Urban Watershed Management Research Terminology

Below are Terms, Abbreviations and Acronyms frequently used in Urban Watershed Management. For a more general and complete glossary of environmental terms visit the Terms of Environment.

SECTION A

Acute Toxicity

1. A deleterious response (e.g. mortality, disorientation, immobilization) to a stimulus observed in ninety-six (96) hours or less.

2. The discernible adverse effects induced in an organism within a short period of time (days) of exposure to a substance or material. For aquatic animals this usually refers to continuous exposure to the substance or material in water for a period of up to four (4) days. The effects (lethal or sub-lethal) occurring may usually be observed within the period of exposure with aquatic organisms.

Advanced Wastewater Treatment

Any treatment of sewage that goes beyond the secondary or biological water treatment stage and includes the removal of nutrients such as phosphorus and nitrogen and a high percentage of suspended solids. ( See: primary, secondary treatment.)

Advective Transport

Physical transport of water and associated concentrations from higher to lower hydraulic potential, exclusive of dispersion / mixing.

Agricultural Pollution

Farming wastes, including runoff and leaching of pesticides and fertilizers; erosion and dust from plowing; improper of animal manure and carcasses; crop residues; and debris.

Anti-Degradation

Policies that are part of each state's water quality standards. These policies are designed to protect water quality and provide a method of assessing activities that may impact the integrity of the waterbody.

Assimilative Capacity

The capacity of a natural body of water to receive wastewaters or toxic materials without deleterious effects and without damage to aquatic life or humans who consume the water.

SECTION B

BAT

Best Available Technology.

BCT

Best Conventional pollutant Technology.

Benefits

A good, service, or attribute of a good or service that promotes or enhances the well-being of an individual, an organization, or a natural system.

Best Management Practices (BMPs)

Schedules of activities, prohibitions of practices, maintenance procedures, and other management practices to prevent or reduce the pollution of waters of the United States. BMPs also include but are not limited to treatment requirements, operating procedures, and practices to control plant site runoff, spillage or leaks, sludge or wastewater disposal, or drainage from raw material storage.

Bioaccumulation

The process by which a contaminant accumulates in the tissues of an individual organism. For example, certain chemicals on food eaten by a fish tend to accumulate in its liver and other tissues.

Bioavailable

The state of a toxicant such that there is increased physicochemical access to the toxicant by an organism. The less the bioavailability of a toxicant, the less its toxic effect on an organism.

Biochemical Oxygen Demand (BOD)

An operational measure of potential for depletion of dissolved oxygen by the biological and chemical degradation of organic material by bacteria.

Biologic Diversity

A measure of the variety of biological organisms.

Biological Treatment processes

Means of treatment in which bacterial or biochemical action is intensified to stabilize, oxidize, and nitrify the unstable organic matter present. Trickling filters, activated sludge processes and lagoons are examples.

Blockage

Any obstruction that interferes with the movement of a fluid. This term is most often used to describe an unwanted flow obstruction in a pipe, channel, hydraulic combined sewer overflow regulator or other regulator, tide gate, orifice, storm inlet grating, filter bed, screen, or other location where flow occurs.

BPT

Best Practicable control Technology.

SECTION C

Catch Basin

A chamber or well, usually at the street curb line, for the admission of surface water to a sewer or sub-drain, having at its base a sediment sump to retain grit and below detritus the point of overflow; whereas, a stormwater inlet does not have a sump and does not trap sediment.

Categorical Pretreatment Standard

A technology-based effluent limitation for an industrial facility discharging into a municipal sewer system. Analogous in stringency to Best Availability Technology (BAT)for direct discharges.

Chlorophyll A

A green pigment found in photo synthetic organisms; used as an indicator of algal biomass.

Chronic

A stimulus that lingers or continues for a relatively long period of time, often one-tenth of the life span or more. Chronic should be considered a relative term depending on the life span of an organism. The measurement of a chronic effect can be reduced growth, reduced reproduction, etc., in addition to lethality.

Chronic Toxicity

Toxic effects from long-term exposure to chemicals; usually nonfatal to organisms. Effects often consist of reduced growth and reproduction, as well as other physiological impacts.

Clean Water Act (CWA)

The Clean Water Act (formerly referred to as the Federal Water Pollution Control Act or Federal Water Pollution Control Act Amendments of 1972), Public Law 92-500, as amended by Public Law 96-483 and Public Law 97-117, 33 U.S.C. 1251 et seq.

Coastal Zone

Lands and waters adjacent to the coast that exert an influence on the uses of the sea and its ecology, or whose uses and ecology are affected by the sea.

Combined Sewer

A sewer receiving intercepted surface (dry- and wet-weather) runoff, municipal (sanitary and industrial) sewage, and subsurface waters from infiltration.

Combined Sewer Overflow (CSO)

Discharge of a mixture of stormwater and domestic waste when the flow capacity of a sewer system is exceeded during rainstorms.

Composites

Water samples comprised of a series of equal volume samples taken over interval time periods (temporal composite) or spatial locations (spatial composite).

Computer Model

A model in which the mathematical operations are carried out on a computer.

Concentration-Based Limit

A limit based on the relative strength of a pollutant in a wastestream, usually expressed in milligrams per liter (mg/l).

