Vegetation in rivers, flood plains, and lakes plays an important role in maintaining the ecosystem and wildlife of rivers, in particular fish habitat. On the other hand, non-submerged vegetation in river bed and flood plains produce high resistance to flow and have a large impact on water levels in rivers, particularly during flood events. To avoid damages as result of floods, ineffective vegetation removal in rivers are sometimes implemented. For this reason, calibration and validation of river hydraulic models using a factual value for the friction factor that varies with flow and vegetation conditions is an important aspect of work dealing with floods and river management and remedy. Experiments are conducted on artificial rigid and flexible vegetation models to estimate drag coefficient and resistance to flow in vegetated waterways and flood plains under non-submerged condition. A momentum approach was considered for the drag measurements, and drag coefficient similarity was a fundamental for modeling. The effect of flexibility of vegetation on drag coefficient was of particular interest. A combination of individual vegetation foliage density (vegetation specie density index) and the spatial distribution of vegetation in a canopy led to introduce a new method for estimation of vegetation density effect. Correlation of drag force verses flow velocity resulted a non-linear relation with powers of 1.83 and 1.23 for rigid and flexible models, respectively. This reveals a significant increase of drag force and drag coefficient for the rigid models in comparison with the similar density flexible models. Also, the drag force and drag coefficient increased with increase of the vegetation specie density index and canopy density as well as the relative flow depth.
Keywords: Density index, Non-submerged vegetation, Flow resistance, Drag coefficient.