TY - JOUR A1 - Stolle, Amelie A1 - Schwanghart, Wolfgang A1 - Andermann, Christoff A1 - Bernhardt, Anne A1 - Fort, Monique A1 - Jansen, John D. A1 - Wittmann, Hella A1 - Merchel, Silke A1 - Rugel, Georg A1 - Adhikari, Basanta Raj A1 - Korup, Oliver T1 - Protracted river response to medieval earthquakes JF - Earth surface processes and landforms : the journal of the British Geomorphological Research Group N2 - Mountain rivers respond to strong earthquakes by rapidly aggrading to accommodate excess sediment delivered by co-seismic landslides. Detailed sediment budgets indicate that rivers need several years to decades to recover from seismic disturbances, depending on how recovery is defined. We examine three principal proxies of river recovery after earthquake-induced sediment pulses around Pokhara, Nepal's second largest city. Freshly exhumed cohorts of floodplain trees in growth position indicate rapid and pulsed sedimentation that formed a fan covering 150 km2 in a Lesser Himalayan basin with tens of metres of debris between the 11th and 15th centuries AD. Radiocarbon dates of buried trees are consistent with those of nearby valley deposits linked to major medieval earthquakes, such that we can estimate average rates of re-incision since. We combine high-resolution digital elevation data, geodetic field surveys, aerial photos, and dated tree trunks to reconstruct geomorphic marker surfaces. The volumes of sediment relative to these surfaces require average net sediment yields of up to 4200 t km–2 yr–1 for the 650 years since the last inferred earthquake-triggered sediment pulse. The lithological composition of channel bedload differs from that of local bedrock, confirming that rivers are still mostly evacuating medieval valley fills, locally incising at rates of up to 0.2 m yr–1. Pronounced knickpoints and epigenetic gorges at tributary junctions further illustrate the protracted fluvial response; only the distal portions of the earthquake-derived sediment wedges have been cut to near their base. Our results challenge the notion that mountain rivers recover speedily from earthquakes within years to decades. The valley fills around Pokhara show that even highly erosive Himalayan rivers may need more than several centuries to adjust to catastrophic perturbations. Our results motivate some rethinking of post-seismic hazard appraisals and infrastructural planning in active mountain regions. KW - fluvial response KW - sediment yield KW - earthquakes KW - Nepal KW - Himalaya Y1 - 2018 U6 - https://doi.org/10.1002/esp.4517 SN - 0197-9337 SN - 1096-9837 VL - 44 IS - 1 SP - 331 EP - 341 PB - Wiley CY - Hoboken ER - TY - THES A1 - Stolle, Amelie T1 - Catastrophic Sediment Pulses in the Pokhara Valley, Nepal T1 - Katastrophale Sediment Ablagerungen im Pokhara Tal, Nepal N2 - Fluvial terraces, floodplains, and alluvial fans are the main landforms to store sediments and to decouple hillslopes from eroding mountain rivers. Such low-relief landforms are also preferred locations for humans to settle in otherwise steep and poorly accessible terrain. Abundant water and sediment as essential sources for buildings and infrastructure make these areas amenable places to live at. Yet valley floors are also prone to rare and catastrophic sedimentation that can overload river systems by abruptly increasing the volume of sediment supply, thus causing massive floodplain aggradation, lateral channel instability, and increased flooding. Some valley-fill sediments should thus record these catastrophic sediment pulses, allowing insights into their timing, magnitude, and consequences. This thesis pursues this theme and focuses on a prominent ~150 km2 valley fill in the Pokhara Valley just south of the Annapurna Massif in central Nepal. The Pokhara Valley is conspicuously broad and gentle compared to the surrounding dissected mountain terrain, and is filled with locally more than 70 m of clastic debris. The area’s main river, Seti Khola, descends from the Annapurna Sabche Cirque at 3500-4500 m asl down to 900 m asl where it incises into this valley