Grain shear stress and bed-load transport in open channel flow with emergent vegetation

•The impact of bedform generated around stems on bedload transport was investigated.•Form drag of bed needs to be accounted for in the calculation of grain shear stress.•The grain shear stress model can reasonably predict the bedload rate. The prediction of bedload transport capacity in riverbed with aquatic vegetation is a pressing need for river restoration. Laboratory experiments were conducted to investigate the weak bed-load transport rates with emergent vegetation for different energy slopes and vegetation densities. Such rate was observed to increase as the energy slope increased and the vegetation density decreased. The bed shear stress calculated by subtracting the vegetation drag from the total flow driving force was unable to predict the measured bed-load transport rate. On the other side, the estimation of the grain shear stress using the Einstein partitioning method considering the form drag provided a reasonable prediction. On the basis of these results, an explicit formulation for estimating grain shear stress was developed to predict the bed-load transport rate in a vegetated channel by considering the superimposition of forces. In addition, a simple relationship between grain shear stress and total shear stress was established by the boundary layer model. It was inferred that bed-load transport rate decreases with the increase of vegetation blocking factor while the total shear stress remains constant. Comparatively, the proposed models were more accurate than literature models, improving the understanding of the bed-load transport in a vegetated flow.