Continuous Discharge

A discharge that occurs without interruption throughout the operation hours of the facility, except for infrequent shutdowns for maintenance, process changes, or other similar activities.

Control Authority

A POTW with an approved pretreatment program or the Approval Authority in the absence of a POTW pretreatment program.

Control Structure

Any device intended to alter or control the flow of a fluid. Typical control structures include, but are not limited to, weirs, dams and fabridams, combined sewer overflow regulators, culverts, gates and floodgates, levees, hydrobrakes and orifices, nozzles, down shafts, stand pipes, inlets, and flow splitters.  This term typically refers to structures used in the control of open-channel flow, but can also refer to controls of pressurized-fluid flows. Control structures are used to control flow in conduits, channels, tanks, basins, vessels, and natural water.

Conventional Pollutants

Statutorily listed pollutants understood well by scientists. These may be in the form of organic waste, sediment, acid, bacteria, viruses, nutrients, oil and grease, or heat.

Cost-Benefit Relationship

The relationship between unit cost to unit benefits usually represented as a curve.

Cost-Effective Solution

A solution to a problem that has been identified as being financially optional (e.g., the solution associated with the knee-of-the-curve of a cost-benefit relationship).

Created Wetland

A wetland intentionally created from a non-wetland site to produce or replace natural habitat (e.g., a compensatory mitigation project). These wetlands are normally considered waters of the United States or waters of the state. (See restoration, enhancement, constructed wetland.)

Critical Design Conditions

Environmental and flow conditions chosen to represent the conditions under which compliance with water-quality standards, criteria, or objectives is desired.

SECTION D

Designated Uses

Those water uses identified in state water quality standards that must be achieved and maintained as required under the Clean Water Act. Uses can include cold water fisheries, public water supply, irrigation, and others.

Detention

The slowing, dampening, or attenuating of flows either entering the sewer system or within the sewer system by temporarily holding the water on a surface area, in a storage basin, or within the sewer itself.

Detention Time

The time period that flow is detained in a storage/sedimentation basin or tank.

Deterministic Mathematical Model

A mathematical model designed to produce system responses or outputs to temporal and spatial stimuli or inputs.

Detritus

A combination of relatively heavy, coarse solids comprised of sand, grit, and organic matter usually having a Stokes'-law settling velocity equal to or greater than a particle with a 0.2 mm effective diameter and a 2.65 specific gravity.

Direct Runoff

Water that flows over the ground surface or through the ground directly into streams, rivers, and lakes.

Discharge Monitoring Report (DMR)

The EPA uniform national form, including any subsequent additions, revisions, or modifications, for the reporting of self-monitoring results by permittees. DMRs must be used by approved states as well as by EPA.

Discharge

Flow of surface water in a stream or canal or the outflow of groundwater from a flowing artesian well, ditch, or spring. Can also apply to discharge of liquid effluent from a facility or to chemical emissions into the air through designated venting mechanisms.

Discharge Permit / Consent

A discharge permit is formal permission to release water, stormwater, combined sewer overflow, or wastewater to a receiving water body obtained by an individual, commercial or industrial business entity, institutional entity, or governmental entity from an agency that regulates discharges to waters. A permit can also be issued to a group of dischargers; this type of permit is referred to as a general permit. Regulating agencies that issue discharge permits are typically governmental or privatized agencies. The term discharge permit may apply to either the concept of permission or to the actual document that confers permission. Discharge consent is used synonymously with discharge permit.

Disinfection

The killing or inactivation of human-disease-causing microorganisms or pathogens.

Dispersion

Pollutant or concentration mixing due to turbulent physical processes.

Dissolved Oxygen Deficit

Difference between saturated dissolved oxygen and ambient concentrations.

Distributed Model

A model in which the physical heterogeneities of the catchment are included.

DMHRF

Dual-Media High-Rate Filtration.

Dry-Weather Deposition / Atmospheric Deposition

The settling of particulates from the atmosphere onto water or land surfaces in the absence of precipitation.

Dry-Weather Duration

The interval of time between precipitation (rainfall and snowfall) events. As it pertains to combined, sanitary, and storm sewerage systems dry-weather duration is the interval of time between instances of stormwater entry into the aforementioned systems.

Dry-Weather Flow (DWF)

1. Usually referred to as the flow in a combined sewer system without stormwater.

2. As it pertains to combined and sanitary sewerage, or stormwater drainage systems, is the flow in a system that occurs during dry weather, without a stormwater component. In these systems dry-weather flow may include one or all of the following: sanitary wastewater; pre-treated industrial wastewater; unauthorized industrial wastewater; groundwater that has infiltrated or leaked into the system; latent or delayed stormwater flows through the vadose zone that have leaked into the system; chemical and sanitary landfill leachate; lawn irrigation runoff; foundation drainage; washwater such as from cars and industrial sites; unauthorized disposal of oil and hazardous chemicals; and other miscellaneous entries.

Dry-weather Overflow / Discharge

An overflow or discharge from a combined or sanitary sewerage system or storm drainage system that is not the result of wet-weather flows into the system. These flows may be the result of a variety of processes (see Dry-weather flow). Dry-weather overflows from combined sewer systems are generally not permitted.

Dual Treatment

Those processes or facilities designed for operating on both dry- and wet-weather flows.

