Impact of floating vegetation island on velocity distribution in open channel flow

Floating vegetation island (FVI) are widely being adopted in various river restoration projects as they can help in decontaminating pollutants. On the other hand, FVI can cause many problems in open channels because they affect the flow movement and flow structure. The knowledge of the hydrodynamic nature of flow in the presence of FVI with varying canopy heights (the gap between channel bed and canopy) is limited. The present study conducted a three-dimensional numerical modeling technique using FLUENT software. The Reynolds stress turbulence model was adopted to imitate the flow behavior, and the validation of the numerical model was performed by conducting a physical laboratory experiment in a glass-sided flume. The results identified that the adopted numerical model can successfully replicate the flow characteristics. Different FVI parameters (canopy height, vegetation density, and placement in the form of symmetrical and asymmetrical arrangement) were tested while investigating the detailed velocity distribution. The vertical profile of streamwise velocity was characterized by three zones i.e., adjustment zone, developing zone, and fully developed zone. Two mixing layers around the interfacial regions (one at the vertical interface of the root canopy while the other at the lateral interface) were identified. The inflectional instability or the gradient in the velocity profile became stronger with the reduction in canopy height with reference to the channel bed. The variation in flow resistance strength is found to be prominent with the height of the canopy parameter. The asymmetrical arrangement of FVI showed the highest flow relaxation i.e., a 76% increase in flow velocity, in the alternate (free) region compared to that in the vegetation region. A low-density root canopy case experienced relatively less drag i.e., 37%, in the canopy column and ultimately the impact of increasing velocity in the gap region beneath the canopy was reduced by 54%. The outcomes of the study highlighted the importance and management of unanchored root canopy in open channel flows.