Permeable Surfaces

Smart solutions for green infrastructure

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Moving Traffic While Slowing Water
The vast expanses of impermeable asphalt surfaces blanketing the country and crisscrossing every urban center and thoroughfare from the east to west add up to a total of 4.12 million miles—enough to circle the Earth’s equator 157 times over. While it might be easy to take impermeable pavement for granted as the historical norm for load-bearing surfaces from roadways to parking lots and storage depots, impervious pavement has recent origins and we are only recently becoming aware of its full costs.

Speed and efficiency were the watchwords in engineering as far back as Roman times, whether talking about moving vehicles, commerce, or water. However, allowing stormwater from roadways and other impermeable surfaces to sluice uncontrolled into receiving waterways has more recently been recognized as a source of pollution and an impediment to well-functioning ecological systems. Stormwater runoff can introduce contaminants such as heavy metals, petroleum byproducts from leaky or malfunctioning vehicles, combustion byproducts, nitrates, solvents, organic waste, and litter directly into storm sewer systems, lakes, and rivers. Regulations such as the National Pollutant Discharge Eliminating System permitting process and rules requiring property owners to maintain predevelopment hydrology for new construction have emerged as we become aware that the very same civil engineering practices that have given rise to vibrant cities and efficient transport can also cause damage. In many areas of life, people are seeing the value of slowing things down; the same goes for water, even keeping it from going anywhere at all and instead allowing it to percolate into the soil as close as possible to where the rain falls.

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Removing impervious surfaces is one solution to slowing down the water to allow natural infiltration to run its course. However, for surfaces that will be required to bear traffic, simply leaving them unpaved is not viable. Installing alternative pavers that allow water to pass through rather than passing over presents one of the most practical solutions to the dilemma of keeping traffic moving while slowing down the water.

Making Hay
When developers began planning a complex of new home sites on a sprawling meadow in the Shenandoah Valley near Winchester, VA, stormwater regulations emphasized managing the quantity of water rather than quality. Tim Painter of engineering firm Painter-Lewis PLC remembers consulting in 2002 with the owners as they considered various options for developing the prime acreage adjacent to Route 11, one of the Shenandoah Valley’s major arterial highways. They entertained development prospects ranging from an apartment complex to a retirement community. “We did a floodplain analysis of Hiatt Run, which flows to the south of this project. We had to stay out of the floodplain and obviously the floodway, and we had a riparian buffer that we had to stay away from, so the design was geared around all of those things,” recalls Painter.

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The county rejected the first apartment plan. “They didn’t want that kind of high-density development at the time,” says Painter. Then the concept of a retirement community on the site got stuck at the drawing board stage because of declining economic conditions.

The property itself went through a couple of changes in ownership during the economic downturn, while farmers literally made hay on the sun-drenched fields. After more than a decade with the property in limbo, a new builder obtained development rights and in a brighter economy received the go-ahead from county planning for 120 units of luxury housing on the site with a project named for Hiatt Run. In the interim, stormwater quality had edged its way into the spotlight as a critical regulatory priority.

“Quality became very critical in the design, when at first pretty much we had to take into account stormwater quantity. Of course, we had to protect Hiatt Run, but as we got into this new phase of development, stormwater quality became part of the design criteria,” says Painter.

“On the northern side of the property we used some open-air detention to handle some of the water that came off the Virginia Department of Transportation right of way and from some of the adjacent properties. We also developed two small constructed wetlands next to the Hiatt Run streambed to protect that area,” he says. But the bulk of the stormwater management system is being provided by Aqua-Bric Type 4 “L” permeable pavers, part of the Eagle Bay SWM PAVE Stormwater System.

Aqua-Bric Type 4 “L” is an L-shaped permeable paver whose shape helps eliminate faulting and rotation. It is practically transparent to rainwater with the ability to capture from its surface 450 inches of rain per hour, many times more than the most torrential downpour can produce, far exceeding the 1/2-inch-per-hour absorption capacity of well-drained soils. The pavers have a joint void system that “carries forward the structural and mechanical connection of the block while also providing a void for maintenance of the system,” says Bob Bridges, director of sales with SWM PAVE. “All stormwater BMPs, whether proprietary or public, require maintenance,” he notes, and SWM PAVE’s Aqua-Bric component is designed with the full product life cycle, including maintenance requirements, in mind.

