Lexington, KY – Urban stormwater runoff is a serious challenge requiring our immediate attention. In undeveloped areas, rainfall soaks into the soil, replenishing the groundwater while entering streams slowly. Vegetation absorbs water and transpiration cools the air.
In urban areas, a lot of the rain never makes it into the soil. Our cities are covered in impervious surfaces. Our drainage and stormwater systems move runoff into our streams and rivers as quickly as possible, taking oil and grease, trash, fertilizers, herbicides, pesticides, and pet waste right along with it. What was clean water becomes untreated waste water.
And there’s more. Many streams are fed by groundwater, and impervious surfaces block rain water from contributing to the groundwater supply. This can result in lower stream flows during the dry season. A sudden rush of contaminated stormwater runoff causes more frequent flooding, stream banks to erode, and shallower channels due to sedimentation. This results in a loss of wildlife habitat -
as impervious surfaces increase, the number and diversity of aquatic life decreases.
An average single-family dwelling property has 2,500 square feet of impervious surfaces -
rooftops, sidewalks, driveways and patios. But that’s small potatoes compared to parking lots and roads: 60 to 70 percent of all impervious surfaces are related to the automobile.
Parking lots and roads are not going away any time soon. That’s a concrete fact. The National Ready Mix Concrete Association (NMRCA) would like to suggest a concrete solution. Pervious concrete, that is.
Pervious concrete (also called porous or permeable concrete) is made with water, cement and aggregate particles, with little or no sand. This mixture looks like a Rice Krispies treat. It has a 15 – 25 percent void structure, allowing 3 – 8 gallons of water per minute to pass through each square foot – much more than is generated in most rain events. It can be used for projects ranging from simple sidewalks, driveways and patios, to major pedestrian plazas and multi-acre parking lots. (Permeable pavers are also an option for patios and pedestrian areas. They can also be used for light vehicular traffic, but would require much more engineering for such an application.)
According to the NMCRA, pervious concrete pavement is “a unique and effective means to address important environmental issues and support green, sustainable growth. By capturing stormwater and allowing it to seep into the ground, porous concrete is instrumental in recharging groundwater, reducing stormwater runoff, and meeting U.S. Environmental Protection Agency (EPA) stormwater regulations.”
Trees flourish in the presence of pervious concrete, since the natural infiltration process allows both moisture and oxygen to reach the roots of the trees.
Pervious concrete improves the quality of the water that runs through it by capturing and aerobically degrading many of the pollutants that seep from parked cars. Remaining pollutants are then absorbed by the soil, where they are digested by plants, fungi or microbes.
Concrete has a higher albedo (reflectivity) than asphalt, so it lowers the urban heat island effect. Pervious concrete, with its lower density, also absorbs less solar heat and cools more rapidly.
However, porous concrete requires the work of a properly trained installer to be effective, according to Brett Ruffing, an education and technology specialist with the Kentucky Ready-Mixed Concrete Association (KRMCA) -
a Frankfort non-profit trade association representing the ready-mix concrete industry in Kentucky. “Pervious concrete is harder to manage than regular concrete,” he said. “The installer has only 15 minutes to roll the material then get it covered with plastic for proper curing.” The KRMCA website (www.krmca.org) maintains a list of graduates of their certified technician program.
Clogging in the void spaces will cause the installation to fail. Clogging problems are mainly an issue of design. If a natural area with grass or exposed soil is allowed to drain stormwater across a pervious concrete pavement, fine material can be introduced into the system causing localized clogging. Ruffing advises that pressure washing will restore most of the porosity of clogged pervious concrete to nearly new conditions. “Many longstanding installations remain in excellent condition with little to no maintenance,” he said.
Pervious parking lot surfaces make sense business-wise. They reduce the need for large detention ponds because the pavement acts as a detention area. Using pervious concrete allows increased utilization of commercial properties because the land ordinarily devoted to costly stormwater management practices can now be developed. And, after water passes through the pavement, it can be collected and used for other purposes such as flushing toilets and watering landscape plantings.
Reducing stormwater runoff requires a multi-faceted solution. Pervious concrete seems to have a place in answering this call to action. As the NRMCA says: Pervious concrete puts rainwater back in the ground where it belongs.