Quantifying the Behavior of Porous Asphalt Overlays with Respect to Drainage Hydraulics and Runoff Water Quality

Abstract

Porous pavements are gaining popularity in urban settings on highways based on their water quality benefits, noise reduction characteristics, and the reduced splash and spray they yield in wet weather. Over time, porous pavements can become clogged with sediment, resulting in a decrease in porosity and hydraulic conductivity as well as a loss of drainage benefits. This article provides an overview of water quality benefits and methodology for measuring hydraulic conductivity specifically for permeable friction course (PFC). PFC is a layer of porous asphalt ranging from 2.5 to 5.0 cm thick placed as an overlay on conventional impervious roadways. Research studies show a reduction in total suspended solids and total metals in runoff from PFC surfaces compared to runoff from conventional pavements. Monitoring results were mixed for nutrient removal from PFC. Porosity and hydraulic conductivity data collected over multiple years from different roadways in Austin, TX, are presented. The porosity values of PFC specimens ranged from 0.12 to 0.23. The non-linear flow relationship observed during hydraulic testing requires analysis of the Forchheimer equation, as opposed to the typical Darcy's law. Hydraulic conductivity values ranged from 0.02 to 3.0 cm/s with significant variability. Furthermore, hydraulic modeling of PFC is useful for design purposes and determines when the layer becomes saturated and surface runoff is expected. Flow through PFC is modeled as an unconfined aquifer with an underlying sloping impervious boundary. The use of porous pavements in an urban environment not only improves driving conditions but also helps reduce the adverse impacts of urbanization on surface water and groundwater quality.

Hydrogeological Impacts of Urbanization

“…The spring orifice is probably preserved only because of its historical significance. localized recharge, reduce water quality threats, and improve road safety (Klenzendorf et al, 2012).…”

Filtering Capability of Porous Pavements

“…Table 2 shows the concentrations of pollutants from highway runoff in different places in Europe [22], in western Washington State [25] and in Texas, in the United States [1,26]. Cadmium (Cd) n/a (1) 0.3-13.0 0.9-2.8 n/a (1) Zinc ( Nickel (Ni) n/a (1) 4.0-70.0 8.6-12.9 n/a (1) Chromium (Cr) n/a (1) 6.0-50.0 n/a (1) n/a (1) Iron (Fe) n/a (1) n/a (1) 2429.0-10,300.0 (2) 361.0-2606.0…”

“…The infiltration capacity was obtained by using Equation (1). It was assessed for the three asphalt slabs and the two models.…”

“…The urbanization process and the disorganized occupation of urban centres to supply the needs of human beings promoted the transformation of the environment, which resulted in modifications and interferences in ecosystems [1]. The development of the road network combined with the growth of cities has led to impervious surfaces.…”

“…Porous pavements are specifically designed to promote the infiltration of stormwater through the paving and base courses where it is filtered through the layers [1,4,[6][7][8]. This particular pavement may also result in a reduction in the amount of pollutants entering the groundwater by filtering the stormwater [5,9].…”