DCGGEOPHYS · Subsurface conditions in Himalayan glaciers – implications for outburst flood risk prediction

Subsurface conditions in Himalayan glaciers – implications for outburst flood risk prediction

In mountain regions hazardous moraine-dammed glacial lakes are becoming increasingly common as a result of climate warming and increased glacier melt. Water volumes can reach several millions of cubic metres, which can be released catastrophically if the dam fails. Numerous glacial lake outburst floods (GLOFs) have already occurred in the Himalaya, many of which have caused extensive damage. In countries such as Nepal, the local economy and limited infrastructure severely inhibit the possibility of implementing such strategies. To allow directed timely mitigation to be applied where the risk is greatest, it is necessary to develop predictive tools to allow the early identification of lakes with a significant GLOF risk. Currently it is not possible to precisely predict where or when significant hazards will develop, thus, lakes identified as potentially hazardous are large fully formed moraine-dammed lakes. The main project aims were PA1) to understand the processes influencing glacier lake expansion and, PA2) to improve tools to investigate the subsurface properties of debris-covered glaciers and moraine dams. Innovative geophysical techniques were employed in combination with remote sensing and mapping methods, moving towards the development of fully quantitative predictive models of glacial lake outburst. The project focused on two main locations. First, development and testing of techniques was undertaken on easily accessible analogous moraine systems and debris-covered glaciers in Svalbard (Fig.1), close to extensive technical and logistical support. Second, the project then focused on Ngozumpa Glacier, located in eastern Nepal with a terminus elevation of 4800 m. This glacier is one of the largest in the country and the lower 15km is covered in a thick rock debris layer. In the early 1990s a system of lakes began to form close to the terminus of Ngozumpa Glacier. The lake system entered a major period of expansion after 2001 when it increased in area by ~10% per year (Fig. 2). To better understand the processes influencing glacier lake expansion a combination of stereo high resolution satellite imagery, field mapping and lake sonar data were collected for Ngozumpa Glacier. The main results of this work are; i) the generation and differencing of three digital elevation models from stereo remote sensing images of the ablation area of the glacier allowing identification of patterns of and main controls on, ice loss from debris covered glaciers (Fig. 3), illustrating; - calving from exposed ice cliffs is the dominant process of glacier ice loss, on Ngozumpa Glacier ice cliffs cover ~5% of the lower area, but account for almost 40% of the ice loss (Fig. 4). - the continually evolving glacier drainage system plays a major role in preconditioning lake formation and rapid expansion, particularly by the collapse and enlargement of drainage channels within the ice. - in addition to ice lost from the glacier surface, significant melt occurs within the glacier, in relation to ephemeral lake drainage and through enlargement of existing drainage channels. ii) lake surveys, combining areal mapping and sonar bathymetric mapping, allow the processes influencing expansion and deepening to be investigated and thresholds in lake behaviour identified; - the 2009–2014 is one of relatively slow lake expansion, despite a period of dramatic expansion between 2001 and 2009. - lake evolution is highly complex with areal expansion and deepening in some locations but reductions in both area and depth in others. Two main processes can be identified: (1) reduction in the number of ice cliffs around the lake periphery and (2) deposition of sediment into the lake. - sediment redistribution can act as an important brake on the growth of supraglacial base-level lakes, delaying their transition to full depth lakes. - parts of Spillway Lake dropped in elevation during the study period. The pattern and magnitude of lake level change indicated the adjustment of lake basins more recently integrated into the Spillway Lake complex to the hydrological base level (Fig. 5). The results and conclusions of PA1 have been presented at the European Geoscience Union conference in Vienna (Apr. 2015) and published in the peer reviewed Journal of Glaciology and the article concludes that it is likely that rapid lake growth will resume in the near future. Ice loss at the surface on the lower glacier averages ~1 m per year. Approximately 700 000 m2 of the glacier has an elevation within 10 m of the lake base level. Combined with the 2014 lake area of 240 000 m2, this suggests lake expansion to an extent of 1 km2 within the next 10 years. Because of its low gradient and stagnant nature, if a fully formed moraine dammed lake does develop in the future, it is possible that the lake may eventually become up to 7 km long if the moraine dam remains in place. To develop and improve tools to investigate the subsurface properties of debris-covered glaciers and moraine dams, geophysical techniques were applied at sites in Svalbard and Ngozumpa Glacier. Geophysical techniques employed to investigate subsurface properties of ice cored moraines and debris covered ice all utilise different physical principles. This project focused on electrical resistance tomography (ERT) and Ground penetrating radar (GPR). ERT exploits the variable resistance of different materials to conduct an electrical current and a significant increase in resistivity occurs at freezing point. GPR utilises an electromagnetic pulse and boundaries between glacier ice and bedrock or debris, as well as the transition between frozen and unfrozen sediments, show as major reflections in GPR data. By using the combination of both methods in interpretation the strengths of each can be incorporated. The main results of this work are; i) Circular ERT survey at an ice wedge polygon in Adventdalen Svalbard, illustrating the - strong influence of surface properties on the subsurface resistivity magnitude - anisotropic nature of subsurface current flow - optimised survey set up and data inversion for all later surveys. ii) Circular and grid based 3D ERT survey design and inversion (Fig. 6), allowing the - precise nature of the distortion of current flow caused by topography (usually very difficult to model) to be more accurately included in the inversion process. iii) GPR surveys over variable ground on Longyearbreen, Svalbard and Ngozumpa Glacier - provides a benchmark for signal interpretation and is invaluable for GPR interpretation in areas of complex debris cover. iv) Joint collection and interpretation of 3D ERT and GRP data at Longyearbreen, Svalbard and Ngozumpa Glacier (Fig. 7). - provides a significantly improved interpretation of the subsurface characteristics, using the precise interface reflectors of the GPR data to better constrain the blurred transitions between areas of divergent resistivity in the ERT data. This work is ongoing and the results and conclusion of the jointly interpreted 3D ERT and GPR surveys from Longyearbreen Glacier are currently under preparation as a methodological article for submission to Journal of Geophysical Research. The results and conclusion of the 3D ERT and GPR from Ngozumpa glacier are under preparation with additional data from the temperature thermistor and DEMs for submission to the Journal of Glaciology in the coming months.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз