Retention & detention overview

Retention and detention overview

These are SuDS components designed to either provide storage, through the retention of surface water runoff, or attenuation through the detention of surface water runoff. Retention is primarily provided on the surface through ponds, and underground through tanks, more commonly geocellular tanks. Detention is often provided by detention basins.

Retention storage within ponds as well as helping manage flood risk is also useful in providing water treatment. However, there should be upstream components or treatment stages before surface water is conveyed to ponds (and wetlands).

Detention is often useful in attenuating the peak flow from a rainfall event, but it also allows filtering and sedimentation to take place, which contributes to water quality improvement. Retention and detention components included in this classification include:

A well designed SuDS scheme should have most of the storage and water treatment performed by upstream source control components of the SuDS scheme. These ponds operate optimally when flows are managed and they’re providing a final polish to remove any pollution. Silt should also be removed from the runoff before it enters geocellular storage.

Detention basins (or ponds) are open, usually flat areas of grass that are normally dry, except after major storm events. In heavy rainfall they are used to store water for a short time and so they can fill with water. These can be designed to be multifunctional (the playing fields in the Manor in Sheffield and children’s play area in Lamb Drove provide this function). Effective operation requires that sediment and debris is removed upstream.

Retention ponds are open areas of shallow water, designed so they can accommodate rainfall and provide temporary storage for excess water. The water level rises temporarily when it rains, but there is always a permanent pool of water. They are similar to wetlands, but they are more useful for storing excess water. The design of retention ponds permanently storing water is what differentiates from detention basins.

Geocellular tanks with a high void ratio, have begun to replace underground pipes or tanks that would normally store water. They can also be used to convey or infiltrate surface water runoff into the ground (but they can’t provide water quality treatment). Underground storage features attenuate an agreed volume with a control structure to limit the discharge rate. Structural design must be provided to ensure integrity of the box, pipe or tank under loading (See CIRIA C680). Geocellular storage used on its own is unlikely to be regarded as a SuDS scheme, as it should incorporate source control.