Paraglacial Environments Research Articles

What controls river widening? Comparing large and extreme flood events

AbstractExtreme (i.e., centennial‐scale) floods are by definition rare and can therefore be difficult to study. As a result, case studies of response to extreme floods can provide unique insights. On 15 November 2021, the Nicola River in British Columbia, Canada, experienced a 200‐year flood that increased the average width of the Nicola River by more than 50% (35 m), leaving the only highway in the region impassable for nearly 12 months. This research assesses the spatial variability of erosion along a 71‐km‐long segment of the Nicola River during a period with a large flood (2015–2018) and a second period containing the extreme flood in 2021 (2018–2021). We use a random forest statistical analysis to explore the most important valley and channel characteristics affecting relative widening during both periods. Unit stream power, gradient and valley confinement were the primary determinants of erosion during the extreme flood, whereas vegetation cover, channel pattern and surficial material were less important. Erosion during the large flood event was not related to these variables, with channel pattern providing a better indication of widening potential. As the Nicola River is flanked by erodible glacial deposits, this work provides important insight into river response in the paraglacial environments that are common throughout much of Canada and the northern United States, as well as Europe and Asia, but less intensely studied. In these settings, the geomorphic response to large floods may not be representative of the potential erosion (and infrastructure impacts) that can occur during extreme events, which destabilize glacial terraces in confined reaches. Historical observations should therefore be used with caution in paraglacial settings as extreme events may not be captured in available data, creating a perception of stability in confined reaches that may have the potential to erode dramatically during extreme events.

Deglaciation patterns in the Upper Zemmgrund, Austria: An exploration of clean-ice disintegration scenarios

The European Alps are rapidly losing glacier mass due to climatic warming and are anticipated to be largely ice-free by the year 2100. Long-term glacier monitoring in the Alps provides a record of anthropogenically-driven climate change since the Little Ice Age maximum in ~1850. Understanding these long-term glacier changes provides a basis for mitigating hazards (e.g., mass movements) associated with a transition to a paraglacial environment and for predicting future scenarios. Here, we present a post-Little Ice Age-maximum record of glacial landscape changes in the Upper Zemmgrund, Austria, utilising a multi-method framework integrating multi-decadal geomorphological mapping, historical imagery and remote sensing of glacier change. This study contributes a high-resolution, quantifiable record of the transition from a glacial to paraglacial landscape. We find that individual glacier response to climatic change varies within the Upper Zemmgrund, attributed to glacier characteristics such as hypsometry and size. Nonetheless, all glaciers show signs of growing instability such as an increase in crevasses and the collapse of circular tension structures, which have dammed meltwater at the terminus of one glacier, posing a substantial hazard to downstream communities. Future glacier disintegration is anticipated to accelerate in the Upper Zemmgrund, which may result in an ice-free landscape within the next ~40–60 years.

Surface Instability Mapping in Alpine Paraglacial Environments Using Sentinel-1 DInSAR Techniques

Surface Instability Mapping in Alpine Paraglacial Environments Using Sentinel-1 DInSAR Techniques

Targeting the source of fine sediment and associated geochemical elements by using novel fingerprinting methods in proglacial tropical highlands (Cordillera Blanca, Perú)

