Experimental Investigation of Flow-Turbulence Behaviour in a Channel with Vegetated Mid-Bar

Compared to straight and meandering rivers, turbulent flow features around a mid-channel bar in braided rivers are far more complex. It is believed that the continuous expansion of these bar deposits causes the commencement of braiding. This article will therefore address the turbulent flow characteristics around mid-channel bars. In an experimental flume, a scaled-down physical model of a mid-channel bar deposit is built to examine the flow structure. Although the geometry of mid-channel bar deposits is not uniform, an elliptical shape is suggested here for simplicity. Rigid cylindrical vegetation is planted over the bar with submergent and emergent flow condition. An Acoustic Doppler Velocimeter (ADV) has been used to measure the three dimensional velocity component. Results indicate that the fluvial process associated with the flow is greatly influenced by flow velocity and turbulent kinetic energy. A flow divergence above the bar head and a flow convergence at the bar tail make up the bulk of the flow structure. Near the upstream end of the bar, adjacent regions are where the flow is perceived to accelerate. The bar's existence reduces the flow area nearby, leading to increased velocity in parts close to the upstream end of the mid-channel bar. The velocity was reduced by 15 to 20% from upstream section to downstream section of the bar. The velocity incriment was observed to be about 10% in vegetated conditon as of non-vegetated condition. Additionally, it has been observed that the momentum exchange is higher in the top of canopy and gradually reduces as it approaches the lower canopy and bar level. The findings obtained from this study can further utilized to examine braid bar occurrences in alluvial rivers to build an appropriate response through training techniques.

Keywords: Braiding, Flume, Mid-Channel Bar, Turbulent Flow, Rigid Vegetation