fill. Humans began to settle on this extensive fan surface in the 1750’s when the Trans-Himalayan trade route connected the Higher Himalayas, passing Pokhara city, with the subtropical lowlands of the Terai. High and unstable river terraces and steep gorges undermined by fast flowing rivers with highly seasonal (monsoon-driven) discharge, a high earthquake risk, and a growing population make the Pokhara Valley an ideal place to study the recent geological and geomorphic history of its sediments and the implication for natural hazard appraisals. The objective of this thesis is to quantify the timing, the sedimentologic and geomorphic processes as well as the fluvial response to a series of strong sediment pulses. I report diagnostic sedimentary archives, lithofacies of the fan terraces, their geochemical provenance, radiocarbon-age dating and the stratigraphic relationship between them. All these various and independent lines of evidence show consistently that multiple sediment pulses filled the Pokhara Valley in medieval times, most likely in connection with, if not triggered by, strong seismic ground shaking. The geomorphic and sedimentary evidence is consistent with catastrophic fluvial aggradation tied to the timing of three medieval Himalayan earthquakes in ~1100, 1255, and 1344 AD. Sediment provenance and calibrated radiocarbon-age data are the key to distinguish three individual sediment pulses, as these are not evident from their sedimentology alone. I explore various measures of adjustment and fluvial response of the river system following these massive aggradation pulses. By using proxies such as net volumetric erosion, incision and erosion rates, clast provenance on active river banks, geomorphic markers such as re-exhumed tree trunks in growth position, and knickpoint locations in tributary valleys, I estimate the response of the river network in the Pokhara Valley to earthquake disturbance over several centuries. Estimates of the removed volumes since catastrophic valley filling began, require average net sediment yields of up to 4200 t km−2 yr−1 since, rates that are consistent with those reported for Himalayan rivers. The lithological composition of active channel-bed load differs from that of local bedrock material, confirming that rivers have adjusted 30-50% depending on data of different tributary catchments, locally incising with rates of 160-220 mm yr−1. In many tributaries to the Seti Khola, most of the contemporary river loads come from a Higher Himalayan source, thus excluding local hillslopes as sources. This imbalance in sediment provenance emphasizes how the medieval sediment pulses must have rapidly traversed up to 70 km downstream to invade the downstream reaches of the tributaries up to 8 km upstream, thereby blocking the local drainage and thus reinforcing, or locally creating new, floodplain lakes still visible in the landscape today. Understanding the formation, origin, mechanism and geomorphic processes of this valley fill is crucial to understand the landscape evolution and response to catastrophic sediment pulses. Several earthquake-triggered long-runout rock-ice avalanches or catastrophic dam burst in the Higher Himalayas are the only plausible mechanisms to explain both the geomorphic and sedimentary legacy that I document here. In any case, the Pokhara Valley was most likely hit by a cascade of extremely rare processes over some two centuries starting in the early 11th century. Nowhere in the Himalayas do we find valley fills of comparable size and equally well documented depositional history, making the Pokhara Valley one of the most extensively dated valley fill in the Himalayas to date. Judging from the growing record of historic Himalayan earthquakes in Nepal that were traced and dated in fault trenches, this thesis shows that sedimentary archives can be used to directly aid reconstructions and predictions of both earthquake triggers and impacts from a sedimentary-response perspective. The knowledge about the timing, evolution, and response of the Pokhara Valley and its river system to