Imagine a huge, concrete mixing truck parked atop a black, asphalt-looking parking lot begins to unload a deluge of water, the stream hurtling towards the ground. But instead of pooling into puddles, the water vanishes. The truck keeps unloading water at a relentless pace, spilling over 1,000 gallons in a single minute, but the water never rests on the parking lot surface for longer than a few seconds before disappearing.
Sound like magic? Not when the parking lot is paved with a type of permeable concrete that can absorb 1,056 gallons of water every 60 seconds.
Permeable paving materials aren’t new — architects and urban designers have been using the material for more than a decade to pave pedestrian areas — but Topmix Permeable, a new paving material created by British building materials manufacturer Tarmac, is one of the first permeable pavements that has a practical application as concrete, meaning it could ostensibly hold more weight than early permeable pavements used for low-weight pedestrian environments. And since concrete is the second most-used product in the world, behind water, that opens up the potential for Topmix to reshape the way that cities pave their surfaces — and deal with stormwater.
As the world’s population continues to shift from rural to urban areas, natural drainage systems are being replaced with impermeable surfaces — mostly concrete — that hinder the environment’s ability to drain rainwater. In a forest, for instance, somewhere between 80 and 90 percent of rainwater is absorbed back into the ground — in urban areas, that absorption can fall to just 10 percent of rainwater. Humans have dealt with this by creating our own system of infrastructure — stormwater drainage systems and sewer systems — but much of this infrastructure is becoming increasingly outdated and unable to keep up with an increase in precipitation events linked to climate change.
“Permeable paving sources are extremely important. Otherwise, water gets concentrated into systems that were designed in ways that are becoming increasingly expensive,” Dana Buntrock, an architecture professor at the University of California, Berkeley, told ThinkProgress.
When extreme precipitation events overwhelm a city’s available infrastructure, flash floods become an increasingly damaging threat. In 2007, intense floods throughout the United Kingdom caused some $4.8 billion in damage — but only 12 percent of flooding incidents were related to an overflow from rivers. The rest were caused by an overflow of surface water and inadequate drainage.
“These days, storms are breaking records,” Buntrock said. “With the volume of water that’s coming out of the sky, [dealing with stormwater] is going to be even more critical.”
In the United States, many of the nation’s oldest sewage systems were built to combine sewage with stormwater, sending the noxious mix out into whatever body of water was nearest for draining. Beginning in the 20th century, engineers figured out that this wasn’t the best way to deal with sewage, and began installing treatment plants at the end of their combined lines, treating stormwater and sewage together. But many of the country’s oldest cities — concentrated along the East Coast, where heavy precipitation events have been increasing most noticeably over the past 50 years — are still saddled with a legacy of antiquated water systems that dump a mix of stormwater and sewage into bodies of water during extreme rainfall events.
Beginning in the 1990s, Buntrock explained, the EPA began requiring cities to split new systems, and began promoting ways to address old problems with existing systems. Some cities, like Washington, D.C. and Minneapolis, have separated their old combined lines. Other cities, like Chicago, have invested billions in new infrastructure meant to guard against heavy precipitation events. But as the United States’ water infrastructure reaches a critical point in its life cycle, and as climate change threatens to overwhelm existing stormwater drainage systems with increasingly extreme precipitation events, architects see something like Tarmac’s Topmix Permeable as an intriguing solution.
Traditional impermeable concrete, usually sand-based, only needs to absorb 300 millimeters of water an hour — just enough to safely handle a major storm event every 100 years, according to Tech Insider. But Topmix employs something called no-fines concrete — a concrete that, instead of “fine” material like sand, is made of tiny pieces of crushed granite.
When rainwater falls on Topmix, it drains through the porous concrete and a base layer of gravel, filtering out pollutants like motor oil in the process. Eventually, the rainwater percolates down into the ground, recharging natural aquifers. In times of intense rainfall, the process helps keep stormwater infrastructure from becoming overwhelmed by effectively acting as a reservoir — pulling water out of the surface and into the ground, where it takes a while to seep back into the environment naturally.
Depending on the area that is being paved, Topmix offers three different design options, ranging from an option that allows all water to permeate back into the soil to an option that includes an impermeable membrane that allows water to be captured and used for things like irrigation or flushing toilets. If the ground isn’t capable of absorbing large amounts of rainwater, that water can simply be directed elsewhere — whether to a recycling system or into existing infrastructure.
“You wouldn’t have to build a $3 billion tank if you were actually just trickling it into the ground at the source,” Buntrock said of the technology’s potential appeal. “Cities don’t have a lot of money for their infrastructure, and they don’t want to spend it on things they don’t have to.”
Buntrock said that she envisioned a time when permeable surfaces are as common place as low-flow toilets or low-wattage lightbulbs — a sustainable material made commonplace through regulations.
“Part of what I find fascinating is the way regulations are constantly moving us into innovating materials. Fire-resistant lumber comes about because we’re trying to reduce collective disasters,” she said. “At some point, governments that are really under pressure are going to say you can’t pave with non-permeable paving on this area, because the sewer systems are under stress.”
But don’t expect permeable concrete to become the default paving material just yet — so far, Topmix is only available in the United Kingdom, and the technology comes with its fair share of caveats. In the product literature, Tarmac warns against using Topmix to pave a heavily trafficked areas, suggesting it might be more appropriate for places like parking lots and driveways rather than highways. And although Tarmac claims the product has “excellent freeze-thaw” ability due to the amount of space between particles (allowing space for water to freeze), it’s unclear how the material would hold up in really cold climates.
It also wouldn’t offer much protection against sea level rise, which threatens to overrun coastal cities’ infrastructure with more frequent floods.
“That kind of technology is great for reducing rain-driven flooding and fluvial flooding (from rivers), which is a big issue. But it won’t make any difference to sea level rise,” Kristina Hill, an associate professor at the University of California, Berkeley, told ThinkProgress in an email. “Nearer to sea level, groundwater could actually come up as sea level rises, and there won’t be anywhere for the rainwater to go as it lands on that permeable pavement. The soil will already be saturated.”
Still, with climate change driving sea level rise and greater rain events, city planners, engineers, and architects are starting to think more about water as they plan for the future.
“In general, for a long time when we thought about environmental consequences in buildings, water wasn’t one of the big ones [that regulatory authorities] thought about — we thought about fire, we thought about human health and air quality,” Buntrock said. “Water is getting more attention now.”