Understanding Permeable Paving
Permeable paving is one of the biggest trends in landscaping right now. An increasing number of landscapers, regulators and regular people are becoming aware of the benefits of permeable paving options.
More traditional paving materials such as concrete or asphalt are impermeable surfaces. While these are both highly durable materials that have served landscapers well for years, they have been found to adversely affect the natural environment [1].
Permeable paving materials such as aggregates stabilized with products like Organic-Lock™ have been recognised as a more sustainable option by organizations such as ‘Leadership in Energy and Environmental Design’ (LEED) [2].
Higher environmental standards often result in higher costs, but this is not the case with permeable pavers. With the right material choice, you can save money and help the environment!
Especially if you are new to permeable paving, it is important that you begin by fully understanding permeability in landscaping.
Let’s get straight into it and look at what permeability is and how it is measured, before looking at how this benefits the environment and outlining the paving options available to you.
What is Permeability?
Permeability is a term commonly used in landscaping that is borrowed from the engineering field of fluid dynamics.
In this context, it refers to materials that allow liquids to move through it. This process of water moving through paving and into the ground is called infiltration [3].
When discussing the permeability of materials, they are seperated into one of the following categories:
- Pervious – These materials allow for the highest flow of water. Examples include loose or compacted sand and gravel.
- Semi-Pervious – With these materials, permeability is lower. Examples include sandstone, very fine sand or clay.
- Impervious – These materials are completely impermeable and do not allow for water to pass through. Solid bedrock such as granite, or man made materials like concrete are impermeable.
It is now commonly expected that landscape designers maximize for permeability when planning paved areas. LEED certification even specifies levels of permeability in a project to meet their guidelines [4].
Gaining a high level certification for is a great way to show your commitment to the environment, but it also could qualify you for substantial tax breaks.
In most cases, the liquid that will be moving through your paved area will be water generated by natural rainfall, although you also need to account for other water sources such as irrigation or run off from nearby impervious surfaces entering your project area [5].
Regardless of the source of the water flows you need to manage, you will need a way to quantify your level of permeability to gain certification. So how exactly do you measure permeability?
Measuring Permeability
Paving materials can be broadly classified as permeable or impermeable, but exact rates of infiltration can only be ascertained by laboratory testing. This is because there is a high level of variance between natural paving materials, even of the same type [6].
The exact mathematics of this laboratory testing is not something you need to know unless you are an engineer, but it is worth knowing how the test works.
A sample of the material is set up under a rainfall simulator and this is typically set at the incline that will be present in the area it is to be used [7].
The test is then run for a length of time to measure how the material performs when dry and then how the infiltration rate is affected once it has been saturated by rainfall. The final results are expressed as an infiltration rate in inches per hour [8].
An example of this is the testing done to show the permeability of loose aggregates combined with the stabilizer Organic-Lock™.
Under the testing conditions, Organic-Lock™ showed an infiltration rate as high as 1.90 inches/hour when dry. falling to only 1.13 inches/hour after 360 minutes of exposure to water.
These example results show a high rate of infiltration, thus proving the permeability of this combination. The same test run with more traditional paving materials would show a much lower level.
If your project is small, it is usually acceptable to use a proven permeable material without going to the trouble of laboratory testing. However, if you are working on a large project with complex drainage systems or very specific environmental requirements, it may be necessary in the planning phase.
Permeable Paving and Sustainable Landscaping
Urban planners have now realized the importance of permeable paving when creating urban environments. In traditionally designed built up areas, rates of infiltration are low.
Low rates of natural infiltration lead to high levels of surface runoff, which can cause flooding and requires complex and expensive storm water systems to manage.
Now there are a number of initiatives such as the ‘Sponge City’ in China, which aims to address this problem and make more permeable cities that flood less [9] [10].
Permeable surfaces are key to the vision of more permeable city designs. The key benefits of using permeable paving are outlined below.
Recharging Aquifers
Aquifers, also referred to as ground water or the water table, require natural infiltration to refill or ‘recharge’. If too many impervious surfaces are placed on top of the catchment area for an aquifer, it will not be able to recharge and will be depleted over time.
This can impact the water supply of areas that rely on groundwater wells, or it can cause land to sink, damaging buildings, roads and footpaths. Researchers have found that it only takes a 20-30% reduction in natural permeability to negatively affect groundwater recharge [11].
