Stormwater Glossary
Below is a list of stormwater terms and definitions for reference throughout this process:
303(d) List
Section 303(d) of the Clean Water Act (CWA) requires states to develop a list of waters not meeting water quality standards or that have impaired uses. Listed waters must be prioritized, and a management strategy or total maximum daily load (TMDL) must subsequently be developed for all listed waters.
Aquifer
Geologic formation that is saturated and that transmits large quantities of water.
Base Flow
Flow in a channel due to soil moisture or ground water.
Best management practices
Activities or structural improvements that help reduce the quantity and improve the quality of stormwater runoff. BMPs include treatment requirements, operating procedures, and practices to control site runoff, spillage or leaks, sludge or waste disposal, or drainage from raw material storage.
Bioretention
A water quality practice that utilizes landscaping and soils to treat stormwater by collecting it in shallow depressions and then filtering it through a planting soil media.
Build-out
The total percentage of development that will be allowed in a watershed based on current zoning.
Catch basin
An entryway to the storm drain system, usually located at a street comer.
Conduit
Any channel or pipe used to transport flowing water.
Conveyance
The process of water moving from one place to another.
Culvert
Pipe or concrete box structure that drains open channels, swales or ditches under a roadway or embankment, typically with no catch basins or manholes along its length.
Design Storm
A specified amount of storm rainfall, with its areal and temporal distribution, used to estimate a design discharge. Typically, the design storms are based on a return period (refer to definition below) of 2-years, 10-years, 25-years, or 100-years.
Detention
A stormwater system that delays the downstream progress of stormwater runoff in a controlled manner, typically by using temporary storage areas and a metered outlet device.
Detention facility
A facility that collects water from developed areas and releases it at a slower rate than it enters the collection system. The excess of inflow over outflow is temporarily stored in a pond or a vault and is typically released over a few hours or a few days.
Discharge
The volume of water (and suspended sediment if surface water) that passes a given location within a given period of time.
Drainage
The collection, conveyance, containment, and/or discharge of surface and storm water runoff.
Drainage facility
A constructed or engineered feature that collects, conveys, stores or treats surface and storm water runoff. Drainage facilities shall include but not be limited to all constructed or engineered streams, pipelines, channels, ditches, gutters, lakes, wetlands, closed depressions, flow control or water quality treatment facilities, erosion and sedimentation control facilities, and other drainage structures and appurtenances that provide for drainage.
Drainage Divides
Boundary that separates sub-basin areas according to direction of runoff.
Erosion
Removal of soil particles by wind and water. Often the eroded debris (silt or sediment) becomes a pollutant via stormwater runoff. Erosion occurs naturally but can be intensified by human activities such as farming, development, road-building, and timber harvesting.
Flood
A temporary rise in flow or stage of any watercourse or stormwater conveyance system that results in stormwater runoff exceeding its normal flow boundaries and inundating adjacent, normally dry areas.
Flow control facility
A drainage facility designed to mitigate the impacts of increased surface and storm water runoff generated by site development pursuant to the drainage requirements. Flow control facilities are designed either to hold water for a considerable length of time and then release it by evaporation, plant transpiration, and/or infiltration into the ground, or to hold runoff a short period of time and then release it to the conveyance system.
Floodplain
Areas adjacent to a stream or river that are subject to flooding or inundation during severe storm events (often called the 100-year floodplain, it would include the area or flooding that occurs, on average, once every 100 years). Sometimes referred to as the “FEMA Floodplain.”
Grading
The cutting and/or filling of the land surface to a desired slope or elevation.
Groundwater
Underground water usually found in aquifers. Groundwater usually originates from infiltration. Runoff can seep into the soil and recharge groundwater that supplies drinking wells and springs.
Household Hazardous Waste
Common everyday products that people use in and around their homes—including paint, paint thinner, herbicides, and pesticides—that, due to their chemical nature, can be hazardous if not properly disposed.
Hydrologic cycle
The circuit of water movement from the atmosphere to the earth and return to the atmosphere through various stages or processes such as precipitation, interception, runoff, infiltration, percolation, storage, evaporation, and transpiration.
Illicit connection
Any discharge to a municipal separate storm sewer that is not composed entirely of stormwater and is not authorized by an NPDES permit, with some exceptions (e.g., discharges due to firefighting activities)
Illicit discharges
Discharges of non-stormwater to the storm drainage system. Examples are discharges from internal floor drains, appliances, industrial processes, sinks, and toilets that are connected to the nearby storm drainage system. These discharges should be going to the sanitary sewer system, a holding tank, an on-site process water treatment system, or a septic system.
Impervious surface
Hard ground cover that prevents or retards the entry of water into the soil and increases runoff, such as asphalt, concrete, rooftops.
Infiltration
The portion of rainfall or surface runoff that moves downward into the subsurface rock and soil
Invert
Bottom of a channel or pipe.
Major drainage system
A group of major streams that carry runoff from large storms when the minor drainage system is full.
National Pollutant Discharge Elimination System (NPDES)
National Pollutant Discharge Elimination System, the two-phased surface water quality program authorized by Congress as part of the 1987 Clean Water Act. This federally mandated system is used for regulating point source and nonpoint stormwater discharge. The second phase of the program requires local governments to implement the following six minimum measures:
- Public Education and Outreach
- Public Participation/ Involvement
- Illicit Discharge Detection and Elimination
- Construction Site Runoff Control
- Post-Construction Runoff Control
- Pollution Prevention/Good Housekeeping
Natural conveyance system elements
Swales and small drainage courses, streams, rivers, lakes, and wetlands.
