<p>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.</p> <p>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.</p> <p>A combined methodological approach to parameterize key boundary conditions of an Alpine proglacial outwash plain (Jamtal valley, Austria) with an area of 0.035 km<sup>2</sup> 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.</p> <p>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.</p>
<p>High mountain environments have shown substantial geomorphological changes forced by rising temperatures in recent decades. As such, paraglacial transition zones in catchments with rapidly retreating glaciers and abundant sediments are key elements in high alpine river systems and promise to be revealing, yet challenging, areas of investigation for the quantification of current and future sediment transport. In this study, we explore the potential of semi-automatic image analysis to detect the extent of the inundation area and corresponding inundation frequency in a proglacial outwash plain (Jamtal valley, Austria) from terrestrial time-lapse imagery. We cumulated all available records of the inundated area from 2018-2020 and analysed the spatial and temporal patterns of flood flows. The approach presented here allows semi-automated monitoring of fundamental hydrological/hydraulic processes in an environment of scarce data. The pixel classification based on greyscale values from oblique hourly recordings returned plausible results of the spatial and temporal variability of surface runoff in the investigated glacier forefield. The image sets, processed in ImageJ, allowed geo-rectification to produce inundation frequency maps. Meteorological and discharge data from downstream measuring stations was consulted to interpret our findings. Runoff events and their intensity were quantified and attributed to either pronounced ablation, heavy precipitation, or a combination of both. We also detected an increasing degree of channel concentration within the observation period. The maximum inundation from one event alone took up 35% of the analysed area. About 10% of the observed area presented inundation in 60-70% of the analysed images. In contrast, 60-70% of the observed area was inundated in fewer than 10% of the analysed period. Despite some limitations in terms of image classification, prevailing weather conditions and illumination, the derived inundation frequency maps provide novel insights into the evolution of the proglacial channel network.</p>
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