ACCESS STREET: The lowest order street in the hierarchy of streets, it conducts traffic between individual dwelling units and higher order streets.

ALTERNATIVE LOT WIDTHS: Site design which utilizes a combination of narrow and wide lots to offer a varied streetscape.

ALTERNATIVE PAVERS: permeable surfaces that can replace asphalt and concrete and can be used for driveways, parking lots and walkways.

ANGLED Z LOT: A lot design where units are tilted at a 30 to 45 angle relative to the street.

ANTI-SEEP COLLAR: An impermeable diaphragm, usually of sheet metal or concrete, constructed at intervals within the zone of saturation along the conduit of a principal spillway to increase the seepage length along the conduit and thereby prevent piping or seepage along the conduit.

ANTI-VORTEX DEVICE: A device designed and placed on the top of a riser or at the entrance of a pipe to prevent the formation of a vortex in the water at the entrance.

AQUATIC BENCH: A 10 to 15 foot wide bench which is located around the inside perimeter of a permanent pool and is normally vegetated with aquatic plants; the goal is to provide pollutant removal and enhance safety in areas using stormwater pond stormwater practices.

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.

AQUIFER: A permeable geologic formation capable of storing and yielding groundwater to wells and springs.

AS-BUILT: Drawing or certification of conditions as they were actually constructed.

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.


BAFFLES: Guides, grids, grating or similar devices placed in a pond to deflect or regulate flow and create a longer flow path.

BANKFULL FLOW: The condition where streamflow 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.

BARREL: The closed conduit used to convey water under or through an embankment: part of the principal spillway.

BASE FLOW: The stream discharge from ground water.

BASIN: 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.

BERM: A shelf that breaks the continuity of a slope; a linear embankment or dike.

BEST MANAGEMENT PRACTICE (BMP): A structural or non-structural device designed to temporarily store or treat urban stormwater runoff in order to mitigate flooding, reduce pollution and provide other amenities. (Also called STORMWATER PRACTICE.)

BIOFILTERS: Grass depression areas such as engineered channels or swales that are used to collect and filter urban stormwater. This term was developed in the Pacific Northwest.

BIORETENTION: A water quality practice that utilizes landscaping and soils to treat urban stormwater runoff by collecting it in shallow depressions, before filtering through a fabricated planting soil media.

BROWNFIELDS: Abandoned or under-used industrial and commercial sites where future expansion or redevelopment can be directed after site remediation for possible contamination.

BUFFER: An area adjacent to a shoreline, wetland or stream where development is restricted or prohibited.

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.


CITIZENS ADVISORY COMMITTEE( CAC): A group of citizens that oversee the implementation of a watershed plan and ensure that all stakeholders are involved in the process of a watershed plan.

CATCHMENT: The smallest watershed management unit, defined as the area of a development site to its first intersection with a stream, usually as a pipe or open channel outfall.

CBD: Acronym for commercial business district.

CHANNEL: A natural stream that conveys water; a ditch or channel excavated for the flow of water.

CHANNEL STABILIZATION: Erosion prevention and stabilization of velocity distribution in a channel using jetties, drops, revetments, structural linings, vegetation and other measures.

CHECK DAM: A small dam construction in a gully or other small watercourse to decrease the stream flow velocity (by reducing the channel gradient), minimize channel scour, and promote deposition of sediment.

CHUTE: A high velocity, 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 millimeter 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)

CLUSTER OR OPEN SPACE DEVELOPMENT: The use of designs that incorporate open space into a development site. These areas can be used for either passive or active recreational activity or preserved as naturally vegetated land.

COCONUT ROLLS: Also known as coir rolls, these are rolls of natural coconut fiber designed to be used for streambank 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.

COMBINED SEWER: Collection systems having only one sewer pipe network to collect domestic wastewater, industrial wastes, and stormwater runoff water.

COMBINED SEWER OVERFLOW (CSO): Excess flow (combined wastewater and stormwater runoff) discharged to a receiving water from a combined sewer network when the capacity of the sewer network and / or treatment plant is exceeded, typically during storm events.

COMMUNITY ASSOCIATION: A planned residential condominium, cooperative and/or homeowner group with the primary function of addressing the concerns and needs of residents within a specific geographic area. Community associations usually include fees and their responsibilities may include maintenance, enforcement of allowable 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.

