Glossary of Terms

Glossary of terms

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 303(d) Waterbody : A list of lakes, rivers, and streams that have been designated as impaired or threatened by a pollutant(s) for which one or more TMDL(s) are needed. Impaired means that the water is not meeting State Water Quality Standards.

319 : The section of the Federal Clean Water Act that deals with nonpoint pollution.

Absorption : the filling of pores in a solid. A fluid permeates or is dissolved by a liquid or a solid.

Accelerated Erosion : Erosion in excess of what is presumed or estimated to be naturally occurring levels and which is a direct result of human activities.

Adsorption : the binding of molecules or particles to a surface. Adsorption is a process utilized in stormwater management BMPs to enhance the removal of soluble pollutants.

Algal Bloom : Rapidly occurring growth and accumulation of algae within a body of water, which usually results from excessive nutrients or sluggish circulation within a waterbody. Persistent and frequent blooms can result in low oxygen conditions which is hazardous to aquatic life.

Alternative Pavers : Permeable surfaces (pervious concrete, pervious asphalt and/or pervious brick pavers) that can replace conventional asphalt and concrete and can be used for driveways, parking lots and walkways.

Alternative Site Design : Innovative site design practices have been developed as alternatives to traditional development. Research has demonstrated that alternative site design has the potential to reduce impervious cover, runoff volume, pollutant loadings, and development costs when compared to traditional development. See also- Low Impact Development.

Annual Plant : A plant that completes its entire life cycle in a single growing season.

Anti-Seep Collar : An impermeable diaphragm, usually of sheet metal, plasticized rubber or concrete, constructed at intervals within the zone of saturation along the conduit of a principal spillway to prevent piping or seepage along the conduit.

Anti-Vortex Device : A device placed at the entrance to a pipe conduit structure to help prevent swirling action and cavitation from reducing the flow capacity of the conduit system. Typically used in pond and lake discharge inlets.

Aquatic Bench : A 10 to 15 foot wide bench around the inside perimeter of a permanent pool that ranges in depth from zero to 12 inches. Typically vegetated with emergent plants, the bench can be designed to augment pollutant removal, provides habitats, protects the shoreline from the effects of water level fluctuations, and enhances safety. See also- Safety Bench.

Aquatic Corridor : Areas of land and water that are important to the integrity and quality of a stream, river, or other body of water. An aquatic corridor usually consists of the actual stream or river, the aquatic buffer, and other areas that are a part of the stream’s right-of-way. See also Riparian Corridor.

Aquifer : A subsurface formation of rock, glacial material, or other deposits that contains water and is capable of storing and yielding water to a well or spring.

As-built (drawing) : Record drawings or professional certification of conditions as they were actually constructed.

Atmospheric Deposition : – The process by which atmospheric pollutants reach the land surface either as dry deposition or as dissolved or particulate matter contained in precipitation.

Attenuation : Reduction in magnitude of peak runoff discharge rates, or the reduction of contaminant concentrations through designed best management practices.

Average Daily Traffic (ADT) : The average total number of vehicles that traverse a road on a typical day. For residential streets, the ADT is usually about 10 trips per residence times the number of residences.

Average Particle Size : Average size of suspended solids expected to be exported from a specified drainage area and/or land use by stormwater runoff.

Backwater : Water upstream from an obstruction or reduced channel opening, which is deeper and typically lower velocity than it would normally be without the obstruction.

Baffle : Guides, grids, grating, berms or similar devices placed in a BMP to deflect or regulate flow and create a longer flow path from the inlet to the outlet structure.

Bankfull Flow : The condition where stream flow just fills a stream channel up to the top of the bank and at a point where the water begins to overflow onto a floodplain.

Base Flow : The portion of stream flow that is not direct rainfall runoff and results from seepage of water from the ground into a channel slowly over time. The primary source of running water in a stream during dry weather.

Basin (area) : The largest single watershed management unit for water planning that combines the drainage of a series of subbasins. Often have a total area more than a thousand square miles.

Basin (structure) : A facility designed to impound stormwater runoff.

Beaver deceiver : A constructed flow control device that reduces beaver damming activities. It is a non -lethal beaver management technique.

Benches : Surface configurations added to stormwater basins that create flat edges, usually installed for safety and to minimize erosion. Benches can be designed as Aquatic Benches.

Benthic : Pertaining to occurrence on or in the bottom sediment of a waterbody’s aquatic ecosystems, including wetlands.

Berm : A mound of earth formed to control the flow of surface water. Berms can be either above or below the water surface.

Best Management Practice (BMP) – nonstructural : Strategies implemented to control stormwater runoff that focus on pollution prevention such as alternative site design, zoning and water quality or quantity ordinances, education, and good housekeeping or maintenance measures.

Best Management Practice (BMP) – structural : Engineered devices implemented to control, treat, remove and/or prevent stormwater runoff pollution from reaching offsite water bodies. BMPs can be applied singularly, in series, or in parallel as necessary to meet Federal, State, and Local laws, codes and ordinances. Ideally, BMPs can be used to convey, treat and store stormwater runoff.

Better Site Design : A collection of site planning, design, and development strategies that help reduce adverse impacts to the natural environment by recreating, to a certain extent, the original hydrology and plant community of the predevelopment site. See also- Low Impact Development.

Biochemical Oxygen Demand (BOD) : A measure of the oxygen required to oxidize all organic compounds in water. Typically used as an indicator for levels of organic based pollutants, naturally occurring or manmade, within a waterbody.

Biodiversity : The number of species of plants and animals in a defined area. Biodiversity is measured by a variety of indices that consider the number of species and, in some cases, the distribution of individuals among species.

Bioengineering : Restoration and stabilization techniques that use plants, often native species, and barriers to mimic natural waterbody functions and benefits.

Biofilters : Vegetated depressional areas such as engineered channels, swales, rain gardens, or bioretention areas (singular, in series or in parallel) that are used to collect and filter urban stormwater.

Biofiltration : The use of vegetation (usually native mesic prairie or wetland plants) to physically filter and treat stormwater runoff as it is conveyed through an open channel or swale. Minimal dissolved pollutant removal is expected.

Biofiltration Swale or Bioswale : A long, gently or zero sloped, vegetated ditch designed to filter pollutants from stormwater. Native plants are the most common vegetation, but wetland vegetation can be used if the soil is saturated.

Biological Diversity : The concept of multiple species of organisms living together in balance with their environment and each other.

Biological Monitoring and Biological Indicators : Surveys of aquatic biota in a waterbody where the organisms (plants, macro-invertebrates, and fish) serve as indicators of the quality and characteristics of that waterbody.

Biological Processes : A pollutant removal pathway in which microbes break down organic pollutants and transform nutrients.

Bioretention : A water quality practice that utilizes landscaping and soils to treatstormwater runoff by collecting it in shallow depressions, before filtering through existing or engineered soil media. May include a subsurface infiltration gallery or a subdrain system beneath the soil media.

Brownfield : Abandoned or under-used contaminated industrial and commercial sites where future expansion or redevelopment may be accelerated after site remediation for possible contamination.

Buffer or Buffer Zone : A designated transitional area around a stream, lake, or wetland left in a natural, usually vegetated state so as to protect the waterbody from runoff pollution. Development is often restricted or prohibited in a buffer zone.

Buffer Averaging : A technique for delineating the width of a buffer such that the buffer boundary can be narrower at some points along the stream and wider at others so that its average width meets the minimum criteria.

Build-Out : The total percentage of development in a watershed based on current zoning.

By-Right Open Space Development : A form of development in which the developer does not need to seek special approval from planning boards in order to use open space design at a site.

Catchment : A small watershed management unit or drainage basin, defined as the area of a development site to its first intersection with a stream, usually as a pipe or open channel outfall.

Catch Basin : An inlet chamber to a storm system usually built at the curb line of a street or low area, for collection of surface runoff. These structures commonly have a sediment sump at its base, below the sewer or subdrain discharge elevation to retain solids below the point of overflow. Require regular maintenance for optimal pollutant removal.

Catch Basin Insert : A device installed within a catch basin to treat stormwater through filtration, settling, absorption, adsorption, or a combination of these mechanisms. There are a number of shapes, sizes, and configurations of inserts available.

Channel : A long, narrow natural surface feature or manmade constructed excavation that conveys surface water and is open to the air.

Channel (constructed) : An open surface water conveyance system; also includes reconstructed natural channels.

Channel (natural) : A naturally occurring open surface water conveyance system; also includesa constructed channel that has taken on the appearance of a natural channel.


Channel Erosion : The widening and/or deepening (called channel scour), and upstream cutting of a constructed or natural channel caused by moderate and extreme flow events. Channel erosion is one way that a stream reacts to changes in flow patterns.

Channelization : The creation of a channel or channels typically resulting from faster water flow, a reduction in hydraulic residence time, and less contact between water and solid surfaces in the water body.

Channel Stabilization : Constructed or naturally occurring erosion prevention and stabilization of velocity distribution in a channel using jetties, drops, revetments, structural linings, vegetation and other measures.

Check Dam : A small constructed dam in a channel, gully or other watercourse to decrease the stream flow velocity (by creating backwater), minimize channel scour, and promote deposition of sediment.

Chemical Oxygen Demand (COD) : A measure of the oxygen required to oxidize all compounds, both organic and inorganic, in water.

Chemical Water Quality : The quality of a waterbody determined using chemical rather than physical or biological parameters and methods.

Chute : A high velocity, typically constructed open channel for conveying water to a lower level without erosion.

Clay (soils) : 1. A mineral soil separate consisting of particles less than 0.002 millimeters in equivalent diameter. 2. A soil texture class. 3. (Engineering) A fine grained soil (more than 50% passing the No. 200 sieve) that has a high plasticity index in relation to the liquid limit. (Unified Soil Classification System). Typically have poor infiltration properties.

Clean Water Act (CWA) : A federal environmental law that includes the management of stormwater.

Closed depression : An area which is low-lying and either has no surface water outlet, or has such a limited outlet that during storm events the area acts as a retention basin.

Cluster or Open Space Development : The use of designs that incorporate a concentrated (“clustered”) housing or apartment area that promotes a greater open space area into a development site. These increased open areas can be used for either passive or active recreational activity or preserved as naturally vegetated land. See also – Conservation Design.

Coconut Rolls : Also known as coir rolls, are rolls of natural coconut fiber designed to be used for waterbody bank stabilization.

Coefficient of Permeability : An engineering constant value which is used to measure the capability of a filter media to pass liquid through a given surface area. Typically used to indicate a site’s soil infiltration properties.

Combined Sewer System (CSS) : A sewer pipe network designed to collect and convey residential, commercial, and industrial wastewater and stormwater runoff.

Combined Sewer Overflow (CSO) : A designed relief discharge point within a CSS that allows excesscombined wastewater and stormwater runoff to discharge to a receiving waterbody when the capacity of the sewer network and / or treatment plant is exceeded during or immediately after storm events. Combined sewer overflows are not allowed during dry periods, as defined by the CWA.

Community Association or Homeowners Association (HOA) : A planned residential, cooperative and/or homeowner group with the primary function of addressing the concerns and needs of residents within a specific geographic area and/or development. HOAs usually include fees and their responsibilities may include maintenance, enforcement of allowable land practice uses, and protection of open space areas from encroachment and future development.

