A Physically-based Model for Surface and Subsurface Drainage from Porous Pavement Overlays

A thin layer of porous asphalt is commonly overlain on regular impermeable pavement to reduce splash and spray and improve visibility in wet weather. The porous layer often has a large hydraulic conductivity (>1cm/s) to encourage infiltration and drainage and therefore contains runoff when the rainfall intensity is low. However, under high rainfall intensity, the layer’s capacity is exceeded and drainage occurs both within and on top of the porous pavement. The problem is analogous to hill-slope hydrology of a thin aquifer where infiltration occurs rapidly and sheet flow is generated only when the aquifer is full. Common roadway features such as slope transitions and curvature make the drainage two-dimensional. A computer model was developed to study this coupled, unsteady process. The porous layer is modeled using the Boussinesq equation. The diffusion wave model is used for sheet flow over the pavement surface. This presentation summarizes the model’s development, shows that model results compare favorably to field measurements, and gives a case study in which the porous layer reduces the maximum sheet flow depth by 25% compared to conventional pavement.