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home > using SuDS > SuDS components > infiltration devices

Infiltration devices

What are they?

Infiltration devices drain water directly into the ground. They may be used at source or the runoff can be conveyed in a pipe or swale to the infiltration area. They include soakaways, infiltration trenches and infiltration basins as well as swales, filter drains and ponds. Infiltration devices can be integrated into and form part of the landscaped areas.

Soakaways and infiltration trenches are completely below ground, and water should not appear on the surface. Infiltration basins and swales for infiltration store water on the ground surface, but are dry except in periods of heavy rainfall.

How they work

Infiltration devices work by enhancing the natural capacity of the ground to store and drain water. Rain falling onto permeable (eg sandy) soil soaks into it. Infiltration devices use this natural process to dispose of surface water runoff. Limitations occur where the soil is not very permeable, the water table is shallow, the ground is not stable or the groundwater under the site may be put at risk (through contamination).

Quantity

Infiltration techniques:

• provide storage for runoff. In the case of soakaways and infiltration trenches, this storage is provided in an underground chamber, lined with a porous membrane and filled with coarse crushed rock. Infiltration basins store runoff by allowing temporary and shallow ponding on the surface;
• enhance the natural ability of the soil to drain the water. They do this by providing a large surface area in contact with the surrounding soil, through which the water can pass.

The amount of water that can be disposed of by an infiltration device within a specified time depends mainly on the infiltration potential of the surrounding soil. The size of the device and the bulk density of any fill material will govern storage capacity.

Quality

Runoff is treated in different ways in an infiltration device. These include:

• physical filtration to remove solids
• adsorption onto the material in the soakaway, trench or surrounding soil
• biochemical reactions involving micro-organisms growing on the fill or in the soil.

The level of treatment depends on the size of the media and the length of the flow path through the system, which controls the time it takes the runoff to pass into the surrounding soil. Pre-treatment may be required before polluted runoff is allowed into an infiltration device.

Amenity

Infiltration systems are easy to integrate into a site. They are ideal for use as playing fields, recreational areas or public open space. Infiltration basins can be planted with trees, shrubs and other plants, improving their visual appearance and providing habitats for wildlife. They increase soil moisture content and help to recharge groundwater, thereby mitigating problems of low river flows.

 

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