Bed roughness in gravel-bed rivers: Channel-scale responses to flow and sediment dynamics

Abstract

Understanding flow resistance is fundamental for both scientific research and the development of engineering applications in fluvial systems. Accurate estimation is crucial for assessing flood magnitude, predicting ecological habitats, estimating sediment transport, and analyzing river morphodynamics. However, natural gravel beds exhibit complex three-dimensional organization, shaped by continuous processes of transport, deposition, and sediment reworking during flow events. This study investigates bed roughness behavior under sediment supply suppression in gravel-bed channels, assessing its variation with flow conditions and sediment dynamics. Experiments were conducted in a 10 m-long, 0.6 m-wide rectangular flume with a mobile bed, divided into two phases. In Phase 1, a morphologically stable bed configuration was established, achieving equilibrium and consistent sediment transport under a given discharge. Preliminary bed stabilization proved essential, as material adjustment and fine sediment infilling significantly influenced hydraulic response. In Phase 2, the hydraulic roughness of the stabilized bed was evaluated. Results showed that flow resistance decreases with increasing relative submergence, whereas low-flow and low-submergence conditions led to higher roughness due to bed microtopography and armoring layer development. Empirical equations underestimated resistance under high relative roughness, indicating the need for more specific formulations. Importantly, this is among the first experimental studies addressing bed roughness processes for a Brazilian mountainous river, providing novel insights for similar environments. The study also confirmed classical sediment transport concepts, such as grain-size sorting, armoring formation, and a critical discharge threshold for sediment mobilization. These findings contribute to improved parameterization of numerical models and advance the understanding of fluvial hydraulics.