An analytical model for transverse velocity distributions of overbank flows with submerged and emergent vegetated floodplains

The emergence of floodplain vegetation enhances the local drag and affects the velocity distribution of the overbank flow in a two-stage channel. Numerous analytical models (e.g., the modified SKM models) have been established to simulate the transverse velocity distributions of overbank flows with vegetation. However, the dimensionless eddy viscosity (lambda) was simplified to be a constant in these models when quantifying transverse momentum exchange. In this paper, we develop a novel analytical model to simulate the depth average velocity profiles of overbank flows with floodplain vegetation. Considering a micro-hexahedral element on a vegetated-bed area for a two-stage rectangular channel, the governing equation of the vegetated overbank flow is derived. An improved eddy viscosity model that considers the transverse variation of the lambda is employed to quantify the transverse momentum transfer, and a linear empirical function simplifies the secondary current. The simulated depth average velocities agreed well with the published measurements. In comparison with existing models, the proposed model shows better prediction accuracy, particularly in the shearing layer domain. More importantly, for the optimized model parameters, we found that the dimensionless eddy viscosity plays the most influential role when simulating depth-averaged velocities in the shearing layer domain, whereas the drag force dominates for vegetated floodplains. This study provides a convenient method for predicting the flow velocity in overbank floods with vegetation.