Study on velocity profile and drag coefficient of flow through double layer vegetation

The present paper studies the characteristics of flow that passes through two-layer vegetation in open channels under emergent and submerged conditions for short and tall vegetation respectively. Various discharges and flow depths have been studied to provide a comprehensive sight of flow details through the vegetation layers, which are modelled by cylindrical dowels with 6.35 mm diameter and two heights of 10 mm and 20 mm installed in a 10 mm thickness. Flow velocity in different locations was measured by a Nortek ADV velocimetry. The velocity profiles in single layer vegetation under emergent condition show a uniform constant velocity all over the flow depth. In our study, experimental data indicate that there is more complexity around the edge of short vegetation which could be due to vortex shedding existence. Drag coefficient as a resistance parameter, has a larger value compared with open channel without vegetation. More specifically, in vegetation layers the resistance parameters are found to be dependent of the flow depth and submergence condition, on the other hand channels without vegetation usually has a constant roughness coefficient. When the short vegetation was fully submerged, the drag coefficient reduced through the flow depth. Meanwhile, drag coefficient showed a reverse tendency in mixed layer condition. Velocity profiles measured in different locations and depths indicate that location is an important factor to velocity profiles. The velocity profile above the short vegetation in two-layer vegetation conditions is much larger than that in single layer conditions. Generally, the flow velocity inside the vegetation layer is significantly smaller than that in the surface layer (i.e. non-vegetation layer). A near-constant velocity dominates inside the vegetation layer, and then starts to increase near the interface at the top of vegetation. There is a sudden change in the velocity profile near the top edge of short vegetation. The results also showed that in channels with double layer vegetation, the flow velocity is strongly dependent on locations.