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

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 Darcy9s 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.

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