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

CONSERVATION EASEMENT: Voluntary agreements that allow an individual to set aside private property to limit the type or amount of development on their property. Easements 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.

CONTOUR: 1. An imaginary line on the surface of the earth connecting points of the same elevation. 2. A line drawn on a map connecting points of the same elevation.

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 ROOT ZONE (CRZ) - The area around a tree required for the tree's survival.

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.

CURVE NUMBER (CN): 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 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 or fill areas.

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

COASTAL ZONE ACT REAUTHORIZATION AMENDMENTS (CZARA): 1990 amendments that sought to address the issue of nonpoint source pollution issue by requiring states to develop Coastal Nonpoint Pollution Control Programs in order to receive federal funds.


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

DENSITY COMPENSATION: Granting a credit for higher density elsewhere on a site to compensate for developable land lost due to environmental considerations.

DENSITY BONUS: A form of incentive to promote conservation natural and open space areas. Developers are allowed to build more homes than allowed by local zoning ordinances if such areas are conserved.

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 STRUCTURE: A structure constructed for the purpose of temporary storage of stream flow or surface runoff and gradual release of stored water at controlled rates.

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.

DISCONNECTED IMPERVIOUS SURFACES: Discontinuous impervious surfaces that allow for the infiltration and filtration of precipitation. An example of this is a residential subdivision in which each dwelling's roof top drains through a vegetative strip before reaching the road surface.

DISTRIBUTED RUNOFF CONTROL (DRC): 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 has been removed or altered and, therefore, is susceptible to erosion.

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.

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 (WATERSHED): All land and water area from which runoff may run to a common (design) point.

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

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.

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


EDGE EFFECT: Extensive, well-defined edges between the impervious and pervious surfaces.

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

ENERGY DISSIPATOR: A designed device such as an apron of rip-rap or a concrete structure placed at the end of a water transmitting apparatus such as pipe, paved ditch or paved chute for the purpose of reducing the velocity, energy and turbulence of the discharged water.

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. The following terms are used to describe different types of water erosion:

Accelerated erosion: Erosion much more rapid than normal, natural or geologic erosion, primarily as a result of the influence of the activities of man or, in some cases, of other animals or natural catastrophes that expose base surfaces, for example, fires.

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.

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

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.

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.

EUTROPHICATION: The process of over-enrichment of water bodies by nutrients often typified by the presence of algal blooms.

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.

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 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.


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.

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 STRIPS: A vegetated area that treats sheetflow and/or interflow by removing sediment and other pollutants. The area may be grass-covered, forested or of mixed vegetative cover (e.g. wildflower meadow).

FILTERING PRACTICE: See Stormwater Filtering Practice

FINES (SOIL): Generally refers to the silt and clay size particles in soil.

FLOOD INSURANCE RATE MAP (FIRM MAP): 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.

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: Additional storage space located near a stormwater practice inlet that serves to trap incoming coarse sediments before they accumulate in the main treatment area.

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.

FRENCH DRAIN: A type of drain consisting of an excavated trench refilled with pervious material, such as coarse sand, gravel or crushed stone, through whose voids water percolates and flows to an outlet.

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 two to six rectangular cells filled with small stones. Gabions may be assembled into many types of structures such as revetments, retaining walls, channel liners, drop structures and groins.

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

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.

GRASS CHANNEL: An open vegetated channel used to convey runoff and to provide treatment by filtering out pollutants and sediments.

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

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.

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.

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.


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.

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 (PLANTS): 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).

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

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.

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

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

HYDROLOGIC SOIL GROUP (HSG): 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.

HYDROSEED: Seed or other material applied to areas in order to revegetate after a disturbance.


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

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.

IMPERVIOUS COVER: Any surface in the urban landscape that cannot effectively absorb or infiltrate rainfall.

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

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 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 BASIN: An infiltration practice that stores stormwater runoff in a shallow depression, and allows this runoff to percolate into the ground.

INFILTRATION PRACTICE: See Stormwater Infiltration Practice

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.

INSECTICIDES: Chemicals developed to control or eradicate insects.


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.


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 (LA):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.

LOT: A parcel of undivided land.

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 = 1.486 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 (MOS): 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.

MEMORANDUMS OF UNDERSTANDING (MOUS): 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.

MULCH: Covering on surface of soil to protect and enhance certain characteristics, such as water retention qualities.

