Debris at the edge: sedimentological dynamics of Hintereisferner's evolving terminus and glacial forefield, Austrian Alps

Abstract. Heavy precipitation events (HPEs) can lead to natural hazards (e.g. floods and debris flows) and contribute to water resources. Spatiotemporal rainfall patterns govern the hydrological, geomorphological, and societal effects of HPEs. Thus, a correct characterisation and prediction of rainfall patterns is crucial for coping with these events. Information from rain gauges is generally limited due to the sparseness of the networks, especially in the presence of sharp climatic gradients. Forecasting HPEs depends on the ability of weather models to generate credible rainfall patterns. This paper characterises rainfall patterns during HPEs based on high-resolution weather radar data and evaluates the performance of a high-resolution, convection-permitting Weather Research and Forecasting (WRF) model in simulating these patterns. We identified 41 HPEs in the eastern Mediterranean from a 24-year radar record using local thresholds based on quantiles for different durations, classified these events into two synoptic systems, and ran model simulations for them. For most durations, HPEs near the coastline were characterised by the highest rain intensities; however, for short durations, the highest rain intensities were found for the inland desert. During the rainy season, the rain field's centre of mass progresses from the sea inland. Rainfall during HPEs is highly localised in both space (less than a 10 km decorrelation distance) and time (less than 5 min). WRF model simulations were accurate in generating the structure and location of the rain fields in 39 out of 41 HPEs. However, they showed a positive bias relative to the radar estimates and exhibited errors in the spatial location of the heaviest precipitation. Our results indicate that convection-permitting model outputs can provide reliable climatological analyses of heavy precipitation patterns; conversely, flood forecasting requires the use of ensemble simulations to overcome the spatial location errors.

<p>Heavy precipitation events (HPEs) can lead to natural hazards (floods, debris flows) and contribute to water resources. Spatiotemporal rainfall patterns govern the hydrological, geomorphological and societal effects of HPEs. Thus, a correct characterization and prediction of rainfall patterns is crucial for coping with these events. However, information from rain gauges suitable for these goals is generally limited due to the sparseness of the networks, especially in the presence of sharp climatic gradients and small precipitating systems. Forecasting HPEs depends on the ability of weather models to generate credible rainfall patterns. In this study we characterize rainfall patterns during HPEs based on high-resolution weather radar data and evaluate the performance of a high-resolution (1 km<sup>2</sup>), convection-permitting Weather Research and Forecasting (WRF) model in simulating these patterns. We identified 41 HPEs in the eastern Mediterranean from a 24-year long radar record using local thresholds based on quantiles for different durations, classified these events into two synoptic systems, and ran model simulations for them. For most durations, HPEs near the coastline were characterized by the highest rain intensities; however, for short storm durations, the highest rain intensities were characterized for the inland desert. During the rainy season, center of mass of the rain field progresses from the sea inland. Rainfall during HPEs is highly localized in both space (<10 km decorrelation distance) and time (<5 min). WRF model simulations accurately generate the structure and location of the rain fields in 39 out of 41 HPEs. However, they showed a positive bias relative to the radar estimates and exhibited errors in the spatial location of the heaviest precipitation. Our results indicate that convection-permitting model outputs can provide reliable climatological analyses of heavy precipitation patterns; conversely, flood forecasting requires the use of ensemble simulations to overcome the spatial location errors.</p>

Glacial forefields host a wide array of processes and landforms, which can vary significantly, even within today’s overarching context of rapid melting and recession correlated to anthropogenic climate forcing. Detailed studies of the geomorphology and sedimentology of glacial forefields provide insight regarding sediment transport, meltwater pathways, and the behavior of the ice itself. Patagonia’s glaciers have been inventoried, there is vast knowledge of paleoglacier extent, and remote sensing has focused mainly on calculating geometrical changes and velocities. By comparison, detailed sedimentological analyses are long overdue and landsystems models for the present-day state of these environments require updating. This work focuses on the sedimentary processes that are occurring at modern glacial margins, specifically at selected sites in the Northern Patagonian Icefield and the neighboring Monte San Lorenzo massif to the east. High resolution geomorphological maps generated with photogrammetric data from an uncrewed aerial vehicle are presented. These maps, complimented with sedimentological facies descriptions and stratigraphic logging seek to characterize landsystems for the margins of glaciers in and near the Northern Patagonian Icefield, thus working towards an accurate reading of the sedimentary record and a better understanding of current glacial processes.

