Urban Stormwater Management in the United States (2009)

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Appendix B Glossary Antidegradation: Policies which ensure protection of water quality from a particular waterbody where the water quality exceeds levels necessary to protect fish and wildlife propagation and recreation on and in the water. This also in- cludes special protection of waters designated as outstanding natural resource waters. Antidegradation plans are adopted by each state to minimize adverse effects on water. Best Management Practice (BMP): Physical, structural, and/or managerial practices that, when used singly or in combination, reduce the downstream qual- ity and quantity impacts of stormwater. The term is synonymous with Stormwa- ter Control Measure (SCM). Biofiltration: The simultaneous process of filtration, infiltration, adsorption, and biological uptake of pollutants in stormwater that takes place when runoff flows over and through vegetated areas. Bioinfiltration: A particular SCM that is like bioretention but has more infiltra- tion, and thus would be categorized as an infiltration process. Bioretention: A stormwater management practice that utilizes shallow storage, landscaping, and soils to control and treat urban stormwater runoff by collecting it in shallow depressions before filtering through a fabricated planting soil me- dia. This SCM is often categorized under “filtration” although it has additional functions. Buffer: The zone contiguous with a sensitive area that is required for the con- tinued maintenance, function, and structural stability of the sensitive area. The critical functions of a riparian buffer (those associated with an aquatic system) include shading, input of organic debris and coarse sediments, uptake of nutri- ents, stabilization of banks, interception of fine sediments, overflow during high-water events, protection from disturbance by humans and domestic ani- mals, maintenance of wildlife habitat, and room for variation of aquatic system boundaries over time due to hydrologic or climatic effects. The critical func- tions of terrestrial buffers include protection of slope stability, attenuation of surface water flows from stormwater runoff and precipitation, and erosion con- trol. Stream buffers are zones of variable width that are located along both sides of a stream and are designed to provide a protective natural area along a stream corridor. 567 

568 URBAN STORMWATER MANAGEMENT IN THE UNITED STATES Combined Sewer Overflow (CSO): A discharge of untreated wastewater from a combined sewer system at a point prior to the headworks of a publicly owned treatment works. CSOs generally occur during wet weather (rainfall or snow- melt). During periods of wet weather, these systems become overloaded, bypass treatment works, and discharge directly to receiving waters. Combined Sewer System: A wastewater collection system that conveys sani- tary wastewaters (domestic, commercial, and industrial wastewaters) and stormwater through a single pipe to a publicly owned treatment works for treat- ment prior to discharge to surface waters. Constructed Wetland: A wetland that is created on a site that previously was not a wetland. This wetland is designed specifically to remove pollutants from stormwater runoff. Created Wetland: A wetland that is created on a site that previously was not a wetland. This wetland is created to replace wetlands that were unavoidably de- stroyed during design and construction of a project. This wetland cannot be used for treatment of stormwater runoff. Detention: The temporary storage of stormwater runoff in an SCM with the goals of controlling peak discharge rates and providing gravity settling of pol- lutants. Detention Facility/Structure: An above- or below-ground facility, such as a pond or tank, that temporarily stores stormwater runoff and subsequently re- leases it at a slower rate than it is collected by the drainage facility system. There is little or no infiltration of stored stormwater, and the facility is designed to not create a permanent pool of water. Drainage: Refers to the collection, conveyance, containment, and/or discharge of surface and stormwater runoff. Drainage Area: That area contributing runoff to a single point measured in a horizontal plane, which is enclosed by a ridge line. Drainage Basin: A geographic and hydrologic subunit of a watershed. Dry Pond: A facility that provides stormwater quantity control by containing excess runoff in a detention basin, then releasing the runoff at allowable levels. Synonymous with detention basin, it is intended to be dry between storms. Effluent Limitation: Any restriction imposed by the EPA director on quanti- ties, discharge rates, and concentrations of pollutants that are discharged from 

