@phdthesis{Veh2019,
  author    = {Veh, Georg},
  title     = {Outburst floods from moraine-dammed lakes in the Himalayas},
  doi       = {10.25932/publishup-43607},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-436071},
  school      = {Universit{\"a}t Potsdam},
  pages     = {124},
  year      = {2019},
  abstract  = {The Himalayas are a region that is most dependent, but also frequently prone to hazards from changing meltwater resources. This mountain belt hosts the highest mountain peaks on earth, has the largest reserve of ice outside the polar regions, and is home to a rapidly growing population in recent decades. One source of hazard has attracted scientific research in particular in the past two decades: glacial lake outburst floods (GLOFs) occurred rarely, but mostly with fatal and catastrophic consequences for downstream communities and infrastructure. Such GLOFs can suddenly release several million cubic meters of water from naturally impounded meltwater lakes. Glacial lakes have grown in number and size by ongoing glacial mass losses in the Himalayas. Theory holds that enhanced meltwater production may increase GLOF frequency, but has never been tested so far. The key challenge to test this notion are the high altitudes of >4000 m, at which lakes occur, making field work impractical. Moreover, flood waves can attenuate rapidly in mountain channels downstream, so that many GLOFs have likely gone unnoticed in past decades. Our knowledge on GLOFs is hence likely biased towards larger, destructive cases, which challenges a detailed quantification of their frequency and their response to atmospheric warming. Robustly quantifying the magnitude and frequency of GLOFs is essential for risk assessment and management along mountain rivers, not least to implement their return periods in building design codes. Motivated by this limited knowledge of GLOF frequency and hazard, I developed an algorithm that efficiently detects GLOFs from satellite images. In essence, this algorithm classifies land cover in 30 years (~1988-2017) of continuously recorded Landsat images over the Himalayas, and calculates likelihoods for rapidly shrinking water bodies in the stack of land cover images. I visually assessed such detected tell-tale sites for sediment fans in the river channel downstream, a second key diagnostic of GLOFs. Rigorous tests and validation with known cases from roughly 10\% of the Himalayas suggested that this algorithm is robust against frequent image noise, and hence capable to identify previously unknown GLOFs. Extending the search radius to the entire Himalayan mountain range revealed some 22 newly detected GLOFs. I thus more than doubled the existing GLOF count from 16 previously known cases since 1988, and found a dominant cluster of GLOFs in the Central and Eastern Himalayas (Bhutan and Eastern Nepal), compared to the rarer affected ranges in the North. Yet, the total of 38 GLOFs showed no change in the annual frequency, so that the activity of GLOFs per unit glacial lake area has decreased in the past 30 years. I discussed possible drivers for this finding, but left a further attribution to distinct GLOF-triggering mechanisms open to future research. This updated GLOF frequency was the key input for assessing GLOF hazard for the entire Himalayan mountain belt and several subregions. I used standard definitions in flood hydrology, describing hazard as the annual exceedance probability of a given flood peak discharge [m3 s-1] or larger at the breach location. I coupled the empirical frequency of GLOFs per region to simulations of physically plausible peak discharges from all existing ~5,000 lakes in the Himalayas. Using an extreme-value model, I could hence calculate flood return periods. I found that the contemporary 100-year GLOF discharge (the flood level that is reached or exceeded on average once in 100 years) is 20,600+2,200/-2,300 m3 s-1 for the entire Himalayas. Given the spatial and temporal distribution of historic GLOFs, contemporary GLOF hazard is highest in the Eastern Himalayas, and lower for regions with rarer GLOF abundance. I also calculated GLOF hazard for some 9,500 overdeepenings, which could expose and fill with water, if all Himalayan glaciers have melted eventually. Assuming that the current GLOF rate remains unchanged, the 100-year GLOF discharge could double (41,700+5,500/-4,700 m3 s-1), while the regional GLOF hazard may increase largest in the Karakoram. To conclude, these three stages-from GLOF detection, to analysing their frequency and estimating regional GLOF hazard-provide a framework for modern GLOF hazard assessment. Given the rapidly growing population, infrastructure, and hydropower projects in the Himalayas, this thesis assists in quantifying the purely climate-driven contribution to hazard and risk from GLOFs.},
  language  = {en}
}
@phdthesis{Mey2016,
  author    = {Mey, J{\"u}rgen},