<p>In the Austrian Alps, recent rapid glacier melt affected the glaciers up to the summits. Glacier disintegration, debris flows, rock falls and increased melt rates are challenging glacier monitoring. Apart from the technical and safety issues arising, the scientific question arises how long a glacier fade out can and should be observed.</p> <p>From a technical perspective, direct mass balance monitoring is hampered by destruction of stakes by rock fall and snow pressure mounted on the increasingly steep glacier surfaces. Glacier changes require a frequent repositioning of stakes. Deep parts of the glaciers where stakes were not mounted in the past because the area was heavily crevassed remain longer that the thinner parts where stakes were operated in past decades, for example on Jamtalferner/Silvretta.</p> <p>Area loss meanwhile has a significant impact on glacier wide specific mass balance, but an annual resurvey of glacier area by terrestrial photogrammetry, airborne LiDAR or UAV rises the monitoring costs and effort considerably, while the uncertainty of mapping glacier area from remote sensing images is hampered by resolution and/or debris cover.  </p> <p>Subglacial melt in large cavities which developed during the last years so far is not quantified, but estimated from thickness loss data suggest that the contribution of basal mass loss to total mass loss can be as large as surface mass balance.</p> <p>Common definitions of a glacier imply ice formation from snow and firn, ice dynamics and a runoff system. For several Austrian glaciers, snow and firn cover is entirely gone. There, no new ice can form during the next decades even in case accumulation of snow would take place in future. In addition to that, flow velocities drop to <5 m/year, which is a similar magnitude than that measured for rock glaciers. From a glaciological perspective, those (former?) glaciers rather meet the definition of dead ice, especially when covered by debris.</p> <p>From a hydrological perspective, monitoring of those glacier remnants still makes sense, as they are still part of the local hydrological system. From the perspective of hazard monitoring, glacier remnants are also worth monitoring as they are relevant for generation of debris flows and englacial/thermokarst lakes.</p> <p>To continue local glacier monitoring and contribute to a global database, a joint global monitoring strategy and methodology would be beneficial. For example, a definition of ‘transient glaciers’ in fade out with a specification of monitoring methods would be helpful to prolongate mass balance time series when direct measurements become impossible for the total area. That could be single stakes, geodetic methods when glacier margins are still clear, or hydrological methods.   </p>

Mid-latitude alpine caves preserve a record of past solid precipitation during winter, locally spanning several centuries to millennia. Dating organic macro-remains trapped in ice layers allows the determination of timing and duration of past periods of positive and negative ice mass balance. We present here the largest comparative study of ice cave sites yet published, using Bayesian age-modelling on a database comprising 107 radiocarbon dates, spread over eight caves in the Austrian Alps. We show that periods of positive mass balance coincide with past glacier advances. We find organic and macro-remain rich layers dated to the Medieval Climate Anomaly (between 850 and 1200 CE) marking widespread ice retreat. We demonstrate positive ice mass balance at all studied sites for the Little Ice Age, coinciding with the largest glacier advances in the Holocene between 1400 and 1850 CE. At the sites with records spanning over 2000 years, positive mass balance is also observed during the periods from 300 BCE to 100 CE and 600–800 CE. These subterranean ice deposits show widespread evidence of accelerated negative mass balances in recent years and their record is under imminent threat of disappearing.

Modelling the feedbacks between mass balance, ice flow and debris transport to predict the response to climate change of debris-covered glaciers in the Himalaya

Debris flows typically result from a critical combination of relief energy, water, and sediment. Hence, besides water-related trigger conditions, the availability of abundant sediment is a major control on debris flows activity in alpine regions. Increasing temperatures due to global warming are expected to affect the periglacial environment and by that the distribution of alpine permafrost and the depth of the active layer. This might lead to increased debris flow activity and increased interference with human interests. Here we assess the importance of permafrost on documented debris flows in the past by connecting the modeled permafrost distribution with a large database of historic debris flows in Austria. The permafrost distribution is estimated based on the model PERMAKART 3.0, which mainly depends on altitude, relief, and exposition. The database of debris flows includes more than 4500 debris flow events in around 1900 watersheds in the Austrian Alps. We find that around 10% of documented debris flows occurred in watersheds having a permafrost fraction larger than 5% in their headwaters. Only around 50% of historic debris flow events were documented in watersheds where permafrost is clearly absent. Our results indicate that watersheds without permafrost experience less, but more intense debris flow events than watersheds with modeled permafrost occurrence. We find no trend of increased debris flow occurrence rate from permafrost regions in recent years. Our study aims to contribute to a better understanding of geomorphic activity and the impact of climate change in alpine environments.