APPENDIX B 569 point sources into waters of the United States, the waters of the contiguous zone, or the ocean. Effluent Limitation Guidelines: A regulation published by the EPA Adminis- trator under Section 304(b) of the Clean Water Act that establishes national technology-based effluent requirements for a specific industrial category. Exfiltration: The downward movement of water through the soil; the down- ward flow of runoff from the bottom of an infiltration SCM into the soil. Extended Detention: A stormwater design feature that provides for the gradual release of a volume of water in order to increase settling of pollutants and pro- tect downstream channels from frequent storm events. When combined with a pond, the settling time is increased by 24 hours. Filter Strip: A strip of permanent vegetation above ponds, diversions, and other structures to retard the flow of runoff, causing deposition of transported material and thereby reducing sedimentation. As an SCM, it refers to riparian buffers, which run adjacent to waterbodies and intercept overland flow and shal- low subsurface flow (both of which are usually sheet flow rather than a distinct influent pipe). The term is borrowed from the agricultural world. Flood Frequency: The frequency with which the flood of interest may be ex- pected to occur at a site in any average interval of years. Frequency analysis defines the n-year flood as being the flood that will, over a long period, be equaled or exceeded on the average once every n years. Frequency of Storm (Design Storm Frequency): The anticipated period in years that will elapse, based on average probability of storms in the design re- gion, before a storm of a given intensity and/or total volume will recur; thus, a 10-year storm can be expected to occur on the average once every 10 years. Sewers designed to handle flows which occur under such storm conditions would be expected to be surcharged by any storms of greater amount or inten- sity. General Permit: A single permit issued to a large number of dischargers of pollutants in stormwater. General permits are issued by the permitting authority, and interested parties then submit a Notice of Intent (NOI) to be covered. The permit must identify the area of coverage, the sources covered, and the process for obtaining coverage. Once the permit is issued, a permittee may submit an NOI and receive coverage within a very short time frame. Grab Sample: A sample which is taken from a stream on a one-time basis without consideration of the flow rate of the stream and without consideration of time. 

570 URBAN STORMWATER MANAGEMENT IN THE UNITED STATES Hotspot: An area where land use or activities generate highly contaminated runoff, with concentrations of pollutants in excess of those typically found in stormwater. Hydrograph: A graph of runoff rate, inflow rate, or discharge rate, past a spe- cific point as a function of time. Hydroperiod: A seasonal occurrence of flooding and/or soil saturation; it en- compasses depth, frequency, duration, and seasonal pattern of inundation. Hyetograph: A graph of measured precipitation depth (or intensity) at a pre- cipitation gauge as a function of time. Impervious Surface or Impervious Cover: A hard surface area which either prevents or retards the entry of water into the soil. Common impervious sur- faces include roof tops, walkways, patios, driveways, parking lots or storage areas, concrete or asphalt paving, gravel roads, packed earthen materials, and oiled surfaces. Infiltration: The downward movement of water from the surface to the subsoil. Infiltration Facility: A drainage facility designed to use the hydrologic process of runoff soaking into the ground, commonly referred to as percolation, to dis- pose of stormwater. Infiltration Pond: A facility that provides stormwater quantity control by con- taining excess runoff in a detention facility, then percolating that runoff into the surrounding soil. Level Spreader: A temporary SCM used to spread stormwater runoff uni- formly over the ground surface as sheet flow. The purpose of level spreaders is to prevent concentrated, erosive flows from occurring. Levels spreaders will commonly be used at the upstream end of wider biofilters to ensure sheet flow into the biofilter. Municipal Separate Storm Sewer System: A conveyance or system of con- veyances (including roads with drainage systems, municipal streets, catch ba- sins, curbs, gutters, ditches, man-made channels, or storm drains) owned by a state, city, town, or other public body that is designed or used for collecting or conveying stormwater, which is not a combined sewer and which is not part of a publicly owned treatment works. National Pollutant Discharge Elimination System: A provision of the Clean Water Act that prohibits the discharge of pollutants into waters of the United States unless a special permit is issued by EPA, a state, or, where delegated, a 

