Permeable Pavement Explained: What It Is and Why It's Important

Permeable pavement has been a trend in landscaping for a while. Interlocking bricks and stones that provide a path without flooding a garden are beneficial for both design purposes and for lawn maintenance. Permeable pavement, however, has begun to see increased popularity in other industries as well. Roadways, sidewalk and parking lot construction is seeing more and more requests with permeable pavement. Read on to learn what permeable pavement is, why it’s becoming more popular and the biggest pros and cons with this trend. 

What Is Permeable Pavement?

Permeable pavement refers to using a porous type of concrete, asphalt, paving stones or a form of interlocking pavers such as bricks to create a permeable surface. Most of the time these surfaces include roads, parking lots, pedestrian paths or sidewalks. Many households introduce these surfaces with permeable driveway systems, patios or garden paths. Now, permeable is entering commercial and large-scale construction projects. 

What Does Permeable Pavement Do?

The purpose of permeable pavement is to allow water to seep through the material into the ground below instead of running off into set drain areas. This reduction of runoff is the main reason permeable pavement is used and has gained popularity.

What’s The Problem with Runoff?

Surface runoff – often just called runoff – refers to water that flows along the surface instead of seeping into the ground as it normally would. Runoff happens when it rains or snows. Runoff can also happen around farms and greenhouses after plants and crops are watered. 

Pollution and Runoff

When water flows down roads, sidewalks and across parking lots it can pick up a number of pollutants. Oil, gas, other fluids from cars as well as salt, pesticides or fertilizers are all picked up and carried with the water down to a storm drain or other drain area. Because there is only so much water on earth and the same water is simply cycled through and used constantly by humans, any type of pollutants can be dangerous. These pollutants can be carried off into lakes, streams, rivers and even oceans.

Groundwater and Runoff

Groundwater is water that is stored in underground reservoirs. The water that is found in these reservoirs are naturally cleaned by flowing through clay, sand, rocks and soil to then settle below the earth’s surface. Over 50% of the American population relies on groundwater for drinking water. 98% of all water used by humans comes from a groundwater source. But groundwater is not indefinite and it is running out. 

Groundwater is naturally replaced when water seeps into the ground and passes through the earth’s layers to reach a reservoir below. With cities growing larger and paved surfaces taking up more and more of the earth, this natural permeation isn’t happening like it used to. When water is carried to lakes, rivers or oceans through runoff, it doesn’t have the chance to refill our groundwater reservoirs. 

Since it can take anywhere from 10 to 10,000 years for water to settle as groundwater, the impact of runoff is significant. In fact, the reduction of groundwater resources is already being discovered and has begun to impact global populations.

Water Displacement and Runoff

Every spring storm or snow-melting season brings on headlines of floods. According to the U.S. Geological Survey many floods can be directly related to increased pavement and urban areas in US cities. In fact, it’s said that 40% of land within cities are not permeable. Forcing water to move where it shouldn’t and impeding the natural process of water to fall, seep and sink is impacting out cities in bigger ways than you may imagine. Runoff is helping to flood our cities. 

Flooding can cause water damage to roads, buildings, bridges and vehicles. It can be devastating to communities and commercial buildings.  The average cost of a homeowner to repair flood damage is $2,960 USD. While there are many other factors to flooding, reducing runoff will help to fight against floods in the future.

Increasing Popularity of Permeable Pavement

 For these reasons and many more, permeable pavement is becoming more and more popular in road, commercial and pedestrian construction projects. It’s no longer viewed as something specific to the landscape industry.

How To Make Permeable Pavement

Interlocking bricks and stones naturally form cracks and pathways for water to seep through. For pavement or asphalt, a larger type of aggregate is chosen to purposefully create air pockets within the material. These air pockets are what allow water to soak through and air to pass back and forth. 

One of the biggest differences between regular paving and this type is the subgrade layer used before laying the surface. Normally, a fine grain subgrade is used and then compressed with rollers creates a solid and strong foundation. Compression is actually part of what stops permeation. That’s why a larger grade of aggregate is used for the subgrade of permeable surfaces and there is no compression. However, that means that it can be less stable and strong compared to traditional surfaces. This is one of the arguments against permeable pavement.

Cons for Permeable Pavement

Less stability and strength means that permeable pavement is best suited for flat land. It does, however, make it popular for large flat paved surfaces such as parking lots. There is research being done to see how to tackle the engineering challenges associated with this type of pavement and to make it more flexible for other projects. 

Cost is another con. Permeable pavement can cost 20-50% more than traditional materials. It also requires a builder or contractor who understands how to work with it. 

Finally, permeable pavement simply isn’t best for certain areas. Soil that has a lot of clay won’t allow the natural permeation. It also isn’t good for areas that are prone to pollution such as gas stations. It’s a great solution, but it isn’t a blanket fix for every situation.

Pro’s for Permeable Pavement

Aside for helping with runoff and groundwater preservation, other pros for permeable pavement include cooler surfaces and quick snowmelt. By creating a pathway between the ground and the earth’s surface, heat and air can move more naturally. On hot days, this can actually help to keep any paved or finished surface cooler. This means that permeable pavement may actually help with the Urban Heat Island effect. It can also help to melt snow quicker and reduce flooding during the springtime.

A Permeable Future

Although permeable pavement isn’t applicable in every situation, the growing interest in it as a construction material and solution shows just how creative the industry is getting to help solve global problems. Whether it’s creating a permeable parking lot or educating homeowners in flood-prone areas it’s clear that permeable pavement will be seen in construction a lot more in the future.