VAA Virginia Asphalt Fall/Winter 2021

20 FALL/WINTER 2021 Michael Fitch, Ph.D., Deputy Director / Research Council, Virginia Department of Transportation PERMEABLE PAVEMENT Does It Make the Transportation System More Resilient? It is difficult to look at any recent transportation journal or trade magazine and not find an article or two focusing on resiliency and all its associ- ated challenges. Although significant work has been done for years under the resiliency umbrella, recent prominent weather events have further highlighted the potential damage to infrastruc- ture resulting from what appears to be more fre- quently occurring hurricanes, heat waves, flooding events, etc. As a result, the Virginia Department of Transportation supports a number of research efforts and initiatives to address some of the anticipated changes in both weather and climate to ensure its system is resilient to these changes. Water is the Challenge So exactly how does increased resiliency look? What form does it take? Currently, most of the immediate resiliency challenges facing Virginia are related to water. As the perceived increases related to precipitation intensity, frequency and duration become quantified and the locations most likely to be affected by these increases are identified, the real work begins—making changes to the transpor- tation infrastructure to increase its resiliency. VDOT has long been required to address stormwater runoff coming from its facilities. To meet environmental regulations and design proper drainage for these facilities is a given, so dealing with more water to protect infrastruc- ture certainly seems like a challenge that can be met. Typically, proper management of runoff water is accomplished by one or more structural best management practices (BMPs), also referred to as stormwater control measures. These can take many different forms, such as stormwater basins, underground manu- factured filtering or hydrodynamic devices and other low-impact development (LID) practices. LID practices are a subset of BMPs designed to behave hydrologically similar to undisturbed, natural land surfaces by allowing water to infiltrate into the soil and do so as close as possible to its original source rather than its original source being collected and conveyed offsite. Permeable pavement systems (PPSs) are one type of LID. Permeable Pavements Are Resilient There are several types of PPSs: pervious concrete, permeable pavers and porous asphalt are the most common. Regardless of type, these function similarly by allowing water to infiltrate through the voids in the pavement matrix and then move into the base material. The voids in the pavement structure are created by the absence of fine aggregates, as compared to conventional pavements. In addition to the interconnected voids resulting in the pavement being perme- able, the base for these systems serves as a foundation to support the pavement and as a reservoir for the infiltrating water. As such, the base is typically composed of large, clean aggregate (e.g., No. 57 or No. 2 stone) or something similar, having a void space of approximately 40%. The depth of this base layer is designed to accommodate the needed volume of water that will be treated under certain site-specific conditions. Depending on the type and design of the system, water is allowed to infiltrate from the base into the underlying soil or is discharged to an outlet by way of an underdrain. PPSs help remove nutrients from the runoff and reduce the peak runoff volume by allowing infiltration and then temporary storage of the infiltrated water. This reduction in peak runoff results in PPSs serving as an option for making transportation infrastructure more resilient. VDOT’S Experience VDOT, along with the Virginia Asphalt Association, developed guidelines for the use of porous asphalt pavement structures. These guidelines were developed based on the Virginia Department of Environmental Quality’s (DEQ) Stormwater Design Specification No. 7, Permeable Pavement, Version 1.8 (1). The development of these guidelines allowed VDOT to begin investigating ways that PPSs could be used as yet another stormwater BMP as early as 2013. However, though there was an abundance of research document- ing the pollutant removal efficiency of PPSs and the runoff volume reduction that could be expected from these systems, little was Infiltration Rate (in / hr) Date Mar-13 Oct-13 Apr-14 Nov-14 May-15 Dec-15 Jun-16 Jan-17 Aug-17 500 450 400 350 300 250 200 150 100 50 0 Pavement Average Minimum Allowable FIGURE 1 Average Infiltration Rates and Standard Errors Across All Sampling Sites for the 4-Year Monitoring Period.