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6th September 2021


Metro Rod know it can be overwhelming when a drainage problem disrupts your day-to-day life. However, we know it can be made worse when our professionals use drainage terminology to explain the problems. That's why we have created a glossary on drainage terminology so you can be fully aware of what the situation is and what the jargon we use means.


In this article, we will list our drainage terminology in alphabetical order, along with a paragraph on what the term means in simple terms.



Adoption of sewers- The responsibility transfer of the upkeep and maintenance of a sewer system to a sewerage undertaker.

Aesthetic pollution- Solid sewage materials that are visible but have little impact on the environment.

Air Admittance Valve- Air Admittance Valves or Durgo Valves is a system that opens to allow air to enter a soil vent pipe when there is negative pressure, but remains closed the remaining time.

Anti-flooding device- Specifically designed to be installed in gravity drains or sewers, this device prevents backflow from sewers towards the property.



Balancing pond- A pond designed to reduce flows by storing excess water during the peak flow and releasing it at a controlled rate during and after the peak flow has passed.

Brownfield site- Redevelopment of a site which is often associated with pollution issues.



Catchment- Where surface water meets at a point on a drainage or river system, which can be divided in to sub-catchments.

Cavitation- The implosion of air in water at high velocities, which can cause damage.

CCTV survey- An in-depth investigation of a drainage system, using colour CCTV cameras.

Cess pool- A sealed tank that collects wastewater which must be emptied once full as it has no outlet.

Collection system- A system of generally underground pipes, that receive and convey sanitary wastewater and/or stormwater.

Combined network- A sewer network that collects rainfall from 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 designed to carry foul sewage and surface water in the same pipe.

Combined sewer overflow- A combined sewer designed to prevent the capacity of the sewer/sewage treatment works from being exceeded under storm conditions by directing the discharge of excess diluted sewage to another sewer/tank/watercourse.

Contributing area- The catchment area that contributes storm runoff directly to a sewerage system.

Control structure- Hydraulic device to limit the rate of flow.

Culvert- A covered channel or pipeline wider than 900mm



Depression storage- A natural depression on the surface of the ground that needs to be filled by rainfall before runoff can take place.

Detention basin- A vegetated depression that is usually dry, except post-storm events. It is constructed to store water temporarily to reduce the flow.

Detention tanks- Tanks constructed in a sewerage system to store a volume of water temporarily during peak flow.

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

Domestic foul wastewater- Wastewater from household services including overflows, for example, sinks, toilets and washing machines.

Drain- A pipeline, usually underground, designed to carry wastewater, and/or surface water from a source to a sewer.

Drain tracing – Inspection method to locate blockages, collapsed drains, and concealed manholes in areas that would be difficult to reach.

Drainage- A collection of pipes, channels, and other engineering works designed to divert stormwater from a built-up environment.

Dry weather flow- A flowing drainage system that hasn't been caused by direct rainfall.



Effluent- Wastewater that is partially/completely treated, or in its natural state that flows out of a pipe or a treatment plant.

Erosion- Detachment and/or movement of soil or sedimentary deposits by the flow of water.

Eutrophication- The enrichment of surface waters, particularly non-flowing bodies of water, for example, lakes or ponds, with dissolved nutrients that accelerate the growth of algae and other plants.

Evaporation- The process of the ground surface drying out.

Event (rainfall)- Single occurrence of rainfall before and after a sufficient dry period which can affect some sewerage systems.

Extreme event- An event that happens infrequently, for example, a long drought or big storm.



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

Flood storage pond- A pond made for the purpose of temporarily storing surface runoff that releases the stored water at controlled rates.

Flow regime- Discharge of a waterway that varies over an annual or seasonal period.

FOG: This stands for fats, oils, and grease in relation to the gunk found in drains.

Foul sewage- Waterborne waste of domestic or industrial origin which doesn't include rain or surface water.

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

French/ filter drain- A granular trench filled with stone to convey and infiltrate stormwater runoff.



Gradient- The angle at which the pipe lays, which dictates the capacity and velocity of flow.

Gravity system- A drain or sewerage system where the flow of the drain is caused by the action of gravity.

Greywater- Wastewater from domestic appliances including sinks, washing machines, etc.