A carefully homogenized climate dataset is used to interpret glacier behaviour in the Austrian Alps. Periods of glacier advance are generally more maritime and cooler, with reduced sunshine duration and increased precipitation sum during the ablation period. Periods of retreat are associated with a more continental, warmer climate, with increased sunshine duration and reduced precipitation sum. Three recent sub-periods of Austrian glacier behaviour are documented by direct measurement of glacier mass balance (before 1965 more negative; 1965–81 more positive; since 1982 more negative). A long-term mass-balance series in the eastern part of the Austrian Alps parameterized by snow-depth measurements indicates very clearly that periods of more negative mass balance have a higher correlation to summer air temperature and a lower correlation to winter accumulation. Periods of more positive mass balance are highly correlated to winter accumulation and only slightly correlated to summer temperature. The positive mass-balance period 1965–81 is also characterized by negative North Atlantic Oscillation index values which caused an increased meridional circulation mode, resulting in a northwesterly to northerly precipitation regime during winter.

The stability of a low Reynolds number flow on an inclined plane is investigated with respect to modelling the initiation of transverse wave-like ridges which commonly occur on the surfaces of rock-glacier forms. In accordance with field observations indicating the presence of stratification in rock glaciers, two models of rock-glacier structure are considered, each stratified and possessing a lower layer which is treated as a Newtonian fluid. An upper, less compliant layer is treated, alternatively, as a Newtonian fluid of viscosity greater than that of the lower layer, or as an elastic solid under longitudinal compression induced by a decrease in the slope of the underlying incline. A linear stability analysis is used to examine the behaviour of each of the proposed models, and both are found to generate instabilities at wavelengths comparable to those associated with transverse surficial ridges on rock glaciers. The growth rates of a flow disturbance predicted by the viscous-stratified model appear to be too slow to account fully for the development of wave forms of finite amplitude, suggesting that other mechanisms are involved in the amplification of an initial disturbance. The results of the stability analysis of the elastic lamina model indicate that finite surficial ridges may develop on rock glaciers as a product of a buckling instability in the surface region if there is a decrease in the slope of the underlying incline. Both of the analyses illustrate that transverse ridges can occur on the surface of a rock glacier in the absence of any variations in debris supply to the system. The results further imply that the use of these features in the paleoreconstruction of Holocene climatic conditions must entail an assessment of the relative roles of external climatically driven forcingversusinternal Theologically derived instability.

The stability of a low Reynolds number flow on an inclined plane is investigated with respect to modelling the initiation of transverse wave-like ridges which commonly occur on the surfaces of rock-glacier forms. In accordance with field observations indicating the presence of stratification in rock glaciers, two models of rock-glacier structure are considered, each stratified and possessing a lower layer which is treated as a Newtonian fluid. An upper, less compliant layer is treated, alternatively, as a Newtonian fluid of viscosity greater than that of the lower layer, or as an elastic solid under longitudinal compression induced by a decrease in the slope of the underlying incline. A linear stability analysis is used to examine the behaviour of each of the proposed models, and both are found to generate instabilities at wavelengths comparable to those associated with transverse surficial ridges on rock glaciers. The growth rates of a flow disturbance predicted by the viscous-stratified model appear to be too slow to account fully for the development of wave forms of finite amplitude, suggesting that other mechanisms are involved in the amplification of an initial disturbance. The results of the stability analysis of the elastic lamina model indicate that finite surficial ridges may develop on rock glaciers as a product of a buckling instability in the surface region if there is a decrease in the slope of the underlying incline. Both of the analyses illustrate that transverse ridges can occur on the surface of a rock glacier in the absence of any variations in debris supply to the system. The results further imply that the use of these features in the paleoreconstruction of Holocene climatic conditions must entail an assessment of the relative roles of external climatically driven forcing versus internal Theologically derived instability.

