Shallow waterways such as rapids, tributaries and smaller streams can have important ecological functions in both free-flowing and regulated rivers. As more intermittent renewable energy is introduced to the energy system to reduce CO₂ emissions, the operational conditions of hydropower plants are changing. This implies various flow scenarios that can lead to more locations with shallow depths and larger variations in water levels and velocities, resulting in increased impact on the riverine ecosystem. Accurate predictions of these impacts require an understanding of the flow dynamics near large roughness elements such as boulders or trees in shallow river regions. This study uniquely investigates the effect of relative submergence, i.e., water depth relative to boulder size, on the flow field, turbulence, and potential fish habitats around idealized stone shapes (hemispheres) in shallow open channel flow using time-resolved 3D particle tracking velocimetry. The results indicate that varying relative submergence significantly affects recirculation zones, velocity and vorticity distribution, as well as turbulent kinetic energy. Notably, larger regions of lower velocity downstream of the roughness elements were generated at lower submergences, which might be favorable for fish energy conservation. Valuable insights into ecohydraulic engineering and habitat restoration in shallow waterways can be gained by understanding the fundamental flow mechanisms at low submergence for the flow around large roughness elements.

Boulders in riverbeds significantly influence sediment transport and deposition by altering local flow patterns. This study employs coupled Computational Fluid Dynamics–Discrete Element Method (CFD–DEM) simulations to examine how boulder concentration affects sediment dynamics. Three boulder spacing scenarios, representing isolated, wake-interference, and skimming flow regimes, are evaluated for their impacts on flow structure, bed shear stress distribution, and sediment transport and deposition. The fluid phase is modelled using a large eddy simulation in a finite volume framework, while individual sediment grains are tracked with the discrete element method. The results reveal that increasing boulder concentration transforms the flow regime from isolated wakes behind individual boulders to a more coherent recirculation zone among neighboring boulders. This transition substantially reduces near-bed shear stresses between boulders and leads to a decline in total sediment transport rates. At high boulder concentration, sediment particles preferentially accumulate in sheltered inter-boulder corridors, forming stable depositional belts. In contrast, widely spaced boulders lead to isolated, localized deposition around individual boulders. To our knowledge, this is the first attempt to apply a coupled finite volume–DEM approach to simulate boulder–sediment interactions in open-channel flow, enabling analysis of sediment transport and deposition under varying boulder concentrations.

Turbulent flow around well-submerged, wall-mounted obstacles has been widely studied, but the effects of low relative submergence on the flow field remain less understood. While submergence effects have been examined in experiments, uncertainties persist in the three-dimensional flow characteristics around large obstacles, particularly regarding its role in shaping instream habitats and fish migration pathways. The complex flow generated by such obstacles plays a key ecological role in natural environments (e.g., rapids, tributaries), influencing sediment transport, refuge zones, and turbulence structures that affect aquatic species. However, volumetric time-resolved measurements covering the upstream, side, and wake regions remain scarce. To examine the impact of submergence on the flow field, this study conducts laboratory flume experiments employing 3D Lagrangian particle tracking to assess two submergence levels around a wall-mounted cube. The response of key mean flow parameters, including streamwise and vertical velocities, as well as the production of turbulent kinetic energy, was examined, and time-resolved flow fields were analyzed using power spectra and proper orthogonal decomposition. Major vortical structures around the cube, namely horseshoe and arch vortices, are identified and analyzed. The study reveals that variations in relative submergence and shallow flow conditions strongly influence the flow field, with lower relative submergence being associated with increased flow complexity. These variations affect flow structures that govern habitat formation and fish migration. The insights gained contribute to a better understanding of flow around large obstacles, supporting ecological engineering and habitat restoration efforts in shallow waterways. Graphic