When It Rains, It Drains

westmont1By Matt Baker

Our infrastructure relies on asphalt and concrete. Few materials can hold up as long as they do and none as cheaply. They are so durable in part because of their impregnability, a strength that unfortunately creates a problem.

These impervious surfaces direct rain into the sewer system; during heavy storm events, the sewers are often overburdened and the contents—now stormwater mixed with effluent—often discharge back into streams, rivers and lakes. This poor handling of stormwater also exacerbates flooding and soil erosion.

So how do you fix the flaws of an impenetrable material? Poke some holes in it.

Though mostly used in driveways and parking lots, pervious concrete has been on the market for several years. But suburban Westmont is taking the concept further, as they install the first pervious concrete roadway in the state.

A quarter mile of Cumnor Road, between 59th and 61st Streets, is currently under reconstruction. It’s a quiet residential street, inherited from an annexation in the early 2000s. The road’s condition is substandard; it varies in width, shows signs of base failure and the surface crumbles by hand. “It’s very antiquated, said Steve Nero. “Some folks even called it a driveway. It was that narrow.”

Nero is a Trustee with the Village of Westmont, and chairs a stormwater committee. “It was decided by this committee and [the Village] Board to really go forward with a green infrastructure within the village, and at the same time use conventional methods,” Nero said. “Everybody was really excited about the opportunity to not only solve stormwater issues but to really make a unique experience for the residents and people coming to town.”

Ozinga is providing the pervious concrete and Addison-based R.W. Dunteman will install it. “Originally, the Cumnor Road reconstruction was designed to be a conventional asphalt pavement,” said Noriel Noriega, Public Works Supervisor with the Village of Westmont. “Midway through design, the Village Board learned of this pervious concrete process.”

westmont2Pervious concrete looks and performs differently from its traditional brethren. A cross-section of cured pervious concrete looks similar to a Rice Krispie treat. A smaller aggregate, little to no sand in the mix and a specialized cement recipe all combine to create a concrete with voids that allow water to filter through unimpeded. Those voids can make up to 20% of the concrete’s volume, and allow it to filter water between two and 18 gallons per minute per square foot, according to the National Ready Mix Concrete Association.

In addition to stormwater handling, the negative space in pervious concrete offers another benefit: decontamination. Roads collect a number of pollutants from cars, such as oil and grease. Rain carries these toxins off of standard pavements, and they ultimately are deposited in rivers, creeks, lakes or in the groundwater. In pervious applications, these hydrocarbons get trapped in the voids as the water sluices through or evaporates. Once trapped there, microorganisms can biodegrade these pollutants, limiting their impact on waterways and aquifers.

There are also differences between pervious and traditional concrete when it comes to installation and maintenance. The base, for example, makes use of a larger stone to create more voids and aid water in drawing away from the surface. The soil condition must be closely examined prior to installation. In the presence of heavy clay, an installer may have to put in a drain tile to move excess water to an adjacent site.

And it’s not just what lies beneath the pavement, but also nearby. The design should try to eliminate or minimize runoff from adjacent surfaces that contain large amounts of sediment. Those pores that allow stormwater to drain through could clog up if erosion from a nearby hill, berm or other feature makes its way to the concrete surface.

“Instead of moving water to a drain and forgetting about it, pervious pavements infiltrate water into the base and eventually into the ground and/or sewer,” said Brian Lutey, Vice President of Sustainability and Compliance at Ozinga “The pervious concrete—we call ours Filtercrete—is a filter, and all filters need to be cleaned or replaced.”

Snow removal should be managed by plows equipped with either rubber blades or brooms, not metallic blades. Lutey also recommends against using deicing chemicals in the winter. Permeable concrete also requires routine cleaning to vacuum out any sediment that has made its way into the pores.

Permeable concrete does come with a cost. When the Cumnor Road project was first drawn up, the budget was $1.2 million. This is a complete reconstruction, so that included total removal of the current road, storm sewer replacement and repaving with asphalt. After the Village Board decided to use permeable concrete, the lowest bid came in at $1.5 million, a 20% increase over the original budget.

“That’s the initial shock to the person who is not educated on the issue,” said Nero. “But we’re not just redoing a road here. We’re solving another issue at the same time. That’s the idea. You’re getting the new street, you’re getting new infrastructure, and at the same time you’re improving stormwater conditions.”

Westmont recently passed a .5% sales tax to fund stormwater by referendum. The village only started collecting on it in July so none of that money is available for this project, which is funded by infrastructure improvement bonds that the village obtained two years ago. Westmont officials plan to resurface half of the village streets over a three year period, albeit mostly using traditional materials.

Right now, this is the only pervious concrete project on the books for construction in Westmont, though the village is considering pervious asphalt for a stretch of Wilmette Avenue due to be resurfaced next year. Westmont officials plan to keep an eye on Cumnor Road to see how the pervious concrete performs. “We will definitely monitor it and see if this is a product that the village would like to pursue at other locations,” Noriega said.

The use of permeable material in a public street is a bold move by a village stricken with occasional flooding. The stormwater committee that Nero chairs was formed in 2013, after significant flooding affected not only Westmont, but dozens of Illinois counties. Over a 24-hour period between April 17 and 18 of that year, over six inches of rain fell. In most years, that would be a heavy rain total for the entire month.

While that was a significantly pernicious storm event, triggering a state of emergency declaration in the village, flooding is an ever-present concern for the suburb’s residents and business owners. “In some areas [of Westmont], it’s terrible. In some areas they don’t experience any issues. That’s why, personally, I think the referendum passed. Even though it didn’t flood in their house, they saw the village-wide need of solving these issues,” Nero said. “Now they gave us the tools to go to work and solve these problems.”

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