Sediment Dynamics in Proglacial Zones: Insights from Grain Size Distribution Mapping and Hydrodynamic Modeling

The retreat of alpine glaciers impacts and intensifies geomorphological processes in proglacial zones, driven by increased sediment availability and altered hydrological regimes. These dynamic systems transfer sediment from glacial sources to downstream fluvial networks, profoundly influencing sediment flux and fluvial morphology. This study investigates sediment dynamics within proglacial gravel plains of the Jamtal Valley (Tyrol, Austria), focusing on DEM of difference (DoD) analysis combined with grain size distribution (GSD) mapping as key tools for understanding sediment redistribution. Both are used as input to calibrate hydromorphological models. A multi-method approach was employed, integrating UAV-based photogrammetry, LiDAR surveys and manual ground-truth sampling. High-resolution local ground truth data were upscaled to large areas by applying a Random Forest Regressor, expanding spatial coverage and reducing dependence on labor-intensive field methods. This approach enables efficient and frequent monitoring even in alpine terrain that is difficult to access. Initial results demonstrate the effectiveness of integrating photogrammetry with semi-automated grain size detection algorithms to capture spatiotemporal variations in sediment properties. The temporal changes in elevation and GSD mapped since 2021 offer insights into sediment redistribution mechanisms. Glacial runoff and associated introduction of bed load trigger the transport and redistribution mechanisms in the glacier forefield. Whereas aggradation and surficial coarsening is assumed to occur under sediment supply-dominated conditions, contrasting to periods leading to a net erosion in the forefield. These dynamics underline the role of proglacial zones in buffering and modulating sediment fluxes connecting downstream river reaches. The hydraulic implications of GSD variability were analyzed using multiple roughness models within a 2D hydrodynamic framework, including Manning’s, Nikuradse’s and Ferguson’s roughness model. Applying the approach by Ferguson (2007) accounts for macro-roughness and variable submergence and revealed significant velocity variations and minor changes in flood extents. However, spatially differentiated roughness exhibited limited impact on water levels under varying discharge conditions, highlighting the nuanced influence of surficial sediment distribution on hydrodynamic behavior. To assess the morphodynamic behavior,  the associated 2D sediment transport model is being used to evaluate the use of GSD maps in advancing our understanding of sediment dynamics and hydromorphological feedback in proglacial environments. This work in progress focuses on impacts of various inflow conditions and model setups on shifts in sediment transport and spatial redistribution patterns.