Resistance and boundary shear in a partly obstructed channel flow

In the present study, flow patterns in partly vegetated open channels are analyzed. The presence of emergent vegetation strongly affects the flow hydrodynamic structures, forming a transversal abrupt velocity-transition region at the interface between the obstructed and the unobstructed domains. It is observed that the transversal mean flow velocity distribution, at the interface, almost resembles a boundary layer feature, which made us adapting the universal law of the wall to predict its transversal profile in the fully developed zone. Analogous to what happened with submerged vegetated channels, in particular, a characteristic hydrodynamic roughness height can be determined, based on the universal loglaw, to quantify the resistance of the emergent vegetation. Results show that the hydrodynamic roughness height in addition to other flow properties depend on a contraction ratio, defined as the ratio of the width of the obstructed area to the width of the unobstructed area. Vegetation was simulated using an array of emergent steel-cylinders mounted on a large rectangular channel in the Department of Civil, Environmental, Building Engineering and Chemistry of the Technical University of Bari—Italy. The three components of the flow velocity were measured using a 3D Acoustic Doppler Velocimeter (vectrino-ADV).