According to Painter, Scott ­Rosenfeld, the developer of Hiatt Run, got a call from Bridges about the possibility of using the pavers to promote infiltration as a means of meeting the region’s “predevelopment condition” storm­water mandate.

Credit: SWM PAVE
The Aqua-Bric L-shaped paver

A Regulation Connection
Bridges says his job starts long before the first pavers are ordered. While he believes that all of the various states’ regulations focus on the same worthy goal—”to have some improvement in water quality”—he says the rules are not always consistent nationwide. For instance, even at the most fundamental level, “There are different approved pollutant removal rates state by state.” He adds, “At SWM PAVE, we get to know the regulatory community. Once we know the regulations, we get in touch with the engineering community and provide engineering support in house. Ninety percent of the time the engineers are specifying the SWM PAVE system.”

Painter admits that he was not immediately sold on the idea of pavers for the site and set out to do some homework of his own. “I researched it pretty hard to ensure we could do what we wanted to do using the system,” he says. “I’m always hesitant about using pavers because obviously we’re in a snow area and in the winter there’s a lot of potential for plowing and maintenance. I’m always concerned about the stones being picked up with the blade.” However, Painter says that in researching the system he found “the interlocking pattern of the system allows the plow blades to go right over top of it.” He got in touch with people who had used the system, who confirmed its resilience in the face of winter storms. “There is one other project locally that had used the system before, and it seems to be holding up pretty well,” he says. “We were able to use the SWM PAVE system for an infiltration system in all the paved areas, and we were able to satisfy stormwater quantity and quality by using that system.”

At the same time, the system saved the developer a significant amount of money on the 127,000-square-foot pavement installation project. “We are very competitive with traditional wearing surfaces such as asphalt and concrete,” says Bridges. “SWM PAVE systems can be installed mechanically, from 5,000 to 10,000 square feet per day. It fully satisfies stormwater requirements but also serves as the wearing surface.” By addressing the two priorities of paving and water quality with a single product, SWM PAVE saved the developer more than $100,000, which amounted to about 1% of the entire development’s cost.

Scott Rosenfeld, the developer of the project, notes that the system has some advantages over ordinary pavement. “It installs so fast, as fast as asphalt, and you don’t have to reseal it every couple of years. We get great savings in stormwater management, and it looks good, sounds good, and feels good.”

He adds, “Most people who are in the planning field, they’re altruistic people; they love a better way of doing it for the environment and a better product. It’s quality, so the acceptance has been great.”

A Diamond in the Rough
If there actually were a paved asphalt drive that circled the earth 157 times, a nice place to stop off would certainly be Hawaii. Joel Kurokawa says one of the nicest places to be in Hawaii is Diamond Head Monument, right next to the campus of University of Hawaii Community College. In 1968, Diamond Head was declared a National Natural Landmark, and Kurokawa says the residents of the surrounding Honolulu community are quite fond of the dormant volcanic crater. “It’s a significant landmark, which has been designated its own special district to protect the natural environment.”

Criteria for development in the district are strict. For example, he says, “Every time you move a tree you have to mitigate with a new tree measured at two inches” or more by caliper, “to make sure the natural landmark is preserved.” Kurokawa is president and owner of Ki Concepts, a landscape architecture firm based in Honolulu. His firm provided landscape design services for the new campus of the university’s Culinary Institute of the Pacific in the shadow of Diamond Head on the island of Oahu.

According to Kurokawa, before work could begin on the landscaping, residents of the community were given the opportunity to voice their concerns over any land-use changes at the site as part of a formal review and permitting process that included three public hearings as the project went through development.

Kurokawa says his firm specializes in an ecosystems approach to each of its landscaping projects and is always on the lookout for environmentally friendly products and techniques. “It’s a response to market demands. In Hawaii, all newly constructed state projects are required to qualify for US Green Building Council LEED Silver certification or better,” he says. While a state mandate makes it easier to sell green infrastructure, by tying in the ecosystems approach he can demonstrate to clients that while “the upfront cost may be higher, using green infrastructure costs less if you look at the entire life cycle.”