Gross solids- Solids that float, or suspend, which have a polluting effect on the receiving body of water.

Gully- Usually fitted with a grating and a grit trap, this structure permits the entry of surface water runoff into a sewerage system.



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

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 to determine its performance.

Hydraulic capacity- The maximum flow that a pipe of given dimensions, slope and roughness can carry.

Hydraulic performance- The measure of the capacity for the system.

Hydraulic simulation- The process carried out by a computer model to analyse the behaviour of a system due to an external influence.

Hydrograph- A graph that shows a certain point of a stream or conduit, can determine the discharge, stage, velocity, available power or other property of water with respect to time.



Impermeable surface- Surface that resists the infiltration of water.

Industrial discharge- The outflow from an industrial unit, that varies depending on the factory.

Infiltration- Groundwater unintentionally entering a drainage system.

Infiltration to sewer- When groundwater enters into a drain or sewer system through defective pipes, joints or manholes.

Inflow- The flow generated by rainfall or an industrial discharge that enters a sewer.

Inlet- Used to describe 3 different things: The connection joining the catchment area and a drain for water to enter; A structure that sits at the entrance of a conduit; The upstream end of a structure that water flows through.

Inspection chamber- A structure that offers access to the drain or sewer for servicing through remotely operated equipment.

Interception- Where rainfall may be prevented from reaching the ground for multiple reasons, for example by vegetation.

Interceptor traps- A device installed in drains and sewers that prevents sewer gasses from entering houses, also known as a U-bend.

Invert- The lowest part of a closed conduit that can be found in its internal cross-section.



Jetting- High-pressure water jetting (HPWJ) used for drain unblocking and cleaning.



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

Lining- Installing a resin drain lining that bonds to the inside of the existing pipe, creating a new pipe inside the old one.

Localised Structural Repair(LSR)- The repair of a cracked or disjointed pipe by inserting a patch or a liner into a drain, creating a pipe within a pipe.



Major system- Refers to the route followed by storm runoff when the minor system is either inoperative or inadequate. Mainly regarding roads and major above-ground drainage channels.

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

Minor system- The drainage pipes/roadway channels/etc, that are designed to convey runoff from average storms, to eliminate or minimise inconvenience in the area to be developed.

Misconnection- An incorrect connection of an inlet/drain/sewer that is not designed to carry that element of flow.



Network- A collection of connected manholes and pipes.



Off-line tank- A detention tank that is off the path of flow in a network that only comes into operation during periods of high flows.

On-line tank- A detention tank where the flow of sewage is on the path of flow.

Orifice- A pipeline constriction that controls the rate of flow.

Outfall- Where wastewater discharges from a pipe, channel, drain, etc

Overflow- The intentional or unintentional discharge of sewage into the environment before it has been treated.

Overflow weir- A structure or device which relieves excess water when it reaches full capacity.

Overland flow- The flow of water over the ground before it enters a defined channel or inlet.



Patch lining- A popular and cost-effective method of drain repair, using a resin liner.

Peak discharge- The maximum flow rate at a specific location and time.

Percentage runoff- The percentage of rainfall that enters the stormwater drainage system.

Percolation Test- Involves digging holes and filling them with water to determine the water absorption rate of the soil at your property and whether it is suitable for the installation of a soakaway.

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

Picote- Specialist heavy duty machines designed for drain cleaning where a high-pressure water-jetting is not enough.

Point rainfall- Rainfall rate at a specific location compared to the average for the region.

Pollutant- Material that is washed into and through sewers but when discharged into receiving waters, can cause an adverse environmental impact.

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

Primary treatment- The treatment of wastewater at a dedicated facility.

Private sewer- A sewer collectively owned and maintained by the owner(s) of the building(s) it serves.

Public sewer- A sewer for which responsibility is maintained by the public authority.

Pumping station- A structure containing pumps and electronic equipment for pumping water, wastewater, and other liquids.



Raingauge- A device used to record the level of rainfall in a specific location.

Rational method- A simple method used for calculating the peak discharge in a drainage system for pipe sizing.

Receiving waters- Waterbody like a river or lake that receives flow from sources such as combined sewer overflows.

Regulator-A structure that is installed in a sewer, conduit or channel to control the flow of water.

