Retention & detention facilities

Typical design

The general feature of a retention/detention facility is that it provides a storage volume for water. Thus, it can buffer peak flows (Q): Qstormwater = Qin > Qout.

Retention is one of the most relevant processes in stormwater management and is, among other things, used to minimise downstream flood risks, to reduce flows to downstream (for example underdimensioned) pipe systems, to provide temporal storage in infiltration facilities (since Qstormwater > Qinfiltation), to facilitate water quality treatment processes in treatment facilities (for example sedimentation), etc.

Retention

Detention

The most natural way of designing retention & detention facilities is in the form of (dry) ponds. A retention pond usually has a permanent water surface. It aims to keep water in place indefinitely and can dry out through infiltration and evaporation. A detention pond is usually a dry pond and aims to delay the runoff. In the design (see the figure) the main difference will be the location of the outlet: for a retention pond this will be elevated, so that there is a permanent volume of water below the outlet. For a detention pond there is an outlet located at the lowest point, but this is designed to allow only a limited flow rate to pass through. So low flow rates will pass unhindered, but during larger rain events the outflow becomes a limiting factor, and the pond starts filling up. After the rain ends, the pond will slowly empty. Therefore, detention ponds are also called dry ponds sometimes, while retention ponds with permanent water storage are also called wet ponds.

Detention facilities can also be constructed as underground facilities. These can be storage chambers made by concrete constructions or plastic modules, over-dimensioned pipes or systems filled with coarse gravel (i.e. storage in voids, appr. 30-35%).

Aims in stormwater management

Retention and detention facilities are designed mainly with stormwater quantity in mind. The goal is simply to provide storage volume for stormwater.

Because the water will be standing in the system for some time, there may be some sedimentation. For dry systems however, there is a risk that the sediment will be mobilized again by the next rain. Further, if sedimentation occurs, the sediment has to be removed regularly. This can be especially problematic in sub-surface systems.

Implementation in catchments

Retention and facilities can be placed both throughout the catchment or at the outlet, depending on the size of the catchment (or parts of it) and the required storage volume. The sizing of the pond depends on the size of the catchment and the flow rates that are acceptable to release downstream. Acceptable flow rates depend on where the water will flow: if the water flows into a small creek the acceptable flow rates will be lower than if the water flows into a large river. In some places there are regulations that require, when re-developing an area, that flow rates in the new situation should not be higher than in the existing situation.

Additional benefits

• Ponds can be integrated into a park or green area to contribute to amenity and recreational values.
• For dry ponds the area is available for other activities between rains.