces underground

Green Roof
A green roof is a conventional roof that is covered with a layer of vegetation;  green roofs serve several purposes for a building, such as absorbing rainwater, providing insulation, creating a habitat for wildlife, and helping to lower urban air temperatures  (

Water beneath the surface of the earth which saturates the pores and fractures of sand, gravel, and rock formations

Groundwater Recharge
The flow of water from the surface into deeper layers, which increases the groundwater supply; water continues to move downward until it reaches the groundwater table and replenishes an aquifer (PA Stormwater BMP Manual Sec. 2.2.3)

Growing Greener
The PA Department of Environmental Protection’s Growing Greener Program, enacted in 1999, provides the funding necessary to preserve farmland and protect open space; eliminate the maintenance backlog in state parks; clean up abandoned mines and restore watersheds; and provide new and upgraded water and sewer systems across the Commonwealth

The science of dealing with the properties, distribution, and circulation of water on the surface of the land, in the soil and underlying rocks, and in the atmosphere

Hydrologic Modeling
The use of physical or mathematical techniques to simulate the hydrologic cycle and its effects on a watershed

Impervious Surface
A hard surface area that either prevents or retards the entry of water into the soil mantle or causes water to run off the surface in greater quantities or at an increased rate of flow. Common impervious surfaces include, but are not limited to, rooftops, walkways, patios, driveways, parking lots, storage areas, concrete or asphalt paving, and gravel roads

Movement of water, typically downward, into subsoil layers or porous rock

Infiltration Trench
A stone-lined trench that contains a perforated pipe slightly angled downgrade for the purpose of conveying runoff; runoff seeps into the trench from the surface and flows through the holes in the pipe, into the stone retaining area, where it infiltrates into the ground; the surface layer of an infiltration trench can be grass or gravel (PA Stormwater BMP Manual Sec. 6.4.4)

Invasive Species
Non-indigenous plant or animal species that adversely affect the habitats they invade economically, environmentally or ecologically; characteristics of invasive species include: fast growth, rapid reproduction, high dispersal ability, the ability to alter one’s growth form to suit current conditions, tolerance of a wide range of environmental conditions, and the ability to live off of a wide range of food types

LEED Certification
An acronym for Leadership in Energy and Environmental Design, LEED is the rating system for green building design; the LEED standard focuses on increasing the efficiency of resource use – energy, water, and materials – while reducing building impacts on human health and the environment during the building’s lifecycle, through better siting, design, construction, operation, maintenance, and removal (Wikipedia)

Level Spreader
A berm, curb, or gravel-lined fringe area that reduces erosion by slowing the speed of runoff as runoff flows from pavement to a permeable surface; as runoff slows, it spreads out as sheetflow, which is less erosive than the force of a concentrated flow (PA Stormwater BMP Manual Sec. 6.4.9)

Animals large enough to be seen with the naked eye

Meadow conversion
Planting tall grasses in place of a turf lawn reduces the speed of runoff, which allows the runoff to infiltrate into the subsoil; meadow grasses also provide habitat for wildlife and require less maintenance than turf grass

Native Vegetation
Plants that have grown in a region prior to the arrival of the first European settlers; since these plants are adapted to the soils, pests, and other local conditions, they tend to require less supplemental watering, fertilizer, and other maintenance needs once they are established (

Naturalized Basin
An excavated shallow surface depression planted with specially selected native vegetation to treat and capture runoff

Naturalized/Vegetated Swale
A broad, shallow channel that is densely planted with trees, shrubs or grasses; the channel is designed to slow runoff to allow sediment to settle, promote infiltration, and filter pollutants and sediments in the process of conveying runoff (PA Stormwater Manual 6.4.8)

No-Mow Zone
A maintenance-free area established along a stream or waterway where grasses and other vegetation grow freely; established vegetation prevents streambank erosion and provides habitat for birds, frogs, and other small wildlife; no-mow zones require periodic maintenance to prevent weeds and invasive species  (

Non-Point Source (NPS)
A surface water pollution source that is distributed over an area rather than limited to an identifiable point; pollutants in runoff include: sediment, grease, oils, pesticides, herbicides, road salt, and other materials that are washed away and carried in runoff during a storm

Runoff collection system that is not connected to the storm sewer system

Runoff collection system that is connected to the storm sewer system

A term that describes the flow of water as it passes through the surface through subsoil layers

Pervious Surfaces
Surfaces that allow water to pass through and soak into the ground (infiltrate); runoff that infiltrates into deeper layers of soil recharges the groundwater supply

Pollutants commonly found in runoff include sediment, nutrients (phosphorus and nitrogen) in fertilizer, pesticides, herbicides, road salt, motor oil, and animal waste

Porous Paving
A permeable surface, consisting of porous asphalt, porous concrete, or various porous structural pavers, underlain by a stone bed which provides temporary storage of runoff for peak rate control and infiltration; porous asphalt or concrete use conventional materials, but are mixed without the fine particles, which creates void spaces that allow runoff to seep through the surface. (PA Stormwater Manual Sec. 6.4.1)

Rain Barrel/Cistern
Barrel or large container that collects drainage from roof leaders and stores water for future use, such as watering plants; cisterns are capable of holding large quantities of water, greater than 500 gallons, and are typically made from fiberglass, concrete, plastic, or brick (PA Stormwater Manual Sec. 6.5.2)

