Concrete and asphalt may be necessary for parking lots, courtyards, and sidewalks at your property, but large expanses won’t help your efforts to manage stormwater. These traditional materials are impervious, meaning they resist moisture, so rainwater has nowhere to go but into the sewers. As stormwater fees continue to climb, facility managers are turning to permeable options that allow water to infiltrate into the ground. Learn how these eco-friendly pavement choices can put rainwater to better use.
From Drizzles to Downpours
Often lumped together, porous pavement and permeable pavers are two methods that help rainwater reach the ground. Pervious materials have voids in them that allow moisture to run through, like pouring liquid through a fine mesh sieve. Permeable options use spacing between tiles or pavers to direct water around the surface. The purpose of either choice is to ensure stormwater can penetrate the ground rather than flow to a municipal drain, retention pond, or public waterway.
Ground infiltration is a passive way to remove contaminants from runoff. Think of the pollutants that dirty your pavement – fluids from cars, spilled food, bird droppings, plant debris, salt, sand, trash, and soil erosion. These particulates pose a burden on city treatment plants or can make their way directly into nearby lakes and rivers. Sandy and clay soils can capture these elements, but only if water has a way to percolate through to them.
Public Library Uses Permeable Pavement for Flood Mitigation
Replacing an older building that was destroyed by a flood, OPN Architects wanted the new municipal facility to set a good example of how to manage rainwater. The downtown library is located only a few blocks away from the Cedar River, a watershed that is sensitive to runoff.
“The project wanted to do what it could to limit the amount of rainwater flowing into the city storm system by retaining as much on-site as possible. The more we can limit peak stormwater flow, the less infrastructure costs the city will incur and peak flood crests in the future will be lower,” explains Matthew Stewart, an architect and project manager for OPN. “This prompted the use of pervious pavement, underground detention basins, irrigation storage tanks, and a green roof.”
This systems approach to stormwater management starts with the vegetated roof, which collects rainwater in cisterns for irrigation, says Stacey Hanley, a designer with OPN. Beneath the parking lot are underground stormwater detention chambers with a 5,400-cubic-foot capacity. Overflow from the roof is directed to these chambers via a drain.
Pervious pavement is used over parking stalls while conventional pavement is used for the driving area. A structural bed of crushed rock below “is tied into a granular backfill that surrounds the stormwater collection chambers. Runoff stored in the chambers and adjacent rock is able to infiltrate into the ground during small storms through a separate header row of vertical chambers,” Stewart explains.
A plaza on the opposite side of the building is constructed with pervious pavers, notes Hanley. Subdrains are connected to the tree pits at the site’s edge, ensuring rainwater can reach their root system for optimal health. The total site is 43% pervious between the landscaped areas, green roof, and paving.
Collectively, these stormwater strategies are designed to retain 90% of normal annual rainfall and 100% of all storm events up to 1 inch in a 24-hour period. They also helped the library earned LEED Platinum, with points under SS Credit 6.1, Stormwater Design – Quantity Control and SS Credit 6.2 – Quality Control.
There’s also a possibility that porous pavement can reduce the heat island effect. The theory holds that because the pavement can breathe and moisture isn’t trapped, it will hold in less heat. It’s like the difference between wearing a loose cotton and a heavy polyester shirt.
According to the “Cool Pavements” chapter of the EPA report Reducing Urban Heat Islands: Compendium of Strategies, “wet, [permeable] pavements can lower temperatures through evaporative cooling. The water passes through the voids and into the soil or supporting materials below. Moisture within the pavement structure evaporates as the surface heats, thus drawing heat out of the pavement, similar to evaporative cooling from vegetated land cover. Some permeable pavement systems contain grass or low-lying vegetation, which can stay particularly cool because the surface temperature of well-hydrated vegetation is typically lower than the ambient air temperature.”
More research is needed, however, to understand the effects of these pavements when dry, the guide notes. It’s possible they can limit heat transfer by increasing convection over the surface or releasing heat in the evening. Without adequate documentation, facility managers should address the heat island effect through conventional strategies such as reducing overall pavement areas or installing reflective roofing and use permeable materials for stormwater management.
A Roadway to Success
Permeable pavement is installed on a different base than asphalt or concrete. Rather than gravel, clean crushed stone is used and must be 4-12 inches deep depending on the application and required strength, notes Dave Ouwinga, president of the manufacturer Porous Pave.
The good news is that you won’t typically replace an entire parking lot with a permeable surface. It can be best for areas with only light traffic or in strips, Ouwinga explains.
As there can be a cost premium for these paving alternatives, savings are more readily achieved with new construction because they replace more conventional stormwater management features such as retention ponds and bioswales.
Porous materials can be used in any climate type. They may also be able to withstand freeze/thaw cycles better than conventional surfaces, says Ouwinga. Properties that experience snowfall should stay away from sanding as fine grains can clog the pavement, he adds. Regular snow blowing and salting won’t cause damage.
Depending on the manufacturer’s recommendations, some surfacing products “require regular cleaning with a vacuum sweeping machine to keep the porous nature of the paving functional,” notes Stacey Hanley, a designer with OPN Architects. Some of the void space will inevitably be filled over time, but even a reduced capacity will manage stormwater far better than conventional pavement, she adds.
Beyond reducing stormwater fees, permeable pavement can benefit facilities working toward green building certification, says Ouwinga. Both LEED and Green Globes offer credits for this strategy. Porous surfaces can also help a project achieve net-zero water under the Living Building Challenge, which requires that all stormwater is managed on-site.
Because this green feature may go unnoticed, call attention to it with interpretive signage, Hanley recommends. Not only can you market your building’s sustainability, but you can help teach occupants and guests about the benefits of managing stormwater. Redirect your budget and runoff with permeable pavement rather than idly watching stormwater fees go down the drain.
Jennie Morton [email protected] is senior editor of BUILDINGS.