A Critical Thinking Investigation into Permeable Pavement Systems and Sustainability

Author: Angelica Werth

In Sustainable Landscape Garden Design, Installation and Maintenance class, we have examined several aspects of sustainable design, including some best management practices to implement to increase the sustainability of a site and how those practices benefit the environment (that is, provide one or more ecosystem services). Although most people first think of plants when they think of landscapes, the truth is that a landscape includes many elements, such as nearby buildings, the view in the distance, and hardscaping materials. When considering or designing a sustainable landscape, it is essential to consider all aspects.

One way to include sustainability in the hardscaping aspect of a landscape (and urban design) that we discussed in class and read about in the textbook Sustainable Landscape Management: Design, Construction, and Maintenance by Thomas W. Cook and Ann Marie VanDerZanden that interested me is permeable pavements. These pavements allow water to filter through them into a reservoir, which then drains out into the surrounding soil slowly. The opposite of this is what we typically see in cities and landscapes, which is impervious pavement (meaning water can't infiltrate but instead runs off, creating pollution problems and leading to potential flooding issues). Permeable pavements may be dry laid or interlocking bricks or stones (the actual pavers aren't permeable, but there are spaces in between the pavers), pervious concrete, or porous asphalt pavement. Their proponents make many claims about their numerous benefits, and the claim I am investigating is paraphrased from our textbook: permeable pavers increase the sustainability of a site by decreasing runoff, filtering contaminants, and allowing water and oxygen to infiltrate the soil for plants (Cook & Vanderzanden).

Permeable pavements have been fairly slow to catch on in the US, however interest in them is growing and so is the body of research testing their benefits, so finding recent reports on them wasn't impossible. One authoritative source I found was published by the United States Geological Survey, which had partnered with the Wisconsin Water Science Center to set up a study testing several aspects of the capabilities of two different types of permeable pavers. The study is still in progress and so no conclusions may be drawn yet, however as background information, they published a comprehensive guide to what permeable pavers are, what they do, and some concerns about using them.

One of the primary potential benefits the USGS report discusses is the decrease in runoff, in particular from heavy rain events. This decrease in runoff is desirable because it "restores hydrological balance" (according to the report) and significantly reduces the risk of flooding and erosion. If there is a sufficient amount of permeable pavers, it also reduces or eliminates the need for other stormwater management systems like retention ponds. Additionally, permeable pavements filter out contaminants in three ways: physically (by trapping them in the pavements themselves), chemically (bacteria and microbes break down the contaminants), and biologically (plants that grow in between some pavers trap and store pollutants). The reservoirs also allow the water to cool off before entering the soil, as opposed to runoff, which can sometimes enter water sources at high temperatures, disturbing the ecosystems there. Finally, the report cited researchers in New Hampshire, who found an additional benefit of permeable pavements for areas that receive snow. They found that permeable pavements only need 0 to 25% of the road salt that is applied to regular asphalt. A reduction in road salt benefits vehicle owners, taxpayers (who pay for the salt and its application), and the environment, which suffers from the pollution caused by road salt ("Evaluating the Potential Benefits of Permeable Pavement on the Quantity and Quality of Stormwater Runoff").

As I continued my investigation of permeable pavement systems, I came across a study that supports the idea that permeable pavers decrease contaminants, thereby improving water quality. The study installed two different types of permeable pavements in an urban neighborhood, then measured the total suspended solids, nutrients, and E. coli bacteria in the stormwater collected by the two systems, as well as stormwater runoff in the same neighborhood, and compared the results. Both permeable pavements were shown to be effective, as both significantly reduced the concentrations of total suspended solids, E. coli, ammonia, and phosphorus (Abdollahian, Sam et al). Ammonia is a source of nitrogen, and both nitrogen and phosphorous are nutrients that are essential for algae growth; an excess of either or both often causes algal blooms that block sunlight and compete for oxygen in a body of water, seriously harming the ecological community of that body of water. Therefore, it can be concluded from this study that permeable pavements protect water sources by filtering contaminants and reducing pollution.

The takeaway from both of these sources is that permeable pavements have a number of benefits, including several that would be considered ecosystem benefits. The Sustainable Sites Initiative defines ecosystem benefits as "goods or services of direct or indirect benefit to humans that are produced by ecosystem processes" (The Case for Sustainable Landscapes). It is generally understood that a sustainable landscape should strive to produce as many of these benefits as possible, since they align with the overall goal of sustainability. According to the historical report Our Common Future, sustainable development is "meeting the needs of the future without compromising the ability of future generations to meet their own needs ("Sustainable Development"). The Sustainable Sites Initiative lists 12 ecosystem benefits that contribute to the sustainability of a site. It is evident from the research and work discussed above that permeable pavements provide three of them: water cleansing (removing pollutants from the water), water supply and regulation (by reducing runoff and storing water in reservoirs to be slowly released), and hazard mitigation (reducing vulnerability to damage from flooding) (The Case for Sustainable Landscapes). Thus, it is quite logical to claim that permeable pavements contribute to the sustainability of a site.

Although permeable pavement systems have many benefits, one that many people can easily identify as being very useful to humans is hazard mitigation. Since permeable pavements collect water instead of allowing it to run off, they greatly reduce the risk of flooding from heavy rains. Considering this aspect of permeable pavement systems reminded me of the summer of 2017, when my home state (Wisconsin) dealt with a lot of very heavy rains that led to severe flooding in many areas. In fact, it was so bad in the town where my grandparents live that they called in the National Guard. Where my mother was living at the time, in an urban area, the flooding was also quite significant. At that time, my mom (who is not a materialistic person), had a fairly nice car that she absolutely adored. But as a result of the flooding in her neighborhood, her car was ruined. I can't help but wonder if permeable pavements, had they been commonly in use in those regions, would have prevented or minimized the damage that had occurred, and possibly even saved my mom's car.

Although there are many benefits to permeable pavement systems, there are those who oppose it. It must be admitted that there are some drawbacks to using permeable pavement; the spaces between the pavers become clogged and must be cleaned (how often maintenance is necessary depends on which type of system is used), and installation represents a significant initial investment. Researchers associated with the University of California conducted a survey examining resistance to the use of permeable pavements, and found that the most commonly cited concerns regarding permeable pavements were a high initial cost, cost and frequency of maintenance, and conservatism in the industry (Harvey, J., et al). Another notable concern is durability of permeable pavers, an area that is still lacking in research. However, most people seem open to considering the idea of permeable pavements, especially after receiving some education on their benefits.

Given the multiple ecosystem services that permeable pavements provide, it can safely be concluded that they do in fact contribute to the sustainability of a site. They are an excellent option for stormwater management, reducing and in some cases eliminating runoff, in addition to filtering contaminants, and mitigating the risks of flooding. As a result, I intend to keep up on research in this intriguing area by looking for future studies to be published, and to spread the word about these pavements to those who may be interested in implementing them. Additionally, if I am ever designing a landscape or garden, I will do my best to include them as well.

Sources:

Angelica Werth is a Clemson University Horticulture major in Dr. Vincent's HORT 3080 Sustainable Landscape Garden Design, Installation and Maintenance class.

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