Modelling the Thickness of a Water Film on Road Pavements—Analysis of Existing and New Equations for Flow Resistance Estimation

This study investigates flow resistance in thin water films on road surfaces during rainfall, which is essential for assessing aquaplaning risk. A one-dimensional surface runoff model based on the diffusion-wave approach is used to compare existing equations for the Darcy–Weisbach friction factor and Manning’s roughness coefficient. Laboratory data from three experimental cases support the analysis. The first case assesses the accuracy of existing equations and develops a new regression-based equation. The second case validates this new model for predicting water film thickness. Findings show that many existing equations poorly estimate water film thickness under high-intensity rainfall conditions relevant for aquaplaning analysis, often under- or overestimating it compared to measurements. Results indicate that flow resistance is mainly influenced by the Froude number, which is defined using the mean macro-texture depth of the pavement. The study emphasizes that accurate estimation of flow resistance parameters is critical in water film modelling, as it directly affects the reliability of traffic safety assessments.