Glossary Terms I-Q


Adoption is the process of taking legal responsibility for SuDS devices for their whole life, which includes maintenance responsibilities. There is no definitive guidance on which authorities can adopt certain SuDS as each project will have different requirements. For piped systems, there is definitive guidance set out in Sewers for Adoption, and developers are responsible for constructing systems to specified standards. For SuDS, advice on model agreements for adopting SuDS devices is set out in the Interim Code of Practice for Sustainable Drainage Systems, produced by the National SuDS Working Group.


The adherence of gas, vapour, or dissolved matter to the surface of solids


The quality of place being pleasant or attractive; ie agreeableness.  A feature that increases attractiveness or value, especially of a piece of real estate or a geographic locations


A sub-surface zone or formation of rock or soil containing a body of groundwater


The reduction of peak water flow by spreading it over a longer time period.  This is done by providing storage in sewers, tanks or soft SuDS structures.  The principle of SuDS is to provide flow attenuation in order to manage surface water effectively.  Any form of flow attenuation is a form of SuDS


The sustained flow in a channel or drainage system.


A ground depression that acts as a flow control or water treatment structure that is normally dry and has a proper outfall, but is designed to detain storm water temporarily.   These types of structures include flood plains and detention basins.


Decomposition of organic matter by micro-organisms and other living things.


The diversity of plant and animal life in a particular habitat.

Bioretention area

A depressed landscaping area that is allowed to collect runoff so it percolates through the soil below the area into an under drain, thereby promoting pollutant removal.

Brown roof

A roof that incorporates a substrate (laid over a waterproof membrane) that is allowed to colonise naturally. Sometimes referred to as an alternative roof.


The area contributing surface water flow to a point on a drainage or river system.  Can be divided into sub-catchments.  Characteristics of catchments, such as soil type, urban density, shape and size affect the rate of surface water run-off and hydrological regime.

Combined sewer

A sewer designed to carry foul sewage and surface runoff in the same pipe.  They transport both wastewater from homes and industry but also carry rain derived surface water from gutters, drains and some highways.  Combined systems are generally not permitted for new developments now due to problems with them becoming overloaded with rain during storms.  Many older drainage systems in the UK are, however, combined and can spill dilute sewerage into watercourses during high flows.  Often for new developments, it is a condition of planning that foul flows can only be taken to existing combined systems if the equivalent amount of surface water can be removed from the system.

Combined Sewer Overflow

Heavy or prolonged rainfall can rapidly increase the flow in a combined sewer until the volume becomes too much for the sewer to carry and excess storm sewage is discharged to river or sea via relief “valves” known as combined sewer overflows (CSO’s).  Without CSO’s the overloaded sewers could flood properties and sewerage treatment works.  Although the discharge from CSO’s contain sewerage it is heavily diluted with rain water.  Nevertheless many of these discharges are today considered unacceptable.

Contaminated land

Ground that contains substances which, when present in sufficient quantities or concentrations, are likely to have detrimental effects.

Controlled waters

Waters defined and protected under the Water Resources Act 1991. Any relevant territorial waters that extend seaward for 3 miles from the baselines, any coastal waters that extend inland from those baselines to the limit of the highest tide or the freshwater limit of any river or watercourse, any enclosed dock that adjoins coastal waters, inland freshwaters, including rivers, watercourses, and ponds and lakes with discharges and groundwaters (waters contained in underground strata).For the full definition refer to the Water Resources Act 1991.

Conventional drainage

The traditional method of draining surface water using subsurface pipes and storage tanks.


The movement of water from one location to another.


See definition for Combined Sewer Overflow


A closed channel carrying a watercourse beneath an obstruction such as a road, railway or canal.


Land area within property boundaries.

Design and access statement

Formal documents explaining the design philosophy behind a planning application.

Design Codes

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

Design criteria

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

Design Statement

In the context of a planning application, a written statement to a local authority prepared by an applicant setting out the design principles adopted in relation to a proposed design for a site and its wider context.

Designing for exceedance

An approach that aims to manage exceedance flows during rainfall events such as using car parks during extreme events.

Detention basin

A vegetated depression, normally  dry except after storm events. It is constructed to store water temporarily to attenuate flows.  It may allow infiltration of water to the ground and permits settlement of coarse solids.  Outlets are normally designed to empty over a 24-hour period and the maximum depth of water should not exceed 3m, only occurring for the design storm.

