GLOSSARY OF DRAINAGE TERMS

Drainage Terms & Definitions

'Aco' style channel drain (also known as linear drainage)


These drains are channels which have a grating over the top and are designed to drain surface water from a hardstanding such as a driveway along the entire length instead of from one point like a yard gully. We advise regular maintenance is carried out to these as leaves and other debris can build up which can cause the water to drain away slowly and possibly overflow. Aco




Air Admittance Valve (also known as Durgo Valve)


This one-way air vent is installed on above ground vertical waste pipes such as a stub stack so that when it is drained (e.g. a toilet being flushed), clean air can enter but foul smells and gasses will not escape and is designed to prevent gurgling, etc.




Anti-flood valve


Anti-flood valves are installed in manholes to prevent a back surge from the sewers flooding the property as the flap which allows water running downstream to flow away will not open against pressure from back flow. Annual maintenance is advised to ensure it remains in a satisfactory working condition. These are common in basement drainage, particularly in cities (e.g. London) where colossal flooding can occur. Anti-flood valve




Attenuation Tank


Like soakaways, attenuation tanks are installed to hold rainwater however cover a much larger area. They are designed to hold a high volume of water during very heavy rainfall which in a normal system would overload the main sewer have a flow control chamber which releases the stored water before running into the drainage system.




Back inlet gully


Although this looks like a standard gully at ground level, this design has below ground connections and therefore the connecting pipework does not discharge onto the grating.




Backfall (also known as reverse gradient)


During the installation of some systems, the pipework has not been laid to a good falling gradient, sometimes flat or with the downstream section higher than the direction the water is coming from and where the water is not flowing away it is pooling which causes the accelerated accumulation of debris deposits. These problems are usually impossible or too expensive to remedy and therefore the only option is likely to be regular maintenance. The ideal fall is 1 in 40 for 100mm pipework and 1 in 60 for 150mm.




Branch connection (also known as lateral connection)


Branch (or lateral) connections are drains that are not the main section of pipework running upstream and downstream but a drain that connects to the side of either a manhole chamber or underground section of pipework. Most manholes have connections joining in and this is usually adequate access to carry out a CCTV camera inspection however this is not possible if the connection is underground as the camera unit is unable to navigate bends such as these. Most connections are made with the correct junctions either during the time of installation or whilst carrying out alterations however it is common to see "DIY" jobs where it has been made by simply breaking open the existing pipework and adjoining the connecting drain by means of a cement joint. It is most likely that these type are not watertight.




Buchan trap


Similar to an interceptor trap and inline trap, Buchan traps were installed in Victorian times to prevent the escape of foul smells and vermin from the communal sewers however as they are not accessible from a manhole and only have a rodding access point at ground level, there are usually difficulties clearing blockages which are known to build up in the u-bend of traps. When a Buchan trap is found to be damaged and needs to be replaced, this is usually an expensive job as typically they are between 2.0-3.0m deep and are located in narrow front gardens.




Burns chamber


A way to prevent internal flooding during a blockage was the installation of a Burns chamber which is where a metal plate is secured within the manhole chamber under the cover to stop water rising and overflowing. Burns chambers are only incorporated as part of a cast iron drainage system and are sealed with metal bolts which can be opened when access is needed for cleaning, investigation or repair works.




Catchpit/Catchment Pit


A catchment pit is an empty chamber installed in the surface water drainage system for intercepting silt and debris deposits being carried along the pipework. This works by having a high level inlet pipe which allows the water to rise to discharge through the outlet pipe also positioned at a high level, leaving sediment sitting at the bottom which will require regular cleaning to avoid blockages.




CCTV camera inspection/Drain survey


CCTV camera inspections allow information to be gained regarding the underground system including the pipework diameter and if there are any branch connections, all of which is pertinent when deciding the best drain repair technique. There are a number of reasons a drain survey may be carried out and you can see these here. Find out more about CCTV Drain Surveys in London.




Cess pit


A cess pit is a sealed underground tank with manhole access that wastewater and sewage discharges into instead of connecting to the main sewers. These require regular emptying and the frequency depends on the size of the tank, size of the property and number of occupants.




Circumferential fracture


Many drainage systems are of earthenware or clay construction and although strong, it is a material known to fracture. A circumferential fracture runs around the circumference of the pipe and will allow the loss of water into the surrounding ground area, possibly leading to subsidence. These fractures are usually easy to repair by drain lining/sleeving if there is adequate access but they can deteriorate over time if left and could result in a collapse where an excavation will be required to carry out the repair. Circumferential fracture




Communal drainage


Since the "Transfer of Sewers" Act in 2011, the Local Water Authorities including Thames Water are responsible for the majority of the sewer network. This act superseded the previous 1937 legislation, increasing the level of drainage the Local Water Authority is responsible for. They took ownership of all shared drains even within private property boundary lines with the exception of most PVC drains as well as any that are in public areas such as those that serve just one property but have crossed the boundary and are under the pavement. Private contractors are not permitted to work on these drains and should you be planning to build within 3.0m of these assets, a Build Over Agreement will be required.