SECTION E

End-of-Pipe Impacts

Impacts that occur in the immediate vicinity of an outfall.

Ecological Habitat

The environmental niche in which an organism lives.

Ecosystem

A biological community together with the physical and chemical environment with which it interacts.

Effluent

Wastewater, treated or untreated, that flows out of a treatment plant, sewer, or industrial outfall.

Effluent Guidelines

Technical EPA documents that set effluent limitations for given industries and pollutants.
1. Restrictions established by a state or EPA on quantities, rates, and concentrations in wastewater discharges.

2. National standards for wastewater discharges to surface waters and publicly owned treatment works (municipal sewage treatment plants). EPA issues effluent guideline regulations for categories of dischargers under Title III of the Clean Water Act. The standards are technology-based, i.e. they are based on the performance of treatment and control technologies. They are not based on risk or impacts upon receiving waters.

End-of-Pipe System

ANY device and/or treatment system applied to stormwater, combined wastewater, municipal wastewater and/or industrial wastewater at the outlet of a collection system prior to a receiving water body. The majority of wastewater treatment systems including sanitary and combined wastewater treatment plants and many stormwater treatment schemes such as detention basins are end-of-pipe systems.

Enhancement

In the context of restoration ecology, any improvement of a structural or functional attribute.

Eutrophication

The process of aging whereby the increase of mineral and organic nutrients favors aquatic plants over animal life and results in increasing daily variations in dissolved oxygen concentrations, reduced biologic diversity, and reduced water clarity.

SECTION F

FC

Fecal Coliform.

Feedlot

A confined area for the controlled feeding of animals. Tends to concentrate large amounts of animal waste that cannot be absorbed by the soil and, hence, may be carried to nearby streams or lakes by rainfall runoff.

First Flush

The condition, often occurring in storm-sewer discharges and CSOs, in which a disproportionately high pollutional load is carried to the first portion of the discharge or overflow.

Floatables

Large floating material sometimes characteristic of sanitary wastewater and stormwater runoff.

Flow Cross-Section

The two-dimensional area of flowing fluid measured perpendicular to the direction of fluid flow. In a rectangular channel, flow cross-section is the depth of flowing water multiplied by the width of the channel. Flow cross-section is used to determine flowrate, which is defined as the cross-sectional average flow velocity multiplied by the flow cross-section.

Flow Isolation

The separation of one type of fluid from another type. Examples of flow isolation include separating stormwater from domestic wastewater and separating stormwater leaving roofs and lawns from stormwater leaving roads and parking areas.

Flow Recording

The recording of flowrate measurements, often by an automatic recording device. Typically an instrument is used to measure a physical parameter(s) that can be easily related to flowrate, such as water level, water velocity, or pressure variances in a closed conduit. This parameter(s) is then recorded and used to calculate the flowrate at a given time. For example, in wastewater treatment, combined sewerage systems, and stormwater conveyances one practice is to record the height of the free water surface behind a weir and use this measurement to calculate flowrate using the calibrated weir equation, although other measurements and calculation schemes may be used.

Flux

The flowrate per unit cross-sectional area.

FS

Fecal Streptococcus.

SECTION G

Gravity Flow / Gravity System

The movement of fluids in reaction to the force of gravity on the fluid. All open channels as well as most storm sewers, sanitary sewers, and combined sewers in which the pipes are less than completely full operate based on gravity flow and are thus considered gravity systems. However, segments of a sewer system may at times flow under surcharged conditions whereby the water level is above the crown of the pipe causing pressurized flow in these segments. In addition to sewer systems flowing under surcharged conditions, those systems or portions of systems that utilize pumps to increase fluid pressures and induce flow are not gravity systems.

Groundwater

The supply of fresh water found beneath the earth's surface, usually in aquifers, which supply wells and springs. Because groundwater is a major source of drinking water, there is growing concern over contamination from leaching agricultural or industrial pollutants and leaking underground storage tanks.

SECTION H

Heavy Metals

Metallic elements, such as mercury, lead, nickel, zinc, and cadmium, that are of environmental concern because they do not degrade over time. Although many are necessary nutrients, they are sometimes magnified in the food chain and in high concentrations can be toxic to life.

Hydraulic Loading

The flowrate per unit plan or surface area in sedimentation facilities or cross-sectional area in screening and filtration facilities. (See also flux.)

SECTION I

Indirect Discharge

A non-domestic discharge introducing pollutants to a publicly owned treatment works.

Industrial User (IU)

A source of indirect discharge that does not constitute "discharge of pollutants" under regulations issued pursuant to section 402 of the Clean Water Act.

Infiltration

The process whereby water enters a sewer system and service connections underground through such means as, but not limited to, defective pipes, pipe joints, connections, or manhole walls. Infiltration does not include, and is distinguished from inflow.

In-Line Storage

The use of a portion of the volume of a storm sewer or drain, combined sewer and/or interceptor sewer system that is not being used to transport combined wastewater or stormwater to accommodate the storage of additional stormwater runoff or combined wastewater. This term also applies to a storage facility, such as a tank, basin, or other reservoir, which is connected to a sewer system in such a way that all flow in the system passes through the storage facility. In the latter usage, inline storage is differentiated from offline storage, which is connected in such a way that excess flow can be diverted to the storage facility, but normal flows bypass the facility. See also Off-Line Storage.