APPENDIX B 571 tribal government on an Indian reservation. The permit applies to point sources of pollutants to ensure that their pollutant discharges do not exceed specified effluent standards. The effluent standards in most permits are based on the best available pollution technology or the equivalent. Nonpoint Source: Diffuse pollution source, but with a regulatory connotation; a source 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. Some common nonpoint sources are agriculture, forestry, min- ing, dams, channels, land disposal, and saltwater intrusion. Nonstructural SCM: Stormwater control measure that uses natural measures to reduce pollution levels, does not require extensive construction efforts, and/or promotes pollutant reduction by eliminating the pollutant source. Peak Discharge Rate: The maximum instantaneous rate of flow during a storm, usually in reference to a specific design storm event. Point Source: Any discernible, confined, and discrete conveyance, including but not limited to any pipe, ditch, channel, tunnel, conduit, well, discrete fixture, container, rolling stock, concentrated animal feeding operation, landfill leachate collection system, vessel, or other floating craft from which pollutants are or may be discharged. Pollutant: A contaminant in a concentration or amount that adversely alters the physical, chemical, or biological properties of the natural environment. Dredged soil, solid waste, incinerator residue, filter backwash, sewage, garbage, sewage sludge, munitions, chemical wastes, biological materials, radioactive materials (except those regulated under the Atomic Energy Act of 1954, as amended), heat, wrecked or discarded equipment, rock, sand, cellar dirt and industrial, mu- nicipal, and agricultural waste discharged into water (EPA, 2008). Polutograph: A graph of pollutant loading rate (mass per unit time) as a func- tion of time. Predevelopment Conditions: Those conditions that existed at a site just prior to the development in question, which are not necessarily pristine conditions. Pretreatment: The removal of material such as gross solids, grot, grease, and scum from flows prior to physical, biological, and chemical treatment processes to improve treatability. The reduction of the amount of pollutants, the elimina- tion of pollutants, or the alteration of the nature of pollutant properties in waste- water prior to or in lieu of discharging or otherwise introducing such pollutants into a publicly owned treatment works [40 C.F.R. § 403.3(q)]. Pretreatment may include screening, grit removal, stormwater, and oil separators. With re- 

572 URBAN STORMWATER MANAGEMENT IN THE UNITED STATES spect to stormwater, it refers to techniques employed in stormwater SCMs to help trap coarse materials and other pollutants before they enter the SCM. Recharge: The flow of groundwater from the infiltration of stormwater runoff. Recharge Volume: The portion of the water quality volume used to maintain groundwater recharge rates at development sites. Retention: The process of collecting and holding stormwater runoff with no surface outflow. Also, the amount of precipitation on a drainage area that does not escape as runoff. It is the difference between total precipitation and total runoff. Retention/Detention Facility: A type of drainage facility designed either to hold water for a considerable length of time and then release it by evaporation, plant transpiration, and/or infiltration into the ground, or to hold stormwater runoff for a short period of time and then release it to the stormwater manage- ment system. Runoff: The term is often used in two senses. For a given precipitation event, direct storm runoff refers to the rainfall (minus losses) that is shed by the land- scape to a receiving waterbody. In an area of 100 percent imperviousness, the runoff equals the rainfall. Over greater time and space scales, surface water runoff refers to streamflow passing through the outlet of a watershed, including base flow from groundwater that has entered the stream channel. Soil Stabilization: The use of measures such as rock lining, vegetation, or other engineering structure to prevent the movement of soil when loads are applied to the soil. Source Control: A type of SCM that is intended to prevent pollutants from entering stormwater. A few examples of source control are erosion control prac- tices, maintenance of stormwater facilities, constructing roofs over storage and working areas, and directing wash water and similar discharges to the sanitary sewer or a dead end sump. Stormwater: That portion of precipitation that does not naturally percolate into the ground or evaporate, but flows via overland flow, interflow, channels, or pipes into a defined surface water channel or a constructed infiltration facility. According to 40 C.F.R. § 122.26(b)(13), this includes stormwater runoff, snow melt runoff, and surface runoff and drainage. Stormwater Control Measure (SCM): Physical, structural, and/or managerial measures that, when used singly or in combination, reduce the downstream qual- ity and quantity impacts of stormwater. Also, a permit condition used in place 