As these features are normally dry, they may be used for other purposes when not required for drainage.  Where further conveyance of flow is required, the base of the detention basin should be designed to drain freely to ensure that standing water is not left on the basin floor.

See also extended detention basin.

Detention pond/tank

A pond or tank that has a lower outflow than inflow.  Often used to prevent flooding.

Diffuse pollution

Pollution that comes from non-point source contamination in urban and rural land-use activities spread out across a catchment or sub-catchment.   Examples of true non-point sources are sheet run off from fields or seepage of nutrients from soil into ground water.  Examples of minor point sources are field drains or surface water drains in urban areas.  Diffuse sources are often individually minor, but collectively significant.

Drainage Catchment

See “Catchment”.


Free of water under dry weather flow conditions.


All living things, such as trees, flowering plants, insects, birds and mammals, and their habitats.


A biological community of interacting organism, and their physical environment.

Ecosystem services

Services provided by the natural environment that benefits society.


Both the natural environment (air, land water resources, plant and animal life) and habitats.

Environmental Footprint

A measure of environmental impact based on the distance that resources for a development are transported.

Environmental management

A management agreement for an area or project set up to plan and make sure the declared management objectives for the area or project are met. Environmental Management Plans are often undertaken as part of an environmental impact assessment and are set out in several stages with responsibilities clearly defined and environmental monitoring procedures in place to show compliance with the plan.


The group of natural processes, including weathering, dissolution, abrasion, corrosion, and transportation, by which material is worn away from the Earth’s surface.


The process by which the Earth's surface or soil loses moisture by evaporation of water and by uptake and then transpiration from plants.

Extended detention basin

A detention basin in which the runoff is stored beyond the time normally required for attenuation.  This provides extra time for natural processes to remove some of the pollutants in the water.  It is important to ensure that these areas are extremely shallow to prevent them from being a hazard to children.


Flood Estimation Handbook (FEH), produced by Centre for Ecology and Hydrology, Wallingford (formerly the Institute of Hydrology) to aid calculationof the possible severity of flooding.

Filter drain

A linear drain consisting of a trench filled with a permeable material, often with a perforated pipe in the base of the trench to assist drainage, to store and conduct water, but may also be designed to permit infiltration.  Filter drains are often used at the side of roads to collect highway run-off and at the toe of embankments to intercept run-off and groundwater flow.  The sides of the trench can be lined with permeable or impermeable lining membranes depending on specific requirements.  The filtration action helps remove suspended solids from the water.

See also French Drains.

Filter strip

A vegetated area of gently sloping ground designed to drain water evenly off impermeable areas and filter out silt and other particulates.


The act of removing sediment or other particles from a fluid by passing it through a filter.

First flush

The initial runoff from a site or catchment following the start of a rainfall event. As runoff travels over a catchment it will pick up or dissolve pollutants and the "first flush" portion of the flow may be the most contaminated as a result. This is especially the case in small or more uniform catchments, however, in larger or more complex catchments pollution wash-off may contaminate runoff throughout a rainfall event.

Flood frequency

The probability of a flow rate being equalled or exceeded in any year.

Flood routeing

Design and consideration of above-ground areas that act as pathways permitting water to run safely over land to minimise the adverse effect of flooding. This is required when the design capacity of the drainage system has been exceeded.  Computer simulation software allows flood routeing to be modelled if the topographical information is available.


Land adjacent to a watercourse that would be subject to repeated flooding under natural conditions.  In Wales, most new development within flood plains is not permitted, subject to the requirements set out in the Welsh Government's TAN 15 guidance.  However many floodplains in Wales have been built upon as they were convenient locations to develop.  As a consequence many thousands of properties in Wales are at risk of flooding.


The plants found in a particular physical environment.

Flow control device

A device used to manage the movement of surface water into and out of an attenuation facility, for example a weir.


A small basin or pond upstream of the main drainage component with the function of trapping sediment.

French Drain

An alternative name for Filter Drain.

Geocellular structure

A plastic box structure used in the ground to create a void which can hold water; often to attenuate runoff.


A plastic fabric that is permeable.

Green Infrastructure

A strategically planned and delivered network of high quality green spaces and other environmental features (often including water features).

