Combined Sewer Overflow (CSO)/ Sewer Overflow – Overflow from a sewer system that occurs when the system is overwhelmed with excessive water – typically occurring during a storm event. The overflow results in a discharge of a mixture of partially treated wastewater and runoff to surface water.

Concentrated Flow – A flow regime where water flows in a channel – the moving water is concentrated, as opposed to being spread out across the landscape, as in sheet flow.

Confluence – The point where one waterbody flows into another.

Constructed Wetland – A wetland that is designed specifically to capture and remove pollutants andcreated on a site that previously was not a wetland.

Contour – A path on the land where the elevation remains constant; describes topography or relief.

Contributing Drainage Area (CDA) – The land area from which surface water drains to a specific point of reference.

Culvert – Pipe or box structure that drains surface water or runoff under a roadway or embankment. Design Element – A term used in reference to a land area input in the Tennessee Runoff Reduction Assessment Tool that has an identified land area and management.

Design Storm – The precipitation depth with a specific return period or frequency which may be used to size and select materials for stormwater treatment.

Designated Use – The use of a water resource as identified by the state. In Tennessee, these include fish and aquatic life, recreation, drinking water supply, irrigation, industrial water supply, livestock and wildlife watering, and navigation.

Detention – Temporary storage of stormwater to decrease peak flow rate into receiving waters. Typically relates to a basin.

Discharge – 1) The release of a water containing pollutants into surface waters, 2) or the volume of water that passes a certain point in time.

Downspout Disconnection – Disconnecting the rooftop impervious surface from the stormwater conveyance system as to reduce total runoff volume by allowing downspouts to drain onto pervious surface or into another retention practice.

Drainage Area – The area that contributes runoff to a point of reference and is enclosed by a ridgeline or divide. See Basin.

Dry Detention – An SCM that provides stormwater flow control designed to temporarily store and release stormwater at an acceptable rate (for permanent stormwater management).

Ecoregion – A recurring pattern of ecosystems associated with characteristic combinations of soil and landform that characterise that region-Brunckhorst, D. (2000). Bioregional planning: resource management beyond the new millennium. Harwood Academic Publishers: Sydney, Australia.

Ecosystem Services – Collectively, the processes that occur in the environment that benefit humans and society. For example, water purification in wetlands, nutrient cycling in streams, and carbon storage in soil.

Effluent – Water that has passed through a treatment process and discharged into the environment.

Energy Dissipater – A structure used to absorb the energy carried by concentrated flow and reduce velocity.

Engineered Media – Engineered mixture of materials (such as sand, soil, clay, granular activated carbon, perilite, zoelite, compost, organic matter, water treatment residuals, etc.) that is designed to infiltrate and filter pollutants from stormwater runoff.

Environmental Site Design – Method of using small-scale stormwater management practices, nonstructural techniques, and better site planning to mimic natural hydrologic runoff characteristics and minimize the impact of land development on water resources. Similar terms include low impact development, green infrastructure, and better site design.

Erosion Control Matting – A sheet-like material that is placed on the soil surface to reduce erosion. These materials are typically used to protect the soil during the germination and growth of the permanent vegetation and are made of various materials.

Eutrophication – The process of being well nourished; an enrichment of nutrients (mostly nitrogen and phosphorus) in surface waterbodies that may result in excessive aquatic plant growth – frequently associated with algal blooms. Eventually may lead to the filling in and loss of the water body.

Evapotranspiration – The sum of evaporation of water from the soil and transpiration of water from plants.

Exceptional Tennessee Waters – 1) Surface waters of the state that satisfy a set of characteristics, including being within state or national parks, wildlife refuges, wilderness or national areas; 2) State or Federal Scenic Rivers; Federally-designated critical habitat; 3) waters within an area designated as Lands Unsuitable for Mining; 4) waters with naturally reproducing trout; 5) waters with exceptional biological diversity or; 6) waters with outstanding ecological or recreational value as determined by TDEC.

Exfiltration – The process in which water moves out of a media, or permeable pavement.

Extended Detention – Design for the temporary storage of stormwater in a detention facility that gradually discharges volume, allowing for increased settling of pollutants and protection of receiving channels. See wet pond.

Filter Strip – An SCM that uses dense vegetation with uniform grade to slow runoff and facilitate deposition of sediment in runoff before runoff reaches surface water.

First Flush – The stormwater that is first to run off a surface and usually carries the largest load of pollutants. Generally considered to be the first inch of rainfall.

Floatables – Litter and debris that will float and travel with water.

Floodplain – The flat area along a stream between the streambank and the valley wall that is periodically inundated by floodwaters.