AbstractIn the Cordillera Blanca, the glaciers delineated by some of the highest peaks in the tropical Peruvian Andes have experienced a fast retreat over the last few decades. At the foot of Artesonraju Peak, the glacier‐fed river conveys fine sediments to Parón Lake, which is enclosed in the proglacial area created by the retreating glaciers of the unique confluence of the Huandoy, Chacraraju, Nevado Caraz and Artesonraju peaks (above 6000 m.a.s.l.). This research addresses the sediment contributions of main glacial landforms to sediment mixtures delivered to the river and lake system (1.6 km2) by applying novel conservativeness and consistency methods for tracer selection and a frequentist unmixing model. A total of 27 surface soil and sediment sources (0–3 cm) were sampled from moraines, alluvial fans and recent colluvium, and 10 composite sediment mixtures, including suspended sediments, were collected to identify the sediment provenance. The unmixing results reveal large variability of the contributing sources related to their predominance and the locations of the mixtures. The largest contributions were from moraines, in percentages varying from 28% to as much as 77%. Alluvial fans contributed between 23% and 47% of the sediments, and colluvium material contributed the least, between 6% and 28%. At the time of sampling, ice melt discharge incising moraines and alluvial fans and eroding riverbanks were the main mechanisms of fine sediment delivery. Runoff over the bare surfaces of recently exposed moraines and the lack of vegetation cover in very active landforms also control sediment mobilization. Overall, highly dynamic changes in this paraglacial environment involve active sediment fluxes accompanying snow and ice melt. Our results indicate that in high tropical mountains, shrinking glaciers expose new surfaces of glacial materials, which deliver fine sediments. Identifying the contributing sources is of interest for water and sediment evaluation and management programs.

Paraglacial Rock Slope Stability Under Changing Environmental Conditions, Safuna Lakes, Cordillera Blanca Peru

Landslides or landslide-induced impact waves in high mountain lakes represent a high hazard for society, calling for realistic assessments of rock slope stability responsible for the process chain initiation. This task is often hampered by complex interplays of triggers, which effects on slope stability may be delayed by decades or even millennia, while historical records describing slope topography or landslide occurrences are usually shorter and incomplete. This article builds on rarely available detailed historical data describing the site of the 2002 rock avalanche in the Cordillera Blanca, Peru. It caused a dangerous impact wave in the Safuna Alta Lake resulting in a minor flood, but ongoing downstream development significantly increased the risk of a comparable event. Pre-2002 and post-2002 failure slope topography, 70 years long history of glaciation and landslide occurrences were combined with non-invasive field geological surveys and laboratory geotechnical analyses to characterize the distinct morphological parts of the failed slope with reliable engineering geological slope models. Slope stability was calculated for a series of environmental scenarios providing insights into the 2002 rock avalanche failure mechanism and dynamics as well as the role of glacier slope support for its stability. Results show that the rock slope stability is governed by discontinuous slip planes where rock bridges represent the most likely additional resisting forces. The effect of glacier support on the slope stability is limited under full-water saturation of the rocks and due to specific morpho-structural conditions. Importance of the long-term, progressive deterioration of the rock slope strength under paraglacial environment and repeated seismic shaking is illustrated by the fact that even the Little Ice Age maximum glacier extend only had minor positive effect on the pre-2002 rock avalanche slope stability. Despite of that, the slope remained without a major failure for decades or possibly even centuries. Its collapse in 2002 caused retrogressive movements of the adjacent slope, which remains highly unstable until now. Therefore the future safety of the lake would largely benefit from the implementation of a reliable slope movement monitoring system.

RECENTES ALTERAÇÕES NAS GELEIRAS E NOS SISTEMAS PARAGLACIAIS, ANTÁRTICA MARÍTIMA

O artigo investiga alterações nos sistemas paraglaciais e interconexões com a retração glacial em setores da ilha Rei George, Antártica Marítima. Foi analisado o ambiente proglacial, resultado da deglaciação recente, e identificados os tipos de formas de relevo em suas diferentes escalas. Estes registros são úteis para a reconstrução de estágios sucessivos evolutivos no sistema paraglacial. Os resultados demonstraram que o ambiente marginal às geleiras e sistemas paraglaciais estão evoluindo na ilha Rei George e há novas paisagens. Algumas geleiras apresentaram uma mudança importante entre 2000 e 2019, onde sua classificação mudou de geleira de terminação marinha para geleira de terminação terrestre (não-marinha). Atualmente há 21 geleiras com término em terra na ilha Rei George (correspondendo a 31% das geleiras) e 11 destas estão localizadas na Baía do Almirantado. 25% destas geleiras eram marinhas em 2000. Os novos ambientes paraglaciais (desde 2000) têm 1,7 km2 ao total de área. Os ambientes marginais às geleiras (como as geleiras Ecology, Wanda, Windy, Anna Sul e Baranowski) mostraram formas de gênese não glacial, como planície de lavagem, depositos de tálus, ravinas e canais fluviais no sistema paraglacial recente. O mapeamento geomorfológico evidencia que os processos paraglaciais se diferenciam entre os ambientes marginais ao gelo das geleiras, não são padronizados.
 Palavras-chave: processos geomorfológicos glaciais, áreas livres de gelo, mudanças climáticas.

