Coupled Surface-Subsurface Model for Simulating Drainage from Permeable Friction Course Highways

Permeable friction course (PFC) is a porous asphalt pavement placed on top of a regular impermeable roadway. Under small rainfall intensities, drainage is contained within the PFC layer; but under higher rainfall intensities, drainage occurs both within and on top of the porous pavement. A computer model—the permeable friction course drainage code (Perfcode)—is developed to study this two-dimensional unsteady drainage process. Given a hyetograph, geometric information regarding the roadway layout, and hydraulic properties of the PFC media, the model predicts the variation of water depth within and on top of the PFC layer through time. The porous layer is treated as an unconfined aquifer using Darcy’s law and the Dupuit-Forchheimer assumptions. Surface flow is modeled using the diffusion wave approximation to the Saint-Venant equations. A mass balance approach is used to couple surface and subsurface phases. Straight and curved roadway geometries are accommodated via a curvilinear grid. The model is validated using steady-state solutions that were obtained independently. Perfcode was applied to a field monitoring site near Austin, Texas, and hydrographs predicted by the model were consistent with field measurements. For a sample storm studied in detail, PFC reduced the duration of sheet flow conditions by 80%. In a second sample storm, PFC prevented sheet flow conditions completely. The model may be used to improve the drainage design of PFC roadways.