Green roof

A roof with plants growing on its surface, which contributes to local biodiversity. The vegetated surface provides a degree of retention, attenuation and treatment of rainwater, and promotes evapotranspiration. (Sometimes referred to as an alternative roof).

Greenfield runoff

This is the surface water runoff regime from a site before development, or the existing site conditions for brownfield redevelopment sites.

To maintain the natural equilibrium of a site, the surface water discharge from a developed site should not exceed the natural greenfield run-off rate.  Environment Agency Wales will advise on allowable discharge rates to watercourses for sites that are to be developed.  However, where possible it is better that surface water is managed at source.


Wastewater from sinks, baths, showers and domestic appliances this water before it reaches the sewer (or septic tank system).


Water that is below the surface of ground in the saturation zone.


The area of environment where an organism or ecological community normally lives or occurs.

Highway authority

A local authority with responsibility for the maintenance and drainage of highways maintainable at public expense.

Highway drain

A conduit draining the highway. On a highways maintainable at the public expense it is vested in the highway authority.


Hydrology of Soil Types (HOST).  A classification used to indicate the permeability of the soil and the percentage runoff from a particular area.


A graph illustrating changes in the rate of flow from a catchment with time.

Hydrological Cycle

See Water Cycle.


Will not allow water to pass through it.

Impermeable surface

An artificial non- porous surface that generates a surface water runoff after rainfall.

Infiltration – to a sewer

The entry of groundwater to a sewer.

Infiltration – to the ground

The passage of surface water though the surface of the ground.

Infiltration basin

A shallow depression in the ground that is usually dry, but during storms is designed to hold surface water for a time while allowing it to soak in the ground.  Infiltration basins and other infiltration devices can promote groundwater recharge, but the use of these devices must also consider the risk of polluting groundwater.

Infiltration device

A device specifically designed to aid infiltration of surface water into the ground.

Infiltration potential

The rate at which water flows through a soil (mm/hour).


Shallow infiltration to the soil, from where it may infiltrate vertically to an aquifer, move horizontally to a watercourse or be stored and subsequently evaporated.

Interim Code of Practice

An agreed provisional document within the existing legislative framework that establishes good practice.


A pond designed for the settlement of suspended solids.

Lateral drain

a) That part of a drain which runs from the curtilage of a building (or buildings or yards within the same curtilage) to the sewer with which the drain communicates or is to communicate; or

(b) (if different and the context so requires) the part of a drain identified in a declaration of vesting made under section 102 or in an agreement made under section 104.

Model agreement

A legal document that can be completed to form the basis of an agreement between two or more parties regarding the maintenance and operation of sustainable water management systems.

Currently (6/2011), unlike conventional piped systems (Sewers for Adoption) there is no one document that sets out standards for adoption of SuDS and who should take the responsibilities for their maintenance.

Natural Capital

The natural resource stocks from which resources useful for livelihoods are derived e.g. water, land, environmental resource.


A substance (such as nitrogen or phosphorus) that provides nourishment for living organisms.


Dry weather flow bypasses the storage area.


Dry weather flow passes through the storage area.

Orifice plate

Structure with a fixed aperture to control the flow of water.

Passive treatment

Natural processes used to remove and break down pollutants from surface water runoff.


 The route by which potential contaminants may reach targets.


Technical name for the road or car park surface and underlying structure, usually asphalt, concrete or block paving. NB the path next to the road for pedestrians (colloquially called pavement) is properly termed the footway.


A measure of the ease with which a fluid can flow through a porous medium. It depends on the physical properties of the medium, for example grain size, porosity and pore shape.

Permeable pavement

A paved surface that allows the passage of water through voids between the paving blocks/slabs.

Permeable surface

A surface formed of material that is itself impervious to water but, by virtue of voids formed through the surface, allows infiltration of water to the sub-base through the pattern of voids, for example concrete block paving.

Pervious surface

A surface that allows inflow of rainwater into the underlying construction or soil.

Piped system

Conduits generally located below ground to conduct water to a suitable location for treatment and/or disposal.


A change in the physical, chemical, radiological or biological quality of a resource (air, water or land) caused by man or man’s activities that is injurious to existing, intended or potential uses of the resource.


Permanently wet basin designed to retain stormwater and permit settlement of suspended solids and biological removal of pollutants.  Ponds include balancing ponds, lagoons and wetlands.

