Assessing water surface profile simulations with bulk flow resistance and composite roughness methods in partially vegetated channels

This study investigates methods for applying bulk flow resistance and composite roughness coefficients to calculate water-surface profiles for partially vegetated channels. The feasibility of each method was evaluated based on reach-scale experiments by comparing water surface elevations simulated by a one-dimensional numerical model with high-accuracy water level measurements. Bulk flow resistance coefficients produced consistently reliable results for scenarios with relatively complex vegetation patch distributions and densely vegetated patches regardless of flow conditions. The composite roughness method, which estimates total flow resistance using a momentum-based model, demonstrated strong agreement with observed data (RMSE < 1 cm and R² > 0.9) under low-flow conditions with a single vegetation patch. For densely vegetated patches, adjusting the drag coefficient further improved the accuracy of simulated water-surface profiles. The findings provide practical and explicit guidance for hydraulic modelers by outlining clear procedures for estimating and applying flow resistance coefficients in river systems characterized by spatially heterogeneous patches of emergent vegetation.