From multi-method monitoring towards numerical simulations of flood flow events in a proglacial outwash plain

Shifting runoff dynamics and highly intensified geomorphic processes are immediate consequences of the evident glacier mass loss in high-alpine headwater catchments. Rapidly retreating glaciers expose unconsolidated sediments to erosion in the proximity of meltwater-fed mountain streams impacting the catchment-scale sediment dynamics. Altering sediment fluxes can have considerable implications for the operation and management of water infrastructure, especially hydro-electric power facilities in otherwise non-regulated glaciated catchments. Bedload-rich outwash plains with typical braided channel networks serve as a deposition area for glacier debris under average runoff conditions. During flood flow conditions, the proglacial areas connect with the downstream catchment, delivering subglacial sediments to lower stream sections.

As such, they represent key elements in high-alpine river systems when considering future discharge and sediment yield from deglaciating catchments. Establishing a numerical model of this important component of the headwater catchment illuminates a data scarce fluvial process domain. Yet, model parametrization and setting boundary conditions for a glacier forefield are challenging. Direct measurements in the paraglacial transition zone of retreating glaciers are usually complicated to achieve, especially since outwash plains are frequently subject to intensive geomorphic processes. Therefore, innovative methods, minimizing labour-intensive and time-consuming manual surveying, are needed to overcome data scarcity in paraglacial environments.

A combined methodological approach to parameterize key boundary conditions of an Alpine proglacial outwash plain (Jamtal valley, Austria) with an area of 0.035 km² and an average channel inclination of 4.8 % is presented. Measuring discharge in situ is difficult since the braided riverbed is not stable due to frequent relocation of sediment. Therefore, close range sensing techniques based on RGB imagery from hand-held and fixed time-lapse cameras used in combination with maximum water level gauges are used directly in the outwash plain to monitor flood runoff events. A conventional discharge gauge (non-contact flow velocity and water level sensor) was realized 3 km further downstream covering the recent hydrologic summers (2019-2022). UAV-borne RGB imagery was used to detect changes in topography, sediment budget and composition.

We present results on key parameters, essential for numerical modelling of hydraulic flood flow conditions, including: (i) multi-annual high-resolution topographic 3-D models of the frequently changing channel geometry, (ii) hydraulic roughness of surface sediments derived from areal grain size distribution maps (i.e., D50, D84) and (iii) spatio-temporal flood flow maps indicating the annual variability in the surveyed proglacial outwash plain. These interrelated survey results are then used to parameterize and calibrate a 2-D numerical model (TELEMAC 2-D) to simulate hydraulic base and flood flow conditions, demonstrating the applicability and robustness of the presented multi-method approach.