Glacial lakes are potentially dangerous sources of glacial lake outburst floods (GLOFs), and represent a serious natural hazard in Himalayan countries. Despite the development of various indices aimed at determining the outburst probability, an objective evaluation of the thousands of Himalayan glacial lakes has yet to be completed. In this study we propose a single index, based on the depression angle from the lakeshore, which allows the lakes to be assessed using remotely sensed digital elevation models (DEMs). We test our approach on five lakes in Nepal, Bhutan, and Tibet using images taken by the declassified Hexagon KH-9 satellite before these lakes experienced an outburst flood. All five lakes had a steep lakefront area (SLA), on which a depression angle was steeper than our proposed threshold of 10° before the GLOF event, but the SLA was no longer evident after the events. We further calculated the potential flood volume (PFV); i.e., the maximum volume of floodwater that could be released if the lake surface was lowered sufficiently to eradicate the SLA. This approach guarantees repeatability to assess the possibility of GLOF hazards because it requires no particular expertise to carry out, though the PFV does not quantify the GLOF risk. We calculated PFVs for more than 2000 Himalayan glacial lakes using visible band images and DEMs of ASTER data. The PFV distribution follows a power-law function. We found that 794 lakes did not have an SLA, and consequently had a PFV of zero, while we also identified 49 lakes with PFVs of over 10 million m3, which is a comparable volume to that of recorded major GLOFs. This PFV approach allows us to preliminarily identify and prioritize those Himalayan glacial lakes that require further detailed investigation on GLOF hazards and risk
To better understand recent rapid recession of marine-terminating glaciers in Greenland, we performed satellite and field observations near the calving front of Bowdoin Glacier, a 3 km wide outlet glacier in northwestern Greenland. Satellite data revealed a clear transition to a rapidly retreating phase in 2008 from a relatively stable glacier condition that lasted for >20 years. Ice radar measurements showed that the glacier front is grounded, but very close to the floating condition. These results, in combination with the results of ocean depth soundings, suggest bed geometry in front of the glacier is the primary control on the rate and pattern of recent rapid retreat. Presumably, glacier thinning due to atmospheric and/or ocean warming triggered the initial retreat. In situ measurements showed complex short-term ice speed variations, which were correlated with air temperature, precipitation and ocean tides. Ice speed quickly responded to temperature rise and a heavy rain event, indicating rapid drainage of surface water to the bed. Semi-diurnal speed peaks coincided with low tides, suggesting the major role of the hydrostatic pressure acting on the calving face in the force balance. These observations demonstrate that the dynamics of Bowdoin Glacier are sensitive to small perturbations occurring near the calving front
This study used Corona KH-4A and Advanced Land Observing Satellite (ALOS) PRISM images to generate digital terrain models (DTMs) of the distal part of Imja Glacier, where a few supra-glacial ponds (~0.07 km 2 ) expanded into the large Imja Glacier Lake (Imja Tsho, ~0.91 km 2 ) between 1964 and 2006. DTMs and subsequently derived topographical maps with contour intervals of 1 m were created from the high-resolution images (Corona in 1964 and ALOS in 2006) in the Leica Photogrammetric Suite (LPS) platform. The DTMs and topographic maps provided excellent representation of the elevation and micro-topography of the glacier surface, such as its supra-glacial ponds/lake, surface depressions, and moraine ridges, with an error of about +/-4 m (maximum). The DTMs produced from the Corona and ALOS PRISM images are suitable for use in studies of the surface change of glaciers. The topographical maps produced from the Corona data (1964) showed that part of the dead ice in the down-glacier area was even higher than the top of the lateral moraine ridges, while the glacier surface in the upglacier area was noticeably lower than the moraine crests. This suggests more extensive melting of glacier ice in the up-glacier area before 1964. The average lowering of the glacier surface from 1964 to 2006 was 16.9 m for the dead-ice area west of the lake and 47.4 m for the glacier surface east of the lake; between 1964 and 2002, the lake surface lowered by 82.3 m. These figures represent average lowering rates of 0.4, 1.1, and 2.2 m/year for the respective areas.
Calving glaciers are rapidly retreating in many regions under the influence of ice‐water interactions at the glacier front. In contrast to the numerous researches conducted on fjords in front of tidewater glaciers, very few studies have been reported on lakes in which freshwater calving glaciers terminate. To better understand ice‐water interactions at the front of freshwater calving glaciers, we measured lakewater temperature, turbidity, and bathymetry near Glaciar Perito Moreno, Upsala, and Viedma, large calving glaciers of the Southern Patagonia Icefield. The thermal structures of these lakes were significantly different from those reported in glacial fjords. There was no indication of upwelling subglacial meltwater; instead, turbid and cold glacial water discharge filled the region near the lake bottom. This was because water density was controlled by suspended sediment concentrations rather than by water temperature. Near‐surface wind‐driven circulation reaches a depth of ~180 m, forming a relatively warm isothermal layer (mean temperature of ~5–6°C at Perito Moreno, ~3–4°C at Upsala, and ~6–7°C at Viedma), which should convey heat energy to the ice‐water interface. However, the deeper part of the glacier front is in contact with stratified cold water, implying a limited amount of melting there. In the lake in front of Glaciar Viedma, the region deeper than 120 m was filled entirely with turbid and very cold water at pressure melting temperature. Our results revealed a previously unexplored thermal structure of proglacial lakes in Patagonia, suggesting its importance in the subaqueous melting of freshwater calving glaciers.
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