Opposing Water-Level Biases of Distributed and Bulk Manning’s Roughness Parameterizations in 2D Modeling of Partly Vegetated Channels: A Patch-Configuration-Dependent Assessment

Vegetation is often retained in river channels for habitat conservation and restoration and to facilitate nature-based flood management, and its hydraulic effects are typically represented using flow resistance coefficients in two-dimensional (2D) models. This study evaluated two Manning’s roughness parameterizations in HEC-RAS 2D against large-scale experimental data for grouped and isolated willow patches under high- and low-flow conditions. A distributed approach, which represents spatially varying total resistance using a momentum-based resistance formulation, was compared with a spatially uniform bulk coefficient applied to the entire reach. Because the bulk coefficient was back-calculated from the measured experimental data, it served as an observation-based benchmark rather than an independently derived prediction. A sensitivity analysis established a terrain resolution of 0.001 m and a mesh size of 0.25 m as appropriate for the simulations. The distributed approach consistently underestimated water levels, exhibited an incomplete but directionally correct response to changes in the drag coefficient for grouped patch configurations, and was negligibly sensitive to changes in the drag coefficient for isolated patch configurations. In contrast, the bulk approach reproduced water levels more closely but overpredicted them for grouped patch configurations and could not resolve local flow structures. Patch arrangement appeared more influential than vegetation density; however, their individual effects could not be separated because the grouped layout was also the densest. These findings provide guidance for selecting resistance parameterizations in 2D models of partly vegetated channels.