Permeable Surfaces and Filter Drains

Filter drains and permeable surfaces have a volume of permeable material below ground to store surface water. Runoff flows to this storage area via a permeable surface. The advantage of permeable surfacing is that the water drains from the surface quickly and reduces the likelihood of ponding on the surface.

Examples of Permeable Surfaces:-

• Grass (if the area will not be trafficked)
• Reinforced grass (if trafficked)
• Gravel areas
• Solid paving blocks with gravel in the joints to allow water flow through the joints
• Porous paving blocks with a system of voids within the unit
• Porous recycled tyre surfacing (made from recycled tyres bound together to make a flexible surface)

The water passes through the surface to the permeable fill. This allows storage, treatment, and infiltration of water. Both the surface and the sub-base of a pavement must allow the passage of water for these methods to be feasible. Ground investigation is usually undertaken to determine the infiltration rate of the soil and hence the suitability for this type of SuDS. If the ground investigation concludes that it is not feasible then this type of system would ultimately have to formally discharge into a watercourse or sewer system.

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, a drain, 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.

The permeable fill or sub-base traps sediment, thereby cleaning up the surface water runoff. This type of SuDS also provide some treatment for other pollutants, such as oil and petrol.