Concealed manhole


Although manholes are for providing access to the drainage system, some properties have built over them and therefore rendered the chamber useless. This is not advisable as in the event of an emergency blockage, engineers would have difficulty clearing it without full access. These also prevent inspections and maintenance being carried out to any connecting pipework. In numerous cases, expensive kitchen floors have to be broken up to access these chambers.




Dancutter lateral cutting machine


This is a piece of machinery predominately used to reopen branch connections with a robotic arm after a structural soft felt liner has been installed. You can read more about this here.




Drain survey/CCTV camera inspection


CCTV camera inspections allow information to be gained regarding the underground system including the pipework diameter and if there are any branch connections, all of which is pertinent when deciding the best drain repair technique. There are a number of reasons a drain survey may be carried out and you can see these here.




Double sealed manhole cover


As foul smells and sewage from a blockage can escape through the edges of a manhole cover, the standard design is not permitted to be used internally and Building Regulations state a bolt down, double sealed cover and frame needs to be used as it is airtight.




Durgo Valve (also known as Air Admittance Valve)


This one-way air vent is installed on above ground vertical waste pipes such as a stub stack so that when it is drained (e.g. a toilet being flushed), clean air can enter but foul smells and gasses will not escape.




Dye testing


If the configuration of the drainage prevents passage of the camera, fluorescein dye is dropped into the system to confirm a connection. You can read more about this method here.




Electronic sonde tracing


Whilst CCTV camera inspections allow for us to see what is happening underground, it is not always possible to determine where the pipework is. Tracers are inserted into the pipework and tracked above ground and are used to pinpoint certain areas such as a concealed manhole or a collapsed section. You can read more about this service here.




Foul water


Foul water is waste water serving a bathroom, kitchen or utility room and is discharged into communal sewers, septic tanks or cess pits. New drainage systems are separated into foul water and surface water where possible to help with the level of waste that needs to be treated before being released back into the environment and therefore combined systems which take both are mainly found in older properties. Foul water cannot discharge to a surface water system or soakaway due to its contaminated waste and the Environmental Agency and Local Water Authorities can issue correction notices and fines if these illegal, unhygienic misconnections causing pollution are found. So called "grey water" which only serves sink wastes, etc. is also classed as foul water and cannot be discharged into a separate surface water system or soakaway.




Gap between sections


A gap between sections is where the original male and female ends of an earthenware pipe connect at a joint but have not been fully adjoined, leaving a space between the two sections. It is not possible to tell from a CCTV camera drain survey if these joints are leaking as the cement fillet of the joint may still be in place.




Gully


Gullies are often found at the base of above ground sections of pipework such as a kitchen sink waste pipe or rainwater pipe connecting to the underground system with a trap to prevent the escape of foul smells and rats. They are also used in large external spaces to drain surface water from the ground into the drainage. Newer gullies are designed with a rodding access which allows for cleaning and investigation works to be carried out beyond the trap should the need arise. Gullies are usually identified by the small circular or square grid at ground level. Gully




Inline trap (also known as running trap)


Similar to an interceptor trap and Buchan trap, inline traps were installed to prevent the escape of foul smells and vermin from the communal sewers however as they are not accessible from a manhole and do not have rodding access, there are usually difficulties clearing blockages which are known to build up in the u-bend of traps.




Interceptor trap


Interceptor traps are usually found in the outlet of the last manhole chamber and this is the favoured design to prevent the escape of foul smells and passage of vermin from the communal sewers as there is incorporated rodding access. This is beneficial as it is quite common for the traps to block up but these are usually cleared with ease. Interceptor Trap




Lateral connection (also known as branch connection)


Lateral (or branch) connections are drains that are not the main section of pipework running upstream and downstream but a drain that connects to the side of either a manhole chamber or underground section of pipework. Most manholes have connections joining in and this is usually adequate access to carry out a CCTV camera inspection however this is not possible if the connection is underground as the camera unit is unable to navigate bends such as these. Most connections are made with the correct junctions either during the time of installation or whilst carrying out alterations however it is common to see "DIY" jobs where it has been made by simply breaking open the existing pipework and adjoining the connecting drain by means of a cement joint. It is most likely that these type are not watertight. Lateral Connection




Linear drainage (also known as 'Aco' style channel drain)


These drains are channels which have a grating over the top and are designed to drain surface water from a hardstanding such as a driveway along the entire length instead of from one point like a yard gully. We advise regular maintenance is carried out to these as leaves and other debris can build up which was cause the water to drain away slowly and possibly overflow. Linear Drainage




Longitudinal fracture


Many drainage systems are of earthenware or clay construction and although strong, it is a material known to fracture. A longitudinal fracture runs down the length of the pipe usually from a joint and will allow the loss of water into the surrounding ground area, possibly leading to subsidence. These fractures are usually easy to repair by drain lining/sleeving if there is adequate access but they can deteriorate over time if left and could result in a collapse where an excavation will be required to carry out the repair.




