Experimental Insights into Flow Turbulence with Vetiver Grass in Mobile-Bed Channels

Abstract

Vegetation naturally occurs in river systems, where it plays a vital role in creating complex flow structures. Therefore, understanding the impact of natural emergent vegetation on flow dynamics is essential for advancing eco-hydraulic modeling and improving sediment management in open-channel systems. This study provides a novel experimental measurement of streamwise and vertical velocities were used to quantify lateral and vertical variations in turbulent kinetic energy (TKE), Reynolds shear stress (RSS), and coherent turbulent events through quadrant analysis in a partially vegetated mobile-bed narrow channel, a configuration rarely explored for natural emergent vegetation. Experiments were conducted in a laboratory flume with an aspect ratio of 4.615 under steady flow conditions in a mobile bed channel, where Vetiver grass was embedded to replicate realistic field-scale vegetation properties. Velocity measurements were systematically recorded by using a 5 cm down-looking micro ADV at different hydraulically distinct position: within the vegetated zone, at the interface of vegetation and non-vegetation zone, and on the fully non-vegetated zone. The findings determine that emergent vegetation induces strong velocity attenuation, alters vertical shear profiles, and produces a sharp velocity gradient across the interface due to intensified momentum exchange. TKE and RSS analysis reveals localized turbulence amplification at the interface of vegetation and non-vegetation zone, while quadrant analysis uncovers the dominance of outward interactions and sweep events in vegetated and interface regions, indicating intensified bursting activity that promotes near-bed momentum exchange and sediment entrainment, which are quantified for natural emergent vegetation in mobile-bed conditions. The novelty this study lies in integrating high-resolution turbulence event classification with real-plant laboratory experiments on a deformable bed, enabling a more physically grounded understanding of flow-vegetation-sediment interactions. These outcomes provide a nature-based solution for improving predictive models and guiding river engineering and ecological restoration approaches.