<p>The alpine infrastructure of trails and huts is an essential asset for summer tourism in the Austrian Alps. Every year, around five million people use the trail network for hiking and other mountaineering activities. Mass movements such as shallow landslides, debris flows and rockfalls cause significant damages to the alpine infrastructure and may block access to certain mountain areas for weeks or even months. Such damages require repair and increased maintenance activity or even rerouting of trails. Climate change will exacerbate the problem as more frequent and severe mass movements can be expected. Therefore, the Alpine associations have to take natural hazards into account for their trail and hut management.</p><p>A promising opportunity for assessing the impact of natural hazards on alpine infrastructure arises through the new generation of Earth observation (EO) satellites of the European Copernicus programme. The high spatial and temporal resolution allows the detection of mass movements with an impact on trails and huts.</p><p>Therefore, we initiated the project <em>MontEO</em> (<em>The impact of mass movements on alpine trails and huts assessed by EO data</em>) to investigate the opportunities for EO-based mass movement mapping and hazard impact assessment for alpine infrastructure. We start with a user requirements analysis that describes the demand for consistent and appropriate information on mass movements for alpine infrastructure management. We perform interviews with the Alpine associations and other relevant stakeholders. They help us to identify significant mass movements, their impact on the alpine infrastructure, and the actions that trail keepers and hut facility managers take to deal with the impacts. Based on this, we assess the suitability of EO-derived mass movement information for alpine infrastructure management, and define requirements for its production and delivery.</p><p>Based on the user requirements, we develop a multi-scale approach and combine optical and synthetic aperture radar (SAR) satellite data (e.g. Sentinel-1/2, Pléiades) to comprehensively map mass movements and to detect mass movement hotspots. Further, we integrate the EO-based mapping results with ancillary data for landslide susceptibility mapping, and for modelling and simulating rockfalls and debris flows. Finally, we analyse the network of trails and huts in relation to the obtained mass movement information and thereby assess their impact on alpine infrastructure, i.e. identify the trails and huts that are (potentially) affected by mass movements.</p><p>We demonstrate the concept and methods for three study areas in the Austrian Alps: Großarl and Kleinarl Valley in Salzburg, Karwendel in Tyrol, and the Salzkammergut in central  Austria. For these areas, we will create EO-based mass movement inventory maps, hotspot maps, and hazard impact maps. We validate our results in close collaboration with the users and analyse their usefulness for alpine infrastructure maintenance and management. The outcomes of <em>MontEO</em> will contribute to improved maintenance efficiency and will lead to a safer alpine infrastructure with an increased value for hikers, the tourism industry and the society.</p>

<p>Processes like flash floods or debris flows, which typically occur in small headwater catchments, represent a substantial natural hazard in alpine regions. Due to the entrainment of sediment, the discharge of debris flows can be up to an order of magnitude larger compared to 100-year fluvial flood events in the same channel, which poses a great threat to affected communities. Besides the triggering rainfall, the initiation of debris flows depends on the watershed’s hydrological and geomorphological susceptibility, which makes it hard to predict and understand where and when debris flows occur.</p><p>In this study we aim to quantify the influence of geomorphologic characteristics and long-term sediment dynamics on debris flow activity in the Austrian Alps. Based on a database of debris-flow events within the last 60+ years, a geomorphological assessment of active and non-active sub-catchments in different study regions is carried out. In a first step, we derive geomorphological characteristics, such as terrain roughness, Melton number as well as weathering potential of geological units found within the watersheds. Based on the findings of the terrain shape analysis, a set of representative watersheds will be selected for systematic monitoring of surface elevation changes over the project period of three years. This will be achieved by comparing digital surface models based on photogrammetric UAV surveys and monitoring of channel reaches with cameras.</p><p>In order to project these findings onto a larger regional scale, the derived terrain parameters will be used to integrate and extend a previously designed hydro-meteorological debris-flow susceptibility model (Prenner et al., 2018) with a sediment-disposition-model. This will form the basis for an advanced debris flow forecasting tool and help to better assess the impact of climate change on the magnitude and frequency of future debris flows.</p><p> </p><div><span>

