Flow Resistance due to Aquatic Vegetation in Streams and Flumes

Although several studies regarding flow resistance due to aquatic vegetation for small-and full-scale are available, the scaling of small-scale results to full-scale conditions still needs to be investigated. In this paper, a theoretical flow resistance equation for open channels was tested using literature measurements performed for full-scale field channels and small-scale experimental flumes with aquatic vegetation. At first, the relationship between the scale factor Γ of the velocity profile, the Froude number and the channel slope was calibrated by using 27 field measurements by Nikora et al. (2008). This relationship was also tested by 39 field experimental series by Okhravi et al. (2022). Then, the proposed relationship for estimating Γ was coupled with the theoretical flow resistance law to assess the performance in the estimate of the Darcy–Weisbach friction factor values. The results demonstrated that the proposed approach gives a more accurate estimate of the Darcy–Weisbach friction factor as compared to that obtained applying the literature relationships. The relationship between Γ, the Froude number and the channel slope obtained for the field condition was recalibrated by flume data changing only the scale coefficient. The analysis demonstrated that, for known hydraulic conditions (slope, Froude number), for scaling the Γ values estimated by field data (streams, rivers), ΓF, to values corresponding to laboratory conditions (flumes) ΓL, a scale factor of 0.5356 must be used. In conclusion, comparing the same hydraulic conditions, streams or rivers are characterized by friction factor values approximately twice those of a laboratory flume.