Predicting flow resistance in open-channel flows with submerged vegetation

In vegetated flows, hydrodynamic parameters, such as drag coefficient, frontal area and deflected canopy height, influence velocity distributions, mean velocity and flow resistance. Previous studies have focused on flow-structure interaction in sparse vegetation, dense vegetation or transitional canopies, respectively. To date, a unifying approach to estimate hydrodynamic properties of submerged vegetated flows across the full vegetation density spectrum is missing. Herein, published data sets across a wide range of vegetation conditions were re-analysed using a previously proposed four-layer velocity superposition model. For the investigated vegetation conditions, the velocity model was able to match measured velocity distributions and depth-averaged mean velocity. The contribution of each velocity layer to the mean velocity was analyzed, showing that the mixing layer is dominant in transitional canopies with shallow submergence, and that the log-law layer is dominant in denser canopies with deeper submergence. Based upon velocity distributions, an explicit equation for the Darcy-Weisbach friction factors was deduced that is able to predict flow resistance as function of relative submergence. While each velocity distribution could be well described with the four-layer model across the range of vegetation conditions, some data scatter in model parameters was observed. To improve predictive capabilities of the model, future research should focus on detailed velocity measurements with high spatial resolution.