What is a wadi (infiltration swale)?
A wadi is a lowered, usually grassed area where rainwater is collected temporarily and then slowly soaks into the ground. In the Netherlands the name is often explained as an acronym of Water Afvoer Drainage Infiltratie (water discharge, drainage, infiltration). The word originally comes from Arabic, where a wadi is a dry riverbed that only carries water after rain. In English the closest terms are infiltration swale or bioswale.
How does it work?
- Collection: rainwater from roofs and paving flows via gutters, an open channel or a pipe into the wadi.
- Storage: the water stands for a while in the shallow, gently sloping hollow.
- Infiltration: the water seeps through the planted topsoil and a permeable layer of sand or gravel into the ground. Plants and soil life filter out part of the pollution.
- Drainage and overflow: under the wadi there is often a drain pipe in a gravel trench that removes water where the soil drains poorly. An overflow to the sewer or a ditch prevents flooding during extreme rainfall.
In practice
Wadis are used in housing estates, along roads and in gardens to disconnect rainwater from the sewer and limit flooding during heavy rain. Tips if you build one yourself:
- Test the soil. Dig a hole, fill it with water and see how fast it drains. On clay or peat with a high water table, a wadi only works with a drain or combined with other measures.
- Keep it away from the house, so no water runs towards foundations or crawl space; slope the ground away from the building.
- Make the side slopes shallow, for safety and easy mowing.
- Size it based on the connected roof area and the soil; an infiltration calculation helps.
- Do not drain dirty water into it, such as water with oil or soap from a driveway where cars are washed.
- Maintenance: mow, remove leaves and litter, and keep inlet and overflow clear.
- Check with your local authority for rules and any schemes for disconnecting rainwater; they differ by area.
Related terms
- Infiltration
- Infiltration in your own garden
- Infiltration calculations
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