An active large rock slide in the Andean paraglacial environment: the Yerba Loca landslide, central Chile

A ca. 2.5 million m3 landslide occurred in August 2018 in the Yerba Loca valley, Andes Main Cordillera (33° 15′ S), at about 4000 m a.s.l. The Yerba Loca landslide is a multirotational slide, with a main scarp and failure surface developed in a volcanic rock mass, with secondary scarps and tilted blocks disturbing the colluvial soil cover. No clear trigger could be identified, although the failure took place some weeks after the largest winter precipitation and a sequence of snowfall and snowmelt, in the context of a severe drought. Inspection of optical satellite images suggests that the landslide suffered slow deformation for at least 15 years, increasing in the months prior to the failure. To corroborate these precursor deformations, InSAR analyses were performed at two time and spatial scales. For over 3 years, deformation in the landslide area was detected, while the local, short-term analysis from the 7 months before failure shows line-of-sight deformation rates at the landslide site of over 10 cm/year. Deformation continues after the failure with decreasing speed, with indications of further activity and expansion of the failure zone. This implies a hazard of rock avalanche, debris flows and/or river damming and subsequent outburst floods that may endanger communities downstream. The Yerba Loca landslide is an example of rock slope failure in paraglacial conditions and the influence of climatic factors in the context of climate change for the central Andes. This event represents an opportunity for learning on landslide mechanisms, remote sensing monitoring and hazard assessment of slow, large volume landslides in the Andean highlands.

Paraglacial adjustment of sediment slopes during and immediately after glacial debuttressing

Daily time lapse imagery and pixel tracking was used to monitor and track spatial and temporal changes in sediment-mantled hillslopes, during and immediately after glacier retreat in the Fox Glacier/Te Moeka o Tuawe valley, New Zealand. Observations from 2014 to 2018 of the Fox Glacier and surrounding hillslopes show hillslope failure is primarily coincident with, and triggered by, glacier retreat with rainfall accelerating movement of the hillslope. During glacier retreat, failure of the hillslope primarily occurred through sediment sliding, internal deformation of the sediment and occasional surficial debris falls, flows and avalanches delivering approximately 9.2 M m3 of sediment directly to the underside of the glacier over the study period with a maximum daily averaged movement of 0.4 m per day of the main sediment mass. Following debuttressing, hillslope failure became dominated by localised rainfall-induced debris flows which initiated gullying of the sediment-mantled slope. Ongoing instability of the slope and associated movement is maintained by toe erosion from the Fox River and melting dead ice, while continued rapid failure is facilitated through localised debris flows. The tracking of temporal and spatial changes of sediment-mantled hillslopes during glacier retreat has shown broad-scale hillslope response to occur quickly within days of rainfall or accelerated glacier retreat, particularly during summer. Debris flows, commonly thought to be a dominant erosion process within paraglacial environments, only occur following complete debuttressing and are supply-limited, only occurring after sediment sliding has occurred. Unlike many other case studies, sediment connectivity immediately following glacier retreat is high due to a lack of storage space and high rainfall inducing mass movements, efficiently delivering hillslope sediments to the proglacial stream channel. Attempts to quantify displaced volumes of sediment from paraglacial systems are likely underestimated due to a) a lack of focus on the early and latter stages of debuttressing b) a reliance on debris flows being a primary transport mechanism, and c) sediment being delivered sub-glacially rather than supra-glacially, further enhancing connectivity.

