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Permeable surfaces and filter drains

What are they?

Filter drains and permeable surfaces are devices that have a volume of permeable material below ground to store surface water. Runoff flows to this storage area via a permeable surface. This can include:

• Grass (if the area will not be trafficked)
• Reinforced grass
• Gravelled areas
• Solid paving blocks with large vertical holes filled with soil or gravel
• Solid paving blocks with gaps between the individual units
• Porous paving blocks with a system of voids within the unit
• Continuous surfaces with an inherent system of voids

How they work

The water passes through the surface to the permeable fill. This allows the storage, treatment, transport and infiltration of water. Both the surface and the sub-base of a pavement must allow the passage of water - for this reason, porous asphalt laid on a conventional impermeable base is not a permeable pavement.

Quantity

The amount of water stored depends on the voids ratio of the permeable fill or sub-base, the plan area and depth. Water can be disposed of by infiltration, an underdrain, or pumped out. Overflow can be via a high level drain or controlled surface flow. In some situations the water should not be stored for extended periods as it can affect the strength of the surrounding soil.

Quality

The permeable fill or sub-base traps sediment, thereby cleaning up runoff. Recent research shows that they also provide some treatment for other pollutants, such as oil.

Amenity

The variety of surfaces is wide enough for the landscape architect to select a hard landscape style to suit the style of the development. By their nature, filter drains and permeable surfaces ensure an efficient use of space.

(July 2005)

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