Macerator/Saniflo


The main purpose of a macerator such as a Saniflo is to break up solid waste with blades, making it easier to pump the liquid sewage uphill or vertically. They are most commonly used in basement toilets where it is not possible to install a gravity system. In our opinion, although they have improved in recent years, they are not always reliable and need replacing every couple of years.




'Multi-Kwik'


A 'Multi-Kwik' is a plumbing connection for a WC pan and although it contravenes Building Regulations, we sometimes find these underground. One reason these are not allowed is that is reduces the bore of the pipework.




Multiple fracturing


Many drainage systems are of earthenware or clay construction and although strong, it is a material known to fracture. Multiple fractures are likely to have developed over a long period of time and will allow the loss of water into the surrounding ground area, possibly leading to subsidence. These fractures are usually easy to repair by drain lining/sleeving if there is adequate access but this is likely to deteriorate further if left and could result in a collapse where an excavation will be required to carry out the repair. Multiple fracturing




Obstruction


Obstructions in the drainage system restrict the free flow of waste and the camera unit will be unable to pass it when carrying out a drain survey. Water usually holds behind an obstruction which prevents a clear view of the cause however it can be anything from a build up of non-disposable wipes and nappies, a roots mass or collapsed section of pipework. You can see what else causes blockages here.




Offset joint


An offset joint occurs when two sections of earthenware pipework are connected but were not aligned properly during installation. An offset joint does not necessarily leak as the cement fillet of the joint may be intact.




Picote cannon lining machine


This is a method of drain repair that seals leaking pipework to a watertight condition. The flexible liners are cut to length and are used to cover numerous defects in one section with bends that would prevent a standard structural soft felt liner being installed. You can read more about this technique here.




Pitch fibre pipework


Pitch fibre was a material used predominately in the 1960's and was subsequently found to be inherently defective, becoming deformed and blistered over a period of years. Advanced stages of the "squashing" cause the pipework to block completely, requiring excavation and renewal. You can read more and see photographs of deformed sections here.




Private drainage


Property owners are only responsible for the drains within their boundary lines that serve only their property. Once these sections of drainage cross the boundary line either under the pavement or into a neighbouring garden, etc., they fall under the ownership of the Local Water Authority.




Radial fracture


Many drainage systems are of earthenware or clay construction and although strong, it is a material known to fracture. A radial fracture runs partly around the pipe and will allow the loss of water into the surrounding ground area, possibly leading to subsidence. These fractures are usually easy to repair by drain lining/sleeving if there is adequate access but they can deteriorate over time if left and could result in a collapse where an excavation will be required to carry out the repair. Radial Fracture




Rat blocker


Marine grade steel rat blockers are usually installed in place of interceptor traps to prevent the passage of vermin from the communal sewers into the private drainage system. These do require regular maintenance by way of an annual service to ensure that the hinged flap continues to operate satisfactorily. These are usually used as an economic alternative to replacing the defective interceptor trap.




Recessed manhole cover


Standard manhole covers are not aesthetically pleasing and therefore it may be decided that a recessed cover is used as they allow for bricks or slabs to be installed to match the surrounding area, making it blend in to the floor but still allowing full access.




Redundant drain


During house renovations, it is possible that some drains no longer have a use and we often find that these have just been left open or poorly capped at the end of the run with a builder's plastic rubble sack. We advise these runs are sealed at the manhole chamber as rats prefer to nest in these sections and can escape inside houses.




Reforming/rerounding


Pitch fibre pipework is prone to "squashing" and therefore rerounding is a technique used to return the drain to its original circular shape. You can read more about the use of pitch fibre pipework here.




Rest bend


Vertical downpipes that do not discharge to a gully are connected to the underground system with a rest bend. This 90° bend has a long sweeping angle to allow the waste to flow smoothly and usually has a footing to help absorb the force of the flushed water as it hits the base.