Assessing paraglacial processes at Nexpayantla Gorge (Popocatépetl volcano, Central Mexico) using OSL and14C

AbstractWe evaluate the paraglacial activity in Nexpayantla, a subtropical mountainous gorge in Popocatépetl volcano (Central Mexico), fully deglaciated in the 20th century. Glacial advances are evidenced by the presence of moraines. Fluvio‐glacial terraces and an alluvial megafan resulted from the gorge deglaciation. Current reworking of the glacigenic material is done by landslides and debris flows produced on the moraines and terraces. To study the different phases of mobilization of glacigenic sediment, we used an approach based on the study of the optically stimulated luminescence (OSL) signals obtained from a portable OSL (POSL) reader in samples extracted from both glacigenic and paraglacial deposits. The luminescence (POSL) results obtained at moraines increase as altitude decreases, which is expected for deglaciated valleys where the oldest moraines are located at lower elevations. We evaluate the grade of luminescence signal reset of the glacigenic sediments during the proglacial stage, and the subsequent deglaciation phases. Our results indicate that there is a marked transition between glacial and fluvially dominated processes at Nexpayantla Gorge. We find that the grade of luminescence signal resetting in the paraglacial deposits is a good indicator to trace paraglacial stages and the beginning of exhaustion of the paraglacial activity in mountain areas. OSL ages confirm that the oldest fluvio‐glacial terraces found at the middle sector of Nexpayantla Gorge are ~2 ka, which is also supported by an AMS14C age. OSL dating was found challenging, since quartz grains have low sensitivity because of their volcanic origin; POSL signals, however, are in good agreement with the location and distribution of geomorphic markers. We propose that luminescence data obtained from the POSL unit can be useful to provide information about sediment mobilization in paraglacial environments during different climatic pulses – even for the case where mineral grains have low sensitivity, such as in volcanic sediments. © 2020 John Wiley & Sons, Ltd.

Paraglacial coasts: challenges for coastal conservation in the Anthropocene

A challenge for coastal conservation over the next decades is to predict and then effectively manage the outcomes of ongoing climate change in the context of the Anthropocene. Paraglacial coasts are those on or adjacent to present or formerly glaciated terrain and which are still influenced by glacigenic processes. Ongoing response of paraglacial coasts to the influences of glaciation can give rise to diverse regional-scale coastal responses that may variously amplify or suppress any effects caused by global climate change. Here we provide insight into the richness of landforms and coastal system responses from two contrasting paraglacial environments, in Svalbard where glaciers are still present but are actively retreating, and in Ireland where late Pleistocene glaciers melted away around 14,000 years ago. Svalbard and Ireland exhibit different paraglacial coastal responses which reflect long-term variations in sediment supply from inland source areas to the coast, and variations in sediment residence times and storage areas within the coastal zone. The conservation of paraglacial coasts in the context of Anthropocene global warming requires an understanding of regional glacial history and longer term coastal responses to paraglacial relaxation.

Frozen ground and periglacial processes relationship in temperate high mountains: a case study at Monte Perdido-Tucarroya area (The Pyrenees, Spain)

Seasonally frozen ground, mountain permafrost and cryogenic geomorphological processes are important components of the Pyrenean high mountains. This work presents the results of a study on the distribution of frozen ground in a marginal and paraglacial environment of temperate mountains. An inventory was made of landforms and indicators of frozen ground, and frozen ground was mapped accordingly. During 2014 and 2016 ground temperatures and thermal regimes were monitored, basal temperatures of snow-cover (BTS) were measured and a thermal map was drawn. Differential thermal behaviours were detected among different elevations and slope orientations. Periglacial processes are the most widespread, in which frost weathering and nivation, together with gelifluction and cryoturbation, are the most efficient processes; the latter two are generally linked to the presence of frozen ground. The fall in air and ground temperatures with altitude, slope orientations, and snowpack thickness and evolution determine ground thermal regimes. In the study area, three types of thermal regimes were established: climate-controlled, snowcover-controlled, and frozen ground-controlled. Seasonally frozen ground occurs across a broad range of elevation between 2650 and 3075 m asl, whereas possible permafrost only occurs above 2750 m asl.