Interceptor

A sewer designed to receive dry-weather flow from a number of transverse combined sewer trunks and additional quantities of intercepted surface runoff during low-flowing sanitary sewage periods and to convey such waters to a point for treatment.

Irrigation Return Flow

Surface and subsurface water that leaves a field following the application of effluent water.

Irrigation

Applying water or wastewater to land areas to supply the water and nutrient needs of plants.

SECTION K

Knee-of-the-Curve

The point along a cost-benefit curve at which there is a noticeable change in the quantity of cost for an increment of benefit.

SECTION L

Lagrangian Framework

A mathematical modeling framework based on a moving reference point in contrast to a fixed coordinate reference point. (Named after the French mathematician J. L. Lagrange.)

Land Application

Discharge of wastewater onto the ground for treatment or reuse. (See irrigation)

Landfills

1. Sanitary landfills are disposal sites for nonhazardous solid wastes spread in layers, compacted to the smallest practical volume, and covered by material applied at the end of each operation day.

2. Secure chemical landfills are disposal sites for hazardous waste, selected and designed to minimize the chance of release of hazardous substances into the environment.

Leachate

Water that collects contaminants as it trickles through wastes, pesticides, or fertilizers. Leaching can occur in farming areas, feedlots, and landfills and can result in hazardous substances entering surface water, groundwater, or soil.

Leachate Collection System

A system that gathers leachate and pumps it to the surface for treatment.

Load Allocation (LA)

The portion of a receiving waters loading capacity that is attributed either to one of its existing or future nonpoint sources of pollution or to natural background sources. Load allocations are best estimates of the loading, which can range from reasonably accurate estimates to gross allotments, depending on the availability of data and appropriate techniques for predicting the loading. Wherever possible, natural and nonpoint source loads should be distinguished. (40 CFR 130.2(g))

Loading Capacity (LC)

The greatest amount of loading that a water can receive without violating water quality standards.

Lumped Model

A model in which the physical characteristics of the catchment are assumed to be homogeneous.

SECTION M

Margin of Safety (MOS)

A required component of the TMDL that accounts for the uncertainty about the relationship between the pollutant loads and the quality of the receiving waterbody (CWA section 303(d)(1)(C)). The MOS is normally incorporated into the conservative assumptions used to develop TMDLs (generally within the calculations or models)and approved by EPA either individually or in state/EPA agreements. If the MOS needs to be larger than that which is allowed through the conservative assumptions, additional MOS can be added as a separate component of the TMDL (in this case, quantitatively, a TMDL = LC = WLA + LA + MOS)

Mass-Based Standard

A discharge limit that is measured in a mass unit such as pounds per day.

Mitigation

Actions taken to avoid, reduce, or compensate for the effects of environmental damage. Among the broad spectrum of possible actions are those which restore, enhance, create, or replace damaged ecosystems.

Model

Any representation of a system by something other than the system itself.

Model Calibration

Refinement of mathematical model parameters and coefficients through comparison to data by making scientifically consistent and rational adjustments.

Model Parameter

A quantity that cannot vary in a particular model run.

Model Variable

A quantity that can vary in a particular model run.

Monitoring

Periodic or continuous surveillance or testing to determine the level of compliance with statutory requirements and/or pollutant levels in various media or in humans, plants, and animals.

Monte Carlo Probabilistic Simulation

A statistical modeling approach used to assess the expected frequency and magnitude of water-quality impacts by running repetitive simulations using statistically selected inputs for rainfall, pollutants, flows, temperature, and so on.

SECTION N

National Pollutant Discharge Elimination System (NPDES)

The national program for issuing, modifying, revoking and reissuing, terminating, monitoring, and enforcing permits, and imposing and enforcing pretreatment requirements, under Sections 307, 402, 318, and 405 of the Clean Water Act.

Nonpoint Source

Diffuse pollution sources (i.e., without a single point of origin or not introduced into a receiving stream from a specific outlet). The pollutants are generally carried off the land by stormwater. Common nonpoint sources are agriculture, forestry, urban, mining, construction, dams, channels, land disposal, saltwater intrusion, and city streets.

SECTION O

Off-Line Storage

A type of storage that requires detention facilities (basins or tunnels) and facilities for pumping storm flow either into or out of the detention facilities.

Open-Channel Flow

Fluid flow where the bottom and sides of the flow are confined by solid surfaces and the upper surface is in contact with the atmosphere and is at atmospheric pressure. Open-channel flow occurs in rivers, streams, canals, channels, swales, and ditches, and in pipes, sewers, and culverts that are less than completely full. See also gravity flow.

SECTION P

Pathogen

A disease-causing microorganism.

Pathogenic Bacteria and Viruses

Bacteria and viruses capable of causing disease in humans.

Peak-and-Shape Parameter

A model parameter that strongly influences the peak, rising, and receding legs of a runoff hydro graph.

Permit

An authorization, license, or equivalent control document issued by EPA or an approved state agency to implement the requirements of an environmental regulation; e.g., a permit to operate a wastewater treatment plant or to operate a facility that may generate harmful emissions.

Point Source

Any discernible confined and discrete conveyance, including, but not limited to, any pipe, ditch, channel, tunnel, conduit, well, discrete fissure, container, rolling stock, concentrated animal feeding operation, landfill leachate collection system, vessel, or other floating craft from which pollutants are or may be discharged. This term does not include return flows from irrigated agriculture or agricultural stormwater runoff.

