Hydrodynamics of four moments in the life of a floodplain forest in compound channels

Abstract

The dynamics of compound channels are of critical importance to various fields, including flood management, environmental conservation, and hydraulic engineering. This thesis investigates the hydrodynamics of compound channel flows, with a particular focus on the interaction between floodplain forest vegetation and flow dynamics. Forested floodplains play a crucial role in biodiversity and ecosystem services, yet their natural succession and interaction with hydraulic processes pose challenges for flood risk management and navigability. Moreover, assessing the natural composition and distribution of forests in floodplains is essential to better understand hydrodynamics under natural conditions. This work addresses these challenges by scaling a real floodplain forest in the Upper Rhine and conducting laboratory experiments to examine how forest succession, management practices, and flood intensity influence the hydrodynamics of a compound channel flow. Data from a comprehensive field survey conducted in the “Rastatter Rheinaue” floodplain forest, in the Upper Rhine in southwest Germany, served as the basis for scaling vegetation characteristics in the laboratory experiments. The experiments were designed to replicate key conditions observed in the field, with controlled variations in vegetation composition, density, and flow discharge. Four stages of the secondary forest succession were investigated (Bare or non vegetated, Early, Mid and Late), alongside the impact of management practices, such as selective plant removal. Additionally, the effect of flood intensity was analyzed under both intermediate and deep flow conditions. Measurements of water levels, velocities, and solute dispersion were collected to characterize the hydrodynamics of a compound channel flow with nature-based floodplain vegetation, corresponding to the above-mentioned scenarios (succession ages and management options). These results provide insights into flood risk, navigability, hydraulic modeling, and the lateral exchanges of mass and momentum. The heterogeneous distribution of vegetation introduced local velocity effects that challenge conventional descriptions of depth-averaged streamwise velocity profiles. To address this, a model was developed to account for these local effects, incorporating the characteristics of the mixing layer and assigning physical meaning to key modeling parameters. This model enabled the description of flow dynamics and the analytical solution of important flow regions, such as the mixing layer at the transition between the floodplain and the main channel. A primary objective of this research was to understand how forest age and structure affect hydraulic resistance and flow separation, which are critical for managing flood risks and ensuring navigability. The results indicate that forest aging must be considered in river management, as it has impacts on the upstream flood risk and has significant changes in the hydraulic roughness. Management practices, such as the removal of young trees, can change the characteristics of the floodplain forest, for instance modifying its hydraulic behavior from Mid age to Late age forest. This research shows that such management options have impact in upstream flood risk. This thesis also examines the complexities of the mixing layer formed at the transition between the main channel and the floodplain, which is crucial for sediment transport, nutrient exchange, and habitat connectivity. The presence of vegetation in the floodplain was found to increase the extent of the mixing layer within the floodplain and overall widen the total mixing layer width. These changes affect the transverse exchanges of mass and momentum, which have important implications for ecosystem functioning, as they influence the movement of solutes, organic matter, and aquatic organisms across the channel cross-section. Additionally, the movement of dissolved solutes was investigated, providing insights into dispersion patterns and tracer cloud development, and their relationship with the presence of a mean transverse flow. A conceptual model is introduced to describe the different flow regions in the transition from a compound channel without forested floodplain to a forested floodplain, which vary in terms of energy levels and mean flow. The characterization of this transition region in terms of flow development and their influence on lateral exchanges of mass and momentum are demonstrated. Understanding these zones can offer valuable insights into sedimentation patterns and nutrient availability, which in turn affect forest growth and the development of heterogeneous habitats. In conclusion, this thesis advances our understanding of the hydrodynamics of compound channel flow with forested floodplains, with significant implications for both hydraulic engineering and environmental management. By highlighting the role of forest succession in shaping flow characteristics, it provides valuable insights for improving flood risk mitigation, navigability, and ecosystem resilience in riverine environments. Key Points: • First research to hydrodynamically access natural vegetation distribution on floodplain, resembling floodplain forest composition in the Upper Rhine section. • Effects of forest aging, management, and flood intensity on river hydrodynamics and flood protection are highlighted. • A model was developed to describe the mean flow profile under the influence of local effects caused by vegetation. • Forest-induced mean transverse flow impacts lateral exchanges in compound channel flow with vegetated floodplains are shown. • For the first time, it was shown that forest age and management impact mass and momentum exchanges between floodplain and main channel. • A conceptual model of the flow in a compound channel with vegetated floodplain and its consequences to the mixing layer is proposed.