Attenuation is the process of water retention on site and slowly releasing it in a controlled discharge to a surface water or combined drain or watercourse. The amount of discharge will vary depending whether it is a brown or greenfield site. For brownfield sites the developer must determine the likely run off and agree an acceptable discharge with the environment agency or water authority. For greenfield sites the environment agency normally limits discharge to 5 l/s per hectare (10,000m2) which only represents about 2-3% of run off.


The area contributing surface water flow to a point on a drainage or river system.


Construction Industry Research and Information Association.


Environment Agency.


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

Flow control device

A device used to manage the movement of surface water into and out of an attenuation facility.

Gravity drainage

Drainage which runs through pipework installed to a fall, and not therefore under pressure.


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


Will not allow water to pass though it.

Impermeable surface

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


Infiltration or soakaway is the temporary storage of water to allow it to naturally soak away into the ground. Because water soaks into the ground gradually, reduces the risk of flooding downstream. Infiltration may be used where there is no surface water sewer or where existing systems are at full capacity. Infiltration helps to recharge natural ground water levels.


A measure of the ease with which a fluid can flow through a porous medium. It depends on the physical properties of the medium.

Rainwater harvesting

Collection and Re-use or recycling of rainwater for the purpose of garden irrigation, car washing, toilet flushing etc.


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

Siphonic Roof Drainage

Rainwater systems engineered on the the concept of full bore (fill rate of 100%), implying that rainwater flows at high speed through small diameter pipework, at normally zero gradient.


Sustainable (urban) drainage systems.


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

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