Measuring the flow resistance of submerged grass

Through laboratory experiments conducted in a grass-lined flume, the hydraulic resistance of grass is measured and quantified. For the grass examined, it is found that Manning's n value is greater than those recommended in well-established texts such as Chow (1959. Open Channel Flow. McGraw-Hill: Singapore), relatively lower than those predicted by n-UR methods, but corresponds well with the value found from calibration studies of two- and three-dimensional numerical models. The assumption of a uniform Manning's n value with flow depth, which is often made in numerical modelling, may be invalid depending on the relative submergence of the vegetation. Drag coefficients are evaluated for a method applicable to three-dimensional numerical models. Further detailed experimental investigation and application of these approaches within a numerical modelling framework is now recommended. Copyright © 2002 John Wiley & Sons, Ltd.