Aquifers also help maintain river levels and depleting aquifers can have a massive knock on effect on the local waterways and the wildlife that relies on them.
Managing Run-Off
Run-off is damaging to both the natural and built environment. It can introduce harmful chemicals to local waterways and also cause flooding if not managed correctly.
On hard and impervious surfaces, pollutants, chemicals or even concentrated dirt can build up over time. When there is then a period of rainfall, this can then be transported into stormwater systems and local waterways with the surface run off [12].
The Maryland Department of Natural Resources has conducted research that shows a healthy stream can see a wholesale die off wildlife due to pollution when just 20% of the watershed is covered in impermeable surfaces [13].
To stop run off causing flooding there is a hefty financial cost. Complex drainage and stormwater systems are necessary and this is often reflected in municipal taxes. You can save money on your drainage and taxes while also helping the environment when you install permeable paving.
LEED Certification
Although you can benefit the environment with permeable paving, you can also use it to help balance the budget!
It can be time consuming to gain LEED certification, but in most states and municipalities it is well worth the hassle. Depending on your jurisdiction, you can expect to get significant tax breaks and even reduced ongoing maintenance fees for stormwater systems due to your lower usage [14].
LEED certification also signals the commitment you and your client have to sustainable practices which is well received by customers.
New Orleans Museum of Art – Organic-Lock™ was used to create sustainable and environmentally friendly pathways and common areas around the museum.
A Brief Guide to Permeable Paving
The benefits of permeable paving really speak for themselves. But how do you choose the right paving type for your next project?
Below is a brief overview of the most popular permeable paving solutions that includes their strengths and weaknesses to help you make an informed decision.
Stabilized Aggregates
Stabilized aggregates are the next big thing in landscaping. Aggregates, like decomposed granite, have been around for a long time, but it is the introduction of stabilizers like Organic-Lock™ that have been the game changer.
Organic-Lock™ is an all natural, laboratory tested compound that hardens when wet. When mixed with the loose aggregate, it fixes it in place, stopping erosion and damage.
The best thing is that using a stabilizer does not impact the natural permeability of the aggregate, making it one of the best ways to create large hard surfaces while still meeting environmental standards.
Laying a stabilized aggregate path is a cheap and simple process when compared to more complex permeable surfaces. They are also easily repaired with basic hand tools, lowering the lifetime cost significantly.
Porous Concrete
Traditional concrete formulations are the go to material for pathways and other paved areas, but it is a rigid material that lacks permeability. That is is why a porous variant has been created that allows water to infiltrate.
Infiltration rates through porous concrete are much improved over traditional variants, but are still much less than natural rates. There is also a need for complex drainage systems, as any water lingering under the rigid surface of the concrete can cause significant damage [15].
There is also a need to regularly clean porous concrete in order to avoid clogging and maintain its porosity.
As this case study for a parking lot at UC Davis shows, the maintenance costs of a porous concrete surface can add tens of thousands of dollars to the lifetime cost of a project using this material [16].
Porous Asphalt
The National Asphalt Pavement Association estimates that as much as 350 million tons of asphalt surfaces are laid in the USA every year [17]. This makes asphalt one of the most popular hard surfaces in the country.
Traditional asphalt is impermeable though, which makes it unsuitable for modern projects that are prioritizing permeability. This has led to the creation of new asphalt formulations that allow for infiltration.
Permeable asphalt costs significantly more than traditional asphalt in materials and installation due to the levelling and subgrade required [18]. Due to porous asphalt still being a rigid surface, complex drainage is typically required to move the infiltrated water away from it to avoid damage.
The ongoing costs of porous asphalt can also be high, as it needs to be regularly cleaned. Without this regular maintenance, the surface can become ‘clogged’ and lose its permeability permanently [19].
Choosing the Right Permeable Paving Material
While all of the permeable paving materials discussed have their place in increasing infiltration and reducing our reliance on drainage systems, stabilized aggregates are by far the most popular.
There is a good reason for this. An aggregate mixed with a stabilizer like Organic-Lock™ strikes the perfect balance between price and performance.
These surfaces are cheap and easy to install and the ongoing maintenance is simple and cheap, without a need for specialized equipment or knowledge.
