Glossary Terms A-D


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.


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