Pollutant

A contaminant in a concentration or amount that adversely alters the physical, chemical, or biological properties of the environment. The term includes pathogens, toxic metals, carcinogens, oxygen-demanding materials, and all other harmful substances. With reference to nonpoint sources, the term is sometimes used to apply to contaminants released in low concentrations from many activities that collectively degrade water quality. As defined in the federal Clean Water Act, pollutant means dredged spoil; solid waste; incinerator residue; sewage; garbage; sewage sludge; munitions; chemical wastes; biological materials; radioactive materials; heat; wrecked or discarded equipment; rock; sand; cellar dirt; and industrial, municipal, and agricultural waste discharged into water.

Pollution

Generally, the presence of matter or energy whose nature, location, or quantity produces undesired environmental effects. Under the Clean Water Act, for example, the term is defined as the man-made or man-induced alteration of the physical, biological, chemical, and radiological integrity of water.

Pretreatment

The reduction of the amount of pollutants, the elimination of pollutants, or the alteration of the nature of pollutant properties in wastewater to a less harmful state prior to or in lieu of discharging or otherwise introducing such pollutant into a publicly owned treatment works.

Primary Treatment

A basic wastewater treatment method that uses settling, skimming, and (usually) chlorination to remove solids, floating materials, and pathogens from wastewater. Primary treatment typically removes about 35 percent of biochemical oxygen demand (BOD) and less than half of the metals and toxic organic substances.

Privately Owned Treatment Works

Any device or system that is (a) used to treat wastes from any facility whose operator is not the operator of the treatment works and (b) not a POTW.

Public Comment Period

The time allowed for the public to express its views and concerns regarding action by EPA (e.g., a Federal Register notice of a proposed rule-making, a public notice of a draft permit, or a Notice of Intent to Deny).

Publicly Owned Treatment Works (POTW)

Any device or system used in the treatment (including recycling and reclamation) of municipal sewage or industrial wastes of a liquid nature that is owned by a state or municipality. This definition includes sewers, pipes, or other conveyances only if they convey wastewater to a POTW providing treatment.

SECTION R

Receiving Waters

Natural or man-made water systems into which materials are discharged

Restoration

Return of an ecosystem to a close approximation of its condition prior to disturbance.

Riparian Areas

Areas bordering streams, lakes, rivers, and other watercourses. These areas have high water tables and support plants that require saturated soils during all or part of the year. Riparian areas include both wetland and upland zones.

Riparian Vegetation

Hydrophytic vegetation growing in the immediate vicinity of a lake or river close enough so that its annual evapotranspiration represents a factor in the lake or river regime.

Riparian Zone

The border or banks of a stream. Although this term is sometimes used interchangeably with flood plain, the riparian zone is generally regarded as relatively narrow compared to a flood plain. The duration of flooding is generally much shorter, and the timing less predictable, in a riparian zone than in a river flood plain.

Risk Assessment

The objective scientific assessment of the expected adverse effects of environmental stressors. The process includes hazard identification, dose-response assessment, exposure assessment, and risk characterization. Adapted from: Thompson, Anthony J., "Federal Risk Management Policy: Where are the problems?" In the proceedings: Risk Based Decision Making in Water Resources VI, ASCE, 1993. P.1. Risk assessment itself does not generate a decision. It only provides an unbiased evaluation of the effects. The risk assessment often includes uncertainty analysis to clarify the nature of the science. One National Risk Management Research Laboratory's (NRMRL) research focus areas is "characterizing pollutant sources that require management." US EPA, 1997 Update to ORD's Strategic Plan, EPA/600/R-97/015, April 1997.

Risk Management

The public process of selecting the actions to take after determining risks exist. Risk Management integrates risk assessment with social, economic, engineering, and political factors that may influence the decision. Adapted from: Thompson, 1993. While the science of risk assessment helps risk management, it does not necessarily drive the decision. Other considerations may overwhelm the scientific contribution. A given community may elect to accept a condition that introduces large environmental stress at one location, but find a lesser induced stress unacceptable at another location. The second NRMRL research focus area is "identifying, developing, and evaluating tools and technologies for prevention, control, restoration, and remediation of environmental problems that are high risk, high cost, or that lack effective management alternatives. "US EPA, 1997 Update to ORD's Strategic Plan, EPA/600/R-97/015, April 1997.

SECTION S

Saint-Venant Equations

The equations of continuity of mass and momentum used to model flow in a channel or over a surface.

Satellite Facilities

CSO storage/treatment facilities at remote locations upstream of the dry-weather flow sewage treatment plant and usually at the regulator /overflow site.

Secondary Treatment

The second step in most publicly owned waste treatment systems, in which bacteria consume the organic parts of the waste. It is accomplished by bringing together waste, bacteria, and oxygen in trickling filters or in the activated sludge process. This treatment removes floating and settleable solids and about 90 percent of the oxygen-demanding substances and suspended solids. Disinfection is the final stage of secondary Treatment. (See: primary, tertiary treatment.)

Sediment Oxygen Demand

Biochemical consumption of dissolved oxygen in overlying waters by decaying sediments across the water-sediment surface.

