Estimación y evaluación de los procesos de inestabilidad de laderas mediante el uso de técnicas en sensoramiento remoto en el Brazo Norte del lago Argentino (Patagonia Austral, Argentina)

Abstract

The present study aims at estimating and evaluating the hazards related to the slope instability processes following the retreat of the ice masses into the glacier valleys located at the Northern Branch of Argentino Lake (49º45´S-50º15´S, 73º24´O-73º0´O), in Los Glaciares National Park, Santa Cruz, Argentina. During the end of the 20th and the beginning of the 21st centuries, glaciers from all over the world have undergone a decrease in thickness and area, as well as a recession of the frontal position, covering the valley slopes with morainic deposits. With the passing of time and the combination of conditioning and triggering factors, this material becomes potentially unstable, contributing to the generation of paraglacial processes, related to slope instability. Also, the natural landscape has a great tourist attraction, especially in the summer season when thousands of tourists visit the area to go on the lake tourist circuits. The presence of people and infrastructure generates greater exposure and vulnerability to the occurrence of threats associated with slope instability. These events are a potential threat not only to tourism activities but also to the conservation of Los Glaciares National Park. Based on the motivation described above, the main objective of this research is to identify, analyze and evaluate the slope instability processes through the implementation and combination of techniques based on remote sensing and statistical methods, and Geographic Information Systems. The methods used were optimal for obtaining estimates of glacier surface velocities, identifying and analyzing the paraglacial geomorphology associated with the processes of instability, analyzing the space-time evolution of slope surface changes, zoning of the susceptibility resulting from the processes of instability, and finally, for characterizing the potential vulnerability of the area that includes the Agassiz subaerial landslide (Upsala Channel), which occurred in February 2013. In the first place, we consider the recent glacial activity that has favored the slope destabilization of the Upsala glacier catchment. In addition, the relationship between the frontal positions and the variations of the surface velocities of the Upsala Glacier was analyzed, where two episodes with increased velocities were detected, coinciding with a greater recession rate of the glacier. Thus, glacial activity has contributed to the generation of paraglacial processes, making the slopes of the Upsala glacier catchment notoriously unstable. Surface velocities estimated on the Upsala Glacier and its tributary glaciers ranged from 0.22 to 2.93 md-1 on average for the period between January and March 2013, and from 0.12 to 0.12 md-1 on average for the period between March and April 2014, respectively. When analyzing the fluctuations and velocities, two moments with maximum values were detected, the first episode occurred during the period between 1993 and 2002, and the second episode occurred between 2009 and 2010. The year 2008 is of great importance in this analysis, since the velocity values increased significantly that year, simultaneously with the fast retreat of the frontal position of Upsala Glacier during the present century. However, knowing the characteristics of the paraglacial geomorphology of the area completes the integral vision of the present thesis. The paraglacial geomorphology is influenced by the combination of several terrain morphometric factors; among them, it considers ground elevations greater than 700 m ASL, average slopes ranging from 25º to 45º, the orientation of the eastern-nor-western slopes with higher sunlight exposure, the concave curvature of the slopes, and slope thickness ranging from low to moderate. Among the conditioning factors, the morainic material deposited by the glaciers, rocky outcrops, and vegetation cover are considered in the area. Finally, triggering factors affect the area in general through the provision of subterranean water by means of infiltrating water from proglacial lakes, and of superficial water by rainfall, ice melting, and runoff; the fluctuation of air and soil temperature; the variation of the lake water level and the seismic activity. In order to detect instability processes, techniques based on statistical, mutivariate, and bivariate methods were applied, dividing the zone in testing areas (TASn) that showed destabilization features. The principal components (PC1 and PC2) explained 76% of the total variability, a percentage that represents the reliability of the relationships between the cases (TASn) and the applied variables. The TASn showed a dissimilar behavior; the distribution of data occurrence is different during those years when instability events represented through Gaussian distributions were recorded. Susceptibility zoning for slope instability processes in the area was carried out implementing weight weighting techniques, contributing to the knowledge of slopes prone to destabilization. The slopes showing high and very high susceptibility degrees are: (i) the western slope of the Upsala Channel, (ii) the slopes of Bertacchi and Cono tributary glaciers, (iii) and the slope of the Moyano and Norte valleys. These susceptibility degrees match the geographical location of the inventory of slope instability processes in the area. Finally, the description, characterization and analysis of the Agassiz subaerial landslide that occurred in February 2013 are further developed. The western slope of the Upsala Channel has shown signs of activity according to records for approximately two decades. The event displaced a large volume of water that generated a tsunami, impacting downstream on various sites and infrastructure present in the area. The valleys surrounding the slopes of the Uppsala channel have a higher degree of susceptibility that is associated with the risk of the area. This reveals the importance of further studies as the basis for future vulnerability and risk research, decision making, and as mechanisms for prevention and mitigation of a potential geological hazard associated with this type of natural process.