Glossary of Drainage Terminology

adoption of sewers

The transfer of responsibility for the maintenance of a system of sewers to a sewerage undertaker.

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.


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

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.


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


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.

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.

consented discharges

Term used in the UK for discharges that meet conditions imposed by the appropriate public authority for potentially polluting flow to a watercourse or into the ground.

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.


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

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).


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.


A pipeline, usually underground, designed to carry wastewater, and/or surface water from a source to a sewer, a pipeline carrying land drainage flows or surface water from a highway.


A collection of pipes, channels and other engineering works designed to convey stormwater way from a built up environment.

dry weather flow

All flow within a drainage system excepting that caused directly by rainfall.


Wastewater or other liquid, partially or completely treated, or in its natural state, flowing out of a pipe or a treatment plant.


Detachment and movement of soil or sedimentary deposits by the flow of water, such as over the ground surface or within a pipe or channel.


The progressive enrichment of surface waters, particularly non-flowing bodies of water such as lakes or ponds, with dissolved nutrients, such as phosphorous and nitrogen compounds, which accelerate the growth of algae and higher forms of plant life.


The drying out process of the ground surface, which constitute a minor part of the losses taken into account within rainfall runoff loss models.

event (rainfall)

Single occurrence of a rainfall period before and after which there is a sufficient dry period to define its effect on the sewerage system.

extreme event

Single occurrence of an event that the likely to occur very infrequently (e.g. long drought or big storm etc.).

first foul flush

The initial discharge of active sediments and pollutants of a generally higher than average concentration of pollutant caused by rainfall.

flap gate

A gate that opens to let water out but prevents water entering back into the system.

flood storage pond

A pond constructed for the purpose of temporary storage of stream flow or surface runoff, which releases the stored water at controlled rates.

flood studies report

Landmark report in the UK for catchment hydrology. (institute of Hydrology 1975)

flow regime

Typical variation of discharge of a waterway usually over an annual or seasonal period.

foul sewage

Waterborne waste of domestic or industrial origin excluding rainwater and surface water.

foul system

A drain or sewerage system that has been designed to carry only foul sewage.

french drain/ filter drain

The use of a granular trench filled with stone to convey and infiltrate stormwater runoff.


The number of occurrences of a certain phenomenon per unit time.


The angle of inclination (of pipe), which dictates its capacity and velocity of flow.

gravity system

A drain or sewerage system whereby flow is caused by the action of gravity and where the pipeline is designed to operate partially full.

greenfield/greenfield site

New development, usually at the periphery of existing urban areas. This creates increased rainfall-runoff and has an impact on existing sewer systems and watercourses.


Wastewater from sinks, baths, showers and domestic appliances.

gross solids Solids, usually organic in nature, either floating, suspended or deposited, which have a polluting effect on a receiving water. Often restricted to visible solids with one dimension in excess of 25mm.
groundwater Water that is below the surface of the ground within the saturation zone.


A structure to permit the entry of surface water runoff into a sewerage system. It is usually fitted with a grating and a grit trap.


The relationship between a discharge rate and the water level causing the discharge.


Any road, track, bridleway or public-footpath in private or public ownership that is not associated with an individual property.

highway drainage system

A drain or sewer constructed for the purpose of draining a highway.

hydraulic analysis

Assessment of the hydraulic behaviour of a system. Simulation hydraulic modelling of a sewerage network to determine its performance.

hydraulic capacity

The maximum flow that a pipe of given dimensions, slope and roughness can carry (often quoted as pipe-full capacity, which is a little less than the maximum capacity).

hydraulic performance

The measure of the capacity of the system or part thereof.

hydraulic simulation

The computational process carried out by a computer model to analyse the behaviour of a system (sewer network) due to an external influence (rainfall).


A graph showing, for a given point on a stream or conduit, the discharge, stage, velocity, available power or other property of water with respect to time.

impermeable surface

Surface that resists the infiltration of water. Usually a measure of roof and road surfaces in simulation modelling.

industrial discharge

Outflow from an industrial unit, which varies enormously depending on the processes carried out in the factory.


