Adapting Full-Depth Permeable Pavement for Highway Shoulders and Urban Roads for Stormwater Runoff Management
With most of the drainage infrastructure in place, positive impacts on stormwater management will be accomplished by retrofitting rather than only through new construction. Highways and roads (as well as parking lots) comprise at least 25% of all impervious surfaces, and this percentage is often higher in urbanized areas. As these pavements age, they see rehabilitation, and this presents opportunities for stormwater retrofitting. Therefore, meaningful reduction of stormwater volumes and pollutant mass loading from such pavements requires some outside-the-box thinking. Permeable pavement—either in the road, as parking lanes, or as road shoulders—presents an efficient method of treating and infiltrating runoff at its source.
The challenge to transforming impervious to pervious or permeable surfaces/systems is institutional as well as technical. Transportation agencies, not stormwater agencies, control the road infrastructure. Therefore, identifying gatekeepers and providing reliable (and data-supported) technical design guidelines for permeable pavements are essential for transforming attitudes and institutions. Technical (and institutional) change occurs in transportation agency design guidelines or specifications when such data and practical experience are demonstrated.
Local road and state highway agencies have stormwater management requirements, and such systems are external to the pavements. The technical challenge lies in making the pavements themselves part of the drainage system especially as pavements are rebuilt, whether as highways or parking lots. This challenge is heightened when traditionally water has been directed away from pavements because it can damage pavement bases and underlying soils. Permeable pavements challenge that notion by managing the extent to which water enters a pavement base and soil subgrade.
Permeable pavements (i.e., pervious concrete, porous asphalt, and permeable interlocking concrete pavements) have been used in many parking lots, mainly in commercial and residential areas throughout the United States that receive light and low-speed traffic. Several guidance manuals and publications have been prepared to document their performance. For high-speed pavements, several departments of transportation have used a porous friction course (PFC, also called open-graded friction course or OGFC) (Figure 1) for noise reduction and increased safety during rain events. PFC is usually composed of 15- to 25-millimeter-thick hot-mix asphalt (open graded) (HMA-O) constructed as surface overlay over an existing impervious asphalt pavement. With the application of PFC, the surface water penetrates vertically within the OGFC and then laterally moves toward the shoulder and still needs to be collected or treated. Besides the noise and safety benefits, several research studies have also shown that there is a water-quality benefit with modest amount of pollutant reduction. However, little or no volume reduction is achieved with PFC pavements.
A more experimental and environmentally beneficial approach is the full-depth permeable pavement shoulder design illustrated in Figure 2. Under this proposed design, the entire amount or majority of runoff water from the highway or road surface will be retained within the shoulder, and there won’t be a need for additional treatment. The use of permeable pavement shoulders in highly urbanized areas would be especially beneficial, because finding sufficient land area to implement infiltration and temporary detention basins is difficult. Even when space is found, collecting and treating large volumes of polluted runoff in urban areas is often cost prohibitive. A full-depth permeable pavement shoulder retrofit in highways or urban roads is an alternative solution that can be cost effective in many cases.
While permeable pavements are effective for stormwater runoff volume control in parking lots and low-speed residential roads, their uses have not been adequately tested for heavy vehicle loads and moderate speed. At present, significant technical barriers exist in applying this effective low-impact-development tool for treating stormwater runoff from heavily used urban roads and highway shoulders. To adapt full-depth permeable pavement shoulder design, reliable information is needed on structural design and performance specifications under traffic exceeding that from parking lots and residential roads. Toward this goal, the University of California Pavement Research Center (UCPRC) has recently completed laboratory and modeling investigations to evaluate the structural and hydraulic performance of permeable pavement as highway shoulders that can be subject to heavy loads.
As part of the UCPRC investigation, numerous hydraulic and structural simulations were performed in 2009 and 2010 under a wide range of parameters, including surface open-graded pavement type, surface layer open-graded pavement thickness, subgrade permeability, rainfall amount and recurrence period, design traffic index, and design truck speed. The laboratory and simulation studies performed at UCPRC showed that retrofitting existing or new highway shoulders is technically feasible. In addition, a preliminary life-cycle-cost analysis showed that the use of full-depth permeable pavement shoulder retrofit is economically advantageous in urban areas compared with construction and maintenance of the conventional best management practices. The hydraulic performance simulation portion of the UCPRC study will be published in the July 2012 issue of Journal of Environmental Engineering.
In addition, the National Cooperative Highway Research Program (NCHRP) recently issued a request for proposal to further explore the technical feasibility of permeable pavement highway shoulders for runoff management. The initial phase of the NCHRP exploratory investigation relies heavily on literature search rather than field investigation. Ultimately, any simulated design must be constructed and tested before implementation in highway or road environments. For full field implementation, the current conceptual or simulated design must be tested under controlled conditions using heavy vehicle simulator (HVS) test tracks and/or a pilot investigation on a local road under heavy truck and passenger bus traffic. The results obtained from HVS or field pilot study can then be used to design and construct one or more test sections under highway environment for further evaluation and developing design and maintenance criteria for full implementation. This approach can build confidence among transportation agencies regarding structural as well as hydrologic performance. In addition, maintenance and management protocols can be developed.
Adapting the full-depth permeable pavement shoulder and road design is the focus of a forum on August 22 from 8 to 9:30 a.m. at the StormCon conference in Denver, CO. This forum includes five panel members representing Environmental Protection Agency, departments of transportation, pavement industries, consulting firms, and academic institutions. Each panel member will discuss key technical and institutional challenges, and ideas to overcome them. We expect some out-of-the box, yet technically, feasible ideas. The panel will then open the forum for public discussion and exchange of ideas. We encourage all conference participants to join us for this timely and stimulating forum.
