Infiltration facilities

Typical design

Infiltration facilities is rather a process than a specific type of facility. Infiltration facilities can cover a range of different types and designs. Their primary aim is to allow water to infiltrate into the soil so that it does not become runoff.

For all infiltration facilities the soil characteristics and the groundwater level must be considered. Some soils such as sand have high infiltration capacities, while others such as clay have much lower infiltration capacities. For any soil, the groundwater level must also be sufficiently far below the infiltration system.

Since the infiltration capacity is nearly always lower than the incoming stormwater flow, infiltration facilities have to provide a storage volume. This is filled up during the storm event when the inflow exceeds the infiltration rate. After the rain, the water continues to infiltrate and the storage is emptied.

A few ways in which infiltration facilities can be designed are:

• Roof top disconnections: instead of connecting a roof gutter or downspout to the sewer, the water is instead lead onto the surrounding grass area. *Soak-away: an underground storage volume filled with a coarse material with large pore volume so it can hold a specific volume of water. Water is led into the soak-away by e.g. connecting a downspout or a street inlet directly to it.
• Infiltration trenches intercept water running along the surface in a gravel-filled ditch which provides the required storage volume in its pore volume
• Permeable pavement comes in the form of a special asphalt or concrete mixture or as grid pavers that are designed to leave gaps between them. Below the pavement is a gravel filled storage volume, similar to an infiltration trench. From this, the water infiltrates into the surrounding soil.
• Vegetated infiltration systems, such as infiltration swales or basins. These provide a depression storage volume on top of the system.

Aims in stormwater management

Infiltration facilities are primarily aimed at reducing runoff volumes and flow rates. Groundwater re-charge is also a common aim. As a side effect there may be some removal of pollutants, but this is not the primary aim. Sediment accumulated in/on the system may reduce infiltration capacities (so called clogging) and has to be removed regularly. Because the water is infiltrated into the underlying soil and can eventually reach ground water, there is a risk for groundwater contamination.

Implementation in catchments

Infiltrating large volumes of water also requires a relatively large storage volume, or area, depending on the infiltration capacity of the underlying soil. Infiltrating a lot of water in one location also creates the risk of increasing the groundwater level locally, which could limit infiltration rates. Therefore, infiltration facilities are best spread out throughout the catchment. This also has the benefit of reducing the amount and/or size of sewer pipes or other transport systems (swales, for example) that are required.

The total area required for infiltration systems is highly dependent on the local conditions. For example, the maximum infiltration rate in sand can be more than 10 cm/hr, while for clay soils it can be less 0.1 cm/hr. Clogging commonly reduces the infiltration rate over time.

Additional benefits

• Infiltrating more water can help restore groundwater levels. Low groundwater levels can cause problems with the stability of soil and can lead to damage to wooden building foundations, which are found in some historical cities, for example in the Netherlands, many historical buildings have such foundations.
• Water infiltrated into the soil can become available to plants so that they can grow. This can reduce irrigation needs in urban areas, and when plants transpirate the water this contributes to cooling and reducing the urban heat island effect.