Glacier decline in the Central Andes (33°S): Context and magnitude from satellite and historical data

Central Andes (33°S) represent a water-scarce region. During arid years, glacier runoff may constitute the main hydrological input at warm season and hence a steadfast deglacierization may represent a decrease in the regional water-budget. Ice-retreat enables landscape transitions from proglacial towards a paraglacial environment, allowing the formation of newly formed cryogenic deposits. Ice-surface changes in the Central Andes (33°S), including the high-mountain areas from Aconcagua, Mendoza and Maipo basins (Argentina and Chile), were studied using digitalized maps, aerial photographs, Landsat (1–8) and Sentinel-2A data for the period between 1956 and 2015. Band ratio and Normalized Difference Snow Index (NDSI) methods were tested using Landsat 8 and Sentinel-2A data for comparison. Geomorphological changes were assessed at Monos de Agua catchment (2750–4000 m a.s.l.) in the Aconcagua basin (Chile) as a regionally representative landscape transition case. Regional glacier shrinkage of 46 ± 5% between 1956 and 2016 was observed for the Central Andean sub-basins in both Argentina and Chile at 33°S. Overall, 107.1 ± 5 km2 of newly exposed surfaces are subject to permafrost conditions. Such insights raise concern in terms of current and future environmental assessments for newly formed cryospheric elements in water scarce regions.

Geomorfología paraglacial asociada a la inestabilidad de laderas en el Brazo Norte del Lago Argentino, Patagonia, Argentina

El retroceso y adelgazamiento de los glaciares a nivel mundial en las últimas décadas inciden directamente sobre la estabilidad de las laderas. La cuenca del glaciar Upsala y los glaciares del Brazo Norte del lago Argentino, han sufrido un marcado retroceso generando valles con pronunciadas pendientes y cubiertas con depósitos morrénicos inestables. Las laderas se encuentran fuertemente desestabilizadas, favoreciendo la generación de procesos geomorfológicos paraglaciales. El objetivo de este estudio es identificar y analizar la geomorfología paraglacial, asociada a los procesos de inestabilidad. Se realiza un análisis geomorfológico del terreno a través de la combinación de parámetros morfométricos y factores intervinientes que condicionan y desencadenan estos procesos, mediante el uso de imágenes de satélite. Los resultados muestran que la geomorfología paraglacial se encuentra influenciada por la combinación de: (i) parámetros morfométricos del terreno, entre ellos, elevaciones del terreno superiores a 700 m, pendientes promedios con un rango entre 25º-45º, orientación de las laderas este-noroeste con mayor insolación, curvatura cóncava del terreno y rugosidad leve a moderada (0,40-0,65); (ii) factores condicionantes, material morrénico depositado por los glaciares, afloramientos rocosos meteorizados y cobertura vegetal; (iii) factores desencadenantes, aporte de agua subterránea por infiltración de lagunas proglaciales y aporte superficial por precipitaciones, deshielo y escurrimiento, variación de temperaturas del aire y del suelo y variación del nivel del lago. En síntesis, las laderas que presentan mayores procesos geomorfológicos paraglaciales, producto de procesos de remoción en masa, son las que se encuentran en contacto directo con los glaciares Upsala, Bertacchi y Cono, la ladera occidental del canal Upsala y algunas zonas de los valles Moyano y Norte. El área se caracteriza por una combinación de ambientes glacial y paraglacial, siendo cada uno parte integral de la evolución del entorno.