Septic System

An on-site system designed to treat and dispose of domestic sewage. A typical septic system consists of a tank that receives waste from a residence or business and a system of tile lines or a pit for disposal of the liquid effluent (sludge) that remains after decomposition of the solids by bacteria in the tank; must be pumped out periodically.

Sewer

A channel or conduit that carries wastewater and stormwater runoff from the source to a treatment plant or receiving stream. "Sanitary" sewers carry household, industrial, and commercial waste. "Storm" sewers carry runoff from rain or snow. "Combined" sewers handle both.

Sewer Flushing

Flushing applied to combined sewer systems during dry-weather flow periods to remove settled material periodically or as it accumulates, and to hydraulically convey it to the treatment facilities, thus preventing resuspension and overflow of a portion of the solids during storm-flow periods and lessening the need for CSO treatment.

Sewer Storage

See Inline storage.

Sewerage

System of piping, with appurtenances, for collecting and conveying wastewaters from source to treatment and/or discharge.

Source Control

A method of abating storm-generated or CSO pollution at the upstream, upland source where the pollutants originate and/or accumulate.

Spatial Discretization

The level of detail in describing the changes in space of a system, such as a catchment or sewer system, for modeling purposes.

Static Head

Also called piezometric head or elevation head, at a point is the level to which a liquid would rise in a piezometer, or open-ended tube, connected to the liquid at the point of interest. In a fluid that has a free water surface (a surface at atmospheric pressure) the static head at a point below the water surface can be measured as the vertical distance between the point and the free water surface. Static head ( in meters (m)) is defined as the elevation of a point in a fluid relative to some datum (m) plus the fluid pressure at the point ( Newtons (N) / m² ) divided by the specific weight of the fluid ( N / m³ ).

Static Storage

The portion of the total storage of a reservoir that is continuously occupied and manifests a relatively constant water level, and is thus not available for temporary storage. For example, when this term is used in reference to wet detention ponds for stormwater storage and treatment, the static storage is the portion of the total volume of the pond that is occupied by water during dry weather.

Stilling well

An enclosure mounted inside a reservoir, channel, or natural water body so as to intersect the free water surface of the larger water body and provide a relatively calm area to facilitate measurement of the position of the free water surface. The inside of a stilling well contains the same fluid as the larger reservoir and is hydraulically connected to the larger reservoir. Thus fluid level in a stilling well is the same as the level outside of the well but is relatively free from waves and ripples that make measurement of the water surface level difficult in the main reservoir, channel, or natural water body. A stilling well is constructed with perforated walls or is open to the main water body at the bottom of the well to ensure that water level in the well accurately represents that of the reservoir or channel. Water level gauges and recorders are often mounted inside stilling wells. When a float is used to gauge the position of the water level the alternative term used is float chamber.

Stochastic Mathematical Model

A mathematical model designed to produce a series of random responses (e.g., runoff) that have the statistical characteristics of historical responses of the system.

Storage Basin / Storage Tank

A reservoir intended to hold stormwater, combined, sanitary, or industrial wastewater when temporary storage of the fluid is needed, typically when the flowrate or volume of flow from an event exceeds the carrying capacity of a pipe, channel, system, or treatment facility, or an allowable flowrate or total volume into a receiving water body is exceeded.

Storage Capacity

The volume of fluid that can be stored in a system. For storm drainage and sewerage systems, storage capacity refers to the volume available for the temporary storage of excess storm flow or wastewater flow in a pipe, channel, basin, tank, or other facility, or in the system as a whole.

Storm Sewer

A sewer that carries intercepted surface runoff, street wash and other wash waters, or drainage, but excludes domestic sewage and industrial wastes except for unauthorized cross-connections.

Storm-Sewer Discharge

Flow from a storm sewer that is discharged into a receiving water or to a sewer system.

Stormwater

Stormwater runoff, snow melt runoff, and surface runoff and drainage; rainfall that does not infiltrate the ground or evaporate because of impervious land surfaces but instead flows onto adjacent land or watercourses or is routed into drain/sewer systems.

Stream Restoration

Various techniques used to replicate the hydrological, morphological, and ecological features that have been lost in a stream due to urbanization, farming, or other disturbance.

Streeter-Phelps Equation

Closed-form analytical mathematical solution for steady-state calculation of a dissolved oxygen in non-dispersive stream systems.

Surface Runoff

Precipitation, snow melt, or irrigation water in excess of what can infiltrate the soil surface and be stored in small surface depressions; a major transporter of nonpoint source pollutants.

Surface Water

All water naturally open to the atmosphere (rivers, lakes, reservoirs, ponds, streams, impoundments, seas, estuaries, etc.) and all springs, wells, or other collectors directly influenced by surface water.

SECTION T

Technology-Based Limitations

Industry-specified effluent limitations applied to a discharge when it will not cause a violation of water quality standards at low stream flows. Usually applied to discharges into large rivers.

Technology-Based Standards

Effluent limitations applicable to direct and indirect sources that are developed on a category-by-category basis using statutory factors, not including water quality effects.

Tertiary Treatment

Advanced cleaning of wastewater that goes beyond the secondary or biological stage, removing nutrients such as phosphorus, nitrogen, and most biochemical oxygen demand (BOD) and suspended solids.

