Hydropower's regulatory capacity is crucial in balancing the increasing integration of intermittent renewable energy sources, such as wind and solar power, into the energy system. This is anticipated to result in frequent start-stop cycles in hydropower operations, leading to increased flow fluctuations, commonly known as hydropeaking, which may negatively impact the riverine ecosystem. According to the European Union’s Water Framework Directive, all water bodies in the EU should achieve good ecological status. Therefore, it is essential to predict the ecological effects of hydropower using hydraulic modeling tools.

In rapids, tributaries, and small streams, where the bed structure is in comparable scale to the water depth, the flow fields become highly disturbed. Such variations in bathymetry can play important ecological roles. Regions characterized by large bed roughness and fluctuating discharge can support multiple aquatic species simultaneously, forming so-called complex habitats. These conditions render empirical assumptions of flow resistance, such as the Gauckler-Manning coefficient, inadequate for accurately describing hydraulic behavior in these streams, leading to inaccurate predictions of water depth and local flow properties. To improve flow resistance formulation in shallow areas with large bed roughness, a broader understanding of the flow field response to changes in flow depth is required. 

This study investigates the effect of relative submergence (i.e., water depth relative to boulder size) on flow field characteristics through flume experiments. Idealized stone shapes (hemisphere and cube) are used to resemble varying river conditions. A Lagrangian Particle Tracking (LPT) method, commonly known as 3D Particle Tracking Velocimetry (PTV), is employed to capture time-resolved volumetric data with high spatial resolution, enabling measurements close to objects. The results provide insights into fundamental hydrodynamics and support ecohydraulic measures, such as river restoration efforts and solutions for fish passage. Additionally, qualitative discussions explore the potential effects on fish habitats due to variations in submergence and additional roughness elements.

These experiments also serve as reference cases for validating Computational Fluid Dynamics (CFD) models. The findings of this work serve as a foundation for future research involving larger, more natural boulders and lower relative submergences in wider flumes, improving our understanding of ecohydraulic processes at larger spatial scales.

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.