Glossary Terms I-Q


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.


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