Estimating drag coefficient for arrays of rigid cylinders representing emergent vegetation

Flow resistance due to vegetation is of interest for a wide variety of hydraulic engineering applications. This note evaluates several practical engineering functions for estimating bulk drag coefficient ( ) for arrays of rigid cylinders, which are commonly used to represent emergent vegetation. Many of the evaluated functions are based on an Ergun-derived expression that relates to two coefficients, describing viscous and inertial effects. A re-parametrization of the Ergun coefficients based on cylinder diameter (d) and solid volume fraction (φ) is presented. Estimates of are compared to a range of experimental data from previous studies. All functions reasonably estimate at low φ and high cylinder Reynolds numbers ( ). At higher φ they typically underestimate . Estimates of utilizing the re-parametrization presented here match the experimental data better than estimates of made using the other functions evaluated, particularly at low φ and low .