Investigation of the flow structure affected by three vegetation patch densities in a cobble-bed river

The impact of submerged vegetation patch on hydraulic processes such as flow resistance and sediment transport is significant. However, a few researches have been reported for this issue in literature. Flow within and just above vegetation behaves similar to mixing layer rather than the boundary layer. This study applies the mixing layer theory to quantify the interaction between flow and submerged vegetation patch, emphasizing the effect of developing flow over small patches in the canonical mixing layer theory. Accordingly, it is essential to combine a canonical mixing layer model and modified equations to quantify evolving area along the patch. Field experiments were conducted over vegetation patch with three different densities in Beheshtabad cobble-bed River located in Chaharmahal-Bakhtiari province. The results reveal that there are reasonable agreement between the measured values of velocity and Reynolds stress profiles and the estimated ones by evolving mixing layer equations. However, the spreading coefficient of this model decreases by increasing the canopy density thanks to the limitation of vertical development of eddies. Quadrant analysis over vegetation patch shows the dominate event under the crest of canopy is “ejection” and over it is “sweep”. Moreover, the contribution of “sweep” event increase slightly at downstream of vegetation patch.