Permeating Issues

I’ve taken up a fair amount of my column space in WaterShapes with discussions of the wise use of water, and for good reason: What could be more important to watershapers than knowing how to make the best possible use of the material that defines our profession? And what could be better than the fact that it’s possible to approach the subject in positive ways that bode well for the future?

The common thread in all of this coverage – whether it’s about conservation, constructed wetlands or rainwater harvesting – is that, ultimately, our aim must be to preserve the integrity of water, to cleanse it for return to the groundwater system and to use what we need and no more. Pursuing these goals is good for the planet, our businesses and, when all is said and done, for our overall quality of life as residents of this planet.

One major topic I’ve yet to mention has to do with one of the single greatest threats to our waterways, whether they be lakes, rivers, streams or oceans: The plain fact is that much of the surface water that now flows into them is loaded with pollutants.

With multiple generations of residential, commercial and industrial construction, we’ve steadily displaced soil, wetlands and woodlands and have replaced them with hard, impermeable surfaces in the form of roofs, parking lots, driveways, roadways and other surfaces onto which we drip oil, antifreeze, transmission fluid and other toxins that wash off into our storm sewers and flow into our waterways.

To compensate, we need to think in terms of rainwater harvesting and green roofs and other water-controlling techniques – one of which, these days, has to do with getting water to flow through pavement rather than over it.

PERMEABLE SURFACING

Some of you are doubtless familiar with the array of permeable pavements now available in the marketplace, but my experience in the field tells me that many of us in the trades are not yet up to speed with this exciting surfacing technology. As I see it, this is something we should all get familiar with right away: It’s the sort of concept we should be mentioning in every conversation we have with clients; should be pushing in every meeting with local officials; and should include in every proposal we advance to general contractors and engineers.

They function just as advertised: Instead of forming solid surfaces, permeable pavements allow water to pass through them and either percolate into the soil or flow into subgrade drainage systems for controlled release into waterways.

The past few years have seen lots of breakthroughs in this arena, and it’s even reached the point where formerly “solid surface” products including asphalt and concrete can now be made to allow water to pass through instead of running off. At this point, these materials are typically used for low-speed traffic areas including parking areas and driveways and they’re even installed in familiar ways – the exception being that the base material contains little (if any) stone dust mixed in with the crushed stone or crusher run.

According to one supplier, permeable concrete with 20 percent voids can store up to an inch of rainfall within its structure, and the further claim is that, with a proper base of six to twelve inches of dust-free crushed stone, the capacity is significantly higher and helps to alleviate any issues with water retention in the concrete and the frost damage that might result in winter.

Beyond these direct substitutions for solid surfacing materials, there are also the permeable pavers that have become familiar in most markets. Most of us, in fact, are sometimes painfully familiar with the prolific number of concrete paver systems that have been offered for about 30 years now: Interestingly, these systems started out by offering a less-expensive method for installing pavements with the added advantage that the pavers could be pulled up and replaced fairly seamlessly if subsurface work needed to be done.

The original edge of these concrete pavers was their interlockability: Indeed, early on many were shaped to resemble I-beams or keyholes to speed installation and make any single unit in a finished area unmovable without moving an entire section. As such, this was an alternative to the herringbone patterns that brought four-way locking to bricks and conventional concrete pavers.

Until recently, these systems were also sold on the basis that, as individual units, these pavers formed flexible surfaces that could withstand both vertical and lateral forces without failing because individual units could actually “roll” with applied weight changes. As a result, they became a popular choice for roadways and public access ways in any situation where there were below-grade utilities to which access might be needed.

Where poured materials such as concrete and asphalt will always betray where a repair has been done, dry-laid concrete pavers can be lifted up, stacked out of the way and reset after repairs are made with no apparent surface changes.

ANOTHER LEVEL

Through the years, these paver systems have become much more sophisticated, with lots of attention being paid to aesthetics. As they gained popularity and marketability, more and more design options were introduced, including various shapes, tumbled products and even styles that approximated the look of real stone.

To be sure, many of these products pose aesthetic challenges and will never find use in high-end projects, but through the years, plenty of pavers have come along that do a fair job of resembling real brick and stone and have proved themselves worthy in a broad range of applications. And they do hold up well to vehicular traffic, making their use as driveways and parking areas both viable and reliable.

The key with these projects when it comes to permeability is the way they are installed: Many concrete pavers are designed with small spacer bars or nubs on their sides that create gaps of approximately an eighth of an inch between pavers. Thus, after the pavers are set into place, the tamping process drives bedding sand back up in between the pavers to lock them in place.

This small gap is these pavers’ route to permeability: Rather than all of the rainwater reaching these surfaces running off, a good portion of the water instead flows through the gaps into the ground. We still pitch any paving to allow surface water to run off, regardless of permeability, but the fact is that this is a step in the right direction relative to solid surfacing.

Seeing an opportunity in the desire for increased permeability, manufacturers have recently begun producing pavers that are designed and engineered for installation with significantly larger gaps to be filled with coarse sand or finely crushed stone (but still no stone dust, which tends to cake and become impermeable). The idea is to capture that much more water within the system before it can run off the surface.

Again, to enjoy the full benefits of this increased permeability, the base must include a bedding course (typically a crushed stone with no dust or fines) set atop a permeable base layer. This way, almost all of the water that penetrates the surface flows down through the base and either flows into a catchment system (for example, a French drain set below the base layer or a large-scale subterranean storage area) or percolates into the ground itself.

To be sure, some pollutants might pass through the gaps in these systems, but much of the load would be captured in the bedding and base systems and would be significantly diminished as it moved through the system. Even in simple relative terms, this is far better than running all of the surface water to a catch basin or storm sewer for direct introduction to our waterways, unchecked and completely unfiltered.

GAINING TRACTION

The advantages of these permeable paver systems are so apparent that they’re being embraced by more and more municipalities and organizations. In one example, the City of Chicago has installed permeable pavers in miles of alleyways; before, these alleys would flood during storms, but now they are completely dry soon after the rain stops falling.

That sort of experience is being backed up by research. Indeed, a study done by the Toronto & Region Conservation Authority (TRCA) in 2005-2006 found that a permeable-paver test plot showed a runoff of less than 10 percent of the water from a two-and-three-quarter-inch storm of five hours’ duration. By comparison, an asphalt test plot experienced runoff approaching 100 percent under the same conditions.

TRCA also conducted a water-quality analysis to test for retention of heavy metals, oils, grease and other pollutants. Many of these tests have shown lower concentrations of pollutants including zinc and lead compared to the asphalt test area, and additional studies are aimed at quantifying these advantages. Suffice it for now to say that stopping the water before it runs unchecked into fragile waterways offers a better (if not the best) solution.

To be sure, I’m still a major proponent of using raingardens, constructed wetlands and bio-rention swales to manage the runoff from parking lots, but I recognize that not all sites are suitable for such systems, whether it’s due to pre-existing conditions or space constrictions. In these cases, the use of permeable pavements is a viable means of minimizing the need for catch basins that empty directly to storm sewers.

This is also an approach that can be used with hardscape playgrounds, large driveways, patios and even substantial pathways. Indeed, while I recognize that permeable surfaces are not the only solution to runoff issues, I see that in the right context and the right setting, they represent another option in a growing spectrum of ways we can make the best and wisest use of our most precious natural resource.

Bruce Zaretsky is president of Zaretsky and Associates, a landscape design/construction/consultation company in Rochester, N.Y. Nationally recognized for creative and inspiring residential landscapes, he also works with healthcare facilities, nursing homes and local municipalities in conceiving and installing healing and meditation gardens. You can reach him at [email protected].