Velocity Distribution and Reynolds Shear Stress Characteristics in a Narrow Gravel-Bed Open Channel with Submerged Rigid Vegetation Patches

The abundance of vegetation in natural and manmade streams has a substantial impact on flow dynamics, nutrient distribution, and aquatic habitat condition (Nepf, 2012; Richardson et al., 2007). To understand the impact of vegetation, laboratory experiments have been conducted in a flume with rigid, continuous submerged vegetation. The vegetation is arranged along both boundaries with a non-vegetated section along the centre of an open channel. The size of the experimental flume is 10 m (length) × 0.4 m (width) × 0.6 m (depth). To simulate continuous vegetation patches, hard cylindrical plastic rods are used and evenly spaced along the boundaries of the channel. A flow tracker, ADV Lab-II, was used to measure the velocity of the water at different depths in the upstream and downstream non-vegetated sections as well as inside the vegetated area. Data was collected at a sampling rate of 5 Hz. Velocity data were recorded at 24 vertical measurement points in each location. Approximately 6000 instantaneous velocity points were recorded at each vertical location over 20 minutes (Kashyap and Barman, 2025). Measurements were conducted under a constant flow discharge of 0.0153 m³/s, and the submergence ratio of the vegetation was 0.43.

The findings showed that the presence of vegetation caused considerable changes in the velocity profiles. The non-vegetated upstream segment showed a logarithmic velocity pattern, which is typical of open channel flow. The velocity profiles of the vegetated zone are compared to the non-vegetated conditions. Specifically, the presence of vegetation reduced near-bed velocities while increasing velocities in the upper water column. The presence of vegetation visibly increases the Reynolds shear stress both within and downstream of the vegetation patches. This suggests that stem flow interactions led to more momentum exchange and turbulence eddies (Ghisalberti and Nepf, 2006). These effects spread outside the vegetated zone and have a prolonged effect on the turbulent structures of the flow. These results indicate the need for consideration of vegetation structure at different patch scales for hydraulic modelling.

Keywords: ADV, Reynolds shear stress, Rigid vegetation, Vegetation patches 

References:

Ghisalberti, M., & Nepf, H. (2006). The structure of the shear layer in flows over rigid and flexible canopies. Environmental Fluid Mechanics, 6(3), 277-301.

Kashyap, S.N., & Barman, B. (2025). Turbulent flow characteristics over gravel bed channel with submerged vegetation patches. Physics of Fluids, 37(3), 035123.

Nepf, H.M. (2012). Flow and transport in regions with aquatic vegetation. Annual Review of Fluid Mechanics, 44(1), 123-142.

Richardson, D.M., Holmes, P.M., Esler, K.J., Galatowitsch, S.M., Stromberg, J.C., Kirkman, S.P., Pysek, P., & Hobbs, R.J. (2007). Riparian vegetation: degradation, alien plant invasions, and restoration prospects. Diversity and Distributions, 13(1), 126-139.