<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>
For morphodynamic modelling, riverbed survey data are essential as the basis for the evaluation of temporal riverbed development, mesh creation, and model calibration. To study the effects of uncertain geometry input on these issues, datasets of different spatial resolutions were analysed. As a result, cross-profile data were derived from high-resolution survey data, which are available for a river reach in the Upper Danube in Bavaria for several periods. Finally, the prediction quality of simulations based on cross-profile and high-resolution spatial data was assessed. The analysis of both datasets shows continuous riverbed erosion but of different magnitudes. However, complex riverbed geometry due to, e.g., scours, is depicted poorly by cross-profile data. In more homogenously characterised reaches, cross-profile data significantly more closely represents the riverbed geometry than the high-resolution spatial data base. Local misinterpretation of riverbed geometry by cross-profile data leads to deviations of calibration parameters in the entire study area. Consequently, these deviations in calibration outcome effect the model predictions. In this case, cross-profile calibration generally induces higher transport capacities, leading to more erosion in the study area compared to the model based on high-resolution spatial calibration. The general shape of predicted riverbed geometries is found to be similar but with local deviations, which are not limited to areas with complex river geometry.
<p>The presented work is part of the optimization of the sediment management at the hydroelectric powerplants in Reutte/H&#246;fen in Austria. The weirs of the two powerplants are situated at the alpine river Lech, located about 3 km upstream of the Lechaschau gauge (A=1012.2 km&#178;). Totally five sluice gates and a fixed overflow weir are controlling the upstream reservoir, being subjected to high rates of coarse bed load material. In frame of a coupled approach of physical and numerical modelling, different options to (i) avoid/minimize sediment deposition and (ii) allow improved sediment flushing were tested and optimized. Besides a lowering of energy losses (reduced spilling times) the reduction of depositions downstream close to the turbine outlet were considered.</p><p>The physical model covers the hydropower and weir system of both power plants within a stretch of 400m / 150m using a model scale of 1:25. Investigated situations covered periods of reservoir sedimentation, flushing of the reservoir and typical flood flow situations (e.g. HQ1 and an unsteady HQ5 event). For model parametrization, sediment samples to quantify size distribution were taken in the field. Sediment inputs to the model were realized dynamically and were required (due to scaling effects) to exclude cohesive fractions having a minimum particle size of 0.5 mm. The full-area surface measurement of the river bed was made by means of airborne laser bathymetry and echo sounding.</p><p>As part of an optimization of the overall sediment management strategy, the focus of the presented research is on the western located runoff power plant H&#246;fen. Via a lateral water intake, a maximum design flow of 15&#160;m&#179;/s is withdrawn causing that the intake structure is subjected to sediment depositions. Within the described scale model (1:25) and a partial scale model (1:15) covering the western side, several management options and configurations of sediment guiding walls were tested. Erosion and deposition as well as the transported material are assessed by 3D laser scanning and permanent monitoring of transported sediment load entering and leaving the scale model.</p><p>Complementary, a 2D hydro numerical model using a layer based multi fraction approach for sediment transport is set up for an extended area to simulate the morpho-dynamic behavior. The numerical model covers the whole weir system and 750 m of the upstream part of the Lech. The simulations made were realized at nature scale and allowed to mimic the erosion and deposition pattern obtained within the physical modelling for different tested options. Regardless of a chosen guiding wall setup, the results showed that each one is compromise between sediment defense and the effectiveness of the subsequent flushing processes.</p>
<p>Morphodynamic modelling relies on different types of riverbed surveys. Surveys are essential as the basis of the evaluation of temporal river bed development, mesh creation, and model calibration. Spatial data, for example, obtained by topo-bathymetric airborne laser scanning (ALB) or sonar surveys results in a dense point cloud, providing detailed information on the river bathymetry. However, data gaps can occur due to restrictions in data acquisition (e.g. high water turbidity or water depth for ALB, low water for boat-mounted sonar). In contrast, cross-profiles contain only limited information on the bathymetry strongly dependent on the cross-profile and point spacing.</p><p>To assess the effect of the two survey data types on river bed development and morphodynamic predictions, the temporal evolution of a river stretch in the upper Danube at Donauw&#246;rth was analysed. The study area contains homogeneous river sections and sections with complex river geometry due to scours, bridge foundations, and river mouths.&#160; Spatial sonar and ALB surveys were conducted from 2013 to 2020 and give detailed documentation of the river bed development. Cross-profiles with a cross-profile spacing of 200 m were derived from the spatial data. The spatial and cross-profile datasets show continuous river bed erosion. However, in this case, cross-profile data overestimate the overall erosion compared to spatial data. The geometry of homogeneous river stretches is depicted very similarly in the two datasets. For cross-profile data two cases exist for reaches with more complex river bed geometry: (i) The geometry lies in between two cross-profiles and it is missed entirely. (ii) The geometry is covered by a cross-profile and the resulting geometry is smeared in between the cross-profiles due to the interpolation process. Both possibilities result in an unsatisfactory depiction of the riverbed geometry.</p><p>To analyse the effect of morphological developments two morphodynamic models based either on the spatial or cross-profile datasets were set up. The models were calibrated against the datasets from 2013 to 2020 by adjusting the Strickler value for river sections with a length of 200 m. The Strickler values differ over the entire river stretch and not only in sections where complex river bed geometry occurs, meaning that the calibration errors propagate through the entire study area. Consequently, the deviations in calibration outcomes affect the model predictions, which simulate 7 years. In this case, the general shape of the predicted riverbed is similar, but due to the overestimation of riverbed erosion by cross-profile data, the morphodynamic model overestimates the erosion compared to the spatial data. However, the obtained error is for river reaches with low local variability within an acceptable range. If a project demands a highly accurate depiction of the river bed and the river geometry is known for having complex features, the use of spatial data is strongly advised.</p>
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