Bed shear stress model for rough bed flows through rigid emergent vegetation

Abstract Hydrodynamics of vegetated flows plays an essential role in fluvial morphodynamics since vegetation leads to significant changes in local bed load fluxes. The interactions between bed roughness and vegetation modify the near-bed flow field, hampering the application of bed load transport models, which rely on accurate estimation of bed shear stress. This paper addresses the impact of sparse arrays of rigid emergent vegetation on the near-bed flow structure and proposes a method to estimate bed shear stress based on the similarities between the near-bed layer of rough bed open channel flows with and without vegetation. A set of laboratory experiments considering two different vegetation densities and a bare bed reference scenario are used to describe the changes and similarities in the near-bed layer flow structure in light of vegetation density. Bed shear stress was estimated through combined measurements with a shear plate and drag force sensors. Double-averaged characterization of the flow hydrodynamics was made through velocity measurements carried out with a Laser-Doppler Anemometer. The results show that the near-bed layer of rough bed flows covered with vegetation resembles the well-known logarithmic law of non-vegetated open channel flows. This finding is the basis for the development of a model to estimate bed shear stress in flows with rigid emergent vegetation. The proposed model is validated with external datasets, which showed very good agreement. The model can be applied in experimental research dealing with vegetation simulated as rigid emergent stems and promote and contribute to further understanding vegetation's impact on morphodynamics.