What Is Permeable Pavement and How Can It Prevent Flood Damage?

If your house has a patio or a large driveway with a parking area, watch what happens when it rains. Then you’ll understand why paving paradise and putting up parking lots has a destructive effect on the environment.

Instead of water soaking into the ground and nourishing the soil, it streams away. You can’t always see where it goes, but it often ends up swelling waterways and collecting in low-lying areas.

Now imagine this happening on a massive scale in American cities, and ask yourself what happens to all that water. That’s right: It causes major erosion and flooding problems in rainy places around the Gulf of Mexico, along the Atlantic seaboard and elsewhere.

I’m a big fan of pavers for patios and walkways partly because I think they’re more attractive, but there’s another reason. They allow water to soak into the ground underneath, reducing the likelihood of eroded borders and mosquito-breeding ponds after heavy rain.

Permeable pavement works like pavers. But it needs to be more sophisticated to solve a major urban problem.

What Is Permeable Pavement?

In short, pavement is permeable if it absorbs water and lets it pass through to the ground underneath.

Unlike compacted earth or turf, pavement doesn’t turn muddy during heavy rain, so it provides more solid footing than sand or gravel. To make it an effective water reservoir, the pavement needs two layers: a porous surface and a thick bedding of crushed rock.

The surface layer is the visible part, but what’s underneath makes permeable pavement work. The underlayment, 12 inches thick or more, absorbs and holds water that passes through the surface layer. The underlayment is crucial. So if you’re planning to install permeable pavement on a patio or driveway, you need to excavate to this depth.

How Does Permeable Pavement Work?

When rain falls on permeable pavement and soaks through the surface covering, the bed of crushed rock absorbs and holds it so it can’t run off underground and form soggy depressions nearby. Similar to a dry well, the bed acts like a reservoir. It holds the water so it can slowly seep into the soil and replenish the groundwater.

Permeable pavement absorbs water surprisingly well, as this video demonstrates. Because water can’t collect on the surface, you never have to worry about puddles and standing water attracting mosquitoes, and ice rarely forms in the winter.

Types of Permeable Pavement

All permanent pavement tends to feature a 12-inch or thicker underlayer of 1/2- or 3/4-in. crushed rock. The composition of the surface layer varies, with the following the most common.

Pervious concrete

Concrete generally consists of sand, stone aggregate, Portland cement and water. Eliminate the sand to allow water to seep between the rocks, and you have pervious concrete. It isn’t as strong as regular concrete.

Porous asphalt

The top layer of a porous asphalt permeable surface consists of two courses. The top one is asphalt mixed with little to no sand or dust, containing about 16% voids. Underneath that you’ll find a two-inch filter layer of 1/2-in. crushed stone aggregate. The filter layer stabilizes the asphalt layer.

Permeable interlocking concrete pavers (PICP)

Interlocking pavers look like regular pavers from the surface, only with tabs on the side that maintain a uniform space between them during installation (typically 10-mm or 0.4-inches).

In addition, several plastic paving systems are available. They consist of interlocking plastic grids that lay on a flat surface and fill with sand or gravel. These are best used for walking surfaces, but some are suitable for limited vehicular traffic.

Benefits of Permeable Pavement

As an ecological alternative to the conventional pavement for parking lots, driveways, patios and walkways, permeable pavement has a number of upsides:

• Helps prevent flooding: As long as the falling rain doesn’t overwhelm the underlying reservoir’s capacity, water soaks into permeable pavement rather than running off and swelling local waterways.
• Prevents ice formation: Because water doesn’t stand on permeable pavement, it can’t freeze. That reduces the need to spread salt, which has a negative environmental impact.
• Controls mosquitoes: Standing water is a favorite breeding ground for mosquitoes. They won’t find any on permeable pavement.
• Nourishes the water table: Instead of running off, water gets filtered as it seeps through the reservoir, replenishing the water table.
• Reduces the heat island effect: Conventional pavement absorbs heat and radiates it back into the atmosphere, making hot days even hotter in urban areas. Permeable pavement allows some of that heat to radiate into the soil, which has a cooling effect on the atmosphere.

Disadvantages of Permeable Pavement

Permeable pavement isn’t a perfect solution in every situation. Here are some of its downsides:

• Not as strong as regular pavement: Pervious concrete and porous asphalt can crumble under heavy vehicular traffic. This makes them unsuitable for busy thoroughfares, highways and airport runways.
• Expensive to install: Installation involves excavating to a depth of 12 inches or more and filling it in with rocks.
• Requires maintenance: Like any drainage system, a permeable pavement system can clog. Sand and fine particles must be removed periodically by vacuuming or power washing.
• Only works on level ground: If you install permeable pavement on a slope, water will run down it and collect at the bottom.

How Does Permeable Pavement Prevent Flood Damage?

Statistics suggest pavement occupies as much as 40% of the land surface in some urban areas. When it rains, all the water falling on the pavement runs off into storm drains and sewers, introducing pollutants into treatment plants and slowly contaminating the water supply. Excessive runoff also creates erosion problems and flood damage.

Because permeable pavement absorbs the water, runoff and pollution become less of a problem.