The site for the culinary institute, adjacent to the slope of the volcano cone, formerly housed a country club that had been razed by fire. The project was designed to take advantage of the terraced slope left over from its prior use. Although Kurokawa says he was keen to preserve the verdant beauty and ambience of the hillside, safety regulations required the construction of a permanent fire lane within 50 feet of the building. His vision for this lane was to provide it with the structural integrity required by fire code regulations but to also integrate it into “the pedestrian amenities,” designed to provide enjoyable experiences for students and faculty as they work or play on the campus. With this objective in mind, a traditional asphalt paved tarmac was out of the question, but there were alternatives.

Cooking With Grass
He elaborates, “We have in the plan the ability to plant gardens along the path as an integrated component of the design.” However, he notes, “It’s a roadway that has to support a fire truck of a certain weight,” and requires a certain structural integrity. GrassPave from Invisible Structures would provide a solution. Kurokawa says he chose the product, in part, because of the company’s extensive history in the field and its ability to provide in-depth support, ranging from numerous case studies to learn from to its broad network of suppliers, including a distribution center in Honolulu.

The lane is built on a structural base course of crushed gravel over the existing subgrade, taking the drainage to 2% across the fire lane. “We have a lot of expansive clay soils,” he says. “We’re designing that top layer skin grid structure that allows drainage. It’s a very low-profile open-cell product. It has two-inch openings and voids that can be filled with sand and compost while sitting on a compacted base course. When the grass grows, you don’t see it at all.”

Kurokawa notes Honolulu’s ongoing challenges in addressing pollutants from impervious surfaces getting into the bay, threatening coral habitat and other aquatic ecosystems. Whether accompanying roadways, parking lots, or other structures, building new gray infrastructure for detention and storage could help alleviate some of that. “In Hawaii you need to manage a 10-year storm over a 24 hour period, to maintain that predevelopment site condition for a 10-year storm onsite for a particular period before releasing it to storm drainage.”

However, he says, “Landscape architects are trained to be interdisciplinary. If you’re going to store that water, let’s use it for other purposes.” For example, the culinary institute design includes rainwater capture and reuse features on rooftops. He believes the systems approach to managing water is gaining traction across disciplines. “Civil engineers have begun to understand the idea of low impact development [LID]—don’t put in impervious surface if you don’t need it. The cheaper way is to infiltrate the water, if you don’t have the space for storage.”

Credit: SWM PAVE
Installation of pavers at Hiatt Run

He adds, “We look at ways to reduce costs. We look at ways to get the water into the ground and not into pipes.” GrassPave provides two additional benefits: “Because it is not a hard paved surface, it combats the heat island effect as well as restoring the natural areas.”

Ultimately, the fire lane at the culinary institute runs just the length of two classrooms, and Kurokawa hopes it will never see its intended use. “It was very short. It’s a small contribution to a single project, but if you look at all the things that are going on and the new requirements being codified, as a whole, they have a huge impact.”

Draining an Intersection
The Chattanooga, TN, region has become popular with travelers who enjoy the outdoor arts of camping, hiking, watercraft, and fishing. To an adventurous hiker, there can be nothing better than fording a waist-deep creek in the backwoods with knapsack held high overhead, or finding a hidden swimming hole just a few yards beyond the trail to frolic in crystal clear water. However, when hikers get back to town itching for hot showers and a warm comfy bed, wading or driving through ankle-deep water pooled at an intersection can seem like a nightmare, confirming the adage that what people do for fun they would often prefer not to do as a necessity.

When the proprietors of a local hostel called the Crash Pad, which caterers to adventure travelers, began working on plans to redevelop some of the neighboring properties in the commercial district surrounding their hotel, they noticed a gaping problem. At this particular intersection, “There was no stormwater drainage system in place, and whenever it would rain it would flood. It made passing through the area kind of uncomfortable. It wasn’t very deep—only about six inches—but the whole intersection would get covered, and the pavement suffered a lot because of that as well,” says Mark Heinzer, engineering manager for drainage and flood control for the City of Chattanooga.

The owners’ redevelopment plans included the construction and operation of a new restaurant. To obtain a permit for building on the site, they would need to conform to stormwater regulations that require developers in Chattanooga to capture the first inch of rain on the property.

From the city’s point of view, there was also a need to address the deteriorating infrastructure at the intersection of Johnson and Passenger Streets, where the Crash Pad and the proposed new Flying Squirrel Restaurant would be located. In addition, the city of Chattanooga has a combined sewer system that sometimes results in raw sewage discharge—combined sewer overflows—to the Tennessee River after large rain events. “We were going to need to replace those roads anyway. So the opportunity presented itself to do more than just pave them; we were able to include some stormwater improvements as well,” says Heinzer.