Flow Path – The path water takes as it moves across land surfaces.

Forebay – A separate segment within a stormwater basin used to trap sediment, chosen to facilitate maintenance and removal of the sediment. Use of a forebay is intended to facilitate sedimentation and thus protect other unit treatment processes.

Green Infrastructure (GI) – Using natural hydrologic features and open space to manage water and provide environmental and community benefits. Similar terms include low impact development and environmental site design.

Green Roof – A rooftop that is covered with beds or a single bed of soil and vegetation and designed to infiltrate precipitation. An extensive application uses media depths between 4-6 inches deep, while an intensive application is deeper than 6 inches. Also known as a vegetated roof.

Groundwater – Water below the ground surface.

Heavy Metals – Elements, such as zinc, mercury, lead, and copper. These elements can become dissolved in stormwater and are prone to accumulate in urban areas due to anthropogenic activities (mainly automobile use).

Hotspots – A term used to describe land use or activities that pose a relatively higher potential threat to surface or groundwater pollution due to the nature of contaminants that are associated with the operations and land use. Some examples are, but not limited to, gasoline stations, trash collection areas, mulching operations, chemical storage facilities, car washes, nurseries, golf courses, etc.

Hydraulic Residence Time (HRT) – The time that a volume of water is held within a defined area. Generally, the longer the HRT, the more effective the treatment of stormwater pollution.

Hydric Soils – A soil that is saturated, flooded, or ponded long enough during the growing season to develop anaerobic conditions that favor the growth and regeneration of water-loving vegetation.

Hydrocarbons – Organic chemical compounds that are made up of solely carbon and hydrogen. Predominantly used as combustible fuel and, as solid state, asphalt. A pollutant of concern in urban areas due to their contribution to ground level ozone and smog.

Hydrologic Unit Code (HUC) – A standardized watershed classification system created by the USGS. See Section II B for more on Tennessee HUCs.

Illicit Discharge – Any discharge to a municipal separate storm sewer that is not entirely composed of stormwater, except discharges authorized under an NPDES permit and discharges resulting from fire fighting activities.

Impaired Water – A segment of surface waters that has been identified as failing to support its designated uses.

Impervious Surface – 1) A hard surface that either prevents or limits the movement of water into the soil as would naturally occur in a pre-development condition. 2) A hard surface that causes water to runoff in greater quantities than that occurring under natural or pre-development conditions, usually associated with building structures or pavement.

Infiltration – The movement of water into the ground surface.

Infiltration Area – An SCM used with downspout/impervious surface disconnection where an area’s vegetation and soil are sized and designed to receive and infiltrate runoff from impervious surfaces.

Infiltration Trench – A runoff reduction facility that captures stormwater in a long and narrow pit that is filled with porous material (usually a sand mixture) and holds water until it has time to soak into surrounding native soils; usually used in areas with poor draining soils.

Inlet – The location where water flows into a structure or facility.

Interflow – The rapid lateral flow of water below the ground surface and above the water table.

Karst – Geological formations shaped by the dissolution of soluble rock, usually carbonate rock like limestone or dolomite.

Load – 1) A measurement of total mass of a constituent in water. 2) The product of the concentration and the water volume

Low Impact Development (LID) – A style of development that incorporates techniques to minimize impacts to natural resources, preserves ecosystem services, and implements best management practices to mimic natural hydrology.

Management – A clearly defined state of soil and vegetation that provides the desired degree of infiltration and contaminant removal under the design conditions. This design condition is considered to be 15 years following stabilization, when the site has reached a reasonable level of maturity and is undergoing only gradual changes. The management is the defined desired endpoint, and depends on a series of techniques to get from the current disturbed condition to that endpoint. A management has clearly defined specifications including vegetation type and density, soil hydrologic characteristics, etc. The term Cover is sometimes used to describe a management that has minimal inputs.

Managed Vegetated Area (MVA) – An SCM that describes and area that sustains a determinant density and type of vegetated cover such that it intercepts rainfall, covers the soil surface and has an established root matrix.

Maximum Extent Practicable (MEP) – Technology-based discharge standard for MS4s to reduce pollutants in stormwater discharges as established by the CWA. MS4 operators shall develop and implement their programs to reduce the discharge of pollutants to the maximum extent practicable, including management practices, control techniques and system, design and engineering methods, and such other provisions appropriate for the control of pollutants.

Minimum Control Measure – Stated requirements from the USEPA for permitted groups to be in compliance and include the implementation of selected BMPs to minimize stormwater and related pollutants into surface waters.