Total Maximum Daily Load (TMDL)

The sum of the individual wasteload allocations (WLAs) for point sources and land allocations (LAs) for nonpoint sources and natural background. If a receiving water has only one point sources discharger, the TMDL is the sum of that point source WLA plus the LAs for any nonpoint sources of pollution and natural background sources, tributaries, or adjacent segments. TMDLs can be expressed in terms of mass per time, toxicity, or other appropriate measure that relates to a state of water quality standards. If best management practices (BMPs) or other nonpoint source pollution control actions make more stringent load allocations practicable, WLAs can be made less stringent. Thus, the TMDL process provides for nonpoint source control trade-offs. (40 CFR 130.2(I))

Total Maximum Daily Load (TMDL) Process

The approach normally used to develop a TMDL for a particular waterbody or watershed. This process consists of five activities, including selection of the pollutant to consider, estimation of the waterbody's assimilative capacity, estimation of the pollution from all sources to the waterbody, predictive analysis of pollution in the waterbody and determination of total allowable pollution load, and allocation of the allowable pollution among the different pollution sources in a manner that ensures that water quality standards are achieved.

Toxic Pollutants

Materials that cause death, disease, or birth defects in organisms that ingest or absorb them. The quantities and exposures necessary to cause these effects can vary widely. Those pollutants listed by the Administrator under section 307(a) of the Clean Water Act.

Trunk

A sewer, also known as a main sewer, that receives the discharge of one or more sub main sewers.

SECTION U

Urban Runoff

Surface runoff from an urban drainage area that reaches a stream or other body of water or a sewer/channel. Urban watersheds include (1) the 405 Bureau of Census Metropolitan Statistical Areas (MSA) considered either urban or urban fringe, and (2) water entering these MSAs (surface and ground waters entering the MSAs from upstream or up-gradient sources, source waters supplying the MSA population, and rain or snow events). Adapted from: US EPA, Risk Management Research Plan for Wet Weather Flows, EPA/600/R-96/140, November 1996. The urban watershed is a subset of the total watershed. Unlike the total watershed, topology does not strictly define the urban watershed. The urban watershed will often have one or more flows entering from outside the urban watershed and may have multiple effluents. Because the urban watershed commonly has influent streams, the up-gradient conditions can strongly affect the water quality. An urban area may include more than one watershed. Based on population demographics, the location of the existing and future urban watersheds shifts with social structure. The national trends measured by the Bureau of Census will define these areas. Two prominent changes that are evolving are the shrinking urban core surrounded by the growing suburban population density and the increasing in near-coastal populations.

SECTION V

V-notch Weir

A weir is an obstruction placed across the entire width of an open channel or conduit to interrupt flow and cause the fluid level upstream of the weir to rise until fluid flows over the top of the weir. With a properly constructed weir, critical flow will exist near the weir crest. Thus only one depth of flow can exist for a given discharge, and an equation can be developed that relates volumetric fluid flowrate over the weir with height of the surface of the fluid pooled upstream of the weir. With the use of such an equation a weir can be used as a simple flow measurement device for open channels. A weir that takes the form of a thin plate and is beveled on the downstream side of the upper surface so that a sharp upstream edge exists is called a sharp-crested weir, while other weirs are broad-crested weirs. Sharp-crested weirs are used where flow measurement is a primary concern, while the more durable broad-crested weirs are used where durability is of greater concern than flow measurement. Equations that describe the flow over weirs are based on the flow over the weir taking the form of a nappe, or jet of flowing fluid, that springs clear from the downstream face of the weir resulting in contact between the fluid and the weir only along the upper edge of the weir crest. At low flows the nappe will not spring free of the weir face and thus flow cannot be considered true weir flow. A sharp-crested weir may be equipped with a notch in the upper surface through which flow will pass. When this notch takes the form of the letter V, with the narrow point of the V pointing downward, the weir is called a v-notch weir. The v-notch weir is used for measuring flow in channels or conduits that experience variable low flowrates for two reasons. First, at low flowrates the nappe from a v-notch weir will flow clear of the weir face more readily than with other weir designs creating conditions that meet the assumptions used in the development of weir rating equations and allowing their use to compute flowrate over the weir. Second, the narrow opening near the bottom of the v-notch causes, at low flowrates, the water behind the weir to back up to a higher level than with a rectangular notch. This results in a greater difference in water levels between a zero-flow condition and a low, but non-zero, flow condition and permits more accurate calculations of low-rate flows over the weir.

The fundamental equation for a v-notch weir is
Q = C_d(8/15) (2g)^0.5 tan(x/2)H^5/2
where:
Q = flow (m³ / s)
Cd = coefficient of discharge that relates actual flow to theoretical flow
g = the acceleration due to gravity = 9.81 m /s²
x = the angle between the two sides of the v-notch
H = the height of the water upstream of the weir relative to the weir crest, measured at a point at least 4H upstream of the weir.

Other types of sharp-crested weirs include rectangular weirs and trapezoidal weirs, which are named for the shapes of their notches, and weirs with a combination of a v-notch inside a larger rectangular notch. The constants associated with discharge equations for weirs of standard designs can be found in reference texts.

SECTION W

Wasteload Allocation (WLA)

The portion of a receiving water s loading capacity that is allocated to one of its existing or future point sources of pollution. WLAs constitute a type of water quality-based effluent limitation (40 CFR 130.2(h).

