Rainfall effects on overland flow resistance

• The reliability of theoretically deduced flow resistance equations was assessed. • The flow resistance component due to rainfall was evaluated. • Introducing rainfall intensity into the model improves the equation performance. • Rainfall component significantly affects total flow resistance only for rough beds. Rainfall characteristics remarkably influence soil erosion processes, and the evaluation of rainfall effects on overland flow resistance is a significant topic that needs attention. In this paper, overland flow measurements available in the literature, obtained for different values of rainfall intensities and roughness conditions, were used to assess the reliability of theoretically deduced flow resistance equations and evaluate the flow resistance component due to rainfall. In detail, the relationship between the velocity profile parameter Γ, the bed slope, and the flow Froude number was firstly calibrated. Then, the performance improvements in the Darcy–Weisbach friction factor f estimate due to the introduction of a dimensionless group representing rainfall intensity effects (rain Reynolds number Re i ) into the relationship were evaluated. Finally, the f component due to rainfall, f rain , its contribution to total flow resistance, and its relationship with bed roughness height were investigated. The results highlighted that, even if the relationship neglecting the influence of Re i gives a good estimate of the Darcy–Weisbach friction factor, introducing Re i into the model significantly improves the performance in the f estimate. The f rain value is negligible for almost all the measurements performed for the smooth arrangement, while, for the rough bed measurements, it varies from 0.15 to 15.4% of the total flow resistance and is higher than 5% for 72.8% of the cases. The analysis also showed that f rain increases with increasing bed roughness height. In conclusion, this study confirmed that including rainfall intensity in the flow resistance law improves its performance and demonstrated that the rainfall component significantly affects total flow resistance only for rough beds.