Effect of Stiff, Emergent Vegetation on Flow Resistance in Open Channel Systems

Abstract

This study investigates the hydraulic resistance that emergent, stiff vegetation causes in open channel flows. By selectively using such vegetation as a flood mitigation strategy, ecosystem restoration and sustainability can be enhanced. On the other hand, vegetation that grows asymmetrically inside channels raises hydraulic resistance, which dissipates energy and lowers flow capacity, potentially leading to operational difficulties.

Earlier experimental studies have extensively examined the resistance caused by uniformly distributed vegetation stems. In this research, the vegetation is simulated using rigid rods that mimic tree stems. Flow velocities were measured using three-dimensional Acoustic Doppler Velocimeters (ADV), employing both upward- and downward-facing probes. In the wake region behind the vegetation stems, the results show a considerable decrease in longitudinal velocity, although increased turbulence resulted in elevated transverse and vertical velocity components. The study confirms that flow resistance is influenced by factors such as flow depth, stem height, diameter, and vegetation density. It was also observed that higher vegetation density corresponds to a reduction in discharge rate. Experimental data from the National Institute of Technology Rourkela (NITR) and prior studies were used to develop a new mathematical model for roughness, expressed in terms of dimensionless parameters. This newly proposed equation demonstrates a strong correlation with experimental results and outperforms traditional resistance formulas.