(a) the unintended ingress of groundwater into a drainage system ( also termed parasitory flow in some countries).

(b) The introduction of rainwater runoff into the ground.

infiltration (to sewer)

The ingress of groundwater into a drain or sewer system through defects in pipes, joints or manholes.


Flow which enters the sewer; this can be generated by rainfall or an industrial discharge or other particular connection.

initial loss

In hydrology, rainfall preceding the beginning of surface runoff. It includes interception, surface wetting and infiltration.


(1) A connection between the catchment area and a drain or sewer for the admission of surface or stormwater.

(2) A structure at the entrance end of a conduit.

(3) The upstream end of any structure through which water may flow.

inspection chamber

A structure that offers access to the drain or sewer for servicing by means of equipment remotely operated from ground level: no personnel access.


The relationship between rainfall intensity (amount per unit of time), rainfall duration (total time over which rainfall occurs) and frequency (return interval) at which the intensity-duration relationship is expected to recur.


The process by which rainfall may be prevented from reaching the ground, for example by vegetation.

internal drainage boards

These manage ordinary watercourses in areas know as internal drainage districts.


The floor, bottom or lowest portion of the internal cross-section of a closed conduit.

land use

Catchments zoned based on ergonomic, geographic or demographic use of land, such as residential, industrial, agricultural and/or commercial.


A private drain or sewer that carries drainage flows from a property to a public sewer.

Lloyd-Davies method

An adaptation by Lloyd-Davies of the Rational Method for storm drainage design.

major system

In the context of major and minor drainage, this refers to the route followed by storm runoff when the minor system is either inoperative or inadequate. It generally refers to roads and major above ground drainage channels.


A structure that provides access for personnel to the drain or sewer for servicing.

Manning’s equation

An equation developed by Manning to relate flows in conduits to their size, shape, the gradient and the conduit roughness.

minor system

The drainage pipes, roadway channels, enclosed conduits an roof connections designed to convey runoff from “normal” storms, to eliminate or minimise inconvenience in the area to be developed. See major system.


An incorrect connection of an inlet or drain to a drain or sewer that is not designed to carry that element of flow (eg foul sewage entering a surface water system or surface water entering a separate foul system).


A series of mathematical equations in a computer developed and used with he aim of replicating the behaviour of a system.

modified rational method

A modification of the Lloyd-Davies method introduced by the Wallignford Procedure whereby the coefficient of runoff was split into two entities (HR Wallingford and Institute of Hydrology, 1981b).
monitoring The procedure of measuring effluent characteristics such as flows or pollutants by means of instruments.

Muskingum-Cunge routeing method

A method of routeing flows in channels and pipes, first applied on the Muskingum river in the USA and subsequently modified by Cunge.


In the context of sewers, a collection of connected nodes and links, manholes and pipes.

off-line tank

Detention tank that is off the normal path of flow in a network, which comes into operation during periods of high flows.

on-line tank

A detention tank through which the flow of sewage is normally conveyed.


A constriction in a pipeline to control the rate of flow.


The point, location or structure where wastewater or drainage discharges from a pipe, channel, were, drain or other conduit.


The intentional or unintentional discharge of sewage to the environment before it has been treated.

overflow weir

Any device or structure over which any excess water or wastewater beyond the capacity of the conduit or container is allowed to flow.

overland flow

The flow of water over the ground or paved surface before it enters some defined channel or inlet, often assumed to be shallow and uniformly distributed across the width.

peak discharge

The maximum flow rate at a point in time at a specific location resulting from a given storm condition.


A measure of the sharpness of a rainfall profile; that is, the ratio of the maximum to the mean rainfall intensity.

peaking factor

The multiple of dry-weather flow used for design of pipe sizes and gradients.

percentage runoff

The percentage of the rainfall volume falling on a specified area that enters the storm water drainage system.


The percentage of occurrences within a stated range; also applied to rainfall profiles (see peakedness).

pervious surface

A type of ground surface that allows infiltration of water, although some surface runoff may still occur.

point rainfall

Rainfall rate at a location, in contrast to the average for the region or surrounding area.