Rain Garden
Also bioinfiltration; rain gardens use a planting mix of sand and soil to promote infiltration of collected runoff; vegetation in the basin serves to filter pollutants out of runoff (water quality) and transpire runoff (water quantity), and plant root systems can enhance infiltration. The plants absorb and store pollutants; the soil medium filters out pollutants and allows storage and infiltration of stormwater runoff; and the bed provides additional volume control. See also bioretention. (PA Stormwater Manual Sec. 6.4.5)

Reduce Impervious Cover
Retaining permeable surface areas on a site performs valuable stormwater functions, including: increasing infiltration, decreasing stormwater runoff volume, improving water quality by decreasing the pollutant loading of streams, improving natural habitats by decreasing the deleterious effects of stormwater runoff, decreasing the concentration and energy of stormwater. Increased imperviousness alters an area’s hydrology, habitat structure, and water quality. Stream degradation has been witnessed at impervious levels as low as 10-20% (Center for Watershed Protection, 1995)

Restore Natural Areas
Re-establishing a natural landscape of woodlands or meadow, as these landscapes are more effective at reducing runoff volumes than maintained landscapes, such as lawns; native vegetation should be planted (PA Stormwater Manual Sec. 5.6.3)

Retrofit Design
Design based around already existing features

The process of correcting environmental degradation

Describes areas adjacent to rivers and streams with a high density, diversity, and productivity of plant and animal species relative to nearby uplands

Riparian Buffer
Areas of trees and other mature vegetation along streambanks and upslope from streams, wetlands, lakes, and ponds that are designed to intercept surface runoff from upland sources for the purpose of removing or buffering the effects of associated nutrients, sediment, organic matter, pesticides, or other pollutants prior to entry into surface waters (PA Stormwater Manual Sec. 6.7.1)

Rooftop Disconnect
Disconnecting roof leaders and directing rooftop runoff to a vegetated area, dry well, or infiltration trench so runoff can infiltrate into the ground (PA Stormwater BMP Manual Sec. 5.8.1)

Part of the precipitation that flows toward the streams on the surface of the ground or within the ground, and is composed of base flow and surface runoff

Sanitary Sewer System
Sewer system designed to convey domestic and industrial wastewater to a treatment plant

Sediment Trap/Forebay
A small, sometimes vegetated, pond that serves to pre-treat runoff by allowing sediment to settle out prior to the runoff being released to another stormwater BMP (PA Stormwater BMP Manual Sec. 6.6.3)

Separate Storm Sewer System
Infrastructure designed to capture and convey only stormwater runoff; separate storm sewer systems discharge directly to a water body

Soil Compaction
Damage caused to soil, especially topsoil, during construction from the weight of construction equipment; soil compaction reduces the pore spaces between soil particles, which reduces its ability to allow water to infiltrate (PA Stormwater BMP Manual Sec. 5.6.2)

Runoff from rain, snow melt, surface water, and other drainage

Street Sweeping
A regularly scheduled maintenance program is an effective source control measure for removing larger debris material and smaller particulate pollutants, preventing this material from clogging the stormwater management system and washing into streams and other bodies of water  (PA Stormwater BMP Manual Sec. 5.9.1)

Subsurface Detention
Designed to protect against flooding and, in some cases, downstream erosion, by storing water for a limited period of a time in an underground container constructed of recharging pipes, gravel, or loose soil

Subsurface Infiltration
An underground storage area comprised of a bed of rock aggregate or plastic polymer cells designed to temporarily hold runoff that is directed to the bed from the surface, and then allow the captured water to gradually seep into deeper ground layers (PA Stormwater BMP Manual Sec. 6.4.3)

Sustainable Development
Development with the goal of preserving environmental quality, natural resources, and livability for present and future generations; practices that minimize the impact of development on the natural landscape, such as clustering development, site design that retains existing vegetation, reducing soil compaction, and re-vegetating disturbed areas; also called environmentally-sensitive development

Total Maximum Daily Loads (TMDL)
Under the Clean Water Act, a TMDL identifies the amount of a particular pollutant a stream can handle without violating water quality standards

Tree Trench
A vegetated swale planted with trees to reduce the speed of runoff; trees in the trench filter pollutants and allow sediment to settle out of runoff and promote infiltration into the ground; trees also absorb runoff and then release the water into the air via transpiration, which reduces the overall quantity of runoff

Cloudiness in water derived from algae, suspended silt, or other impurities

Vortex Separator
A device that removes sediment and other suspended solids from runoff; the vortex creates a circular flow of water, which causes the suspended particles move to the center of the device where they settle to the bottom (California BMP Handbook)

The specific land area that drains water into a river system or other body of water

A low head, overflow-type dam commonly used to raise the level of a river or stream (Wikipedia)

Wet Pond
Also called retention ponds, wet extended detention ponds; constructed basins that have a permanent pool of water throughout the year. Ponds treat incoming stormwater runoff by allowing settling while stormwater runoff resides in the pool. Nutrient uptake also occurs through biological activity in the pond. Wet ponds are among the most cost-effective and widely used stormwater treatment practices (

A marsh area that is inundated or saturated by surface water or groundwater at a frequency, duration, and depth sufficient to support a predominance of emergent plant species adapted to saturated soil conditions (

Wetland Vegetation
Plants that are tolerant of the moist to wet conditions associated with wetland areas; in any wetland, plant species vary depending upon climate, water levels, landscape position, and other variables; plant species that require conditions of permanent wetland are called obligates (Penn State Cooperative Extension publication “Managing Your Restored Wetland, online at

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