Compaction (Soils) : Any process by which the soil grains are rearranged to decrease void space and bring them in closer contact with one another, thereby increasing the weight of solid material per unit of volume, increasing the shear and bearing strength and reducing permeability. Compacted soils typically have lower coefficients of permeability.

Conduit : Any channel intended for the conveyance of water, whether open or closed.

Consent order or consent decree : A legal injunction mandating a party to perform an action. Typically used by Federal and State agencies as a compliance mechanism with public and private entities.

Conservation Design : Site design that incorporates the conservation of natural site features such as on-site tree, waterbody and topsoil preservation, concentrating or clustering homes or apartments on a limited percentage of the site area. See also – Cluster or Open Space Development.

Conservation Easement : Voluntary landowner agreements that allow private property to be set aside in order to limit the type or amount of site disturbance on the property. Easements can be set to relieve property owners of the burden of managing these areas by shifting responsibility to a private organization or government agency better equipped to handle maintenance and monitoring issues.

Constructed Conveyance System Facilities : Gutters, ditches, pipes, channels, BMPs, and most flow control and water quality treatment facilities.

Constructed Stormwater Wetland : A water quality BMP design to have similar characteristics and functions to a natural wetland, with the specific purpose of treating stormwater runoff through uptake, retention, and settling.

Constructed Wastewater Treatment Wetland (CWWT) : a regulated wastewater treatment facility design to have similar characteristics and functions to a natural wetland, with the specific purpose of treating residential, commercial and/or industrial wastewater prior to discharging to a receiving waterbody. CWWT’s typically are designed as subsurface flow wetlands.

Contour : A line drawn on a map or development plan representing the same elevation. Elevations are typically confirmed through site specific survey techniques.

Conveyance : The process of water moving from one place to another.

Core Trench : A trench, filled with relatively impervious material intended to reduce seepage of water through porous strata.

Cradle : A structure, usually of concrete, shaped to fit around the bottom and sides of a conduit to support the conduit, increase its strength, and in dams, to fill all voids between the underside of the conduit and the soil.

Crest : 1.The top of a dam, dike, spillway or weir, frequently restricted to the overflow portion. 2. The summit of a wave or peak of a flood.

Critical Drainage Area : An area with such severe flooding, drainage, and/or erosion/sedimentation conditions which have resulted or will result from the cumulative impacts of development and urbanization,

Critical Root Zone (CRZ) : The area around a tree required for the tree’s survival. Typically, the dripzone area is protected during construction activities.

Crushed Stone : Aggregate consisting of angular particles produced by mechanically crushing rock.

Cul-de-sac : A local access street with a closed circular end which allows for vehicle turnarounds.

Culvert : Pipe or structure which drains open channels, swales, or ditches under a roadway or embankment typically with no catch basins or manholes along its length.

Curve Number : A numerical representation of a given area’s hydrologic soil group, plant cover, impervious cover, interception and surface storage derived in accordance with Natural Resources Conservation Service methods. This number is used to convert rainfall volume into runoff volume.

Curvilinear Street Pattern : A street design which follows the natural topography of the land and uses curving roads and cul-de-sacs to reduce vehicle speeds and inhibit cut-through traffic.

Cut : Portion of land surface or area from which earth has been removed or will be removed by excavation; the depth below original ground surface to excavated surface.

Cut-and-Fill : Process of earth moving by excavating part of an area and using the excavated material for adjacent embankments, roads, or fill areas.

Cut Off : A wall or other structure, such as a trench, filled with relatively impervious material intended to reduce seepage of water through porous strata.

Dam : A barrier to confine or raise water for storage or diversion, to create a hydraulic head, to create backwater, to prevent gully erosion, or for retention of soil, sediment or other debris.

Dead Storage : The volume available in a depression in the ground below any conveyance system, surface drainage pathway, or outlet invert elevation that could allow the discharge of surface and storm water runoff. Dead storage volume typically only leaves the depression via infiltration, evaporation and/or vegetative evapotranspiration.

Debris Barrier : Any obstruction, typically constructed, designed to catch and hold naturally occurring or manmade solids. Solids can include woody debris, vegetation, solid waste litter/trash or other articles large enough to be easily captured within the barrier. Maintenance, automated or manual, is required for periodic debris removal and disposal.

Deciduous plant : A plant that sheds or loses its foliage at the end of each growing season.

Denitrification : The anaerobic microbial conversion of nitrogen to nitrogen gas.

Density Compensation : Granting a credit for higher density elsewhere on a site to compensate for developable land lost due to environmental considerations. May also be used as an incentive to promote use of Low Impact Development techniques. See also – Density Bonus.

Density Bonus : A form of incentive to promote conservation of natural and open space areas. Developers are allowed to build more units than allowed by local zoning ordinances by incorporating certain levels of Low Impact Development techniques as defined by the regulating planning authority. See also – Density Compensation.

Depression Storage : The amount of precipitation volume that is trapped in low areas on the surface of vegetated and impermeable the ground areas.

Design Storm : A selected rainfall heyetograph of specified amount, intensity, duration and frequency that is used as a basis for design.

Designated Use : Uses specified, typically by the State, by water quality standards for a waterbody.

Detention : The temporary storage of storm runoff in a stormwater practice with the goals of controlling peak discharge rates and providing gravity settling of pollutants.

Detention Basin : A stormwater management facility which temporarily impounds runoff and discharges it through a hydraulic outlet structure to a downstream conveyance system. While a certain amount of outflow may also occur via infiltration through the surrounding soil, such amounts are negligible when compared to the outlet structure discharge rates and, therefore, are not considered in the facility’s design. Since an extended detention basin impounds runoff only temporarily, it is normally dry during nonrainfall periods.

Detention Facility : A facility that collects water from developed areas and releases it at a slower rate than it enters the collection system. The excess of inflow over outflow is temporarily stored in a pond or a vault and is typically released over a few hours or a few days.

Detention Vault : A type of detention facility.

Detritus : Dead plant material that is in the process of decomposition.

Dike : An embankment to confine or control water; for example, one built along the banks of a river to prevent overflow or lowlands; a levee or berm.

Direct Discharge : Undetained discharge from a proposed project to a receiving waterbody.

Discharge : The volume of water that passes a given location within a given period of time.

Dispersed Discharge : Release of storm water runoff from a drainage facility system such that the flow spreads over a wide area and is located so as not to allow flow to concentrate anywhere upstream of a drainage channel with erodible underlying granular soils or the potential to flood downstream properties.

Disconnected Impervious Surfaces : the separation of impervious surfaces that promote infiltration, physical filtration, and stormwater discharge flow rate reduction.

Dissolved Oxygen (DO) : The amount of oxygen that is dissolved in water. It also refers to a measure of the amount of oxygen available for biochemical activity in a waterbody and as indicator of the quality of that water.

Dissolved Pollutant : soluble contaminants that are absorbed by stormwater runoff from both hard and vegetative surfaces. Fertilizer based nitrogen and phosphorous are typical dissolved contaminants of concern.

Disturbed Runoff Control : A stream channel protection criteria that utilizes a non-uniform distribution of the storage stage-discharge relationship within a stormwater practice to minimize the change in channel erosion potential from predeveloped to developed conditions.

Disturbed Area : An area in which the natural vegetative soil cover or existing surface treatment has been removed or altered and, therefore, is susceptible to erosion.

Ditch : A constructed channel with its top width less than 10 feet at design flow.

Diversion : A channel with a supporting ridge on the lower side constructed across the slope to divert water from areas where it is in excess to sites where it can be used or disposed of safely. Diversions differ from terraces in that they are individually designed.

Domestic Wastewater : Wastewater discharged from residences and from commercial, institutional and similar facilities; also known as wastewater or sanitary wastewater.

Drainage : 1.The removal of excess surface water or ground water from land by means of surface or subsurface drains. 2. Soils characteristics that affect natural drainage.

Drainage Area (Basin) : An area of land that contributes stormwater runoff to a designated point. Also called a watershed on a larger scale.

Drainage Facility : A constructed or engineered feature that collects, conveys, stores or treats surface and storm water runoff. Drainage facilities shall include but not be limited to all constructed or engineered streams, pipelines, channels, ditches, gutters, lakes, wetlands, closed depressions, flow control or water quality treatment facilities, erosion and sedimentation control facilities, and other drainage structures and appurtenances that provide for drainage.

Drop Structure : A structure for dropping water to a lower level and dissipating surplus energy; a fall. The drop may be vertical or inclined.

Dryfall : The deposition of atmospheric pollutants on the land surface.

Dry Pond : A stormwater pond design with no permanent pool. Stormwater is detained in the practice temporarily to settle pollutants, protect downstream channels, and prevent flooding. These practices typically provide poor pollutant removal.

Dry Swale : An open drainage channel explicitly designed to detain and promote the filtration of stormwater runoff through an underlying fabricated soil media.

Dry Well : An infiltration practice designed to treat rooftop runoff. Runoff is directed to the trench via a downspout. It is temporarily stored in the voids of the trench, and then percolated into the ground.

Duration : The length of time over which precipitation occurs.

Easement : A grant of one or more of the property rights by the property owner to and/or for the use by the public, a corporation or another person or entity.

Ecosystem : An interactive system that includes the organisms of a natural community together with their abiotic, physical, chemical and geochemical environment.

Edge Effect : Extensive, well-defined edges between the impervious and pervious surfaces.

Embankment : A man-made deposit of soil, rock or other material used to form an impoundment.

Emergency Spillway : A dam spillway designed and constructed to discharge flow in excess of the principal spillway design discharge.

Emergent Plant : A plant with stems and leaves that grows in periodically or permanently flooded areas. Parts of the plant extend through and above the water.

Energy Dissipator : A designed device placed at the end of a water transmitting apparatus for the purpose of reducing the velocity, energy and turbulence of the discharged water.

Environmental Impact Statement (EIS) : A document that discusses the likely significant adverse impacts of a proposal, ways to lessen the impacts, and alternatives to the proposal. It is required by the national and state environmental policy acts when projects are determined to have the potential for significant environmental impact.

Erosion : 1. The wearing away of the land surface by running water, wind, ice, or other geological agents, including such processes as gravitational creep. 2. Detachment and movement of soil or rock fragments by water, wind, ice or gravity.

Erosive Velocities : Velocities of water that are high enough to wear away the land surface. Exposed soil will generally erode faster than stabilized soils. Erosive velocities will vary according to the soil type, slope, structural, or vegetative stabilization used to protect the soil.

Estuary : A semi-enclosed coastal waterbody such as a bay, mouth of a river, salt marsh, or lagoon, where freshwater and saltwater mix. These waters support a rich and diverse ecology.

Eutrophic : A condition of a water body in which excess nutrients, particularly phosphorous, stimulates the growth of aquatic plant life usually resulting in the depletion of dissolved oxygen. Thus, less dissolved oxygen is available to other aquatic life.

Evapotranspiration : The combined processes of evaporation from the water or soil surface and transpiration of water by plants.

Event Mean Concentration (EMC) : A method for characterizing pollutant concentrations in a receiving water from a runoff event often chosen for its practicality. The value is determined by compositing (in proportion to flow rate) a set of samples, taken at various points in time during a runoff event, into a single sample for analysis.