Dissolved or particulate material washed into and through sewers. When discharged into receiving waters, pollutants cause an adverse environmental impact.


The addition to a natural body of water of any material that diminishes the optimal use of the water body by the population which it serves, and that has an adverse effect on the surrounding environment.

PR equation

Usually refers to the Wallingford Procedure runoff equation (HR Wallingford and Institute of Hydrology 1981a).

primary treatment

The first major treatment in a wastewater treatment facility, usually sedimentation.

private sewer

A sewer for which responsibility is not vested in the sewerage undertaker. Generally it is collectively owned and maintained by the owner(s) of the building(s) it serves.

public sewer

A sewer for which responsibility is vested with the sewerage undertaker to maintain it.

pumping station

A structure containing pumps and appurtenant piping, valves and other mechanical and electrical equipment for pumping water, wastewater and other liquids.

rainfall intensity

Amount of rainfall occurring in a unit of time, generally expressed in millimetres per hour (mm/h).

rainfall profile

A series of values of rainfall intensity varying with time; a rainfall event is referred to as a hyetograph.


An instrument used to measure and record the amount of rainfall at an allocated location.

rational method

A simple method, used throughout the world, for calculating the peak discharge in a drainage system for pipe sizing.

reach (river)

A stretch of river between two points, often used where the river characteristics are similar.

receiving waters

Water body (river or lake) that receives flow from point or non-point sources such as combined sewer overflows.


(1) A structure installed in a sewer, conduit or channel to control the flow of water or wastewater at an intake, or overflow or t control the water level along a canal, channel or treatment unit.

(2) The term used in the UK to refer to the Environment Agency and OFWAT due to their legal involvement in controlling water companies.

reservoir storage

The phenomenon by which a volume of flow is stored temporarily on a surface or in a length of pipe or channel as the depth and rate of flow increase; the storage is depleted after the peak of the storm passes.

retention pond

A pond constructed for the temporary storage of surface water runoff, which releases the stored water at controlled rates.

return period

The reciprocal of the average annual probability of exceedence of a specific flow value or event.


Water from precipitation that flows off a surface to reach a drain, sewer or receiving water.

runoff coefficient

The proportion of total rainfall that appears as total runoff volume after subtracting depression storage, infiltration and interception.

Saint Venant equation

An equation developed in the 19th century by a French mathematician, which takes account of all the physical processes of fluid flow such as momentum and inertia to calculate depth for gradually varying flow states.


A device with openings, generally of uniform size, used to retain or remove suspended or floating solids in flowing water or wastewater.


A board or plate that dips below the top water level to retain scum and other floating debris.

sediment concentration

The ratio of the weight of the sediment in a water-sediment mixture to the total weight of the mixture. Sometimes expressed as the ratio of the volume of sediment to the volume of mixture.

sediment transport

The movement of solids transported in any way by a flowing liquid.


The process of deposition and consolidation of suspended material carried by water, wastewater or other liquids, by gravity.

self-cleansing (velocity)

The minimum velocity in sewers necessary to keep solids in suspension, so preventing the deposition and subsequent nuisance from blockages or reduced capacity.

separate system

A drain or sewer system, normally of two pipelines, one carrying wastewater and the other surface water.

septic tank

A structure for the collection and partial treatment of sewage.


Wastewater and/or surface water conveyed by a drain or sewer.


A pipe or conduit that carries wastewater or drainage water serving more than one property.

sewer flooding

The unintentional escape from a sewerage system; the inability of drainage flows to enter a sewerage system because of surcharge.


Alternative term for “drainage collecting system” for foul and surface water systems.

sewerage system

A network of pipelines and ancillary works that conveys wastewater and/or surface water from drains to a treatment works o other place of disposal.

sewerage undertaker

An organisation with the legal duty to provide sewerage services in an area. In England and Wales these services are provided by 10 water service companies, in Scotland by 3 water authorities, and in Northern Ireland by the Water Service of the Department of the Environment for Northern Ireland.

side weir

A diverting weir constructed on the side of a channel or conduit, usually at right angles to the centre-line of the main channel.


