Areas with traffic

Infiltrating water in areas with truck, car or pedestrian traffic provides unique challenges.

Permeable surfaces help to sustainably manage polluted stormwater by filtering, slowing, and infiltrating stormwater as it flows across the landscape especially in areas of high traffic.

Permeable pavements have an upper “wearing layer” that is rigid enough to support vehicles and permeable enough to allow water to pass through pores in the layer. This upper layer is commonly made with grids, pavers, pervious concrete, or porous asphalt. Below the wearing layer are base layers of different sizes of aggregate with pore spaces where the water infiltrating from the surface is temporarily stored. Over time, the water in the storage layers slowly infiltrates into the native soils.

layers in permeable pavements
Layers used in permeable pavements.

Here you will learn the basics about permeable paving options. Click an item below to jump to a topic, or scroll down to view them all.

Refer to the WSU Fact Sheet: Options for Managing Surface Water Drainage (PDF) to learn more.

Permeable pavers 

Like regular interlocking pavers, permeable pavers can be attractive in a home or commercial setting. However, unlike interlocking pavers, permeable paving blocks are spaced to allow water to flow between them into the subsurface.

There are two types of permeable paver systems. The first type uses impermeable blocks that are spaced apart, with unsealed joints filled with specific grades of gravel aggregate so that water can flow between them into the subsurface storage layer. The second type uses permeable blocks that allow water to flow directly through them. In both systems, specific sizes of gravel aggregates or sand are typically used between blocks, and the subsurface is underlain with larger stones to facilitate drainage. There may also be engineered storage capacity or piping depending on the native soils and volume of water received. It’s also important to consider the type and frequency of traffic the area will receive. Pavers are typically designed only to accommodate light traffic conditions – not heavy trucks.

2 types of pavers
Two types of pavers, inset paver is permeable. Credit: C.Thompson

Gravel grids

Gravel grids work with an engineered form of rigid plastic or concrete that has open cells filled with gravel. The form is placed below grade, and then the required size of gravel is inserted in the openings. If heavier traffic is anticipated, an additional subsurface of gravel may be needed depending on the underlying soils. Other grid systems rely on the open cells being filled with soil and planted with grass if traffic is light. Vehicles can drive or park on the grid, and water flows through.

gravel grid
Credit: C. Thompson

Permeable pavement: Pervious concrete and porous asphalt

Permeable paving options are usually found in urban and suburban areas, but rural areas can benefit too. Like all the methods discussed above, proper design and construction are essential for function and long-term maintenance; including ensuring that the site’s underlying soils and subsurface water levels are appropriate for the type of installation.

Traditional asphalt and concrete mixes contain a wide range of particle sizes that interlock and create an impermeable surface. Pervious concrete and porous asphalt mixes, however, use only larger particles, resulting in the creation of pores (or voids) in the pavement. These pores allow water to infiltrate through them, from the surface to the underlying layers. Porous asphalt void space ranges from 16 – 25 percent, and the asphalt has some flexibility to it. Pervious concrete void space typically ranges from 15 – 35 percent and is considered a rigid pavement. These pavements offer other benefits, including avoiding the need for seal coats and allowing water and snowmelt to penetrate, avoiding wear due to the freeze-thaw cycle.

Newly installed permeable pavements may have very high infiltration rates initially, but the infiltration rates often decline somewhat over time. However, as long as the system receives regular maintenance (at least annually) to keep the pore spaces from clogging with sediment and debris, small decreases in infiltration are unlikely to affect overall performance. Watching where the water goes during heavy rain events is an excellent way to ensure permeable pavements are functioning correctly. For example, noticing a significant amount of water flowing across the top of the pavement instead of soaking in indicates that the pavement is likely clogged with sediment.

Pervious concrete (left) and porous asphalt (right). Credit: C. Thompson

Comparing permeable paving options

FeaturesPermeable PaversGravel GridPorous AsphaltPervious Concrete
Appropriate forResidential and light commercial use, trails, pedestrian paths, drivewaysTrails, paths, access roads, driveways, patios, residential use.Parking lots and streets with low-weight axle loads, and light traffic volumes.Heavier traffic loads. Also, commonly used for sidewalks and parking lots.
Construction (min. layer requirements)Pavers on top of an aggregate base that is installed above infiltrating soils.Concrete or rigid plastic grids filled with topsoil/grass or gravel over an aggregate base that is installed over infiltrating soils.4-inch minimum porous asphalt wearing layer on top of 6-inch sand layer, with an aggregate base that is installed over infiltrating soils.6-inch minimum pervious concrete wearing layer on top of a 6-inch sand layer, with an aggregate base that is installed over infiltrating soils.
Maintenance* Sweep or vacuum when surface and debris are dry, 1-2 times annually.
* Occasional pressure washing, as prescribed by manufacturer.
* Replace joint aggregate as necessary.
* May need to replace or replenish gravel if clogged or depleted.* Prevent sediment from collecting on pavement.
* Clean surface twice a year with a combination of suction and sweeping.
* Prevent sediment from collecting on pavement.
* Clean surface annually with a combination of suction and sweeping.

Refer to the WSU Fact Sheet: Options for Managing Surface Water Drainage (PDF) for more details and to compare the systems.