Laboratory investigation of bedload morphological effects and dynamics under varying flow confinement.

Gravel-bedded rivers are crucial to riverine ecosystems worldwide, offering vital ecosystem services and often experiencing significant human impacts. Water, sediment, and vegetation interaction are central to river morphodynamics. Understanding these interactions is essential for effective river management and restoration. Despite advancements in topographic survey technologies and flume experiments, key aspects of bedload transport and riverbed morphology remain unclear, especially regarding the time scales of morphological changes. The spatial patterns of morphological changes and sediment dynamics, including entrainment, transport, and rest stages, are also poorly understood due to the spatial-temporal variability of bedload transport and challenges in direct flood measurements and modelling. Indirect methods such as geophones, tracers, and sediment volume budgeting have addressed these gaps. Morphological methods, based on sediment mass conservation, estimate sediment transport by tracking changes in river morphology. However, these methods are time-intensive, and their accuracy remains uncertain. While useful for rivers with high spatial-temporal variability, further understanding of bedload dynamics is needed to validate these approaches. This thesis utilizes laboratory experiments to address these knowledge gaps to investigate sediment transport dynamics and morphological imprints in a controlled environment. This approach provides deeper insights into the relationship between bedload and its morphological imprint. We investigate how bedload active width and morphological active width differ across various types of gravel-bed morphology and how these active widths change over different temporal scales. It will also explore the underlying mechanisms responsible for the differences between bedload and its morphological imprint. In addition, we analyse the temporal and spatial trends of tracer distributions in response to changes in flow rate within a consistent morphology, aiming to separately assess the influence of flow rate and morphological factors. We explore the influence of the main deposition areas on the path taken by sediments. We find that the difference between the morphological and bedload active width depends on the flow confinement. Morphological active width envelope from high temporal resolution DoDs improve bedload active width estimates, in particular in low confined systems. Not all areas experiencing bedload transport undergo morphological changes, which is central to the distinction between bedload and morphological active widths. This difference is attributed to near-equilibrium transport and the compensation and filtering processes. Compensation processes were observed to increase with discharge with their effects amplified when survey frequency decreases. In a broader context, compensation affects not only the active width but also the volume of sediment transported. The research underscores the critical role of compensation in sediment dynamics, particularly as the time between topographic surveys increases. The intensity of bedload transport in these areas is also substantial. In contrast, the influence of filtering processes shows a slight reduction with increasing discharge and decreasing survey frequency, with bedload activity in these areas being considerably lower than in compensation zones. Tracer displacement exhibited a clear correlation with flow confinement and bed morphology. Tracers are more likely to settle in depositional zones. The spacing between distribution modes and depositional areas offered further insights into tracer behaviour. In particular depositional zones act as attractors resulting in multi-modal distributions. This finding provides valuable insights into the frequency of river observations and has important implications for the application of methods such as the virtual velocity approach, where the volume of eroded sediment is directly proportional to the bedload flow rate. The findings of this research offer significant contributions to understanding the complex interplay between bedload transport, morphological changes, and flow confinement.