Credit: SWM PAVE

The Crash Pad’s owners offered to share the costs of street improvements as a means to address their new construction stormwater requirements and worked with the city to put together a public-private partnership agreement to financially support installation of permeable pavers on the public roadways adjacent to their properties. Heinzer says the owners liked the look of cobblestone and decided to give permeable pavers a try, with the objective of meeting their stormwater requirement and avoiding the cost, in both space and monetary expense, of installing stormwater management infrastructure directly on the premises of the new restaurant. As an additional benefit, replacing asphalt and concrete with colorful pavers would add an attractive provincial element to the commercial landscape of the neighborhood.

In a bold move that went beyond talk, says Heinzer, “The owners of the Crash Pad ­purchased the permeable interlocking concrete pavers [PICP] from Belgard and donated them to the project.”

To begin installation, contractors planned to remove a 2-foot cross section of existing roadbed and fill the cavity with weighted stone. However, the construction process was not without its complications. “This area of the city is old in terms of its development history,” says Heinzer. In a bygone era the area had been devoted to rail and manufacturing land uses, and evaluation of the soil underneath the existing roadbed indicated that some of the existing fill bore traces of industrial contamination. “We actually had to take some fill out. Some of that fill was foundry sand from some of the former foundry industry that was here in Chattanooga. We ended up having to take that to a landfill—for safe keeping, I guess you’d say.”

Heinzer says the soils that remained were fill materials as well, “so we weren’t expecting to get to any really good ­infiltrative soils.” However, to optimize opportunities for infiltration to occur, crews “scarified the bottom of the ­subgrade, as opposed to compacting it like you would normally do for a roadway.”

The project also installed an underdrain system within the pavement. Heinzer explains that using this system, the stone fill provides up to 6 inches of storage that will be allowed to infiltrate. During large storms when the water rises above that level, the excess will flow into the underdrain to be directed to the combined sewer system. “It slows down that peak rate for the combined sewer system,” reducing the potential for an overflow, says Heinzer.

The permeable pavers on Johnson Street, with 3 feet of gravel reservoir underneath, now capture all of the intersection’s excess rainwater. The Johnson Street Green Infrastructure Retrofit Project includes construction of 14,000 square feet of roadway using PICP. The project keeps more than 11,000 cubic feet of rainwater out of the combined sewer system.

A few years after installation of the pavers, says Heinzer, “There have been zero problems with rainwater. It doesn’t stand in the roadway at all. It all goes right through the surface of the pavement and gets underground and is held there. We’ve had absolutely no problems since the project with any kind of flooding.”

Heinzer says the community, impressed by the roadway’s new look, is doing a good job keeping the area clean, and the pavers do a good job keeping litter out of the combined sewer system. “We’re looking at getting a sweeper and a vac truck in there to sweep up all the fines that have made their way into the spaces between the bricks. We do have some areas that are kind of clogged up, but the overall system still works. It’s got such a redundancy in its capacity that it’s still working fine. We’re going to bring in some new sand to fill those spaces and basically rejuvenate the whole thing, and we should be fine for another couple of years after that.”

The City of Chattanooga has endorsed permeable pavement as one LID solution to capture stormwater before it enters the sewer system. PICP is a solution that has shown recent success, with well over 100,000 square feet being used within the city. As an incentive for property owners and developers, Chattanooga will become one of the first cities in the US to set up a stormwater market for property owners who replace their asphalt parking lots with PICP, allowing them to trade stormwater credits to those who can’t make the same improvements.

The City plans on continuing its work refurbishing its sewer and stormwater infrastructure to reduce overflows into the Tennessee River, as mandated by EPA. “Just about any project we get into, we’re looking to see how can we use green infrastructure to minimize the volume of discharge,” says Heinzer. “We’re retrofitting our parks, the parking lots, and recreation facilities—any kind of city facility. We’re looking for ways to reduce the environmental impact from stormwater runoff from those areas.” When it comes to street improvements, he says his department looks at all the options, “Maybe it’s permeable pavements, maybe it’s trees or trying to add some more vegetation along the rights of way. We’re always looking for opportunities to do those types of projects.”  EC_bug_web

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