Sediment (often soil) consisting of particles between 0.002 mm and 0.02 mm in equivalent diameter.


The representation of specific conditions during a specific period in a sewerage system, treatment works, river etc, by means of a computer model.

simulation model

The representation of a physical system and its time-related behaviour by a computer model.

sluice gate

A gate constructed to slide vertically and fastened into or against masonry of dams, (penstock) tanks or other structures under which flow takes place when open.


A pit into which surface water is drained to infiltrate into he ground.


The top of the inside of a pipe or conduit.

soil moisture deficit (SMD)

A measure of soil wetness, calculated by the Meteorological Office in the UK, to indicate the capacity of the soil to absorb rainfall.

source control

The practice of reducing runoff and also pollutants at their source so that they do not enter the drainage system or become significantly delayed and attenuated.

spill event

A period when an overflow discharges to a watercourse/ spill frequency The number of spill events over a given period.

stilling pond

A small basin into which flow is discharged, which is used to either dissipate energy or trap solids.


The impounding of water, either in surface or in underground reservoirs.


An occurrence of a meteorological event, often of rainfall, snow or hail. Used in connection with a phenomenon that is either unusual or of great magnitude, rate or intensity.

storm tanks

Storage tanks designed to hold most of the stormwater in either sewers or treatment works such that downstream flooding or incomplete treatment respectively is minimised.

stormwater overflow

A weir, orifice or other device for permitting the discharge from a combined sewer of the flow in excess of that which the sewer is designed to carry.


The ground surface area drainage directly to one fully or a collection of gullies.

surface washoff

The process whereby the rainfall runoff carries surface sediments and dissolved pollutants into the drain or sewer system.

surface water

Water from precipitation that has not seeped into the ground and is discharged to the drain or sewer system directly from the ground or from exterior building surfaces.

surface water system

A drain or sewer system that has been designed to carry only surface water.

suspended solids

Insoluble solids that either float on the surface of, or are in suspension in, water, wastewater or other liquids.

sustainable drainage

The application of drainage techniques that are considered to be environmentally beneficial, causing minimal or no long-term detrimental impact.


A grass channel for stormwater collection with shallow side slopes, which is normally dry except during rainfall.

synthetic rainfall

Rainfall depths or intensities derived from rainfall statistics and not representing an individual real rainstorm.

synthetic rainfall series

Rainfall time series usually derived by stochastic processes for use in place of a recorded rainfall series.

tank sewer

A length of sewer with a cross-sectional area in excess of that required for the conveyance of the normal sewer flow, the additional volume being used for the storage of storm sewage.

time of concentration

Time between the start of a runoff event and the time when the entire catchment is contributing flow to a specific point in the network.

time of entry

The time taken for surface runoff to reach the entry into the pipe system.

time series rainfall

A continuous or discontinuous record of individual events generated artificially or selected real historical events that are representative of the rainfall in that area.

urban drainage

Pipe systems and other related structures to serve an urban environment.

vacuum sewerage system

A system that operates under negative (sub-atmospheric) pressure to evacuate drainage flows from a property or group of properties; the system may consist of one or more vacuum pumps, a central vacuum reservoir, pipework and interface valves.

vortex overflow

A type of overflow that make use of the spiralling flow in a cortex to retain polluting material within the pipe system.

Wallingford procedure

A design and analysis procedure for urban drainage networks (HR Wallingfor and Institute of Hydrology, 1981a).

washoff (of pollutants)

The transport of pollutant mass from the catchment surface during a rainfall event.


Water used and discharged to drain.

water quality

The chemical, physical and biological characteristics of water with respect to its suitability for a particular purpose.

water quality standards

Standards set by the national legislation or European Community directives and enforced by regulatory authorities in member states.


The surface within soil or rock strata at which groundwater saturation occurs.

Water UK

The organisation representing all water supply companies in the UK.


A natural or artificial channel for passage of water.


An overflow structure across a channel that may be used for controlling upstream surface level, or for measuring discharge, or for both; usually horizontal and constructed as either broad- or sharp-crested.

wet well

The entry chamber in a pumping station from which water is pumped to a higher level.