Evergreen Plant : A plant that remains green and retains its foliage throughout the year.

Exceedence Probability : The probability that an event having a specified volume and duration will be exceeded in one time period, usually assumed to be one year. If a storm has a one percent chance of occurring in any given year, than it has an exceedence probability of 0.01.

Excess Parking : Parking spaces that are constructed over and above the number required or predicted based on the parking demand ratio for a particular land use or activity.

Exfiltrate : The downward movement of runoff through the bottom of a treatment system into the soil layer.

Exfiltration : The downward movement of water through the soil; the downward flow of runoff from the bottom of an infiltration stormwater practice into the soil.

Exotic Species : A plant or animal species that has been intentionally or accidentally introduced and that does not naturally occur in a region.

Extended Detention (ED) : A stormwater design feature that provides for the gradual release of a volume of water over a 12 to 48 hour interval in order to increase settling of urban pollutants and protect downstream channels from frequent storm events.

Extended Detention Basin : A stormwater management facility which temporarily impounds runoff and discharges it through a hydraulic outlet structure over a specified period of time to a downstream conveyance system for the purpose of water quality enhancement or stream channel erosion control. While a certain amount of outflow may also occur via infiltration through the surrounding soil, such amounts are negligible when compared to the outlet structure discharge rates and, therefore, are not considered in the facility’s design. Since an extended detention basin impounds runoff only temporarily, it is normally dry during nonrainfall periods.

Extended Detention Basin (enhanced) : An extended detention basin modified to increase pollutant removal by providing a shallow marsh in the lower stage of the basin.

Extended Detention Control Device : A pipe or series of pipes that extend from the riser of the stormwater pond that are used to gradually release stormwater from the pond over a 12- to 48-hour interval.

Extended Detention Wetland : A wetland system that provides storage for a fraction of the water quality volume by detaining storm flows above the marsh surface.

Extreme Flood (Qf) : The storage volume required to control those infrequent but large storm events in which overbank flows approach the floodplain boundaries of the 100-year flood.

Fascine : Bundled willow cuttings used to stabilize stream banks. Bundling allows otherwise weak green twigs to reinforce each other and resist the forces of stream currents.

Fecal Coliform : Applied to Escherichia coli and similar bacteria that are found in the intestinal tract of humans and animals, and also found in soil. Coliform bacteria are commonly used as indicators of the presence of pathogenic organisms. Their presence in water indicates fecal pollution and potentially adverse contamination by pathogens.

Fibric Peat : Organic material, usually derived from wetland vegetation, in which the undecomposed fibrous organic materials are easily identifiable. Also characterized by low bulk density and a highly porous structure.

Fill : A reference to an area or material that has been placed by mechanical equipment in the process of a grading operation.

Filter Bed Chamber : The section of a constructed filtration device that houses the filter material and the outflow piping.

Filter Fence : A geotextile fabric designed to trap sediment and filter runoff.

Filter Media : The sand, soil, or other organic material in a filtration device used to provide a permeable surface for pollutant and sediment removal.

Filter Strip : An area of vegetation, usually adjacent to a developed area, constructed to remove sediment, organic matter, and other pollutants from runoff in the form of sheet flow.

Fines (Soil) : Generally refers to the silt and clay size particles in soil.

First Flush : The first portion of runoff, usually defined as a depth in inches, considered to containing the highest pollutant concentration resulting from a rainfall event.

Flashy Stream : A stream or river that is characterized by dramatic fluctuations in flow, in which sharply higher flows in wet weather can be followed by very low flows in dry weather.

Floatables : Materials found in runoff that are buoyant, such as polystyrene, plastic, some organic material, or cigarette butts.

Flooding : When the volume or rate flow exceeds the capacity of the natural or manmade conveyance system and overflows onto adjacent lands, causing or threatening damage.

Flood Control Channel : The open portion (often concrete-lined) of the storm drain system

Flood Fringe : The flood fringe occupies the distal parts of the floodplain, outside of the floodway. Complete obstruction of the flood fringe will not significantly increase flood levels. The flood fringe boundary is typically based on an increase in flood level of one foot during the 100-year return frequency flooding event.

Flood Insurance Rate Map (FIRM) : Maps established by the Federal Emergency Management Agency (FEMA) that identify the 100-year floodplain for flood insurance purposes.

Floodplain : Areas adjacent to a stream or river that are subject to flooding or inundation during severe storm events (often called a 100 year floodplain, it would include the area or flooding that occurs, on average, once every 100 years).

Floodplain Management : A process to limit flood damage by prohibiting new development within the boundaries of the 100-year floodplain. In existing developments within the floodplain, management includes maintaining and increasing open space areas along waterways.

Floodway : Part of the floodplain, centered on the stream, that will convey most of the flow during overbank flooding events.

Flow Control Facility : A drainage facility designed to mitigate the impacts of increased surface and storm water runoff generated by site development. Flow control facilities are designed either to hold water for a considerable length of time and then release it by evaporation, plant transpiration, and/or infiltration into the ground, or to hold runoff a short period of time and then release it to the conveyance system.

Flow Control Standards : The level of flow control required is based on the resource value of the receiving system in terms of its hydrology, ecology, geology and water quality.

Flow Restrictor : A control device or a T section with a specifically sized orifice(s) to control release rates.

Flow Splitter : An engineered, hydraulic structure designed to divert a percentage of storm flow to a stormwater practice located out of the primary channel, or to direct stormwater to a parallel pipe system, or to bypass a portion of baseflow around a stormwater practice.

Forebay : Stormwater design feature that uses a small basin to settle out incoming sediment before it is delivered to a stormwater BMP.

Freeboard (Hydraulics) : The distance between the maximum water surface elevation anticipated in design and the top of retaining banks or structures. Freeboard is provided to prevent overtopping due to unforeseen conditions.

Free Water Surface Wetland : a natural or constructed wetland that allows direct open access to any water surface of any portion of the wetland. Naturally occurring wetlands are typically Free Water Surface.

French Drain : A type of drain consisting of an excavated trench filled with pervious material such as coarse sand, gravel or crushed stone, through whose voids water percolates and infiltrates into the soil.

Frequency (Design Storm Frequency) : The recurrence interval of storm events having the same duration and volume. The frequency of a specified design storm can be expressed either in terms of exceedence probability or return period.

Frontage Requirements : A requirement in the subdivision code that mandates that each lot within a particular zoning category have a minimum length that fronts along the street.

Full Build-Out : The total potential development in a watershed based on current zoning plans which includes existing development and expansion potential in the future.

Gabion : A flexible woven wire basket composed of rectangular cells filled with large cobbles or riprap. Gabions may be assembled into many types of structures such as revetments, retaining walls, channel liners, drop structures, diversions, check dams, and groins.

Gabion Mattress : A thin gabion, usually six or nine inches thick, used to line channels for erosion control.

General Permit : A permit issued under the NPDES program to cover a certain class or category of stormwater discharges. These permits reduce the administrative burden of permitting stormwater discharges.

Geographic Information System (GIS) : A database of digital information and data on land-use, land cover, ecology, and other geographic attributes that can be overlaid, statistically analyzed, mathematically manipulated, and graphically displayed using maps, charts, and graphs.

Geotextile Fabric : A synthetic textile of relatively small mesh or pore size that is used to (a) allow water to pass through while keeping sediment out (permeable), or (b) prevent both runoff and sediment from passing through (impermeable). Also known as filter fabric.

Grade : 1. The slope of a road, channel or natural ground. 2. The finished surface of a canal bed, roadbed, top of embankment, or bottom of excavation; any surface prepared for the support of construction, like paving or laying a conduit. 3. To finish the surface of a canal bed, roadbed, top of embankment or bottom of excavation.

Grading : The cutting and/or filling of the land surface to a desired slope or elevation.

Grassed Swale : An earthen conveyance system which is broad and shallow with check dams and vegetated with erosion resistant and flood tolerant grasses, engineered to remove pollutants from stormwater runoff by filtration through grass and infiltration into the soil.

Gravel : 1. Aggregate consisting of mixed sizes of 1/4 inch to three inch particles which normally occur in or near old streambeds and have been worn smooth by the action of water. 2. A soil having particle sizes, according to the Unified Soil Classification System, ranging from the No. 4 sieve size angular in shape as produced by mechanical crushing.

Gravel Filter : Washed and graded sand and gravel aggregate placed around a drain or well screen to prevent the movement of fine materials from the aquifer into the drain or well.

Gravel Diaphragm : A stone trench filled with small, river-run gravel used as pretreatment and inflow regulation in stormwater filtering systems.

Gravel Trench : A shallow excavated channel backfilled with gravel and designed to provide temporary storage and permit percolation of runoff into the soil substrate.

Green Alleys : A network of bioretention basins, infiltration trenches or bioretention filters that provide both redundant water quality management and stormwater conveyance.

Green Parking : Refers to several techniques applied together to reduce the contribution of parking lots to the total impervious cover in a lot.

Green Space : The proportion of open space that is retained in an undisturbed vegetative state.

Greenway : A planning study that creates a linked and linear network of trails, accesses, passive and possibly active recreational facilities along an aquatic corridor.

Gross Density : The maximum number of dwelling units allowed within a particular zoning class, expressed in terms of dwelling units per acre.

Ground Cover : Plants which are low-growing and provide a thick growth which protects the soil as well as providing some beautification of the area occupied.

Groundwater : Water that flows below the ground surface through saturated soil, glacial deposits, or rock.

Groundwater Recharge : Increasing the amount of groundwater in storage via percolating rainwater.

Gully : A channel or miniature valley cut by concentrated runoff through which water commonly flows only during and immediately after heavy rains or during the melting of snow. The distinction between gully and rill is one of depth. A gully is sufficiently deep that it would not be obliterated by normal tillage operations, whereas a rill is of lessor depth and would be smoothed by ordinary farm tillage.

Gully Erosion : The erosion process whereby water accumulates in narrow channels and, over short periods, removes the soil from this narrow area to considerable depths, ranging from 1 or 2 feet to as much as 75 to 100 feet.

Gutter : The edge of a street (below the curb) designed to drain water runoff from streets, driveways, parking lots, etc. into catch basins.

Habitat : The specific area or environment in which a particular type of plant or animal lives and grows.

Hardpan : A cemented or compacted and often clay-like layer of soil that is impenetrable by roots.

Harmful Pollutant : A substance that has adverse effects to an organism including death, chronic poisoning, impaired reproduction, cancer, or other effects.

Head (Hydraulics) : 1. The height of water above any plane of reference. 2. The energy, either kinetic or potential, possessed by each unit weight of a liquid expressed as the vertical height through which a unit weight would have to fall to release the average energy possessed. Used in various terms such as pressure head, velocity head, and head loss.

Headwall : A wall of stone, metal, concrete, or wood at the end of a culvert or drain to protect fill from scour or undermining, increase hydraulic efficiency of conduit, divert flow, retard disjointing of short sectional pipe, or serve as a retaining wall.

Heavy Metals : Metals with high molecular weights that are generally toxic to animal life and human health if naturally occurring concentrations are exceeded. Examples include, arsenic, chromium, lead and mercury.

Hemic Peat : An organic material, usually derived from wetland vegetation that is moderately decomposed, has a moderate bulk density and modest porosity.

