Comparison of Empirical and Analytical Solutions for Open-Channel Flow Velocity with Common Grass Species in Taiwan

Grassed channels utilize the soil stabilization and water infiltration enhancement functions of grass in order to conserve soil and water in drainage systems. The construction processes and hydraulic mechanisms of grassed channels are more complicated, depending on the conditions of both soil and grass. As flow resistance is affected by grass characteristics, giving a single value of Manning’s n for a grass type under different flow conditions may lead to over-conservative designs or safety concerns. In this study, grassed flow experiments were carried out in a flume, with a bed of red soil covered by three grass species and with the flow conditions of three bed slopes. Average flow velocities were evaluated using five methods, including Manning’s equation and an analytical method. Comparison between the methods showed that Manning’s equation was unable to properly reflect the grass characteristic effects on the flow, but the analytical method performed better in estimating the average velocity and velocity profiles. The experimental results will be useful for the verification of mathematical methods, including analytical solutions and numerical models of grassed flow. For application, the relationships of average flow velocity against the grass layer relative height were proposed based on the analytical method as a reference for a hillslope drainage system design in Taiwan