Porous paving

A permeable surface allowing the passage of water through voids within, rather than between, the paving blocks/slabs.

Porous surface

A surface that infiltrates water to the sub-base across the entire surface of the material forming the surface, for example grass and gravel surfaces, porous concrete and porous asphalt.

Potable/mains water

Water company/utility/authority supplied drinking quality water.


A section of a swale designed to detain runoff.


Site design and management to stop or reduce the occurrence of pollution and to reduce the volume of runoff by reducing impermeable areas.

Private Sewer

Private sewers are those owned by either the owner of the land it runs through, the owner(s) of the properties it serves, or an agent such as a Housing Association.  It is the owner’s responsibility to maintain a private sewer.  This includes any decision to connect new or additional drains to the private sewer. This responsibility extends until the pipe connects to the public sewer.  Sometimes in rural areas a private sewer may eventually connect to a privately-owned storage facility like a septic tank, or even to a privately-owned sewage treatment works.

Proper outfall

An outfall to a watercourse, public sewer and in some instances an adopted highway drain. Under current legislation and case law, the existence of a proper outfall is a prerequisite in defining a sewer.

Public sewer

A sewer that is vested in and maintained by a sewerage undertaker.


Rain is liquid precipitation; that is precipitation that falls as water as opposed to snow or hail.

Rainwater butt

A small scale garden water storage device which collects rainwater from the roof via the drainpipe.

Rainwater harvesting

A system that collects rainwater from where it falls rather than allowing it to drain away.  It includes water that is collected within the boundaries of a property, from roofs and surrounding surfaces.

All that is necessary to capture this water is to direct the flow of rainwater from roof gutters to a rainwater storage tank.  By doing this, water can be collected and used for various uses.  The water is filtered prior to entry into a tank.  From here it can be used for domestic applications where water of drinking quality is not required such as in toilet flushing and in washing machines.  Other uses for rainwater include vehicle washing plants and irrigation/garden use.

Rainwater Use System

See Rainwater harvesting.


The addition of water to the groundwater system by natural or artificial processes.

Recurrence interval

The average time between runoff events that have a certain flow rate, e.g. a flow of 2 m/s might have a recurrence interval of two years in a particular catchment.

Retention pond

A pond where runoff is detained (e.g. for several days) to allow settlement and biological treatment of some pollutants.


Open surface water channels with hard edges.


The chance of an adverse event occurring.

Risk Assessment

“A carefully considered judgement” requiring an evaluation of the consequences that may arise from the hazards identified combing the various factors contributing to the risk and then evaluating their significance.


Royal Society for the Prevention of Accidents.


The amount of water from precipitation, which flows from a catchment area past a given point over a certain time period.  Run-off can be defined as rainfall less infiltration and evaporation.  Development of sites often increases the impermeable area, reducing the amount of infiltration and thus increasing the volume of run-off.

Section 102 or 104

Section within the Water Industry Act 1991 permitting the adoption of a sewer, lateral drain or sewage disposal works by the sewerage undertaker. Sometimes referred to as S102 or S104.

Section 106 (Town and Country Planning Act 1990)

A section within the Town and Country Planning Act 1990 that allows a planning obligation to a local planning authority to be legally binding.

Section 106 (Water Industry Act 1991)

A key section of the Water Industry Act 1991, relating to the right of connection to a public sewer.

Section 38

An agreement entered into pursuant to Section 38 Highways Act 1980 whereby a way that has been constructed or that is to be constructed becomes a highway maintainable at the public expense. A publicly maintainable highway may include provision for drainage of the highway. (Drainage of highways is defined in Section 100 (9) of the Highways Act 1980).


Scottish Environment Protection Agency.

Separate sewer

A sewer for surface water or foul sewage, but not a combination of both.


A pipe or channel taking domestic foul and/or surface water from buildings and associated paths and hard standings from two or more curtilages and having a proper outfall.

Sewerage undertaker

This is a collective term relating to the statutory undertaking of water companies that are responsible for sewerage and sewage disposal including surface water from roofs and yards of premises.

Sewers for Adoption

A guide agreed between sewerage undertakers and developers (through the Home Builders Federation) specifying the standards to which private sewers need to be constructed to facilitate adoption.  First published in 1980, Sewers For Adoption is currently in its 7th Edition and is available from the WRc.

Site and regional controls

Manage runoff drained from several sub-catchments. The controls deal with runoff on a catchment scale rather than at source.