  title     = {Intermontane valley fills},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-103158},
  school      = {Universit{\"a}t Potsdam},
  pages     = {xii, 111},
  year      = {2016},
  abstract  = {Sedimentary valley fills are a widespread characteristic of mountain belts around the world. They transiently store material over time spans ranging from thousands to millions of years and therefore play an important role in modulating the sediment flux from the orogen to the foreland and to oceanic depocenters. In most cases, their formation can be attributed to specific fluvial conditions, which are closely related to climatic and tectonic processes. Hence, valley-fill deposits constitute valuable archives that offer fundamental insight into landscape evolution, and their study may help to assess the impact of future climate change on sediment dynamics. In this thesis I analyzed intermontane valley-fill deposits to constrain different aspects of the climatic and tectonic history of mountain belts over multiple timescales. First, I developed a method to estimate the thickness distribution of valley fills using artificial neural networks (ANNs). Based on the assumption of geometrical similarity between exposed and buried parts of the landscape, this novel and highly automated technique allows reconstructing fill thickness and bedrock topography on the scale of catchments to entire mountain belts. Second, I used the new method for estimating the spatial distribution of post-glacial sediments that are stored in the entire European Alps. A comparison with data from exploratory drillings and from geophysical surveys revealed that the model reproduces the measurements with a root mean squared error (RMSE) of 70m and a coefficient of determination (R2) of 0.81. I used the derived sediment thickness estimates in combination with a model of the Last Glacial Maximum (LGM) icecap to infer the lithospheric response to deglaciation, erosion and deposition, and deduce their relative contribution to the present-day rock-uplift rate. For a range of different lithospheric and upper mantle-material properties, the results suggest that the long-wavelength uplift signal can be explained by glacial isostatic adjustment with a small erosional contribution and a substantial but localized tectonic component exceeding 50\% in parts of the Eastern Alps and in the Swiss Rh{\^o}ne Valley. Furthermore, this study reveals the particular importance of deconvolving the potential components of rock uplift when interpreting recent movements along active orogens and how this can be used to constrain physical properties of the Earth's interior. In a third study, I used the ANN approach to estimate the sediment thickness of alluviated reaches of the Yarlung Tsangpo River, upstream of the rapidly uplifting Namche Barwa massif. This allowed my colleagues and me to reconstruct the ancient river profile of the Yarlung Tsangpo, and to show that in the past, the river had already been deeply incised into the eastern margin of the Tibetan Plateau. Dating of basal sediments from drill cores that reached the paleo-river bed to 2-2.5 Ma are consistent with mineral cooling ages from the Namche Barwa massif, which indicate initiation of rapid uplift at ~4 Ma. Hence, formation of the Tsangpo gorge and aggradation of the voluminous valley fill was most probably a consequence of rapid uplift of the Namche Barwa massif and thus tectonic activity. The fourth and last study focuses on the interaction of fluvial and glacial processes at the southeastern edge of the Karakoram. Paleo-ice-extent indicators and remnants of a more than 400-m-thick fluvio-lacustrine valley fill point to blockage of the Shyok River, a main tributary of the upper Indus, by the Siachen Glacier, which is the largest glacier in the Karakoram Range. Field observations and 10Be exposure dating attest to a period of recurring lake formation and outburst flooding during the penultimate glaciation prior to ~110 ka. The interaction of Rivers and Glaciers all along the Karakorum is considered a key factor in landscape evolution and presumably promoted headward erosion of the Indus-Shyok drainage system into the western margin of the Tibetan Plateau. The results of this thesis highlight the strong influence of glaciation and tectonics on valley-fill formation and how this has affected the evolution of different mountain belts. In the Alps valley-fill deposition influenced the magnitude and pattern of rock uplift since ice retreat approximately 17,000 years ago. Conversely, the analyzed valley fills in the Himalaya are much older and reflect environmental conditions that prevailed at ~110 ka and ~2.5 Ma, respectively. Thus, the newly developed method has proven useful for inferring the role of sedimentary valley-fill deposits in landscape evolution on timescales ranging from 1,000 to 10,000,000 years.},
  language  = {en}
}