Neuse River Nutrient Sensitive Waters Management Strategy (i.e. Neuse Rules)
Legislation aimed at reducing nitrogen levels in the Neuse - which passed in 1998. Affected areas, working in cooperation with the North Carolina Division of Water Quality, have been required to develop a model stormwater plan to meet the objectives of the rules. These plans were required to include a 30% nitrogen reduction goal for new developments—achieved through the use of best management practices (BMPs) and planning considerations, public education, identification and removal of illegal discharges, and identification of potential restoration projects.
Nonpoint-source pollutants
Pollutants from many diffuse sources. Nonpoint-source pollution is caused by rainfall or snowmelt moving over and through the ground. As the runoff moves, it picks up and carries away natural and human-made pollutants, finally depositing them into lakes, rivers, wetlands, coastal waters, and even underground sources of drinking water.
Nonstructural BMP
A preventative action to protect receiving water quality that does not require construction. Nonstructural BMPs rely predominantly on behavioral changes in order to be effective. Major categories of nonstructural BMPs include education, recycling, maintenance practices and source controls.
Outfall
The point where wastewater or drainage discharges from a sewer pipe, ditch, or other conveyance to a receiving body of water.
Overland flow
Flow of water across the land surface
Point-source pollutants
Pollutants from a single, identifiable source such as a factory or refinery; also called single-point-source pollution and usually from a permitted outfall.
Pollutant loading
The total quantity of pollutants in stormwater runoff
Precipitation
Water that falls to the earth in the form of rain, snow, hail, or sleet.
Receiving waters
Bodies of water or surface water systems receiving water from upstream manmade or natural systems.
Recharge
Water that infiltrates to an aquifer, usually from above.
Recurrence interval/Return Period
Time interval in which an event will occur once on the average.
Retention
The process of collecting and holding surface and storm water runoff with no surface outflow.
Riparian Buffers
Vegetated areas next to water resources that protect water resources from nonpoint source pollution and provide bank stabilization and aquatic and wildlife habitat.
Runoff
Water originating from rainfall and other precipitation that ultimately flows into drainage facilities, rivers, streams, springs, seeps, ponds, lakes, and wetlands as well as shallow groundwater.
Runoff coefficient
Ratio of runoff to precipitation
Runoff curve number
Parameter used in the Soil Conservation Service method that accounts for soil type and land use.
Sanitary sewer
A system of underground pipes that carries sanitary waste or process wastewater to a treatment plant.
Sediment
Soil, sand, and minerals washed from land into water, usually after rain. Sediment can destroy fish-nesting areas, clog animal habitats, and cloud waters so that sunlight does not reach aquatic plants. Sediment contributes additional nutrients to water. It often comes from construction sites and is North Carolina’s #1 pollutant.
Sheet flow
The portion of precipitation that moves initially as overland flow in very shallow depths before eventually reaching a stream channel
Simulation model
Model describing the reaction of a watershed to a storm using numerical equations.
Storm drain
An opening leading to an underground pipe or open ditch for carrying surface runoff, separate from the sanitary sewer or wastewater system
Storm drainage system
The system built to collect and transport runoff to prevent flooding. This system consists of storm drains, drainage ditches, pipes and culverts. Anything that flows into the storm drainage system flows directly into local creeks and waterways. (Storm water runoff is not treated.) Storm drainage systems are completely separate from those that carry domestic and commercial wastewater (sanitary sewer system).
Storm sewer
System of conduits that carries storm runoff.
Stormwater
Precipitation that accumulates in natural and/or constructed storage and stormwater systems during and immediately following a storm event
Stormwater management
Functions associated with planning, designing, constructing, maintaining, financing, and regulating the facilities (both constructed and natural) that collect, store, control, and/or convey stormwater
Stormwater pollution
Water from rain, irrigation, garden hoses or other activities that picks up pollutants (cigarette butts, trash, automotive fluids, used oil, paint, fertilizers and pesticides, lawn and garden clippings and pet waste) from streets, parking lots, driveways and yards and carries them through the storm drain system and straight to the ocean
Structural BMP
Constructed facilities or measures to help protect receiving water quality and control stormwater quantity. Examples include storage, vegetation, infiltration, and filtration.
Sub-basins
Hydrologic divisions of a watershed that are relatively homogeneous.
Surcharge
Condition in which the water level in a storm sewer rises above the crown of the conduit.
Surface runoff
The portion of rainfall that moves over the ground toward a lower elevation and does not infiltrate the soil.
Surface water
Water that remains on the surface of the ground, including rivers, lakes, reservoirs, streams, wetlands, impoundments, seas, and estuaries
Total maximum daily load (TMDL)
The maximum allowable loading of a pollutant that a designated water body can assimilate and still meet numeric and narrative water quality standards. TMDLs were established by the 1972 Clean Water Act. Section 303(d) of the US Water Quality Act requires states to identify water bodies that do not meet federal water quality standards. In 1996 the states developed (with EPA approval) a list of water bodies that failed to meet section 303(d) standards. These are the focus of TMDLs. Allocation of named pollutants is on percentage basis.
Urban runoff
Stormwater from urban areas, which tends to contain heavy concentrations of pollutants from urban activities
Water quality
The biological, chemical and physical conditions of a waterbody; a measure of the ability of a waterbody to support beneficial uses.
Water surface profile
Plot of the depth of water in a channel along the length of the channel.
Watershed
Geographical area that drains to a specified point on a water course, usually a confluence of streams or rivers. Also known as drainage area, catchment, or river basin.
Wetpond
Drainage facilities for water quality treatment that contain a permanent pool of water. They are designed to optimize water quality by providing long retention times (on the order of a week or more) to settle out particles of fine sediment to which pollutants such as heavy metals adsorb, and to allow biologic activity to occur that metabolizes nutrients and organic pollutants. For wetvaults, the permanent pool of water is covered by a lid which blocks sunlight from entering the facility, limiting light-dependent biologic activity.