Glossary of Sustainable Urban Drainage Terminology


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. The pond always contains water. Also known as a wet detention pond.


Flow control or water treatment structure that is normally dry.


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

bioretention area

A depressed landscaping area that is allowed to collect runoff so that it percolates through the soil below the area in to an underdrain, 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 to a point on a drainage or river system, which may be divided in to sub-catchments.

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 a sewage treatment works from being exceeded under storm flow conditions by allowing the discharge of excess diluted sewage to a watercourse.

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, ponds and lakes with discharges and groundwaters (waters contained in underground strata). For a fuller definition see the Water Resources Act 1991.


Land area within property boundaries.

Design criteria

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

Detention basin

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.

Diffuse pollution

Pollution arising from land-use activities (urban and rural) that are dispersed across a catchment, or sub-catchment, and do not arise as a process effluent, municipal sewage effluent, or an effluent discharge from farm buildings.


Free of water under dry weather flow conditions.

dry weather flow

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

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 process by which soils lose moisture by evaporation of water and uptake and transpiration by 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 within the water.


Flood estimation handbook, produced by the Centre for Ecology and Hydrology, Wallingford (formerly the Institute of Hydrology).

filter drain

A linear drain consisting of a trench filled with a permeable material, often with a perforated pipe within the base of the trench to assist drainage, to store and conduct water, but may also be designed to permit infiltration.

filter strip

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


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

first flush

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

flood frequency

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

flood plain

Land adjacent to a watercourse that would be the subject of repeated flooding under natural conditions (see Environment Agency’s Policy and Practice for the protection of flood plains for further definition.

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.

flow control device

A device used to manage the movement of surface water in to and out of an attenuation facility e.g. a weir.

greenfield runoff

The surface water runoff regime from an undeveloped site, or the prevailing conditions on a brownfield development.

green roof

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


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 the ground within the saturation zone.

Highways Agency

The government agency that is responsible for strategic highways, the motorways and trunk roads.

highway authority

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

highway drain

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


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

HOST (Hydrology of Soil Types)

A classification used to indicate the permeability of the soil and the percentage runoff from a particular area.


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 in to the ground.

infiltration basin

A dry basin designed to promote the infiltration of surface water in to the ground.

infiltration device

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

infiltration potential

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

infiltration trench

A trench, usually filled with permeable granular material, designed to promote the infiltration of surface water in to the ground.


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

That part of the drain that 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 The part of a drain identified in a declaration of vesting made under section 102 or in an agreement made under section 104, of the Water Industry Act 1991.

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.

natural capital

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


Dry weather flow bypassing a storage area.


Dry weather flow passing through a storage area.


Road or car park surface and underlying structure, usually asphalt, concrete or block paving. The path next to the road for pedestrians and known as the “pavement” is properly termed the footway.


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

permeable pavement

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

permeable surface

A surface formed of material that is itself impervious to water, but by virtue of voids formed through the surface, allows the 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 of 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’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.

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.


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.

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 sewer.

public sewer

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

rainwater harvesting or rainwater use system

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 the property, from roofs and surrounding surfaces.

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.

requisition sewer

A sewer built by a sewerage undertaker in response to a “requisition” notice served under Section 98 of the Water Industry Act (WIA) 1991. The requisition may be served on the undertaker by any of the bodies listed in subsection 1c of the Act (including the owner/occupier of the premises or the relevant local authority).


The principle of ensuring that runoff is controlled as close as possible to the source if it cannot be completely dealt with at source.

retention pond

A pond where runoff is detained for a sufficient time to allow settlement and possibly biological treatment of some pollutants.


Water flow over the ground surface to drainage system. This occurs if the ground is impermeable, is saturated or if rainfall is particularly intense.

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 maintained highway may include provision for drainage of the highway. (Drainage of highways is defined within Section 100(9) of the Highways Act 1980.

section 102 0r 104

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

section 106 TCPA 1990

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

section 106 WIA 1991

A section within the Water industry Act, relating to the right of connection to a sewer.


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

The statutory undertaking of water companies that is responsible for sewerage and sewage disposal including surface water from roofs yards and premises.

sewers for adoption

A guide agreed between sewerage undertakers and developers (through the House Builders Federation) specifying the standards to which private sewers need to be constructed to facilitate adoption.

site and regional controls

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


Scottish Natural Heritage.


A subsurface structure into which surface water is conveyed to allow infiltration in to the ground.


Soil Index Value obtained from the WRAP soil classification, used in the Wallingford Procedure to calculate the treatment volume.

source control

The control of runoff at, or near, its source.


A computer model based on equations used in the California Stormwater Best Management Practice Handbook. Used to assess detention basin performance.


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 or sustainable urban drainage systems; a sequence of management practices and control structures designed to drain surface water in a more sustainable fashion than conventional techniques.

surface water management

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

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.


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.


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

WRAP (Winter Rain Acceptance Potential)

Classification used to calculate the permeability of soils and the percentage run-off from a particular area.


Containing water under dry weather conditions.


A pond that has a high proportion of emergent vegetation in relation to open water.