Herbaceous Perennial (Plant) : A plant whose stems die back to the ground each year.

Herbicides : Chemicals developed to control or eradicate plants.

Hi Marsh : A pondscaping zone within a stormwater wetland which exists from the surface of the normal pool to a six inch depth and typically contains the greatest density and diversity of emergent wetland plants.

Hi Marsh Wedges : Slices of shallow wetland (less than or equal to six inches) dividing a stormwater wetland.

High-Input Lawn : A heavily irrigated lawn subject to high usage of chemicals such as fertilizers, pesticides and fungicides.

High Occupancy Vehicle Lanes (HOV) : A reference to highway lanes that are reserved for vehicles with two or more occupants (carpooling).

Homeowners Association (HOA) : A planned residential, cooperative and/or homeowner group with the primary function of addressing the concerns and needs of residents within a specific geographic area and/or development. HOAs usually include fees and their responsibilities may include maintenance, enforcement of allowable land practice uses, and protection of open space areas from encroachment and future development.

Hot Spot : Area where land use or activities generate highly contaminated runoff, with concentrations of pollutants in excess of those typically found in stormwater.


Household Hazardous Waste : Common everyday products that people use in and around their homes (paint, paint thinner, and pesticides) that, due to their chemical nature, can be hazardous if not properly disposed.

Humic : A soil or other material characterized by a high organic content.

Hybrid Street Network : A street layout that incorporates both grid and curvilinear street patterns in a “wheel-and-spoke” design that conserves important natural features but still allows for interconnected roads.

Hydraulics : The physical science and technology of the static and dynamic behavior of fluids.


Hydraulic Engineering Circular-1 (HEC-1) : A rainfall-runoff event simulation computer model sponsored by the U.S. Corps of Engineers.


Hydraulic Gradient : The slope of the hydraulic grade line. The slope of the free surface of water flowing in an open channel.

Hydric Soil : A soil that is saturated, flooded, or ponded long enough during the growing season to develop anaerobic conditions in the upper part.

Hydrodynamic Structure : An engineered flow through structure which uses gravitational settling to separate sediments and oils from stormwater runoff.

Hydrogeology : The study of water flow in aquifers and the characterization of aquifers.

Hydrograph : A graph showing variation in stage (depth) or discharge of a stream of water over a period of time.


Hydrologic Cycle : The circuit of water movement from the atmosphere to the earth and return to the atmosphere through various stages or processes such as precipitation, interception, runoff, infiltration, percolation, storage, evaporation, and transpiration.


Hydrologic Flow Criteria : Data used to assess the hydrologic characteristics of a waterbody to determine how closely it mimics expected conditions or patterns, in order to develop and calibrate models, predict flow events, or protect channel and biological integrity.

Hydrologic Soil Group : A Natural Resource Conservation Service classification system in which soils are categorized into four runoff potential groups. The groups range from A soils, with high permeability and little runoff production, to D soils, which have low permeability rates and produce much more runoff.

Hydrology : The science addressing the properties, distribution, and circulation of water across the landscape, through the ground, and in the atmosphere.

Hydrophytic Vegetation : Vegetation that grows partly or wholly in water whether rooted in the mud, as a lotus, or floating without anchorage, as the water hyacinth.


Hyetograph : A graph of the time distribution of rainfall over a watershed.


Hydroseed : A method by which seed, water, fertilizer, and fiber mulch are blended together in a tank and applied to a prepared area by a spraying hose.

Illicit Connections : Illegal and/or unauthorized connections that result in untreated wastewater discharges into storm drainage systems and receiving waters.

Illicit Discharge : Any discharge to a municipal separate storm sewer system that is not composed entirely of storm water, except for discharges allowed under an NPDES permit or waters used for certain emergency situations.

Impacted Stream or Subwatershed : Stream classification for a subwatershed with 11 to 25% ultimate impervious cover. Urbanization is expected to lead to some permanent degradation to stream quality

Impaired Stream : Stream classification assigned by the governing State to streams that do not meet State Water Quality Standards. Impaired streams are determined every two years and are listed on the State’s 303(d) list.


Impervious : The characteristic of a material which prevents the infiltration or passage of liquid through it. This may apply to roads, streets, parking lots, rooftops and sidewalks.


Imperviousness : The percentage of impervious cover within a development site or watershed.

Impervious Cover : Any surface in the urban landscape that cannot effectively absorb or infiltrate rainfall.

Impervious Surface : A surface that cannot be penetrated by water such as pavement, rock, or a rooftop and thereby prevents infiltration and generates runoff.

Impoundment : An artificial collection or storage of water, as a reservoir, pit, dugout, sump, etc.

Index of Biological Integrity (IBI) : Tool for assessing the effects of runoff on the quality of the aquatic ecosystem by comparing the condition of multiple groups of organisms or taxa against the levels one expects to find in a healthy stream.

Industrial Activity : Any activity which is directly related to manufacturing, processing or raw materials storage areas at an industrial plant.

Industrial Stormwater Permit : An NPDES permit issued to a commercial industry or group of industries which regulates the pollutant levels associated with industrial storm water discharges or specifies on-site pollution control strategies.

Infill Development : A form of development that utilizes vacant lots or other existing undeveloped property in urban areas and directs development away from rural areas.


Infiltration : The process or rate at which water percolates from the land surface into the ground. Infiltration is also a general category of BMP designed to collect runoff and allow it to flow through the ground for treatment.


Infiltration Facility : A drainage facility designed to use the hydrologic process of water soaking into the ground (commonly referred to as percolation) to dispose of surface and storm water runoff.

Infiltration Trench : A stormwater quality treatment practice that consists of a stone-filled reservoir that allows runoff and accompanying pollutants to settle into the soil where further filtering can take place.

Infiltration Rate (Fc) : The rate at which stormwater percolates into the subsoil measured in inches per hour.

Inflow Protection : A water handling device used to protect the transition area between any water conveyance (dike, swale, or swale dike) and a sediment trapping device.

Inflow Regulation : The control, usually by an engineering device, of the inflow into a stormwater practice.

Initial Abstraction : The maximum amount of rainfall that can be absorbed under specific conditions without producing runoff. Also called initial losses.

Insecticides : Chemicals developed to control or eradicate insects.

Integrated Pest Management (IPM) : The practice of using biological and physical measures to control pests while minimizing or eliminating the use of synthetic chemical pesticides.

Intensity : The depth of rainfall divided by duration.

Invasive Plant : A plant that becomes established and spreads aggressively into new areas and environments, often with detrimental effects on native plant species.

Invert : The lowest flow line elevation in any component of a conveyance system, including storm sewers, channels, weirs, etc. Elevation to the inside bottom of the pipe.

Jurisdictional Wetland : A wetland which is regulated by the U.S. Army Corps of Engineers under Section 404 of the Clean Water Act.

Karst : Topography characterized by regions of carbonaceous rock formations typified by limestone caverns and sinkholes.

Lag Time : The interval between the center of mass of the storm precipitation and the peak flow of the resultant runoff.

Lake : An area permanently inundated by water in excess of two meters (7 ft) deep and greater than twenty acres in size as measured at the ordinary high water mark.

Land Development : A manmade change to, or construction on, the land surface that changes its runoff characteristics.

Landscaping : The placement of vegetation in and around stormwater management BMP’s.

Linear Development Project : A land development project that is linear in nature such as, but not limited to, (I) the construction of electric and telephone utility lines, and natural gas pipelines; (ii) construction of tracks, rights-of-way, bridges, communication facilities and other related structures of a railroad company; and (iii) highway construction projects.

Level Spreader : A device for distributing stormwater uniformly over the ground surface as sheet flow to prevent concentrated, erosive flows and promote infiltration.

Load Allocation : The portion of a receiving water’s loading capacity that is estimated to come from either existing or future nonpoint sources of pollution or natural background sources.

Locality : A county, city, or town.

Lot : A parcel of undivided land.

Low Flow Channel : An incised or paved channel from inlet to outlet in a dry basin which is designed to carry low runoff flows directly to the outlet without detention.

Low Impact Development (LID) : Low impact development is a site design strategy intended to maintain or replicate predevelopment hydrology through the use of small scale controls integrated throughout the site to manage runoff as close to its source as possible. The primary goals of LID are to minimize Site Disturbance and maximize onsite stormwater management.

Low-Input Lawn : A lawn that is regularly mowed but is not subjected to a high usage of chemicals and irrigation.

Manning’s Formula (Hydraulics) : A formula used to predict the velocity of water flow in an open channel or pipeline V = 1/n R2/3 S1/2 Where V is the mean velocity of flow in feet per second; R is the hydraulic radius; S is the slope of the energy gradient or for assumed uniform flow the slope of the channel, in feet per foot; and n is the roughness coefficient or retardance factor of the channel lining.

Margin of Safety : A representation of the uncertainty of the relationship between pollutant loads and the existing quality of the receiving waterbody. MOS’s are a required component of TMDL establishment.

Marsh : A wet area, periodically inundated with standing or slow moving water, that has grassy or herbaceous vegetation and often little peat accumulation; the water may be salt, brackish or fresh.

Maximum Extent Practicable (MEP) : A standard for water quality that applies to all MS4 operators regulated under the NPDES Stormwater Program. Since no precise definition of MEP exists, it allows for maximum flexibility on the part of MS4 operators as they develop and implement their programs.

Measurable Goal : An observable, preferably numerical, achievable target or objective selected by a municipality, or other project manager, or the community that guides and measures the success of the selection, design, and operation or maintenance of a stormwater management measure.

Memorandums of Understanding (MOU) : Agreements by local government agencies and other local stakeholders to work together in exploring solutions/alternatives to water quality issues and the creation of a watershed planning strategy.

Micro-Environment : This term refers to the conditions created under which a separate, smaller environment exists distinct from the dominant one, which can affect and be affected by the immediate surroundings.

Micropool : A smaller permanent pool which is incorporated into the design of larger stormwater ponds to avoid resuspension or settling of particles and minimize impacts to adjacent natural features.

Micropool Extended Detention Pond : A stormwater pond that temporarily detains the majority of the water quality volume in dry storage, and incorporates a small permanent pool in the form of a forebay at the inlet and a micropool at the outlet of the pond.

Microtopography : The complex contours along the bottom of a shallow marsh system, providing greater depth variation which increases the wetland plant diversity and increases the surface area to volume ratio of a stormwater wetland.

Minimum Lot Size : The minimum area or dimension of an individual lot within a particular zoning category, as specified within the local subdivision code.

Mitigation : The replacement of functional values lost when an ecosystem is altered. Mitigation can include replacement, restoration, and enhancement of functional values.

Modified Rational Method : A variation of the rational method used to calculate the critical storage volume whereby the storm duration can vary and does not necessarily equal the time of concentration.

Mulch : Any material such as straw, sawdust, leaves, plastic film, loose soil, wood chips, etc. that is spread or formed upon the surface of the soil to protect the soil and/or plant roots from the effects of raindrops, soil crusting, freezing, evaporation, etc.

Multi-Sector General Permit : An NPDES permit that regulates stormwater discharges from eleven categories of industrial activities.

Municipal Separate Storm Sewer System (MS4) : Any pipe, ditch or gully, or system of pipes, ditches, or gullies, that is owned or operated by a governmental entity and used for collecting and conveying storm water.

