Products / Techniques

Infiltration Basins, Swales, Trenches and Soakaways

Infiltration basins are man-made depressions, engineered to hold back surface water and let it infiltrate into the soil, thus reducing the volume of runoff needing to be conveyed to end-of-pipe systems.

Basins can be landscaped to create structures that appear natural and offer an amenity value for the local community. The process of infiltration through the soil removes pollutants and the collected surface water aids in recharging groundwater.

Basins are usually designed ‘off-line’ with surface water being diverted to them by other drainage systems. Pre-treatment of silts and sediments is essential to protect the basin soils from becoming clogged. This can be achieved through construction of a sediment forebay, for example, or by use of a hydrodynamic treatment device such as the Downstream Defender® , where space is limited or for ease of maintenance.

To be effective, infiltration basins require a relatively large amount of space, as well as the right ground and site conditions. One means of providing amenity value could be to combine an infiltration basin with underground attenuation and storage using a geocellular system such as Hydro Stormbloc®.

In selecting an infiltration basin as a SUDS solution, developers should be mindful of the need to educate the local community to expect pooling and ponding on the surface of the basin during peak storm events as an intended function of the drainage system.

Infiltration Swales

Swales are shallow, flat-bottomed drainage features which capture rainwater at-source and act as a conveyance system through the SUDS treatment train. Swales can also be designed to act as infiltration features, by being designed to slow down the flow of water and let it infiltrate into the soil.

In the same way as basins, swales can be planted with grasses and other vegetation and provide a level of pollution control and treatment for smaller storm events.

Where storage capacity is limited, the use of swales could be enabled by the addition of a geocellular storage such as Hydro Stormbloc® to maximise drainage potential.

Infiltration Trenches

Infiltration trenches are narrow excavated strips, filled with gravel or aggregate and designed to accept storm water runoff, often from lateral flows, and allow them to infiltrate into the surrounding soils.

Like swales, they can be designed as a conveyance system for surface-water runoff as part of a treatment train, and to treat pollutants through filtration into the soil.


Perhaps the most familiar infiltration devices, soakaways are excavated underground chambers designed to attenuate surface water and enable it to percolate into the surrounding ground. Soakaways provide source control for small impermeable areas, although they can be combined for larger surface water control e.g. as part of a highways development.

Geocellular infiltration systems can be specified to provide a higher underground storage capacity as part of a soakaway design, for example using modular systems such as Hydro Stormbloc®.