Green Infrastructure

Pervious surfaces are a green infrastructure alternative that allows stormwater to infiltrate through the surface of the pavement to the ground below. There are two general categories of pervious pavement: a permeable paver system and pervious concrete/porous asphalt.

A permeable paver system is a type of stormwater best management practice (BMP) that is used in place of conventional asphalt or concrete pavement. 

Key features:

• Bricks, blocks, or a grid system
• Voids between the units are filled with infill material with a high infiltration rate, typically consisting of sand or fine gravel
• The permeable pavers are underlain by a bedding layer/filter layer consisting of the same sand or fine gravel infill material
• Structural/reservoir layer consists of a layer of washed gravel at least 12 inches thick where captured stormwater runoff is stored. The gravel used for the structural layer should be about 1.5 to 2.5 inches in diameter
• A filter fabric may be installed between the bedding layer and the structural layer. The filter fabric prevents the sand or fine gravel from the paver infill and bedding layer from migrating into the structural layer below
• An underdrain system is required where native soils are not suitable for infiltration.
• An overflow or bypass system may be included to divert large storm events or runoff captured in the underdrain system to a downstream BMP or to a stormwater drainage system
• Although not recommended, some permeable paver systems may receive runoff from pervious areas if those areas are stabilized. In these cases, appropriate pretreatment should be used to remove any sediment from runoff and prevent clogging. Pretreatment may consist of pea gravel or a grass filter strip

Key features:

• The pavement surface layer is permeable, which allows runoff to infiltrate into the pavement for temporary storage
• The surface layer for pervious concrete consists of a special mixture that results in void spaces which allow water to drain through the surface layer. Pervious concrete is typically 6-15 inches thick.
• The surface layer for porous asphalt consists of a mixture of asphalt with less sands or fines compared to conventional asphalt, providing larger void spaces for water to drain through. The porous asphalt layer is typically 4-8 inches thick.
• Both types incorporate a reservoir layer consisting of a 2-4 foot layer of washed uniform gravel below the surface layer. Water is temporarily stored in the reservoir layer before infiltrating into surrounding soils or an underdrain.
• An underdrain system is required where native soils are not suitable for infiltration. The underdrain consists of perforated pipes which collect runoff after it has filtered through the pavement surface layer and reservoir layer.
• Although not recommended, some permeable pavement may receive runoff from pervious areas if those areas are stabilized. In these cases, appropriate pretreatment should be used to remove any sediment from runoff and prevent clogging. Pretreatment may consist of pea gravel or a grass filter strip.
• An overflow or bypass system may be included to divert large storm events to a downstream BMP or to the storm sewer system.

Clogging by sediment is the leading cause of permeable pavement failure.

Without proper maintenance, pervious pavements will not provide the intended water quality and runoff reduction functions. Poorly maintained or failing pervious pavements can also pose a risk to public health and safety, particularly if lack of maintenance leads to flooding during storm events, mosquitos, odors, and undesirable aesthetics (e.g., stagnant water and sediment and trash accumulation).

Property owners are legally responsible for ensuring that the pervious pavements on their site are properly inspected and maintained.
Owners could face enforcement actions and incur liabilities if the system fails.

Work with professionals to properly perform basic inspection and maintenance tasks.