Municipal Stormwater Permit : An NPDES permit issued to municipalities to regulate discharges from municipal separate storm sewers for compliance with EPA established water quality standards and/or to specify specific stormwater control strategies.

National Pollutant Discharge Elimination System (NPDES) : A provision of the EPA Clean Water Act that prohibits discharge of pollutants into waters of the United States unless a special permit is issued by the EPA to a state, or (where delegated) a tribal government or and Indian reservation.

Native Plant : A plant that naturally occurred in an area before disturbance by humans.

Natural Buffer : A variable width area maintained with natural vegetation between a pollutant source and a waterbody that provides natural filtration and other forms of protection.

Natural Conveyance System Elements : Swales and small drainage courses, streams, rivers, lakes, and wetlands.

Natural Onsite Drainage Feature : A natural swale, channel, stream, closed depression, wetland, or lake.

Net Annual TSS Load Reduction : Suspended solids removal efficiency target.

Net Site Density : The maximum number of dwelling units which can be constructed on a site after all unbuildable land areas are subtracted out.

Non-Authorized States : Any State that does not have the authority to regulate the NPDES Stormwater Program

Non-Native Plant : Also called “introduced”, this vegetation has been brought to an area by humans and becomes established. These plants are native to other regions. Some non-native plants become invasive.

Nonpoint Source (NPS) Pollution : NPS pollution occurs when rainfall, snowmelt, or irrigation runs over land or through the ground, picks up pollutants, and deposits them into rivers, lakes, and coastal waters or introduces them into ground water.

Nonstructural BMP : A preventative action to protect receiving water quality that does not require construction. Nonstructural BMPs rely predominantly on behavioral changes in order to be effective. Major categories of nonstructural BMPs include education, recycling, maintenance practices and source controls.

Nonstructural Stormwater Practice : Stormwater runoff treatment techniques that use natural measures to reduce pollution levels, do not require extensive construction efforts, and/or promote pollutant reduction by eliminating the pollutant source.

Non-Supporting Stream or Subwatershed : Stream classification for a subwatershed with more than 25% total impervious cover. These streams are not candidates for restoration.

Normal Depth : Depth of flow in an open conduit during uniform flow for the given conditions.

Notice of Intent (NOI) : An application to notify the permitting authority of a facility’s intention to be covered by a general permit; exempts a facility from having to submit an individual or group application.

Nutrient : A substance that provides food or nourishment, such as usable proteins, vitamins, minerals or carbohydrates. Fertilizers, particularly phosphorus and nitrogen, are the most common nutrients that contribute to eutrophication.

Off-Line : A stormwater management system designed to manage a storm event by diverting a percentage of stormwater events from a stream or storm drainage system.

Oil/Grit Separator : A stormwater treatment practice that briefly detains stormwater in three underground concrete chambers. Stormwater first passes through the sedimentation chamber, which is designed to capture coarse sediment particles. It then passes through a second chamber designed to capture oil and grease, and finally into an overflow chamber and back to the storm drain system.

Oil/Water Separator : A vault, usually underground designed to provide a quiescent environment to separate oil from water. Floatables (e.g., styrofoam) are also removed.

On-Line : A stormwater management system designed to manage stormwater in its original stream or drainage channel.

One Year Storm : A stormwater event that occurs on average once every year, or statistically has a 100% chance on average of occurring in a given year.

One Hundred year Storm : A extreme flood event which occurs on average once every 100 years or statistically has a 1% chance on average of occurring in a given year.

Open Channels : Also known as swales, grass channels, and biofilters. These systems are used for the conveyance, retention, infiltration and filtration of stormwater runoff.

Open Space : A portion of a development site which is permanently set aside for public or private use and will not be developed with homes. The space may be used for passive or active recreation, or may be reserved to protect or buffer natural areas.

Open Space Development : The use of designs which incorporate open areas into a development site. These areas can be used for either passive or active recreational activity or preserved as naturally vegetated land.

Open Space Management : The legal and financial arrangements needed to manage open space according to its prescribed use (i.e., natural areas, recreation).

Open Vegetated Channels : Also known as swales, grass channels, and biofilters. These systems are used for the conveyance, retention, infiltration and filtration of stormwater runoff.

Ordinance : A law, a statute, a decree enacted by a municipal body, such as a city council or county commission. Ordinances often govern matters not already covered by state or federal laws (such as local zoning, safety and building regulations), but may also be used to require stricter standards in local communities than those imposed by state or federal law.

Ordinary Care : The basic level of care that can be expected of reasonably experienced and prudent professional in determining design decisions for roadways.

Organic Filter : A filtering practice that uses an organic medium such as peat or compost in the filter bed to filter stormwater runoff.

Outfall : A point where collected and concentrated surface and storm water runoff is discharged from a pipe system or culvert.

Outlet : The point at which water discharges from such things as a stream, river, lake, tidal basin, pipe, channel or drainage area.

Outlet Channel : A waterway constructed or altered primarily to carry water from man-made structures such as terraces, subsurface drains, diversions and impoundments.

Overflow Chamber : A design feature of some on-line stormwater treatment practices that captures larger flows that are not treated by the practice, and passes them to the storm drain system.

Oxygen Depletion : Deficit of dissolved oxygen in a water system due to oxidation of organic matter.

Palustrine Wetland : All nontidal wetlands dominated by trees, shrubs, persistent emergents, emergent mosses, or lichens; and all such tidal wetlands in areas where salinity from ocean-derived salts is below 0.5 parts per thousand.

Parking Demand : The number of parking spaces actually used for a particular land use.

Parking Lane : A section of the roadway which has been designed to provide on-street parking for residential neighborhoods.

Parking Ratios : An expression of the required parking spaces that must be provided for a particular land use, often stated as a ratio of x spaces per y units.

Parking Stall : The total area needed to accommodate the parking of a single vehicle, extending outward from the curb, and between the stripes.

Pea Gravel Curtain Drain : A thin wall of small, river-run gravel used to convey water to the sides/bottom of bioretention practices.

Pea Gravel Diaphragm : A stone trench filled with small, river-run gravel used as pretreatment and inflow regulation in stormwater filtering systems.

Peak Discharge (Flow Rate) : The maximum instantaneous rate of flow during a storm, usually in reference to a specific design storm event.

Peak Flow : The maximum flow that the collection system is designed to handle, typically associated with a recurrence interval (e.g., 10-yr, 25-yr, 50-y or 100-yr).

Peak Rate (of Runoff) : The maximum instantaneous rate at which runoff is discharged from a site as the result of a precipitation event, usually measured in cubic feet per second.

Percent Area Method : Technique used to evaluate the compliance of a non-structural stormwater practice for meeting recharge requirements by calculating the percent of impervious area effectively treated and comparing to a minimum recharge target percentage for the various soil groups.

Percent Impervious : The specific portion of the contributing drainage area that does not allow stormwater to infiltrate, usually expressed as a percent 0-100%. Examples include asphalt or bituminous surface and other non-porous or compacted surfaces.

Percent Volume Method : Procedure used with structural stormwater practices to evaluate compliance with recharge requirements by assuring that the volume of runoff treated by the practice exceeds the computed recharge volume.

Percolation Rate : The velocity at which water moves through saturated, granular material.

Perennial Plant : Persisting for more than one year. Perennial plant species persist as woody vegetation from year to year or resprout from their rootstock annually.

Perennial Stream : A stream channel that has running water throughout the year.

Performance Standard : An established amount or limit of a specified pollutant that can be discharged from a land-use activity or BMP.

Perimeter Sand Filter : A filtering practice design also referred to as the “Delaware” sand filter. The sedimentation chamber and filtering bed chamber are two vaults in series, parallel to the edge of a parking lot or other impervious surface. Runoff flows into the sedimentation chamber through grates in the parking lot surface, and is then returned to the storm drain system via underdrains at the bottom of the filter bed.

Permanent Seeding : Results in establishing perennial vegetation which may remain on the area for many years.

Permeability : The rate of water movement through the soil column under saturated conditions.

Permissible Velocity (Hydraulics) : The highest average velocity at which water may be carried safely in a channel or other conduit. The highest velocity that can exist through a substantial length of a conduit and not cause scour of the channel. A safe, non-eroding or allowable velocity.

Pervious : Any material that allows for the passage of liquid through it.

Permitting Authority (PA) : The NPDES-authorized state agency or EPA regional office that administers the NPDES Stormwater Program. PAs issue permits, provide compliance assistance, and inspect and enforce the program.

pH : An expression of the intensity of the basic or acidic condition of a liquid. Natural waters usually have a pH range between 6.5 and 8.5. A pH of 7.0 denotes neutrality, higher values indicate alkalinity, and lower values indicate acidity.

Phase 1 Stormwater Permit Program : The Phase I program addressed sources of storm water runoff that had the greatest potential to negatively impact water quality. Under Phase I, EPA required NPDES permit coverage for storm water discharges from “medium” and “large” municipal separate storm sewer systems (MS4s) located in incorporated places or counties with populations of 100,000 or more; and eleven categories of industrial activity, one of which is construction activity that disturbs five or more acres of land.

Phase 2 Stormwater Permit Program : The Phase II Program requires NPDES permit coverage for storm water discharges from certain regulated small municipal separate storm sewer systems (MS4s); and construction activity disturbing between 1 and 5 acres of land.

Phosphorus : An element found in fertilizers and sediment runoff which can contribute to the eutrophication of water bodies. It is the keystone pollutant in determining pollutant removal efficiencies for various BMP’s.

Photic Zone : The area of a water body receiving sunlight.

Physically interconnected MS4 : This means that one MS4 is connected to a second MS4 in such a way that it allows for direct discharges into the second system.

Piping : Removal of soil material through subsurface flow channels or “pipes” developed by seepage water.

Planimeter : An instrument used to calculate land area on a paper map.

Planning Area : A designated portion of the parcel on which a land development project is located. Planning areas must be established by delineation on a master plan. Once established, planning areas must be applied consistently for all future projects.

Plant Community : All of the plant species and individuals occurring in a shared habitat or environment.

Plugs : Pieces of turf or sod, usually cut with a round tube, which can be used to propagate the turf or sod by vegetative means.

Plunge Pool : A small permanent pool at either the inlet to a BMP or at the outfall from a BMP. The primary purpose of the pool is to dissipate the velocity of stormwater runoff, but it also can provide some pretreatment.

Pocket Pond/Wetland : A stormwater wetland design adapted for the treatment of runoff from small drainage areas (< 5 acres) and which has little or no baseflow available to maintain water elevations and relies on ground water to maintain a permanent pool.

Point discharge : The release of collected and/or concentrated surface and storm water runoff from a pipe, culvert, or channel.

Point Source Pollutant : Stormwater discharges are generated by runoff from land and impervious areas such as paved streets, parking lots, and building rooftops during rainfall and snow events that often contain pollutants in quantities that could adversely affect water quality. Most storm water discharges are considered point sources and require coverage by an NPDES permit. The primary method to control storm water discharges is through the use of best management practices.

