Author(s): Randall J. Charbeneau; Jaehak Jeong; Ly Reeder; Wa Seong Chan
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Keywords: Rainfall runoff; Sheet flow; Urban drainage; Roughness coefficient
Abstract: Curved highway segments often use superelevation to help balance centrifugal forces on vehicles. Superelevation transitions have regions with near-zero cross-slope which may promote extended stormwater drainage paths on the pavement surfaces, increasing ponding depth, splash from vehicles, and the potential for hydroplaning. The overall objective of this research project is to develop design guidance on minimum longitudinal grade through superelevation transitions. This manuscript presents results from an experimental study of sheet flow characteristics on rough surfaces under simulated rainfall conditions. A large-scale (13 meter) experimental facility was developed. A rainfall simulator, with its unique rotating system, provides uniform rainfall on the roadway model surface. The roadway model is coated by resin with sand particles to simulate the roughness of asphalt and concrete type pavement surfaces. The support system of the roadway model was constructed as to allow variation of the surface slope. To describe the hydraulics of sheet flow on pavement surfaces, a model for the Manning coefficient is developed and evaluated. Because the pavement roughness is relatively large compared with the thickness of sheet flow, the shear drag, often represented by the friction slope, needs to be defined carefully. The proposed model shows that the roughness coefficient should be larger for low Reynolds number but asymptotically approaches to a constant value as flow rate increases. Furthermore, we observed that the impact of rainfall on the flow resistance for rough surfaces is negligible compared to surface drag.
Year: 2007