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.