Pollution : Any substance that exists in the environment that is undesirable or harmful for that

environment. :


Pollution-Generating Impervious Surface : An impervious surface considered to be a significant source of pollutants in surface and storm water runoff.. Such surfaces include those subject to vehicular use or storage of erodible or leachable materials, wastes, or chemicals, and which receive direct rainfall or the run-on or blow-in of rainfall. Thus, a covered parking area would be included if runoff from uphill could regularly run through it or if rainfall could regularly blow in and wet the pavement surface. Metal roofs are also considered pollution-generating impervious surface unless they are treated to prevent leaching.

Pollution-Generating Pervious Surface : A non-impervious surface with vegetative ground cover subject to use of pesticides and fertilizers. Such surfaces include, but are not limited to, the lawn and landscaped areas of residential or commercial sites, golf courses, parks, and sports fields.

Pollutant Loading : The total quantity of pollutants in stormwater runoff.

Polluted Runoff : Rainwater or snowmelt that picks up pollutants and sediments as it runs off roads, highways, parking lots, lawns, agricultural lands, and other land-use activities that can generate pollutants.

Pollution Prevention Plan : A requirement for some land uses or activities (e.g., industrial sites) that outlines techniques to prevent pollutants from being washed off in stormwater runoff (e.g., spill response, material handling, employee training, etc.)

Pond Buffer : The area immediately surrounding a pond which acts as filter to remove pollutants and provide infiltration of stormwater prior to reaching the pond. Provides a separation barrier to adjacent development.

Pond Drain : A pipe or other structure used to drain a permanent pool within a specified time period.

Pondscaping : Landscaping around stormwater ponds which emphasizes native vegetative species to meet specific design intentions. Species are selected for up to six zones in the pond and its surrounding buffer, based on their ability to tolerate inundation and/or soil saturation.

Pore Space : Open space in rock or granular material; also known as interstices.

Porosity : Ratio of pore volume to total solids volume.

Porous Pavement and Pavers : Alternatives to conventional asphalt that utilize a variety of porous media, often supported by a structural matrix, concrete grid, or modular pavement, which allow water to percolate though to a sub-base for gradual infiltration.

Post-Development : Refers to conditions that reasonably may be expected or anticipated to exist after completion of the land development activity on a specific site or tract of land.

Pre-Development : Refers to the conditions that exist at the time that plans for the land development of a tract of land are approved by the plan approval authority. Where phased development or plan approval occurs (preliminary grading, roads and utilities, etc.), the existing conditions at the time prior to the first item being approved or permitted establishes the pre-development conditions.

Pretreatment : Techniques employed in stormwater practices to provide storage or filtering to help trap coarse materials before they enter the system.

Primary Stormwater Treatment Practice : Stormwater treatment practices that are capable of providing high levels of water quality treatment as stand alone devices; can be grouped into five major catergories – stormwater ponds, stormwater wetlands, infiltration practices, filtering practices, and water quality swales.

Principal Spillway : The primary pipe or weir which carries baseflow and storm flow through the embankment.

Public Turf : Pervious land held in the public domain. Examples include parks, golf courses, cemeteries, median strips and school grounds.

Queuing Street : A narrowed street which contains a single travel lane and which may occasionally require an opposing driver to pull over to allow an oncoming vehicle to pass.

Rain Barrel : A temporary storage device connected to a roof downspout, typically including a hose attachment to allow for reuse of rooftop runoff.

Rainfall Frequency Spectrum : The frequency distribution of cumulative rainfall volume generated by all storm events. This analysis is used to determine how much rain can be treated in a stormwater filter, and how much may be bypassed.

Rain Garden : Functional landscape elements that combine plantings in depressions that allow water to pool for only a few days after a rainfall then be slowly absorbed by the soil and plantings.

Rapid Stream Assessment Technique : A set of protocols developed to provide a simple, quick field-level assessment of stream quality conditions.

Rate-Based Design : Stormwater practice design which uses the discharge in volume per unit of time as a basis for sizing the practice.

Rational Method : Means of computing peak storm drainage flow rates based on average percent imperviousness of the site, mean rainfall intensity, and drainage area.

Reach : A length of channel with uniform characteristics.

Receiving Waters : Bodies of water or surface water systems receiving water from upstream man-made or natural systems.

Recharge : Replenishment of groundwater reservoirs by infiltration and transmission of water through permeable soils.

Recharge Rate : Annual amount of rainfall which contributes to groundwater as a function of hydrologic soil group.

Redevelopment : Any construction, alteration, or improvement on existing development.

Reference Condition : An area in a watershed that is least impacted in comparison to other areas. This area can be used as a baseline to judge the success of future watershed management efforts.

Residential Subdivision : A large land area divided into smaller parcels for the purpose of housing.

Restorable Stream or Watershed : Stream classification that is impacted or non-supporting but has high retrofit or stream restoration potential.

Retention : The amount of precipitation on a drainage area that does not escape as runoff. It is the difference between total precipitation and total runoff.

Retention and Detention (R/D) Facility : A type of drainage facility designed either to hold water for a considerable length of time and then release it by evaporation, plant transpiration, and/or infiltration into the ground, or to hold surface and storm water runoff for a short period of time and then release it to the surface and storm water conveyance system. Also called flow control facilities.

Retention Basin : A stormwater management facility which includes a permanent impoundment, or normal pool of water, for the purpose of enhancing water quality and, therefore, is normally wet, even during nonrainfall periods. Storm runoff inflows may be temporarily stored above this permanent impoundment for the purpose of reducing flooding, or stream channel erosion.

Return Frequency Storm (Rainfall Event) : The average period of time that an observer must wait between the occurrence of an event of a particular statistic probability of a storm of equal magnitude, or larger magnitude occurring. For example, when the interval between observations is a year, a return frequency period of 100 years means that, on the average, an event of this magnitude or greater is expected to occur not more often than once in 100 years.

Retrofit : The creation or modification of a stormwater management practice, usually in a developed area, that improves or combines treatment with existing stormwater infrastructure.

Return Period : The average length of time between events having the same volume and duration. If a storm has a one percent chance of occurring in any given year, than it has a return period of 100 years.

Reverse Slope Pipe : A pipe which draws from below a permanent pool extending in a reverse angle up to the riser and which determines the water elevation of the permanent pool.

Right-of-Way : 1. Right of passage, as over another’s property. 2. A route that is lawful to use. 3. A strip of land acquired for transport or utility construction. 4. The design area of a roadway which includes the pavement width, vegetated strip, sidewalk and space designated for utility location

Rill Erosion : An erosion process in which numerous small channels only several inches deep are formed.

Rim : Elevation at the finish grade.

Rip-Rap : Broken rock, cobbles, or boulders placed on earth surfaces, such as the face of a dam or the bank of a stream, for protection against the action of water (waves); also applies to brush or pole mattresses, or brush and stone, or similar materials used for soil erosion control.

Riparian : The land area which borders a stream or river and which directly affects and is affected by the water quality. This land area often coincides with the maximum water surface elevation of the 100 year storm.

Riparian Buffer : See Buffer

Riparian Corridor : Narrow strip of land, centered on a stream, that includes the floodplain as well as related riparian habitats adjacent to the floodplain.

Riser : A vertical pipe which extends from the bottom of a pond stormwater practice and houses the control devices (weirs/orifices) to achieve the discharge rates for specified designs.

Riverine Wetlands : Wetlands associated with rivers.

Roughness Coefficient (Hydraulics) : A factor in velocity and discharge formulas representing the effect of channel roughness on energy losses in flowing water. Manning’s “n” is a commonly used roughness coefficient.

Routing : A method of measuring the inflow and outflow from an impoundment structure while considering the change in storage volume over time.

Runoff (Hydraulics) : That portion of the precipitation on a drainage area that is discharged from the area in the stream channels. Types include surface runoff, ground water runoff or seepage.

Runoff Coefficient (RV) : A value derived from a site impvious cover value that is applied to a given rainfall volume to yield a corresponding runoff volume.

Runoff Pretreatment : Technique employed in a stormwater practice to retain storage volumes or prevent clogging by trapping coarse materials before they enter the system.

Run-on : The flow of stormwater from impervious cover to pervious cover.

Safety Bench : A flat area above the permanent pool and surrounding a stormwater pond designed to provide a separation from the pond pool and adjacent slopes.

Sand : 1. (Agronomy) A soil particle between 0.05 and 2.0 millimeters in diameter. 2. A soil textural class. 3. (Engineering) According to the Unified Soil Classification System, a soil particle larger than the No. 200 sieve (0.074mm) and passing the No. 4 sieve (approximately 1/4 inch).

Sand Filter : A contained bed of sand which acts to filter the first flush of runoff. The runoff is then collected beneath the sand bed and conveyed to an adequate discharge point or infiltrated into the in-situ soils.

Sanitary Sewer System : Underground pipes that carry only domestic or industrial wastewater to a sewage treatment plant or receiving water.

Superfund Amendments and Reauthorization Act (SARA) : An acronym for the 1980 Congressional legislation regarding the management and cleanup of hazardous waste sites.

SARA 312 Generators : Facilities which are required by the Emergency Planning and Community Right-to-Know Act (EPCRA), also known as SARA Title III, to submit an inventory of the location and amount of hazardous chemicals which are located at a site.

Saturated Soil : Soil in which the pore space is completely filled with water.

Sediment : Solid material, both mineral and organic, that is in suspension, is being transported, or has been moved from its site of origin by air, water, gravity, or ice and has come to rest on the earth’s surface either above or below sea level.

Secondary Stormwater Treatment Practices : Stormwater treatment practices that may not be suitable as stand alone treatment because they are either not capable of meeting the water quality treatment performance criteria or have not yet received the thorough evaluation needed to demonstrate the capabilities for meeting the performance criteria.

Sediment Forebay : A settling basin or plunge pool constructed at the incoming discharge points of a stormwater facility.

Sediment : Soil, sand, and minerals washed from land into water, usually after rain. Sediment can destroy fish-nesting areas, clog animal habitats, and cloud waters so that sunlight does not reach aquatic plants.

Sedimentation : A solid-liquid separation process utilizing gravitational settling to remove soil or rock particles from the water column.

Sedimentation Chamber : This is a section of a stormwater practice that provides for the settling out of large particles from suspension.

Seed Bank : The accumulation of viable plant seeds occurring in soil and available for germination under favorable environmental conditions.

Seepage : 1. Water escaping through or emerging from the ground. 2.The process by which water percolates through the soil.

Seepage Length : In sediment basins or ponds, the length along the pipe and around the anti-seep collars that is within the seepage zone through an embankment.

Sensitive Stream or Subwatershed : Stream classification for a subwatershed with less than 10% impervious cover, that is still capable of supporting stable channels and has good to excellent biodiversity.

Septic System : An onsite wastewater collection system

Setbacks : The minimum distance requirements for location of a structural stormwater practice in relation to roads, wells, septic fields, other structures.

Settable Solids : particles that are typically large enough to be removed through conventional stormwater and/or wastewater sedimentation and/or siltation methods. Typical methods include settling basins and/or stormwater ponds.

Sewer System : The system of pipes and pump stations that collect and transport wastewater from homes and businesses to a wastewater treatment plant.

Shallow Marsh : A wetland that provides water quality treatment entirely in wet shallow marsh.

Shared Parking : A parking strategy which reduces the total number of parking spaces needed by allowing adjacent users to share a parking area during noncompeting hours of operation.

Sheet Erosion : The spattering of small soil particles caused by the impact of raindrops on wet soils. The loosened and spattered particles may or may not subsequently be removed by surface runoff.