Water Level Gage/Gauge

A device that facilitates the measurement of the level, or stage, of the upper surface of water in a reservoir, channel, natural water body, stilling well, or groundwater well. A water level gauge may be as simple as a graduated staff mounted in the water that is to be measured, a float, or it may be a more complicated device that measures (1) water pressure at a point below the surface or the level of the water surface, or (2) travel time of a sound wave and its echos generated and received at a point above the water surface. A water level measurement device that both measures and records the position of the water surface is often termed a water level recorder or water stage recorder.

Water Level/Water Stage Recorder

The stage of a water body is the elevation of the water surface relative to some datum. Stage is synonymous with level. A water stage recorder is a device that measures and records the position of the upper surface of water in a reservoir, channel, stilling well, or natural water body. Some water level recorders utilize a float on the surface of the water which is attached to a moveable gear in the recorder. A rise or fall of the water surface causes the float to rise or fall. This rotates the gear and the movement is recorded. Other water level gauges monitor either relative or absolute pressure at a point under the water surface. This pressure measurement can then be converted to height of the water surface. Still other water level recorders monitor the travel time of sound waves generated and received at a point above the water surface. This travel time can then be converted to the height of the water surface.

Water Quality Criteria

Levels of water quality expected to render a body of water suitable for its designated use. Composed of numeric and narrative criteria. Numeric criteria are scientifically derived ambient concentrations developed by EPA or states for various pollutants of concern to protect human health and aquatic life. Narrative criteria are statements that describe the desired water quality goal. 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 Standard

A law or regulation that consists of the beneficial designated use or uses of a waterbody or a segment of a waterbody and the water quality criteria that is necessary to protect the use or uses of that particular waterbody. Water quality standards also contain an anti-degradation policy. The water quality standard serves a twofold purpose: (a) it establishes the water quality goals for a specific waterbody and ( b) it is the basis for establishing water quality-based treatment controls and strategies beyond the technology-based levels of treatment required by sections 301(b) and 306 of the Clean Water Act, as amended by the Water Quality Act of 1987.

Water Quality-Based Effluent Limitations (WQBEL)

Effluent limitations applied to dischargers when mere technology-based limitations would cause violations of water quality standards. Usually WQBELs are applied to discharges into small streams.

Water Quality-Based Permit

A permit with an effluent limit more stringent than one based on technology performance. Such limits may be necessary to protect the designated use of receiving waters (e.g., recreation, irrigation, industry or water supply).

Water Quality-Limited Segments

Those water segments which do not or are not expected to meet applicable water quality standards even after the application of technology-based limitations required by sections 301(b) and 306 of the Clean Water Act (40 CFR 130.29(j). Technology-based controls include, but are not limited to, best practicable control technology currently available (BPT)and secondary treatment.

Waterbody Use

A waterbody or a segment of a waterbody can have many uses. Typical uses include public water supplies, propagation of fish and wildlife, recreational purposes, agricultural use, industrial use, navigation, and other such uses. EPA does not recognize waste transport as an acceptable use.

Watershed

The land area from which rainfall or snow melt drains into a single waterbody. Ridges of higher ground generally form the watershed boundary. Adapted from US EPA, Compendium of Tools for Watershed Assessment and TMDL Development, EPA 841-B-97-006, May 1997. While all surface drainage in a watershed flows to a single waterbody, subsurface water flow may span one or more watersheds. Infiltrating surface water within a given watershed can, by way of the indirect groundwater routing, become part of the surface water in another watershed. Groundwater interactions with surface water vary with the hydraulic conductivity of the soil and can be significant. Surface water within a watershed can be directly connected to the surface water of another watershed by introducing artificial water routes.

Watershed Management

A holistic approach applied within an area defined by hydrological, not political, boundaries, integrating the water quality impacts from both point and nonpoint sources. Watershed management has a premise that many water quality and ecosystem problems are better solved at the watershed scale rather than by examining the individual waterbodies or dischargers. The Watershed management process identifies existing and potential contamination sources within a watershed to target priority problems. The approach typically uses high stakeholder involvement to integrate solutions and mitigation strategies. The management relies on the expertise and authority of multiple agencies, and measures success through monitoring and other data gathering. Adapted from: 1. OWOW, Watershed Approach--An Introduction. web location: http:/www.epa.gov/owow/watershed/index2.html and
2. Miller, M., A Watershed Management Plan: Steps to Protect Your Water Supply. web location: http:/www.epa.gov/owowwtr1/watershed/Proceed/miller.html.

Watershed Protection Approach (WPA)

The U.S. EPA s comprehensive approach to managing water resource areas, such as river basins, watersheds, and aquifers. WPA has four major features targeting priority problems, stakeholder involvement, integrated solutions, and measuring success.

Watershed-Scale Approach

A consideration of the entire watershed, including the land mass that drains into the aquatic ecosystem.

Wetlands

An area that is saturated by surface water or groundwater with vegetation adapted for life under those soil conditions, as in swamps, bogs, fens, marshes, and estuaries.

Wet-Weather Flow (WWF)

Usually referred to as the flow in a combined sewer system with stormwater, but may also constitute the flow in a separate storm or sanitary drainage system with stormwater.