APPENDIX B 573 of or in conjunction with effluent limitations to prevent or control the discharge of pollutants. This may include a schedule of activities, prohibition of practices, maintenance procedures, or other management practices. SCMs may include, but are not limited to, treatment requirements; operating procedures; practices to control plant site runoff, spillage, leaks, sludge, or waste disposal; or drainage from raw material storage. Stormwater Drainage System: Constructed and natural features which func- tion together as a system to collect, convey, channel, hold, inhibit, retain, detain, infiltrate, divert, treat, or filter stormwater. Stormwater Facility: A constructed component of a stormwater drainage sys- tem, designed or constructed to perform a particular function or multiple func- tions. Stormwater facilities include, but are not limited to, pipes, swales, ditches, culverts, street gutters, detention basins, retention basins, constructed wetlands, infiltration devices, catch basins, oil/water separators, sediment ba- sins, and modular pavement. Structural SCMs: Devices which are constructed to provide temporary storage and treatment of stormwater runoff. Swale: A shallow drainage conveyance with relatively gentle side slopes, gen- erally with flow depths of less than one foot. Biofilter (same as a Biofiltration Swale): A sloped, vegetated channel or ditch that provides both conveyance and water quality treatment to stormwater runoff. It does not provide stormwater quantity control but can convey runoff to SCMs designed for that purpose. Dry Swale: An open drainage channel explicitly designed to detain and promote the filtration of stormwater runoff through an underlying fabricated soil media. It has an underdrain. Wet Swale: An open drainage channel or depression, explicitly de- signed to retain water or intercept groundwater for water quality treat- ment. Technology-Based Effluent Limit: A permit limit for a pollutant that is based on the capability of a treatment method to reduce the pollutant to a certain con- centration. Time of Concentration: The time period necessary for surface runoff to reach the outlet of a subbasin from the hydraulically most remote point in the tributary drainage area. 

574 URBAN STORMWATER MANAGEMENT IN THE UNITED STATES Total Maximum Daily Load (TMDL): The amount, or load, of a specific pol- lutant that a waterbody can assimilate and still meet the water quality standard for its designated use. For impaired waters the TMDL reduces the overall load by allocating the load among current pollutant loads (from point and nonpoint sources), background or natural loads, a margin of safety, and sometimes an allocation for future growth. Volumetric Runoff Coefficient (Rv): The value that is applied to a given rain- fall volume to yield a corresponding runoff volume based on the percent imper- vious cover in a drainage basin. Water Quality-Based Effluent Limit (WQBEL): A value determined by se- lecting the most stringent of the effluent limits calculated using all applicable water quality criteria (e.g., aquatic life, human health, and wildlife) for a specific point source to a specific receiving water for a given pollutant. Water Quality SCM: An SCM specifically designed for pollutant removal. Water Quantity SCM: An SCM specifically designed to reduce the peak rate of stormwater runoff. Water Quality Volume (Wqv): The volume needed to capture and treat 90 per- cent of the average annual stormwater runoff volume equal to 1 inch times the volumetric runoff coefficient (Rv) times the site area. Wetlands: Those areas that are inundated or saturated by surface water or groundwater at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs, and similar areas. This includes wetlands created, restored, or enhanced as part of a mitigation procedure. This does not include constructed wetlands or the following surface waters of the state intentionally constructed from sites that are not wetlands: irrigation and drainage ditches, grass-lined swales, canals, agricultural detention facilities, farm ponds, and landscape amenities. Wet Pond: A facility that treats stormwater for water quality by utilizing a permanent pool of water to remove conventional pollutants from runoff through sedimentation, biological uptake, and plant filtration. Synonymous with a reten- tion basin. SOURCES: Most of the definitions are from EPA (2003), “BMP Design Considerations,” 600/R-03/103, or EPA (2008), “Handbook for Developing Watershed Plans to Restore and Protect Our Waters,” EPA 841-B-08-002.