Sheet Flow : Water, usually storm runoff, flowing in a thin layer over the ground surface.

Short-Circuiting : The passage of runoff through a stormwater practice in a time span or flow path without adequate treatment.

Side Slopes (Engineering) : The slope of the sides of a channel, dam or embankment. It is customary to name the horizontal distance first, as 1.5 to 1, or frequently, 1 ½

Sight Distance : The length of roadway ahead visible to the driver. The minimum sight distance should be long enough to allow a vehicle traveling at or near the speed limit to stop before reaching a motionless object in its path.

Silt : Typically smaller sized, fine grain particles. 1. (Agronomy) A soil separate consisting of particles between 0.05 and 0.002 millimeter in equivalent diameter. 2. A soil textural class. 3. (Engineering) According to the Unified Soil Classification System a fine grained soil (more than 50 percent passing the No. 200 sieve) that has a low plasticity index in relation to the liquid limit.

Siltation : A solid-liquid separation process utilizing gravitational settling to remove fine-grained solids, including soil or rock particles from the water column. See also – Sedimentation.

Silviculture : A branch of forestry dealing with the development and care of forests.

Site : The parcel of land being developed, or a designated planning area in which a land development project is located.

Site Disturbance : any modification to a piece of property. Site disturbances are activities that can include, but are not limited to, grading, excavating, natural resource removal, tree removal, wetland removal, structure demolition, structure building, paving, etc.

Site Plan : A graphical representation of a layout of buildings and facilities on a parcel of land.

Site Planning and Design : Techniques of engineering and landscape design that maintain predevelopment hydrologic functions and pollutant removal mechanisms to the extent practical.

Site Runoff : Any drainage or flood discharge that is released from a specified area.

Smart Growth : Development that uses a variety of strategies to enhance existing communities and protect community character in a way that is compatible with the natural environment and attracts economic development. It encourages more town-oriented, transit-focused, and pedestrian-friendly new development while restoring vitality to existing developed areas.

Site Stormwater Management Plan : Plan describing the potential water quality and quantity impacts associated with a development project both during and after construction. It also identifies selected source controls and treatment practices to address those potential impacts, the engineering design of the treatment practices, and maintenance requirements for proper performance of the selected practices.

Sanitary Sewer Overflow (SSO) : Wastewater entering sanitary sewers may be so great, because of blockage, a lack of capacity, inflow and infiltration, or other reasons, that the collection system or sewage treatment plant cannot handle the increased flow. As a result, untreated sewage empties directly into receiving waters, often from manholes or up through sewer connections.

Smart Growth : Development that uses a variety of strategies to enhance existing communities and protect community character in a way that is compatible with the natural environment and attracts economic development. It encourages more town-oriented, transit-focused, and pedestrian-friendly new development while restoring vitality to existing developed areas.

Soil Science : Science dealing with soils as a natural resource on the surface of the earth including soil formation, classification, mapping; physical, chemical, biological, and fertility properties of soils per se; and these properties in relation to the use and management of soils.

Soil Test : Chemical analysis of soil to determine needs for fertilizers or amendments for species of plant being grown.

Soil Texture : Relative proportion of the physical components of any given soil. For instance, clay is defined as soil having >40% clay, <45% sand and <40% silt.

Sole-Source Aquifer : An aquifer whose recharge area is the only source of drinking water to both public water supplies and private wells.

Source control : Action to prevent pollution where it originates.

Spillway : An open or closed channel, or both, used to convey excess water from a reservoir. It may contain gates, either manually or automatically controlled to regulate the discharge of excess water.

Sprawl Development : Expansion of low-density development into previously undeveloped land.

Stabilization : Providing adequate measures, vegetative and/or structural that will prevent erosion from occurring.

Stage (Hydraulics) : The variable water surface or the water surface elevation above any chosen datum.

Stakeholder : Any agency, organization, or individual that is involved in or affected by the decisions made in the development of a watershed plan.

Steep Slope : An area of a development site that is too steep to (a) safely build on or (b) has a high potential for severe soil erosion during construction.

Stilling Basin : An open structure or excavation at the foot of an outfall, conduit, chute, drop, or spillway to reduce the energy of the descending stream of water.

Storm Drain System : The system of gutters, pipes, streams, or ditches used to carry surface and storm water from surrounding lands to streams, lakes, Storm Sewer – A system of pipes, separate buildings and land surfaces.from sanitary sewers, that only carries runoff from

Storm Sewer System : A system of pipes and channels that carry stormwater runoff from the surfaces of building, paved surfaces, and the land to discharge areas.

Stormwater : Water derived from a storm event or conveyed through a storm sewer system.

Stormwater Basin : A structural facility designed to impound stormwater runoff. Can be for stormwater detention or retention.

Stormwater Credits : A form of incentive for developers to promote conservation of natural and open space areas. Developers are allowed reductions in stormwater management requirements when they use techniques to reduce stormwater runoff at the site.

Stormwater Filtering (or Filtration) : A pollutant removal method to treat stormwater runoff in which stormwater is passed through a filter media such as sand, peat, grass, compost, or other materials to strain or filter pollutants out of the stormwater.

Stormwater Hot Spot : An area where the land use or activities are considered to generate runoff with concentrations of pollutants in excess of those typically found in stormwater. Examples include fueling stations and airport de-icing facilities.

Stormwater Infiltration Systems : Stormwater practices that are designed to percolate runoff into the underlying soil.

Stormwater Management : The programs to maintain quality and quantity of stormwater runoff as required by Federal, State and Local laws, codes, and ordinances.

Stormwater Management Facility : A structural BMP device that controls stormwater runoff and changes the characteristics of that runoff including, but not limited to, the quantity and quality, the period of release or the velocity of flow.

Stormwater Management Plan (SMP) : The document that describes how existing or pre-development site runoff characteristics will be affected by a land development project and the specific methods, techniques, and stormwater Best Management Practices (structural and nonstructural) required to comply with applicable Federal, State and Local laws, codes and ordinances. SMPs are typically required for pre- and post- construction periods, including operation and maintenance activities.

Stormwater Outfall : A discharge point for stormwater runoff which has been collected in a conveyance system.

Stormwater Pollution : naturally occurring and manmade organic and inorganic containments collected within the runoff during and after precipitation events. Contaminants can include, but are not limited to, solid waste, oil, grease, paint, fertilizer, pesticide, lawn and garden clippings, leaves, pet waste, and sediment.

Stormwater Ponds : A land depression created for the detention or retention of stormwater runoff.

Stormwater Pollution Prevention Plan (SWPPP) : A plan to describe the process whereby a facility thoroughly evaluates potential pollutant sources at a site during and after construction activities. The SWPPP identifies and provides a clear outline of the implementation, operation and maintenance measures designed to prevent, treat and control the discharge of pollutants in stormwater runoff.

Stormwater Management Practice : A structural or non-structural process designed to store or treat stormwater runoff in order to mitigate flooding, reduce pollution and provide other amenities. See also – Best Management Practice (BMP).

Stormwater Quality Control : The removal of pollutants from stormwater runoff through the use of stormwater management practices.

Stormwater Treatment Train : BMPs designed in series or in parallel to treat pollutants in a manner that meets Federal, State and Local laws, codes and ordinances. BMP treatment trains allow for various types of pollutant removal techniques that can maximize treatment while minimizing operation and maintenance activities.

Stormwater Utility : A regulated utility established to generate a dedicated source of funding for stormwater management where users pay a fee based typically on equivalent residential units (ERUs) per the land owners specific land-use, area, and anticipated volume contribution of runoff to the utility’s stormwater system.

Stormwater Wetland : A manmade, constructed wetland that utilizes characteristics and functions similar to natural wetlands to treat stormwater. See also-Subsurface Flow Wetland; Free Water Surface Wetland.

Stream Buffers : Zones of variable width which are located along both sides of a stream and are designed to provided a protective natural area along a stream corridor.

Stream Channel Protection (CpV) : A design criteria that requires 24 hour detention of the one year postdeveloped, 24 hour storm event for the control of stream channel erosion.

Streambank Erosion : Removal of soil particles from a bank slope primarily due to water action. Changes in land use, climatic conditions, ice and debris and chemical reactions can also lead to streambank erosion.

Structural BMP : Constructed facilities or measures to help protect receiving water quality and control stormwater quantity. Examples include storage, vegetation, infiltration, and filtration.

Structural Stormwater Practices : Devices that are constructed to provide temporary storage and treatment of stormwater runoff.

Structured Parking : More commonly referred to as parking garages, these are parking facilities which expand vertically to provide parking on various levels. Structured parking allows more parking on sites where space for single level parking lots is no longer available.

Subdivision : A new development that splits an existing tract, parcel or lot into two or more parts.

Subdivision Code or Subdivision Control Ordinance : A set of local requirements that govern the geometric dimensions of a particular zoning category, and also specifies the nature and geometry of roads, drainage, waste disposal and other community services that must be constructed to serve the development.

Subgrade : The soil prepared and compacted to support a structure or a pavement system.

Substrate : Substances used by organisms for growth in a liquid medium. Surface area of solids or soils used by organisms to attach.

Subsurface Flow (SSF) Wetland : Typically a constructed wetland that is designed to prohibit direct open access to any water surface of any portion to the wetland. Designed with porous granular materials, vegetation is typically maintained through a hydroponic environment.

Subwatershed : A smaller geographic section of a larger watershed unit with a drainage area of between 2 to 15 square miles and whose boundaries include all the land area draining to a point where two second order streams combine to form a third order stream.

Succession : The temporal changes of plant and animal populations and species in an area that has been disturbed.

Surface Sand Filter : A filtering practice that consists of a sediment chamber and filter bed chamber both above ground. Stormwater flows from the first chamber to the second chamber, and is then returned to the storm drain system via underdrains at the bottom of the filter bed.

Surcharge : Flow condition occurring in closed conduits when the hydraulic grade line is above the crown of the sewer. This condition usually results localized flooding or stormwater flowing out the top of inlet structures and manholes

Surface Water : Water that flows across the land surface, in channels, or is contained in depressions on the land surface (e.g. runoff, ponds, lakes, rivers, and streams).

Suspended Solids (SS) : small particles, typically less than 1.2 micrometers in size, that resist separation or removal through conventional stormwater and/or wastewater treatment methods.

Swale : A natural or human-made open depression or wide, shallow ditch that intermittently contains or conveys runoff. Can be used as a BMP to detain and filter runoff.

Storm Water Management Model (SWMM) : Rainfall-runoff event simulation model sponsored by the U.S. Environmental Protection Agency.

Tailwater : Water, in a river or channel, immediately downstream from a structure.

Technical Advisory Committee : The group of technical experts that assures quality control over the technical aspects of a watershed plan.

Technical Release No. 20 (TR-20) : A NRCS watershed hydrology computer model that is used to compute runoff volumes and route storm events through a stream valley and/or ponds.

Technical Release No. 55 (TR-55) : A watershed hydrology model developed by NRCS used to calculate runoff volumes and provide a simplified routing for storm events through ponds.

Temporary Seeding : A seeding which is made to provide temporary cover for the soil while waiting for further construction or other activity to take place.

