Drainage Glossary
  • A

  • aesthetic pollution

    Solid sewage-related materials that are visible but create little environmental impact.

  • antecedent conditions

    The condition of a catchment before a rainfall event.

  • antecedent precipitation

    The relevant rainfall that takes place before the point in time of interest.

  • antecedent precipitation index

    An index determined by the summation of weighted daily rainfalls for a period preceding the start of a specific event.

  • anti-flooding device

    A device specifically designed to be installed in gravity drains or sewers to prevent backflow from a sewer towards a property or a group of properties.

  • areal reduction factor

    A factor applied to point rainfall depths or intensities to generate values applicable to an area.

  • attenuation

    Reduction of peak flow and increased duration of a flow event.

  • B

  • balancing pond

    A pond designed to attenuate flows by storing runoff during the peak flow and releasing it at a controlled rate during and after the peak flow has passed.

  • base flow

    Sustained or dry weather flows not directly generated by rainfall. It commonly constitutes flows generated by domestic and industrial discharge and also infiltration.

  • best management practices

    Structural and non-structural measures used to store or treat urban surface water runoff to reduce flooding, remove pollution and provide other amenities.

  • brownfield site

    Redevelopment of a site often associated with pollution issues.

  • C

  • catchment

    The area contributing surface water to a point on a drainage or river system, which may be divided in to sub-catchments.

  • cavitation

    The process of implosion of air in water that is a function of high velocities, which cause damage.

  • Colebrook-White equation

    An empirical equation relating flow to roughness and gradient of a conduit and the viscosity of the fluid.

  • collection system

    In wastewater, a system of conduits, generally underground pipes, which receive and convey sanitary wastewater (domestic and/or industrial) and/or stormwater.

  • combined network

    A sewer network that collects rainfall from impervious surfaces and foul water from domestic and industrial sources.

  • combined sewage

    Foul sewage mixed with surface water, also known as storm sewage.

  • combined sewer

    A sewer design to carry surface water and foul sewage within the same pipe.

  • contributing area

    The area of the catchment that contributes storm runoff directly to a sewerage system.

  • control structure

    A hydraulic device to limit the rate of flow.

  • CSO (combined sewer overflow)

    An outfall from a combined sewer designed to prevent the capacity of the sewer or a sewage treatment works from being exceeded under storm flow conditions by allowing the discharge of excess diluted sewage to another sewer, tank, watercourse or some other disposal point.

  • culvert

    A covered channel or pipeline (defined by the Highway Agency as wider than 900mm)

  • D

  • depression storage

    Natural depression on the surface of the ground that need to be filled by rainfall before runoff can take place.

  • design criteria

    A set of standards agreed by the developer, planners and regulators that the proposed system should satisfy.

  • design storm

    A synthetic rainfall event of a given duration and return period, which is derived by statistically analysing an historical series of rainfall events for a specific location.

  • detention basin

    A vegetated depression which is normally dry, excepting post storm events, constructed to store water temporarily to attenuate flows. May allow infiltration of water in to the ground.

  • detention tanks (balancing tanks)

    Tanks constructed in a sewerage system to store a volume of water temporarily during peak flows (see off-line and on-line tanks).

  • discharge

    The volume of liquid flowing through a cross-section of conduit per unit of time.

  • discharge coefficient

    A coefficient, derived by experiment, applied in a formula, by which the theoretical discharge of a fluid through an oriface, weir or nozzle, can be correctly calculated.

  • domestic (foul) wastewater

    Wastewater from household services including overflows from sinks, toilets, washing machines, etc.