Model of Flow Regimes in Porous Pavement and Porous Friction Courses

Porous pavements (PP) and porous friction courses (PFC) are increasingly recognized as viable alternatives to traditional impervious pavements. PP and PFCs passively provide safety, hydrologic and rainfall-runoff treatment benefits. These benefits are a result of such permeable systems providing hydraulic conveyance and filtration of particulate matter (PM) transported by runoff. With respect to 1) hydrology, these systems re-introduce infiltration, evaporation and storage phenomena; 2) traffic and road safety, increase skid resistance in wet conditions, reduce hydroplaning, reduce splash and spray phenomena; and 3) runoff treatment, as a function as a filter for PM and PM-bound constituents. These phenomena and the models therefore are a function of the hydrodynamics within the porous medium (which are not necessarily laminar) and commonly characterized using the hydraulic conductivity of the porous medium. Within the scope of this study the measured hydraulic conductivity (permeability) of 12 common porous asphalt mixes were examined. As is common for PP and PFCs, a Darcian model for the hydraulic conductivity in the laminar regime of flow was examined. However during testing, hydrodynamic regimes in porous asphalt (PA) that extended beyond laminar flow were identified for these porous mixes. Therefore, the variability of the hydraulic conductivity as a function of applied hydraulic head within a hydrodynamic regime (based on Reynolds number) was measured and modelled. From these results a saturated seepage model valid for laminar and transitional regimes was developed. Implications for the application of standardized permeameter testing of such porous mix designs are reported