Ten Year Storm (QP 10) : The peak discharge rate associated with a 24 hour storm event which exceeds bankfull capacity and occurs on average once every ten years (or has a likelihood of occurrance of 1/10 in a given year).

Terrestrial : Living or growing on land that is not normally flooded or saturated.

Tightline : Typically a continuous length of pipe used to convey flows down a steep or sensitive slope with appropriate energy dissipation at the discharge end.

Time of Concentration : Time required for water to flow from the most remote point of a watershed, in a hydraulic sense, to the outlet.

Toe (of Slope) : Where the slope stops or levels out. Bottom of the slope.

Toe Wall : Downstream wall of a structure, usually to prevent flowing water from eroding under the structure.

Topsoil : Fertile or desirable soil material used to top dress roadbanks, subsoils, parent material, etc.

Tool Box : A term to describe the activities and materials that EPA plans toperform/produce to facilitate implementation of the stormwater program in an effectiveand cost-efficient manner. The eight components include

Total Kjeldahl Nitrogen (TKN) : A measure of the ammonia and organic nitrogen present in a water sample.

Total Maximum Daily Load (TMDL) : A tool for establishing the allowable loadings of a given pollutant in a surface water resource to meet predetermined water quality standards.

Total Suspended Solids (TSS) : The total amount of particulate matter which is suspended in the water column.

Toxic : Poisonous, carcinogenic, or otherwise directly harmful to life.

Toxic substances : Those chemical substances, such as pesticides, plastics, heavy metals, detergents, solvents, or any other harmful materials, which are poisonous, carcinogenic, or otherwise directly harmful to human health and the environment.

Toxicant : A poisonous agent that kills or injures animal or plant life.

Transferable Development Rights : A form of incentive for developers in which the developer purchases the rights to an undeveloped or underdeveloped piece of property in exchange for the right to increase the number of dwelling units on another site. Often used to concentrate development density in certain land areas.

Transition Zone : The area between habitats or ecosystems. Frequently, transition zone is used to refer to the area between uplands and wetlands. In other cases, wetlands are referred to as transitional areas between uplands and aquatic ecosystems.

Transpiration : The transport of water vapor from the soil to the atmosphere through growing plants.

Trash Rack : Grill, grate or other device at the intake of a channel, pipe, drain or spillway for the purpose of preventing oversized debris from entering the structure.

Travel Time : The time required for water to flow from the outlet of a drainage sub-basin to the outlet of the entire drainage basin being analyzed. Travel time is normally concentrated flow through an open or closed channel.

Treatment Train : BMPs designed in series or in parallel to treat pollutants in a manner that meets Federal, State and Local laws, codes and ordinances. BMP treatment trains allow for various types of pollutant removal techniques that can maximize treatment while minimizing operation and maintenance activities.

Tributary : A stream that flows into a larger stream or other body of water.

Truncated Hydrograph : A method of computing the required design infiltration storage volume utilizing the differences from post-developed and pre-developed hydrograph volumes over a specific time frame.

Turbidity : Cloudiness of a liquid, caused by suspended solids; a measure of the suspended solids in a liquid.

Two Year Storm (QP 2) : The peak discharge rate associated with a 24 hour storm event which exceeds bankfull capacity and occurs on average once every two years (or has a likelihood of occurrence of 1/2 in a given year).

Unbuildable Lands : The portions of a development site where structures cannot be located for physical or environmental reasons.

Ultimate Condition : Full watershed build-out based on existing zoning.

Ultra-Urban : A region dominated by highly developed areas in which very little pervious surface exists.

Urbanized Area (UA) : A Bureau of the Census determination of a central place (or places) and the adjacent densely settled surrounding territory that together have a minimum residential population of 50,000 people and a minimum average density of 1,000 people/square mile.

Underdrain System : A perforated pipe system in a gravel bed, installed on the bottom of filtering stormwater practices, which are used to collect and remove filtered runoff.

Underground Sand Filter : A three-chamber filtering practice design completely below the ground surface. Stormwater is diverted to the sedimentation chamber, and then flows to the filter bed chamber. Bypassed flows and treated stormwater are returned to the storm drain system through a third overflow chamber.

Upland : An area that is not an aquatic, wetland, or riparian habitat. An area that does not have the hydrologic regime necessary to support hydrophytic vegetation.

Urban Forestry : The study of trees and forests in and around towns and cities.

Urban Growth Boundaries : Planning tool that establishes a dividing line that defines where a growth limit is to occur and where agricultural or rural land is to be preserved.

Urban Return Flows : Non-stormwater discharges into a watershed caused by activities such as car washing and watering lawns.

Urban Runoff : Stormwater from city streets and adjacent domestic or commercial properties that carries nonpoint source pollutants of various kinds into the sewer systems and receiving waters.

Variance : A special allowance granted to a developer which permits the use of designs different from the requirements of the current code.

Vegetated Open Channels : Also known as swales, grass channels, and biofilters. These systems are used for the conveyance, retention, infiltration and filtration of stormwater runoff.

Vegetated Roof Covers (Green Roofs) : Multilayered, constructed roof systems consisting of a vegetative layer, media, a geotextile layer, and a synthetic drain layer installed on building rooftops. Rainwater is either intercepted by vegetation and evaporated to the atmosphere or retained in the substrate before being returned to the atmosphere through transpiration and evaporation.

Velocity Head : Head due to the velocity of a moving fluid, equal to the square of the mean velocity divided by twice the acceleration due to gravity (32.16 feet per second per second).

Volume-Based Design : A stormwater practice design which uses the volume of runoff as a basis for sizing the practice.

Volumetric Runoff Coefficient (Rv) : The value that is applied to a given rainfall volume to yield a corresponding runoff volume based on the percent impervious cover in a drainage basin.

Vortex : A mass of fluid moving in a circular motion. A vortex creates a vacuum and can cause cavitation in a riser.

Wasteload Allocation : The portion of a receiving water’s loading capacity that is estimated to come from present or future point sources of pollution and is regulated by the NPDES program.

Wastewater : Usually refers to effluent from a sewage treatment plant.

Waterbody : any natural or manmade waterbasins, watercourses and/or wetlands.

Watershed : A geographic area in which water, sediments, and dissolved materials drain to a common outlet, typically a point on a larger stream, a lake, an underlying aquifer, an estuary, or an ocean. A watershed is also sometimes referred to as the “drainage basin” of the receiving waterbody.

Watershed Management Unit : Refers to one of five categories based on typical drainage area. The five categories from smallest to largest are catchment, subwatershed, watershed, subbasin, and basin. Impervious cover influences each unit in varying degrees and corresponding management measures usually differ as well.

Water Surface Profile : The longitudinal profile assumed by the surface of a stream flowing in an open channel; the hydraulic grade line.

Water Quality Criteria : Specific levels of water quality that, if achieved, are expected to render a body of water suitable for its designated use. The criteria are based on specific levels of pollutants that would make the water harmful if used for drinking, swimming, farming, fish production, or industrial processes.

Water Quality Standards : State-adopted and EPA-approved ambient standards for water bodies. The standards prescribe the use of the water body and establish the water quality criteria that must be met to protect designated uses.

Water Quality Swales : Vegetated open channels designed to treat and attenuate the water quality volume and convey excess stormwater runoff. Dry swales are primarily designed to receive drainage from small impervious areas and rural roads. Wet swales are primarily used for highway runoff, small parking lots, rooftops, and pervious areas.

Water Quality Treatment Facility : A drainage facility designed to reduce pollutants once they are already contained in surface and storm water runoff. Water quality treatment facilities are the structural component of best management practices (BMPs); when used singly or in combination, WQ facilities reduce the potential for contamination of surface and/or ground waters.

Water Quality Volume (WQV) : The storage needed to capture and treat 90% of the average annual stormwater runoff volume equal to the 1″ (or 0.9″ in western zone ) times the volumetric runoff coefficient (Rv) times the site area.

Water Quality Design Flow : The flow which is required or desired to be treated by the structural BMP device. Often times the design flow is associated with a rainfall depth of intensity that has a high probability of occurrence or frequency (e.g., 50-90% of all rainfall events in a given year occur at an intensity of 0.2 in/hr).

Water Surface Profile : Longitudinal profile assumed by the surface of a stream flowing in an open channel; hydraulic grade line.

Water Table : Upper surface of the free groundwater in a zone of saturation.

Wattles : Fence or barrier constructed of interwoven twigs and branches used to stabilize soil from erosive forces.

Wedges : Design feature in stormwater wetlands that increases flow path length to provide for extended detention and treatment of runoff.

Weir : A wall or plate placed in an open channel to regulate or measure the flow of water.

Wet Detention Ponds : A BMP consisting of a permanent pool of water designed to treat runoff by detaining water long enough for settling, filtering, and biological uptake. Wet ponds are also often designed to have an aesthetic or recreational value.

Wet Extended Detention Pond : A stormwater pond design that captures some of the water quality volume in the permanent pool, and temporarily detains the remainder of this volume above the permanent pool.

Wetfall : The deposition of atmospheric pollutants on the land surface that are washed out by precipitation.

Wetland : An area inundated or saturated by ground or surface water at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs, and similar areas (U.S. Army Corps of Engineers Regulation 33 CFR 328.3 (1988)).

Wetlands Protection : Activities to protect and restore wetlands that are an integral part of a nonpoint source management program or part of implementation or development of comprehensive estuary conservation and management plans.

Wet Pond : Drainage facilities for water quality treatment that contain a permanent pool of water. They are designed to optimize water quality by providing long retention times (on the order of a week or more) to settle out particles of fine sediment to which pollutants such as heavy metals adsorb, and to allow biologic activity to occur that metabolizes nutrients and organic pollutants. For wetvaults, the permanent pool of water is covered by a lid which blocks sunlight from entering the facility, limiting light-dependent biologic activity.

Wet Vault : Drainage facilities for water quality treatment that contain a permanent pool of water. They are designed to optimize water quality by providing long retention times (on the order of a week or more) to settle out particles of fine sediment to which pollutants such as heavy metals adsorb, and to allow biologic activity to occur that metabolizes nutrients and organic pollutants. For wetvaults, the permanent pool of water is covered by a lid which blocks sunlight from entering the facility, limiting light-dependent biologic activity.

Wetted Perimeter : The length of the line of intersection of the plane or the hydraulic cross-section with the wetted surface of the channel.

Wet Swale : An open drainage channel or depression, explicitly designed to retain water or intercept groundwater for water quality treatment.

Wet Weather Flow : Combination of dry weather flows and stormwater runoff.

Wing Wall : Side wall extensions of a structure used to prevent sloughing of banks or channels and to direct and confine overfall.

Xeriscaping : Landscaping that uses drought-tolerant vegetation instead of turf to reduce the amount of water required to maintain a lawn.

X-Year Storm Event : The storm event that has a probability of recurring on average once every X-years based on records from previous years.

Zero Lot Line : The location of a structure on a lot in such a manner that one or more sides of the structure rests directly on a lot line.

Zipper Lot : Lot design approach in which the rear lot line moves back and forth to vary the depth of the rear yard and concentrate open space on the side of the lot.

Zonation : The development of a visible progression of plant or animal communities in response to a gradient of water depth or some other environmental factor.

Zoning : A set of regulations and requirements that govern the use, placement, spacing and size of buildings and lots within a specific area or in a common class (zone).