Interpreting environmental changes from radionuclides and soil characteristics in different landform contexts of Elephant Island (maritime Antarctica)

AbstractSoils in ice‐free areas of Elephant Island (South Shetland Islands) have been forming since the last deglaciation following the glacial retreat that started in the area probably later than 9.7–5.5 ka. In paraglacial landscapes, landforms and processes in transition from glacial to nonglacial conditions are experiencing rapid environmental adjustments under conditions of climate change. Soils are highly sensitive and can be good descriptors of these transitional changes. A soil sampling campaign was undertaken for characterizing soils developed on moraines and marine platforms, underlain by metamorphic rocks and with distinctive periglacial features. Eight soil profiles were sampled to investigate the processes involved in their development and the relations with main landforms and processes of ice retreat. The stony Cryosols with mosses and lichens coverage are developed in permafrost environment with an active layer depth of 15–150 cm. Soil organic C content (0.16–1.6%) and large variations of P, K and N contents are related to ornithogenic activity. Soils on moraines and platforms show differences that reflect the more recent exposure of moraines that preserve most the characteristics of the parent material. More vegetated soils on platforms show 137Cs and 210Pbex activities (11 and 25 Bq kg−1, respectively) at the topsoil whereas absence of 137Cs and depleted levels of 210Pbex occurred in more recently exposed and less developed soils on moraines. Fallout radionuclides are good tracers for identifying characteristics of soil development and providing information on environmental changes of interest to understand the soil response to actual changes in unstable paraglacial environments.

Little Ice Age mapping as a tool for identifying hazard in the paraglacial environment: The case study of Trentino (Eastern Italian Alps)

The Little Ice Age (LIA) is a well-recognized climatic event during which the glaciers in the Alps advanced and reached their maximum Holocene extent. During their retreat following the LIA, the glaciers left large areas of loose or poorly consolidated glacial deposits in their forelands, which are subject to paraglacial reworking and may represent potential hazards for human infrastructures. In this study, we present a regional scale mapping of the LIA and post-LIA glacial deposits and a reconstruction of the maximum LIA extents of glaciers in the same area. This work is motivated by a local law requiring the classification of areas subject to natural hazards in Trentino (Italian Alps). Results highlight that glaciers shrunk by 63% from the LIA maximum, leaving 30km2 of unconsolidated deposits, which are subject to geomorphic paraglacial processes. Potentially hazardous consequences can occur, in particular, during high-magnitude instantaneous events, causing debris and mud flows, mass wasting from debris-covered ice, and floods from small moraine-dammed lakes.

Contemporary glacier retreat triggers a rapid landslide response, Great Aletsch Glacier, Switzerland

AbstractThe destabilization and catastrophic failure of landslides triggered by retreating glaciers is an expected outcome of global climate change and poses a significant threat to inhabitants of glaciated mountain valleys around the globe. Of particular importance are the formation of landslide‐dammed lakes, outburst floods, and related sediment entrainment. Based on field observations and remote sensing of a deep‐seated landslide, located at the present‐day terminus of the Great Aletsch Glacier, we show that the spatiotemporal response of the landslide to glacier retreat is rapid, occurring within a decade. Our observations uniquely capture the critical period of increase in slope deformations, onset of failure, and show that measured displacements at the crown and toe regions of the landslide demonstrate a feedback mechanism between glacier ice reduction and response of the entire landslide body. These observations shed new light on the geomorphological processes of landslide response in paraglacial environments, which were previously understood to occur over significantly longer time periods.

