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Glossary

Looking for a quick definition of a water or conservation related term or concept? We’ve compiled an extensive list of the most commonly used terms in the biz.

  • Assimilative capacity: refers to the ability of the environment or a portion of the environment (such as a stream, lake, air mass, or soil layer) to carry waste material without adverse effects on the environment or on users of its resources. Pollution occurs only when the assimilative capacity is exceeded.
  • Best Management Practice (BMP): Best Management Practice means a practice or combination of practices, that is determined to be an effective and practicable (including technological, economic, and institutional considerations) means of preventing or reducing the amount of pollution generated by nonpoint sources to a level compatible with water quality goals.
  • Bioretention: Stormwater quality management facility that retains runoff in a shallow basin, usually less than 18 inches in depth and provides infiltration of runoff through designed soil media to filter pollutants. Bioretention basins can be landscaped to be an attractive site amenity. Rain gardens are a form of bioretention.
  • Depression storage: Natural features found in the landscape in the Texas coastal region that retains runoff, effectively storing stormwater and promoting infiltration.
  • Detention: The temporary storage of stormwater runoff (in ponds, under-ground systems, or depressed areas) to allow for controlled discharge at a later time. The outlet structure restricts outflow to pre-development rates.
  • Disconnection: Refers to vegetation, soil, and landscape areas that can disconnect impervious cover surfaces from one another. An example is the flow length across a single-family lawn that “disconnects” the downspout runoff from the street. Thus, runoff is slowed, spread out, and soaked into the ground to reduce runoff volume.
  • Evapotranspiration: The combined amount of evaporation and plant transpiration from the soil surface or from the plant’s vascular system to the atmosphere.
  • Extended detention: A stormwater management facility that can provide water quality and flood reduction benefits. The basin outlet is sized to release the design storm volume over a period of 24 to 72 hours, thus, promoting pollutant settling and slow runoff release to protect the receiving waterbody from scour and accelerated erosion as a result of development.
  • Filtration: The removal of sediment and other pollutants from storm-water runoff by the movement of runoff across a vegetated area and through media.
  • Green Infrastructure: Green infrastructure is a cost-effective, resilient approach to managing wet weather impacts that provides many community benefits. While single-purpose gray stormwater infrastructure—conventional piped drainage and water treatment systems—is designed to move urban stormwater away from the built environment, green infrastructure reduces and treats stormwater at its source while delivering environmental, social, and economic benefits.
  • Impervious cover: Impervious cover is any type of man-made or stone surface that doesn’t absorb rainfall. Surfaces such as stone, rooftops, patios, driveways, sidewalks, roadways, parking lots, and some decks are considered impervious cover.
  • Infiltration: The vertical movement of stormwater through plants and soil. In systems without an under drain or liner, infiltration re-charges groundwater.
  • Low Impact Development (LID): An engineering design approach that manages stormwater runoff by mimicking natural processes that result in infiltration, evapotranspiration or use of stormwater in order to protect water quality and associated aquatic habitat.
  • Municipal Separate Storm Sewer System (MS4): A conveyance or system of conveyances (including roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches, man-made channels, or storm drains):
    • (a) Owned or operated by the U.S., a state, city, town, borough, county, parish, district, association, or other public body (created by or pursuant to state law) having jurisdiction over the disposal of sewage, industrial wastes, stormwater, or other wastes, including special districts under state law such as a sewer district, flood control district or drainage district, or similar entity, or an Indian tribe or an authorized Indian tribal organization, or a designated and approved management agency under the CWA §208 that discharges to surface water in the state;
    • (b) That is designed or used for collecting or conveying stormwater;
    • (c) That is not a combined sewer; and
    • (d) That is not part of a publicly owned treatment works (POTW) as defined in 40 CFR §122.2.
  • Nonpoint Source Pollution: Nonpoint source pollution is water pollution that is caused by widely dispersed sources of pollutants. While most nonpoint source-caused pollution problems are associated with pollutants carried by runoff from rain, other pollutant sources include spills and leaks, atmospheric deposition, and hydrologic modifications. Nonpoint source pollutants affect groundwater and surface water.
  • Non-traditional Small MS4: A small MS4 that often cannot pass ordinances and may not have the enforcement authority like a traditional small MS4 would have to enforce the stormwater management program. Examples of non-traditional small MS4s include counties, transportation authorities (including the Texas Department of Transportation), municipal utility districts, drainage districts, military bases, prisons, and universities.