In this study, energy and mass balance is quantified using an energy balance model to represent the glacier melt of Urumqi Glacier No. 1, Chinese Tian Shan. Based on data from an Automatic Weather Station (4025 m a.s.l) and the mass balance field survey data nearby on the East Branch of the glacier, the “COupled Snowpack and Ice surface energy and Mass balance model” (COSIMA) was used to derive energy and mass balance simulations during the ablation season of 2018. Results show that the modeled cumulative mass balance (−0.67 ± 0.03 m w.e.) agrees well with the in-situ measurements (−0.64 ± 0.16 m w.e.) (r2 = 0.96) with the relative difference within 5% during the study period. The correlation coefficient between modeled and observed surface temperatures is 0.88 for daily means. The main source of melt energy at the glacier surface is net shortwave radiation (84%) and sensible heat flux (16%). The energy expenditures are from net longwave radiation (55%), heat flux for snow/ice melting (32%), latent heat flux of sublimation and evaporation (7%), and subsurface heat flux (6%). The sensitivity testing of mass balance shows that mass balance is more sensitive to temperature increase and precipitation decrease than temperature decrease and precipitation increase.

This study examines the relationship between structural connectivity, forcing conditions and functional connectivity of debris flows in an alpine catchment in the Austrian Alps. We investigate two consecutive rainfall events in the Horlachtal valley in 2022 that triggered 163 and 69 debris flows, respectively, providing a unique opportunity to assess connectivity under different rainfall forcing magnitudes. Using the Index of Connectivity (IC) to represent structural connectivity, spatially distributed precipitation data for forcing and a debris flow–channel proximity metric to quantify functional connectivity, we evaluate how well the IC predicts debris flow–channel coupling with and without incorporating observed forcing information. Our results demonstrate that the IC serves as a robust predictor of debris flow connectivity across different forcing conditions, with strong correlations for both events. While observed rainfall forcing showed moderate correlation with functional connectivity, their inclusion in predictive models provided only marginal improvement (2% additional variance explained) over IC alone. This suggests that topographic and morphological constraints, rather than precipitation patterns, predominantly control debris flow propagation in this setting. Notably, the predictive capability of the IC proved relatively stable despite substantial differences in rainfall magnitude between events. Various regression models were evaluated, with quadratic and beta regression approaches performing best. The proximity metric used in this study offers advantages over binary coupling classifications by providing more nuanced information about functional connectivity, especially valuable when most observed processes do not reach the channel network. These findings empirically validate the IC as a meaningful descriptor of system structure in alpine catchments and suggest that challenges in spatial transferability of IC models likely stem from factors other than forcing variability.

Several heavy precipitation events causing flash floods, debris flows, landslides, and morphological channel changes have occurred in Europe over the last years. In mountain environments, mass movements along the hillslopes are important sources of sediment supply to the rivers, and may enhance the geomorphic effects of floods. The Stolla creek (catchment area: 40 km2) is a confined/partly confined channel of the Dolomites (Easter Italian Alps), that was affected by an extreme flood in August 2017, and by a moderate flood in August 2020. The geomorphic effects of the two floods were investigated in the main channel and along the hillslopes with the aims: to compare the channel changes induced by the two events; to assess the impacts of the lateral sediment connectivity to the channel response. A multi-methodical approach was applied, including radar rainfall estimation, rainfall-runoff modeling, field surveys, remote sensing, geomorphological and statistical analysis. Hillslope and channel processes were mapped by comparing multitemporal orthophotos and Digital Terrain Models. Debris-flow connectivity to the main channel was derived by combining field evidence and geomorphometric analysis. The 2017 flood was caused by rainfall with a short duration (6 hrs) and a rain rate exceeding 45 mm in one hour. More than 600 debris flows were triggered along the hillslopes, among which 23 were connected to the Stolla. Important discontinuities for the sediment flux were represented by the floodplains. The Stolla channel experienced channel widening occurred through bank erosion, and overbank depositions. With ratio (ratio between the channel width after and before the flood) was between 1.3 and 4.9. Widening was accompanied by channel bed aggradation up to 1.2 m or incision up to -2.2 m. Widening through bank erosion was more common in narrower reaches, affected by higher flood power, and presenting higher connectivity with debris flows. Although 294,000 m3 of sediments were eroded in the connected debris flows and 12,380 m3 were transferred to channel from toe erosion processes, limited volumes of sediments (< 1000 m3) were exported to the catchment outlet. The 2020 flood event was characterized by a lower rain rate (max 17 mm h-1) and a long duration (48 hrs) and did not trigger debris flows. The moderate magnitude of the flood peak did not lead to channel widening, but only bed incision (up to -1.4 m) in the reaches where the 2017 event had caused channel-bed aggradation occurred.