Geomorphological features of the Kongsfjorden area: Ny-Ålesund, Blomstrandøya (NW Svalbard, Norway)

Ice retreat and advancement have affected the Svalbard Islands since the Last Glacial Maximum. The area shows a continuous transition, both in space and time, from glacial environments to proglacial, periglacial, and paraglacial ones. Periglacial processes (as well as landslides and slope, fluvial, and coastal processes) are overprinting pre-existing glacial landforms. Our study focuses on the geomorphological features of the paraglacial environment in some selected areas along Kongsfjorden (Svalbard, Norway): Ny-Alesund and Blomstrandoya. The study is based on the geomorphological mapping, carried out over two field seasons (2013 and 2014), integrated by the interpretation of orthophotos from years 1998, 2008, and 2010. The results presented in this work connect the overall morphology of the area and the landform distribution to the main stages of the recent evolution of the Kongsfjorden area. This may contribute to a better understanding of the geomorphological and environmental changes induced by glacier retreat in a very sensitive high Arctic area.

The role of the paraglacial environment in the transformation of glacial wartanian relief in Poland

Abstract The article comprises a justification for the need to distinguish the record of the paraglacial environments in the belt of relief formed as a result of the Warta Glaciation in Poland. The zone covers the foreland of the Weichselian Glaciation, occupying a strip of land from several to more than 200 kilometres in width. Simplified, two periods are marked in the history of views on relief genesis of the area. The first, which covers the first half of the 20th century, was dominated by the idea of glacial genesis of the area. The latter period is characterised by the discussion between advocates of the periglacial character of relief of the Wartanian zone and those who defended the thesis of the good preservation of glacial relief of the area. Taking the paraglacial environment into account in analysing relief transformations introduces a new quality and may result in a more complete and precise evaluation of post-Wartanian morphogenesis. It is also consistent with tendencies in worldwide literature and results from the principle of uniformitarianism.

The role of the paraglacial environment in the transformation of glacial wartanian relief in Poland

The article comprises a justification for the need to distinguish the record of the paraglacial environments in the belt of relief formed as a result of the Warta Glaciation in Poland. The zone covers the foreland of the Weichselian Glaciation, occupying a strip of land from several to more than 200 kilometres in width. Simplified, two periods are marked in the history of views on relief genesis of the area. The first, which covers the first half of the 20th century, was dominated by the idea of glacial genesis of the area. The latter period is characterised by the discussion between advocates of the periglacial character of relief of the Wartanian zone and those who defended the thesis of the good preservation of glacial relief of the area. Taking the paraglacial environment into account in analysing relief transformations introduces a new quality and may result in a more complete and precise evaluation of post-Wartanian morphogenesis. It is also consistent with tendencies in worldwide literature and results from the principle of uniformitarianism.

Lake outburst and debris flow disaster at Kedarnath, June 2013: hydrometeorological triggering and topographic predisposition

Heavy rainfall in June 2013 triggered flash flooding and landslides throughout the Indian Himalayan state of Uttarakhand, killing more than 6000 people. The vast majority of fatalities and destruction resulted directly from a lake outburst and debris flow disaster originating from above the village of Kedarnath on June 16 and 17. Here, we provide a systematic analysis of the contributing factors leading to the Kedarnath disaster, both in terms of hydrometeorological triggering and topographic predisposition. Topographic characteristics of the lake watershed above Kedarnath are compared with other glacial lakes across the north-western Himalayan states of Uttarakhand and Himachal Pradesh, and implications for glacier lake outburst hazard assessment in a changing climate are discussed. Our analysis suggests that the early onset of heavy monsoon rainfall (390 mm, June 10–17) immediately following a 4-week period of unusually rapid snow cover depletion and elevated streamflow was the crucial hydrometeorological factor, resulting in slope saturation and significant run-off into the small seasonal glacial lake. Between mid-May and mid-June 2013, snow-covered area above Kedarnath decreased by around 50 %. The unusual situation of the lake being dammed in a steep, unstable paraglacial environment but fed entirely from snowmelt and rainfall within a fluvial dominated watershed is important in the context of this disaster. A simple scheme enabling large-scale recognition of such an unfavourable topographic setting is introduced. In view of projected 21st century changes in monsoon timing and heavy precipitation in South Asia, more emphasis should be given to potential hydrometeorological triggering of lake outburst and debris flow disasters in the Himalaya.