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Impervious Area

Areas of the land surface that by man's action become blocked or sealed from rainfall causing runoff in excess of the natural rain water runoff of vegetated areas. Examples are parking lots and rooftops.

Impervious Charge

A system for assessing fees for stormwater runoff conveyances and controls, and the operation and maintenance of same based upon the amount of impervious area on the rate payer's property.

Impervious Surface or Cover

The characteristic of a material which prevents the infiltration or passage of liquid through it. This may apply to roads, streets, parking lots, rooftops and sidewalks.


The seepage of groundwater into a sewer system, including service connections. Seepage frequently occurs through defective or cracked pipes, pipe joints, connections or manhole walls.

Infiltration Basin

A shallow impoundment designed to infiltrate stormwater runoff into the soil. Infiltration basins do not release water except by infiltration, evaporation, or emergency overflow.


Rain water discharged into a wastewater sewer system and service connections from such sources as, but not limited to, downspouts, cellars, yard and area drains, foundation drains, cooling water discharges, drains from springs and swampy areas, around manhole covers or through holes in the covers, cross connections from storm and combined sewer systems, catch basins, stormwater, surface runoff, street wash waters or drainage. Inflow differs from infiltration in that it is a direct discharge into the sewer rather than a leak in the sewer itself.


Refers collectively to public works such as electrical and water distribution systems; streets, roads, bridges, and other components of the transportation system; stormwater collection and conveyance systems; and other constructed components that are required for an economy to function.

  • When used in the context of stormwater management, “green infrastructure” or rainscaping ,refers to the use of soil, plants, and other natural features that capture precipitation and use it on-site or allow it to infiltrate. Green infrastructure/rainscaping varies in scale from preserved and restored natural landscape features (such as forests, floodplains, and wetlands) that manage and filter significant stormwater quantities to smaller scale practices including rain gardens, porous pavement, and green roofs. Green infrastructure/rainscaping is designed to mimic natural processes.
  • “Gray infrastructure” refers to all components in traditional systems used to collect and convey stormwater runoff, such as curbs and gutters, storm drains, culverts, and others. This name is derived from the use of concrete to manufacture many of these components.

A surface connection to a storm or combined sewer pipe. A chamber for collecting stormwater with no well below the outlet pipe for collecting grit. Often connected to a catch basin or a basin manhole/cleanout manhole with a grit chamber.

Intercepting Sewer

A sewer that receives flow from a number of other large sewers or outlets and conducts the waters to a point for treatment or disposal.  Also referred to as an interceptor.


A sewer installed to connect two separate sewers. If one sewer becomes blocked, wastewater can back up and flow through the interconnector to the other sewer.


The lowest elevation of a pipe, pond, or drainage facility where water is designed to flow out.