Glossary Terms R-Z


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.


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