| 04 Oct 2025
Grain roughness controls on velocity and bed stress fields around a fully protruding obstacle in supercritical flow
Abstract. Supercritical flows in mountain rivers create complex flow-obstacle interactions that govern infrastructure vulnerability and channel morphodynamics, yet current understanding remains focused mostly on smooth-bed assumptions that poorly represent natural gravel-bed channels, where grain-scale roughness fundamentally alters flow physics near the bed and around obstacles such as bridge piers and in-stream vegetation. This study quantifies how bed surface characteristics control velocity fields, turbulent structures, and bed stress patterns around obstacles in supercritical flow through high-resolution detached eddy simulations coupled with volume-of-fluid free surface tracking. We examined three morphodynamic states representative of natural channel evolution: smooth beds analogous to bedrock channels, rough flat beds representing post-flood recovery conditions, and equilibrium scoured beds representing quasi-steady morphodynamic states. Digital representation of detailed bed surface elevation, including individual sediment grains, was considered using Structure-from-Motion photogrammetry. Numerical simulations reproduced characteristic supercritical flow structures including wall-jet formations, horseshoe vortex systems, and reverse spillage phenomena across all bed configurations. We observed that grain-scale roughness completely transforms flow organization from coherent, predictable vortical structures to chaotic, grain-dominated flow fields. While smooth beds exhibit symmetric stress distributions with organized patterns, rough beds generate highly skewed distributions with extreme spatial variability, where coefficient of variation increases from 37 % to 115 %. Individual grains work as micro-obstacles, creating localized stress concentrations exceeding smooth-bed conditions by factors of 2–3, which can fundamentally alter sediment transport mechanisms. An equilibrium scour hole creates hierarchical flow disturbances where large-scale topographic modifications interact with grain-scale disruptions to produce the most complex stress fields observed. These findings demonstrate that engineering design standards based on smooth-bed assumptions can significantly underestimate the spatial heterogeneity and peak stress magnitudes characteristic of natural rough-bed conditions. The transition from organized stress patterns in smooth beds to grain-scale dominated physics in rough beds necessitates fundamentally different approaches to flow prediction, infrastructure design, and morphodynamic modelling in steep channel environments.
General Comments
This manuscript presents a technically rigorous and scientifically meaningful study into how grain-scale roughness and scour morphology influence flow organization, turbulence structure, and bed stress fields around emergent cylinders in supercritical flow. The authors combine high-resolution Structure-from-Motion topography, grain-resolving computational meshes, and LES-VOF simulations with controlled flume experiments, an impressive methodological framework that allows them to examine supercritical hydraulics in gravel-bed channels at a level of detail that is rarely done.
The contribution is significant. Smooth-bed assumptions still dominate many hydraulic engineering, ecohydraulic, and sediment-transport models, yet are often inappropriate for steep, gravel-bed rivers where supercritical flows and complex roughness elements are common. This paper fills an important gap by documenting how roughness fundamentally alters velocity fields, coherent structures (e.g., horseshoe vortices), and shear stress distributions. The work is timely and well aligned with the growing recognition that grain-scale processes matter for slop e, velocity, roughness relationships in steep rivers.
The manuscript is overall well written and thoughtfully analyzed. However, the narrative is occasionally verbose and sometimes overstates interpretations using subjective wording (e.g., “remarkable,” “critical,” “severely”). The Results section in particular blends results with interpretation, and the Discussion sometimes introduces claims that should be more firmly grounded in existing literature. Organizational improvements, such as adding a table summarizing flow parameters across flume and numerical runs, would help the reader follow the experimental design.
With moderate tightening and clarification, the paper will be a strong contribution.
Specific Comments
Discussion