  • Outfall: A point source at the point where a small MS4 discharges to waters of the U.S. and does not include open conveyances connecting two municipal separate storm sewers, or pipes, tunnels, or other conveyances that connect segments of the same stream or other waters of the U.S. and are used to convey waters of the U.S. For the purpose of this permit, sheet flow leaving a linear transportation system without channelization is not considered an outfall. Point sources such as curb cuts; traffic or right-or-way barriers with drainage slots that drain into open culverts, open swales, or an adjacent property, or otherwise not actually discharging into waters of the U.S. are not considered an outfall.
  • Pervious Cover: A surface that can absorb rainfall. Examples include vegetation, permeable pavers, and permeable concrete.
  • Point Source Pollution: Any single identifiable source of pollution from which pollutants are discharged, such as a pipe, ditch, ship, or factory smokestack. Factories and sewage treatment plants are two common types of point sources
  • Pollutant Loading: The amount of stress placed upon an ecosystem by pollution, physical or chemical, released into it by man-made or natural means.
  • Retention: The storage of stormwater runoff on site and not released at a later time. There is no outlet structure but retained runoff could be used for an additional purpose such as irrigation or a de-sign amenity.
  • Riparian Zone: Riparian zones are the transitional areas between land and water, including the margins of streams, rivers, lakes, and wetlands. They are rich in biodiversity and play an important role in protecting water quality and stream ecosystem health.
  • Small Municipal Separate Storm Sewer System (MS4): A conveyance or system of conveyances (including roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches, man-made channels, or storm drains):
    • (a) Owned or operated by the U.S., a state, city, town, borough, county, district, association, or other public body (created by or pursuant to State law) having jurisdiction over disposal of sewage, industrial wastes, stormwater, or other wastes, including special districts under state law such as a sewer district, flood control district or drainage district, or similar entity, or an Indian tribe or an authorized Indian tribal organization, or a designated and approved management agency under CWA §208;
    • (b) Designed or used for collecting or conveying stormwater;
    • (c) Which is not a combined sewer;
    • (d) Which is not part of a publicly owned treatment works (POTW) as defined in 40 CFR §122.2; and
    • (e) Which was not previously regulated under a National Pollutant Discharge Elimination System (NPDES) or a Texas Pollutant Discharge Elimination System (TPDES) individual permit as a medium or large municipal separate storm sewer system, as defined in 40 CFR §122.26(b)(4) and (b)(7).
    • This term includes systems similar to separate storm sewer systems at military bases, large hospitals or prison complexes, and highways and other thoroughfares. This term does not include separate storm sewers in very discrete areas, such as individual buildings. For the purpose of this permit, a very discrete system also includes storm drains associated with certain municipal offices and education facilities serving a nonresidential population, where those storm drains do not function as a system, and where the buildings are not physically interconnected to a small MS4 that is also operated by that public entity.
  • Stormwater Runoff: Stormwater runoff is generated from rain and snowmelt events that flow over land or impervious surfaces, such as paved streets, parking lots, and building rooftops, and does not soak into the ground. The runoff picks up pollutants like trash, chemicals, oils, and dirt/sediment that can harm our rivers, streams, lakes, and coastal waters.
  • Total Suspended Solids (TSS): The weight of solids remaining after a well-mixed sample is filtered through a standard glass filter and the suspended portion is dried to a constant weight at 103-105 deg C.
  • Traditional Small MS4: A small MS4 that can pass ordinances and have the enforcement authority to enforce the stormwater management program. An example of traditional MS4s includes cities.
  • Urbanized Area (UA): An area of high population density that may include multiple small MS4s as defined and used by the U.S. Census Bureau in the 2000 and the 2010 Decennial Census.
  • Vegetated filter strips: Vegetated sections of land that are relatively flat with low slopes to accept runoff as overland sheet flow and promote filtering and infiltration to improve runoff quality and slow runoff from impervious areas. Vegetated filter strips are a good choice to disconnect impervious areas.
  • Vegetated swales: Stormwater quality management channels that convey runoff and remove pollutants by filtering and infiltration through the soil. Pollutant management is related to the slope, vegetation density, and slope; thus, an optimum swale would have a shallow slope, dense vegetation, and porous soils to maximize water quality treatment benefits.
  • Wetland basins: Constructed shallow basin that creates growing conditions suitable for wetland and marsh plants while provide runoff storage to provide water quality and flood reduction benefits. Constructed wetlands provide physical, chemical, and biological water quality treatment of stormwater runoff.
  • Wet ponds: Stormwater quality control facilities that maintain a permanent wet pool, usually at least four feet in depth and a standing crop of emergent littoral vegetation. Wet ponds are often perceived as a positive aesthetic element in a community and offer significant opportunity for creative pond configuration and landscape design.