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Photograph - Flow control structure

SUDS - Site Contol

Restricting flow from the site

SUDS site control measures aim to restrict the discharge from the whole or a significant part of a site at a single location and to temporarily store the excess water. As well as attenuating the flow this may also improve the quality of the run-off from the site. Site control measures include ponds; detention basins; wetlands and infiltration basins. The run-off from the area served is collected and transported to the site control feature by a conveyance system.

Design criteria

The design of site control measures is covered in 'The SUDS Manual', CIRIA C753, 2015. See also Source Control for further details of SUDS measures.

Some site control techniques are examined in more detail below.

Detention basins

The outflow from a detention basin is via a control such as an orifice which is designed to limit the peak flow passed downstream. Flow in excess of the limiting discharge is stored within the detention basin until such time as the storm has abated and will then be gradually allowed to drain away through the basin's outlet. The design of the outlet control needs to take into account the possibility of blockage due to rubbish or dead vegetation, etc clogging the orifice. Consideration also needs to be given to the operation of the basin during a severe storm which exceeds the basin's design criteria.

An option for sites with restricted space is an underground tank which fulfils the same function as a detention basin but which can be positioned under a road or car park.


Constructed wetlands are in many ways similar to detention basins except that the floor of the basin provides a site for a shallow pond with appropriate vegetation to improve pollution removal and to  provide  a  wildlife  habitat.

Infiltration basins

Again infiltration basins are similar to detention basins but the outflow from the basin is via infiltration into the underlying soil.

Photograph: Detention basin, control structure and reed bed, Hamilton North, Leicester. In this case the control structure is constructed from recycled timber railway sleepers and flow control is via the stainless steel orifice plate. In the event of a blockage or a severe storm which exceeds the basin's design criteria, excess flow will spill over the control structure. An open flow control structure such as this is easy to construct and maintain, is not too unsightly, but is open to vandalism.