Reverse gradient (also known as backfall)


During the installation of some systems, the pipework has not been laid to a good falling gradient, sometimes flat or with the downstream section higher than the direction the water is coming from and where the water is not flowing away it is pooling which causes the accelerated accumulation of debris deposits. These problems are usually impossible or too expensive to remedy and therefore the only option is likely to be regular maintenance. The ideal fall is 1 in 40 for 100mm pipework and 1 in 60 for 150mm.




Rodding access


Rodding accesses are found on interceptor traps, new style gullies and some sections of above ground pipework to allow entry to the drainage system, mainly to attempt to clear a blockage however CCTV camera inspections can also be carried out from these points. They are sealed with caps to prevent the escape of foul smells and vermin and these should be able to be removed when needed and not be cemented into place. Whilst they do provide access, this is usually limited and we do advise there is further access in the form of a manhole.




Running trap (also known as inline trap)


Similar to an interceptor trap and Buchan trap, running traps were installed to prevent the escape of foul smells and vermin from the communal sewers however as they are not accessible from a manhole and do not have rodding access, there are usually difficulties clearing blockages which are known to build up in the u-bend of traps.




Saniflo/Macerator


The main purpose of a macerator such as a Saniflo is to break up solid waste with blades, making it easier to pump the liquid sewage uphill or vertically. They are most commonly used in basement toilets where it is not possible to install a gravity system. In our opinion, although they have improved in recent years, they are not always reliable and need replacing every couple of years.




Soakaway


To ease the burden on sewers, rainwater can discharge to soakaways which is simply pipework connecting to a hole in the ground (at least 5.0m away from the building) filled with crates wrapped in non-woven Geotextile membrane and holds water, allowing it to disperse back into the earth slowly. This is a modern alternative as previously the hole used to be filled with rubble. This prevents water logging in the garden, stops damp seeping into the property and can save you money on your water bills. Because of the limited access, soakaways are difficult to repair and may need to be replaced if damaged.




Soil and vent pipe/stack


As drainage systems need some ventilation to allow the escape of gasses, most properties have a soil and vent pipe where the top of the pipe is open to let air circulate. This usually serves upstairs bathrooms and Building Regulations state that the pipe must extend up to at least 1.0m above eaves level.




Structural patch lining


This is a method of drain repair that seals leaking pipework to a watertight condition. The structural patch liners come in pre-cut lengths and are positioned in the drain to cover specific defects. You can read more about this technique here.




Structural soft felt lining (also known as drain sleeving)


This is a method of drain repair that seals leaking pipework to a watertight condition. The structural soft felt liners are cut to length and are used to cover numerous defects in one section. You can read more about this technique here.




Stub stack


These are short sections of above ground pipework that serve waste pipes and toilets (similar to a soil and vent pipe) but do not extend to the top of the property with a vent.




Surface water


Surface water is clean rainwater which enters the drainage system through guttering and yard gullies, etc. and as it is not contaminated so doesn't require treatment, it can discharge into a soakaway, stream or river. New drainage systems are separated into foul water and surface water where possible to help with the level of waste that needs to be treated before being released back into the environment and therefore combined systems which take both are mainly found in older properties. Foul water cannot discharge to a surface water system due to its contaminated waste and the Environmental Agency and Local Water Authorities can issue correction notices and fines if these illegal, unhygienic misconnections causing pollution are found.




Swan neck


Swan necks are used in vertical downpipes to allow for a difference in the structure of a building such as the gutter on the edge of a roof to the rainwater pipe adjoined to the wall.




Tumbler


Pipework needs to be laid to a good gradient for water to drain away however the fall cannot be too sharp. If the downstream manhole is in a lower ground area, the drain is usually laid to the correct gradient but has a vertical section of pipework with a rodding access within the chamber connecting it to the channel. This is similar to a vertical backdrop with the difference being this vertical section of pipework is inside the chamber walls.




Vertical backdrop


Pipework needs to be laid to a good gradient for water to drain away however the fall cannot be too sharp. If the downstream manhole is in a lower ground area, the drain is usually laid to the correct gradient but has a vertical section of pipework underground outside the manhole walls connecting it to the channel with a rodding access within the chamber. This is similar to a tumbler with the difference being this vertical section of pipework is outside the chamber walls.




Void


Some redundant drains are left open once the original connection has been removed and if they have not been sealed, the surrounding earth will move over time, leaving spaces underground. This will also occur if there is a large hole in the pipework or a collapsed section where the shingle surround falls into the drain and is washed away. These will lead to movement above ground and potentially to a building's structure.




Water testing


Drain surveys are able to see if there are fractures in the pipework however they do not tell us how much water is leaving the system. Carrying out a water test by capping off a run at the manhole then filling the drain with water determines the extent of the damage. You can read more about this investigation here.





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