Simulation of proglacial forefield morphodynamics and their implications for the filtering of subglacial sediment export following glacier retreat

Proglacial forefields are geomorphologically active zones with rivers that may buffer sediment connectivity between glaciers and downstream regions. To date, there are few studies of the magnitude of this process and the role of system-internal perturbations, such as meltwater discharge rates, subglacial sediment supply and changing valley topography (e.g. valley slope, lateral accommodation space) following glacier retreat. Here, we use a physically-based morphodynamic model, calibrated using continuous field-collected data, to investigate the geomorphic response of these fluvial systems to varying topographical, sedimentological and hydrological boundary conditions. Results confirm the central role of the supply to transport capacity ratio in determining both proglacial stream geomorphic response and downstream sediment delivery rates, particularly for bedload particles; while suspended sediment transport is less affected by morphodynamic processes. Simulations also showed that the filtering effect is influenced by the lateral accommodation space and valley bottom slope. These observations not only suggest that the filtering varies between geomorphological settings, but also that the form of a valley (width, slope), as it is revealed following glacier retreat, will also impact filtering. These findings are important for understanding the future evolution of the longitudinal sediment connectivity in deglaciating catchments, in light of likely peak water and peak sediment. In the near-future, the evolution of the proglacial margin filtering will have repercussion for the supply of sediment towards downstream regions, for the management of hydropower plants and for aquatic ecosystems.