Reservoir storage- Where water is stored temporarily on a surface or in a pipe/channel as the flow increases but the storage is depleted after the peak passes.

Retention pond- A pond constructed for the temporary storage of surface water runoff that releases the stored water at controlled rates.

Rodent Valve- A valve with a flap that is inserted into your drain that allows waste to leave your property, but stops rats from entering.

Root ingress – When a crack forms in an underground pipe allowing water to escape. Nearby tree roots will benefit from this, subsequently lead roots to the crack in the pipe, therefore, forcing their way into the pipe, causing further damage.

Runoff- Water from precipitation that flows off a surface to reach a drain.



Screen- A device with openings used to retain or remove suspended/floating solids in flowing water.

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

Sediment transport- The transportation of solids in any way by a flowing liquid.

Sedimentation- The process of deposition and consolidation of suspended material carried by water.

Self-cleansing (velocity)- The minimum velocity in sewers necessary to keep solids in suspension, preventing blockages.

Separate system- A drain or sewer system of two or more pipelines where one carries wastewater and the other surface water.

Septic tank- A structure for the collection and partial treatment of sewage.

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

Sewage treatment plant- Self-contained units normally buried into the ground which treat raw sewage by using good bacteria.

Sewer- A pipe that carries wastewater or drainage water that serves more than one property.

Sewer flooding- The unexpected escape of wastewater from a sewerage system.

Sewerage- Another term for a drainage collecting system for foul and surface water systems.

Sewerage system- A network of pipelines that conveys wastewater/surface water from drains to a treatment works.

Sewerage undertaker- An organisation with the legal duty to provide sewerage services in an area.

Side weir- A diverting weir constructed on the side of the main channel.

Silt- Fine sediment.

Sluice gate- A gate constructed to slide vertically and fastened into or against a tank, that flows when open.

Soakaway- A pit into which surface water is drained to infiltrate into the ground.

Soffit- The top part of the inside of a pipe.

Soil moisture deficit (SMD)- A measure of soil wetness that indicates how much rainfall the soil can handle.

Source control- Reducing runoff and pollutants at their source so that they do not enter the drainage system.

Spill event- The number of spill events/discharges over a specific period.

Stilling pond- A small basin into which flow is discharged used to dissipate energy or trap solids.

Storage- The impounding of surface water or underground reservoirs.

Storm tanks- Storage tanks designed to hold most of the stormwater in either sewers or treatment works.

Stormwater overflow- A weir that permits the discharge from a combined sewer.

Sub-catchment- Where the ground surface area drains directly to one or a collection of gullies.

Surface washoff- Where rainfall-runoff carries surface sediments and dissolved pollutants into the drain/sewer system.

Surface water- Water from precipitation that has not seeped into the ground and is discharged to the drain directly.

Surface water system- A drain that has been designed to carry only surface water.

Suspended solids- Insoluble solids that either float on the surface of water of any kind.

Sustainable drainage- The application of drainage techniques that are considered to be environmentally beneficial.

Swale- A grass channel where stormwater collects which is normally dry except during rainfall.



Tank sewer- A sewer with a cross-sectional area in excess of that required for the normal sewer flow with the additional volume being used for the stormwater storage.

Time of entry- The time taken for surface runoff to enter into the pipe system.

Trap- A U-bend or trap is designed to hold water to prevent unwanted sewer gases and odours from entering buildings while allowing waste materials to pass through.



U-Bend- A U-bend or trap is designed to hold water to prevent unwanted sewer gases and odours from entering buildings while allowing waste materials to pass through.

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



Vacuum sewerage system- A system that operates under negative pressure to evacuate drainage flows from a property.

Vortex overflow- An overflow that makes use of the spiralling flow in a cortex to retain polluting material within the pipe system.



Washoff (of pollutants)- The transportation of pollutant masses from the catchment surface during rainfall.

Wastewater- 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.

Watercourse- A natural or artificial channel for passage of water.

Weir- An overflow structure across a channel that may be used for controlling upstream surface level, or measuring discharge.

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

Talk to you local metrorod specialist

Talk to your local Metro Rod specialist

We are always happy to arrange a free site assessment and no obligation quotations for any work you might need. Alternatively, you can call our emergency hotline number on 0800 66 88 00

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