MEASUREMENT AND MONITORING OF POROUS PAVEMENTS FOR ASSESSING THE PERFORMANCE OF WATER QUALITY BENEFITS

Porous pavements can be used in an urban setting for driver safety and water quality benefits. Both full-depth and surface course porous pavements are gaining popularity in urban planning. Over time, porous pavements can become clogged with trapped sediment resulting in a decrease in porosity and loss of drainage benefits. This presentation discusses multiple methods for in-situ hydraulic conductivity measurement of porous pavements and focuses on improved methods developed specifically for Permeable Friction Course (PFC). PFC is a layer of porous asphalt approximately 5 cm thick placed on top of conventional impervious asphalt roadways. Water quality, porosity, and hydraulic conductivity data collected for PFC over multiple years at different roadways in Austin, Texas is presented. The Texas Commission on Environmental Quality (TCEQ) has recently approved PFC as a stormwater Best Management Practice (BMP) for use over the Edwards Aquifer recharge zone. Therefore, in order to ensure that adequate water quality benefits persist over time, a standard in-situ measurement method for assessing the performance and removal efficiency of PFC is required. Water quality monitoring at three PFC sites in Austin, TX show a reduction in total suspended solids and total metals when compared to stormwater runoff from conventional pavements. Porosity values of PFC core specimens range from 12% to 23% and show a statistical decreasing trend over time. The nonlinear flow effects observed during hydraulic conductivity testing require analysis of the Forchheimer equation as opposed to the typical linear Darcy’s law. Hydraulic conductivity values of PFC core specimens range from 0.02 cm/s to nearly 3 cm/s with much greater variability, but have remained statistically constant over time. Furthermore, hydraulic modeling of PFC is useful for design purposes and in order to determine when the PFC layer is 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 provides safety benefits, but also helps reduce the adverse impacts of urbanization on surface and ground water quality.