A subsurface structure into which surface water is conveyed to allow infiltration into the ground.  Soakaways can be circular perforated manhole structures filled with granular stone or trenches filled with stone, facilitating percolation and separation of solids prior to infiltration.  The procedure for soakaway design is set out in BRE Digest 365.

Source control

The control of runoff or pollution at or near its source.  The principles of SuDS are to mimic as far as possible the natural drainage characteristics of a site to maintain the drainage regime.  By returning water to the natural drainage system as close to where it falls as possible represents effective management of surface water.  Source control devices include Soakaways, Permeable surfaces, Infiltration basins and Swales.


A layer of material on the sub-grade that provides a foundation for a pavement surface.


A division of a catchment, allowing runoff management as near to the source as is reasonable.


The surface of an excavation prepared to support a pavement.


The principle that an issue should be managed as close as is reasonable to its source.


Sustainable drainage systems: a sequence of management practices and control structures designed to drain surface water in a more sustainable fashion than some conventional techniques.

Surface Water

Water that appears on the land surface, ie lakes, rivers, streams, standing water and ponds.

Surface water management

The management of runoff in stages as it drains from a site.

Surface water management Train

This is a useful tool in the development of surface water drainage systems.  The management train starts with the prevention of flooding and pollution.  The train then progresses through a hierarchy of run-off management controls.  Run-off need not pass through all stages of the management train and the higher up the train that run-off can be managed, the more sustainable the solution.

Suspended solids

Undissolved particles in a liquid.


A shallow vegetated channel designed to conduct and retain water, but may also permit infiltration; the vegetation filters particulate matter.


Technical Advisory Note (TAN) –provides technical guidance which supplements the policy set out in Planning Policy Wales.  (The planning area covered is identified by the number).

TAN 15

Technical Advice Note (TAN) 15 provides technical guidance which supplements the policy set out in Planning Policy Wales in relation to development and flooding.  It provides advice on Development advice maps (DAM’s), the nature of development or land use; justifying the location of built development; assessing flooding consequences; surface water run-off from new development; action through Development Plans; and Development Control.  The Development Advice Map (DAM) for use with Technical Advice Note (TAN) 15 is available as an interactive map.


The loss of water vapour through plant leaves.


Improving the quality of water by physical, chemical and/or biological means.

Treatment volume

The volume of surface runoff containing the most polluted portion of the flow from a rainfall event.


The UK Climate Impacts Programme (UKCIP) has developed the UK Climate Projections (UKCP09).  These projections of our changing climate provide information for the UK up to the end of this century.  It is expected that average annual rainfall in Wales could increase by up to 20% within 50 years.  Summers will be drier (with baseflows half their current rate), whilst winter rainfall increases significantly.  Sea levels will also rise partly due to melting of polar ice caps.  Drier summers will cause pollution problems in watercourses with reduced flow and increased periodic liberation of pollutants that have gathered during extended drier periods.


Any substance or object that the holder discards, intends to discard, or is required to discard.

Water Cycle

The continuous circulation of water in systems throughout the planet, involving condensation, precipitation, runoff, evaporation and transpiration.  It is also known as the hydrological cycle.

Water Table

The point where the surface of groundwater can be detected. The water table may change with the seasons and the annual rainfall.


A term including all rivers, streams, ditches, drains, cuts, culverts, dykes, reens, sluices and passages through which water flows.


Containing water under dry weather conditions.


A flooded area where the water is shallow enough to enable the growth of bottom-rooted plants and these exceed the proportion of open water.

What is a detention basin?

A detention basin is normally dry, constructed to store water temporarily to attenuate flows. This permits settlement of coarse solids within the drainage system. Outlets are normally designed to empty over a 24-hour period and the maximum depth of water should not exceed 3m, only occuring for the design storm.

As these features are normally dry, they may be used for other purposes when not required for drainage. A detention basin may also allow infiltration of water to the ground. Where further conveyance of flow is required, the base of the detention basin should be designed to drain freely to ensure that standing water is not left on the basin floor.


Cyfoeth Naturiol Cymru / Natural Resources Wales Welsh Water / Dwr Cymru Welsh Government / Llywodraeth Cymru Atkins Welsh Local Government Association Consumer Council for Water
Home Builders Federation CIWEM Institute of Civil Engineers