If you are in any doubt about which permeable surface to use in your next project, then you should contact a landscape architect with experience in permeable surfaces. They will be best placed to help you make the right choice based on your individual needs.
References
[1] Frazer L. (2005). Paving paradise: the peril of impervious surfaces. Environmental health perspectives, 113(7), pp. 456-62. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1257665/
[2] [4] Abel, K. (2016). Trends in LEED Buildings and their Effects on Urban Permeability. GIS For Water Resources. Accessed: 24 Feb 2019. Available from: https://www.caee.utexas.edu/prof/maidment/giswr2016/Papers/Abel.pdf
[3] Paving Expert (2019). Permeable Paving Overview. http://www.pavingexpert.com/permabl1.html
[5] Shuster, W. D., Bonta, J.,Thurston H., Warnemuende, E., Smith D.R. (2005). Impacts of impervious surface on watershed hydrology: A review. Urban Water Journal, 2(4), pp. 263-275. DOI: 10.1080/15730620500386529
[6] [15] Scholz, M., Grabowiecki, P. (2007). Review of permeable pavement systems.
Building and Environment, 42(11), 3830-3836. https://doi.org/10.1016/j.buildenv.2006.11.016.
[7] [8] Head, K. H., & Epps, R. (1980). Manual of soil laboratory testing (Vol. 1, No. 2). London: Pentech Press. http://www.whittlespublishing.com/uploads/ManSoilErrata.pdf
[9] RoxBurgh, H. (2017). China’s ‘sponge cities’ are turning streets green to combat flooding. The Guardian. Accessed: 25 Feb 2019. Available from: https://www.theguardian.com/world/2017/dec/28/chinas-sponge-cities-are-turning-streets-green-to-combat-flooding
[10] Ka Shun Chan, F., Griffiths, J.A., Higgitt, D., Xu, S., Zhu, F., Tang, Y., Xu, Y., Thorne, C.R. (2018).
“Sponge City” in China—A breakthrough of planning and flood risk management in the urban context.
Land Use Policy, 76, pp. 772-778. https://doi.org/10.1016/j.landusepol.2018.03.005
[11] Frazer L. (2005). Paving paradise: the peril of impervious surfaces. Environmental health perspectives, 113(7), pp. 456-62. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1257665/
[12] Chithra S.V., Harindranathan N., Amarnath A., Anjana N.S. (2015). Impacts of Impervious Surfaces on the Environment. International Journal of Engineering Science Invention, 4(5), pp. 27-31. http://www.ijesi.org/papers/Vol%284%295/E045027031.pdf
[13] Maryland Department of Natural Resources. How Impervious Surface Impacts Stream Health. Accessed: 26 May 2019. Available from: https://dnr.maryland.gov/streams/Pages/streamhealth/How-Impervious-Surface-Impacts-Stream-Health.aspx
[14] Ashley, E. (2008). Using Pervious Concrete to earn LEED points. Concrete in Focus. Accessed: 26 May 2019. Available from: https://www.nrmca.org/research/CIF%20Winter%2008%20Perv%20Conc%20LEED.pdf
[16] Terhell, S.L., Cai, K., Chiu, D., Murphy, J. (2015). Cost and Benefit Analysis of Permeable Pavements in Water Sustainability. ESM 121 Final Paper. Accessed: 24 May 2019. Available from: http://watermanagement.ucdavis.edu/files/5414/3891/2393/A03_Terhell_Cai_Chiu_Murphy_ESM121_FinalReport.pdf
[17] Federal Highway Administration. “Employment Impacts of Highway Infrastructure Investment.” U.S. Department of Transportation, Washington, D.C., 2012. Accessed: 01 May 2019. Available from:http://www.fhwa.dot.gov/policy/otps/pubs/impacts/index.htm
[18] Pavement Interactive (2019). Surface Preparation. Accessed: 01 Mar 2019. Available from: https://www.pavementinteractive.org/reference-desk/construction/site-preparation/surface-preparation/
[19] Babashamsi, P., Yusoff, N.I., Ceylan, H., Ghani, N., Jenatabadi, H.S. (2016). Evaluation of pavement life cycle cost analysis: Review and analysis. International Journal of Pavement Research and Technology, 9(4), pp. 241-254/ DOI: https://doi.org/10.1016/j.ijprt.2016.08.004.
