A study of decadal scale glacier changes of the Lunana glacier system in Bhutan, Himalaya, with considerations to glacial lake outburst floods (GLOFs)

This study assesses changes in glacier area, velocity, and geodetic mass balance for a selection of glaciers in the Lunana glacier system of Bhutan, Himalaya. It takes considerations to Glacial Lake Outburst Floods (GLOFs) by creating a glacial lake inventory of two important potential dangerous glacial lakes, Raphstreng Tsho and Luggye Tsho. Bhutan is located in the eastern parts of the HKH region and is known for its earlier GLOF events. The precipitation in Bhutan is driven by the Indian monsoon resulting in 60% annual precipitation, the high amount of rainfall results in rockfalls that covers large valley glacier tongues with debris. I studied the glacier area changes between 1976, 1996 and 2018 using freely available Landsat satellite imagery, SAR Sentinel 1&2, the SRTM Digital Elevation Model (DEM) and HMA DEM. The geodetic mass balance was calculated between 1976, 2000 and 2018/9 (for selected glaciers) using DEM constructed from high-resolution stereo images, Pléiades and SPOT, granted from the European Space Agency, as well as using the already accessed SRTM DEM and a Hexagon DEM courtesy of King, et al. (2019). The glacier velocity was generated using SAR TerraSAR-X data from 2016 and shows an average yearly displacement over the Lunana glacier system. The glacial lake time series for Raphstreng Tsho and Luggye Tsho where studied between 1993 and 2018 using a stack of freely available Landsat imagery. The results of this study, show a variety of decadal glacial changes over Lunana glacier system, with glaciers lowering on an average by 0.48± 0.08 m a-1 between 1976 and 2018/9 which calculates to a geodetic mass balance of -0.41 ± 0.068 m w.e. a-1. The system had a total average of 12.73% area of reduction for all glaciers, between the same time period. The Lunana glacier system consists of both debris-covered glaciers in the south and debris-free glaciers in the north, and as a result, the glacier changes vary between the two regions. Between 1976 – 2018/9 the southern region had an average surface melt of 0.76 ± 0.07 m a-1 which calculates to a geodetic mass balance of -0.65 ± 0.06 m w.e. a-1 and a 12.65% area of reduction. For the Northern region, the average surface melt was measured to be 1.26 ± 0.07 m a-1 which calculates to a geodetic mass balance of 1.07 ± 0.06 m w.e. a-1 and a 12.80% area of reduction. The glacier velocity was calculated to be at average of 3.05 ± 0.73 m a-1 over the south region and 3.78 ± 0.73 m a-1 over the north region. The Luggye glacier 1, located in the southern parts of Lunana glacier system, is the main input source for glacier meltwater to Luggye Tsho an ice-moraine dam proglacial lake which outburst in 1994 due to hydrostatic pressure. Between 1976 and 2018/9, the Luggye glacier 1 has had a considerable loss in surface elevation by 1.19 ± 0.07 m a-1 which calculates to a geodetic mass balance of 1.01 ± 0.069 m w.e. a-1. The 1994 GLOF event discharged over 18 million m3 of water, destroying infrastructure, flooding villages and houses which killed 21 humans. Today, Luggye Tsho is classified to yield over 1.41 km2 of water, an increase from its former state of 1.12 km2 in 1993, just before the event. This study cannot affirm if PDGLs such as Luggye Tsho is to outburst in the future, but it does affirm its growth in lake area and its input source from glacier melt over Luggye glacier, and that it should be monitored in case of potential outbreak. This can be done by doing repeated analysis of glacier velocity and calculation of glacier mass balance, as this would calculate the input source amount of meltwater to Luggye Tsho.Masteroppgave i geografiGEO350MASV-PHYGMASV-GEOGMPGEOGRMASV-MEH

Geomorphometric analysis and sediment dynamics in mountainous basins: spatial and temporal scales

In this work geomorphometric methods were applied at different spatial and temporal scales for the analysis of the sediment dynamic related to debris flows and bedload sediment transport in alpine environments. The thesis involves two kinds of analysis. The first is aimed at investigating morphological changes occurred in a six years period in two catchments (Gadria and Strimm) of Venosta valley (Eastern Alps, Italy). The study areas were analyzed from both a quantitative (volumetric and areal variations) and qualitative (spatial distribution pattern of erosion and deposition) perspective. The multitemporal analysis was performed by calculating the digital terrain model (DTM) of Difference (DoD) obtained from the comparison of high resolution DTMs (2m), related to both studied catchments, derived from successive LiDAR surveys. A method based on fuzzy logic that takes into account the spatial variability of DTM vertical error was applied to derive the DoD. To evaluate the uncertainty in both pre-event and post-event DTMs, two geomorphometric parameters, i.e., ground point density and slope, approximating the quality of the DTM and the topographic complexity of the study area, respectively, were considered. Volumes of sediment eroded and deposited by events occurred in the analyzed period, as computed by the DoD, were compared to field survey data derived from a database of historical events provided by the Autonomous Province of Bolzano. The use of a spatially variable uncertainty permitted both to recover the information related to low magnitude changes in gentle slope areas that would be lost if a uniform threshold was applied. The analysis also highlighted the possibility to use the DoD for the identification of erosion and deposition processes in uneasily accessible areas and of events that could not be detected through field surveys. The analysis of the relationship between geomorphometric parameters, such as curvature (planform and profile), slope and drainage area, and geomorphologic changes detected by the DoD, improved the qualitative interpretation of surface variations, integrating the volumetric estimates of erosion and deposition. The second analysis involves the investigation of sediment connectivity at different spatial scales both in terms of DTM resolution and geographic extent. The analysis was carried out by using the index of connectivity (IC) proposed by Borselli et al. (2009) and modified by Cavalli et al. (2013) for the analysis of alpine catchments. IC applied to high resolution DTMs allows the spatial characterization of the potential sediment connectivity between hillslope and areas of particular interest (e.g. road, basin outlet, channel). The feasibility of applying IC at regional area (Venosta valley) presenting high topographic and land use variability was tested. In particular, the effect of the DTM resolution on IC results and the variability of the index applied to selected basins of Venosta valley characterized by different shape, size, slope and sediment dynamics, was investigated. The dependence of the sediment connectivity index on the drainage area, mainly due to the downslope component of the index that considers the length of sediment pathways to reach a target or a sink, implies that only basins of similar size can be compared. DTM resolution affects not only mean values of IC but also the spatial distribution of the sediment connectivity both at basin and regional scale. Nevertheless, the obtained results highlight the possibility to apply the connectivity index for a rapid spatial characterization of the sediment connectivity at large scale and in areas characterized by complex morphology and different sediment transport processes such as debris flows and bedload transport. The two analyzed scales, spatial and temporal, even if presented separately in the thesis, can be considered connected. The application of the connectivity index in a basin undergoing glacier retreat (Zinal glacier, Switzerland) allowed the evaluation in a future scenario of the melting process on the potential sediment connectivity after a period of fourty years. Qualitative analysis of the variation of the geomorphologic index suggested that the degree of sediment connectivity is a key factor in controlling the release of sediment between hillslopes and main channel and that future sediment fluxes coming from the melting zone critically depend on the lateral moraines development. As a general conclusion of this study, the high resolution of digital terrain models derived from LiDAR surveys, coupled with the use of suitable tools for geomorphometric analysis, permitted both to evaluate geomorphic changes, caused by multiple events, occurred at basin scale and to create scenario map of the potential sediment connectivity at different spatial scales, in areas characterized by different morphology and sediment transport processes