MUNICIPAL STORMWATER PERMIT: An NPDES permit issued to municipalities to regulate discharges from municipal seperate storm sewers for compliance with EPA established water quality standards and/or to specify specific stormwater control strategies.


NATIONAL POLLUTANT DISCHARGE ELIMINATION SYSTEM (NPDES): Established by Section 402 of the Clean Water Act, this federally mandated system is used for regulating point source and stormwater discharges.

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-STRUCTURAL STORMWATER PRACTICES: 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.

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.

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: The point where water flows from a conduit, stream, or drain.

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.


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.

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 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.

PERENNIAL STREAM: A stream channel that has running water throughout the year.

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.

pH - A number denoting the common logarithm of the reciprocal of the hydrogen ion concentration. A pH of 7.0 denotes neutrality, higher values indicate alkalinity, and lower values indicate acidity.

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.

PLANIMETERING: The process of measuring designated land areas with a planimeter.

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.

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.

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.

POROSITY: Ratio of pore volume to total solids volume.

POROUS PAVEMENT: Permeable pavement surface with an underlying stone reservoir to temporarily store surface runoff before it infiltrates into the subsoil.

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

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.

RAPID STREAM ASSESSMENT TECHNIQUE (RSAT): 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.

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

REDEVELOPMENT: New development activities on previously developed land.

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 SUBWATERSHED: 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.

RETROFIT: The installation of a new stormwater practice or the improvement of an existing one in a previously developed area.

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

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.

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.

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.

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.

RUNON: 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 stormwater quality treatment practice, whereby runoff is diverted into a self-contained bed of sand. The runoff is then strained through the sand, collected in underground pipes and returned back to the stream or channel.

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.

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.

SEDIMENTATION: Soil particles suspended in stormwater that can settle in stream beds and disrupt the natural flow of the stream.

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

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.

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

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 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 timespan or flowpath 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: 1, meaning a horizontal distance of 1.5 feet to 1 foot vertical.

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: 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.

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

SOLE-SOURCE AQUIFER: An aquifer whose recharge area is the only source of drinking water to both public water supplies and private wells.

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.

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: Stormwater treatment methods which utilize an artificial media to filter out pollutants entrained in urban runoff.

STORMWATER HOT SPOTS: Land-uses or activities that generate highly contaminated runoff. 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 to pre-development levels.

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

STORMWATER PONDS: A land depression created for the detention or retention of stormwater runoff.

STORMWATER PRACTICE: A structural or non-structural device designed to temporarily store or treat urban stormwater runoff in order to mitigate flooding, reduce pollution and provide other amenities (also called BMP).

STORMWATER QUALITY CONTROL: The removal of pollutants from stormwater runoff through the use of stormwater management practices.

STORMWATER WETLAND: A shallow, constructed pool that captures stormwater and allows for the growth of characteristic wetland vegetation.

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.

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: 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.

SUBMERGED GRAVEL FILTERS: A filtering stormwater practice which uses a gravel based substrate, supporting a wetland vegetation cover crop, to treat urban runoff.

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.

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.

SPORT UTILITY VEHICLE (SUV): A vehicle typically designed for both on and off-road travel. These vehicles are often larger than the standard automobile.

SWALE: An open drainage channel or depression explicitly designed to detain and promote the filtration of stormwater runoff.


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

TECHNICAL ADVISORY COMMITTEE (TAC): The group of technical experts that assures quality control over the technical aspects of a watershed plan.

TECHNICAL RELEASE No. 20 (TR-20): A Soil Conservation Service (now 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 the Soil Conservation Service (now 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).

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.

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: The total amount of soils particulate matter which is suspended in the water column.

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.

TRANSIT SHARE: The percentage of trips using a particular mode of travel.

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.

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.

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 occurrance 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.

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.

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.


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.

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 (WLA):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.

WATER USE DESIGNATION: State of Maryland water use classification for protection of resources (i.e., Use I-contact recreational use, Use II-shellfish harvest waters, Use III-natural trout waters, Use IV-recreational trout waters).

WATER SURFACE PROFILE: The longitudinal profile assumed by the surface of a stream flowing in an open channel; the hydraulic grade line.

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.

WATERSHED: All the land area that contributes runoff to a particular point along a waterway.

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.

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

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.

WET POND: A stormwater pond design that captures the entire water quality volume in a permanent pool.

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

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

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

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.


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.

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).