earthquake triggered sediment pulses is important to address the seismic and geomorphic risk for the city of Pokhara. This thesis demonstrates how geomorphic evidence on catastrophic valley infill can help to independently verify paleoseismological fault-trench records and may initiate re-thinking on post-seismic hazard assessments in active mountain regions. N2 - Der Transport von Sedimenten in Flüssen ist wichtig, um Landschaftsformen in Gebirgsregionen entstehen zu lassen. Eine erhöhte, plötzliche Sedimentzufuhr, beispielsweise durch Massenbewegungen ausgelöst, kann ein Flusssystem schnell aus dem Gleichgewicht bringen. Innerhalb kurzer Zeit transportiertes Sediment wird häufig an Überschwemmungsflächen abgelagert, was zu instabilen Flussverläufen, erhöhtem Sedimentabtrag und vermehrten Überschwemmungen führen kann. Talverfüllungen, Schwemmmfächer, Flussterrassen und Überschwemmungsebenen sind in diesem Zusammenhang die am häufigsten vorkommenden Landschaftsformen, um große Materialvolumen zu speichern. Weil Wasser und Sediment als Baustoff in ausreichenden Mengen zur Verfügung stehen, sind sie bevorzugte Siedlungsflächen. Diese Dissertation untersucht in drei Studien die Entstehung, geomorphologische und sedimentologische Prozesse, sowie die Anpassung des Flusssystems auf einen erhöhten Sedimenteintrag des heute mit Sedimenten verfüllten Pokhara Tals im zentralen Himalaya. Die Stadt Pokhara liegt am Fuße des bis zu 8000 m hohen Annapurna Massivs auf einem ~150 km2 großen, aus klastischen Sedimentablagerungen bestehender Fächer. Das Tal ist von bis zu 70 m hohen Terrassen gekennzeichnen und auffallend flach im Vergleich zur umliegenden Topographie. Der Seti Khola entwässert das Annapurna Massiv in einer Höhe von 3500-5000 m ü.N.N. und erreicht nach kurzer Distanz den Pokhara Fächer. Erste Bewohner siedelten sich in den 1750er Jahren an als die frühere Handelsroute den Hohen Himalaya mit dem subtropischen Tiefland (Terai) verbunden hat. Seither wächst die Stadt stetig und ist heute, nach Kathmandu, die zweitgrößte Stadt Nepals. Durch die Nähe der geologischen Hauptstörung zwischen dem Hohen Himalaya und dem tiefer liegenden Vorderen Himalaya herrscht ein hohes Erdbebenrisiko im Pokhara Tal. Die Kombination aus hohen Terrassen und tiefen, von schnell fließenden Flüssen ausgespülten Schluchten, machen das Tal zu einem geeigneten Ort, um die kaum untersuchte geologische und geomorphologische Geschichte der in diesem Tal abgelagernten Sedimente zu erforschen. Um Landschaftsveränderungen und -entwicklungen zu verstehen sowie die Reaktion des Flussnetzes auf erhöhte Sedimentzufuhr zu überblicken, ist es unabdingbar, den Ursprung des Materials, die sedimentologischen Prozesse und mögliche Auslöser der Talverfüllung zu analysieren. Daten und Proben aus dem Gelände, die später im Labor datiert, ausgewertet, mit sedimentologischen Aufzeichnungen kombiniert und durch fernerkundliche Methoden ergänzt und analysiert wurden, bilden die Basis der Ergebnisse dieser Dissertation. Da solche massiven, in sehr kurzer Zeit abgelagerten Sedimente auf eine katastrophale Entstehung hindeuten, spielt auch der zeitliche Aspekt eine wichtige Rolle. erschiedene Beweise zeigen, dass mindestens drei Sedimentereignisse das Pokhara Tal verfüllt haben. Ich dokumentiere ein aufschlussreiches Sedimentarchiv, Sedimentabfolgen geochemische Provenienz, Radiokarbonalter und die stratigraphische Beziehung zwischen diesen Ergebnissen. Diese unabhängig voneinander gewonnen Ergebnisse zeigen, dass geomorphologische und sedimentologische Beweise mit den Altersdatierungen konsistent sind und wir mit diesen Untersuchungen die abgelagerten Geröllmassen mit historischen Starkbeben in Verbindung bringen können (~1100, 1255, 1344 AD). Provenienz in Kombination mit den Altersdatierungen lassen uns wiederum die drei Ereignisse in ihren Mächtigkeit unterscheiden und individuel das Volumen bestimmen. Messungen zur Anpassung des Flusssystems ergeben, dass das System noch stark von seinem Gleichgewicht abweicht, da erst 30-70% des über kurze Zeit abgelagerten Materials aus dem Flussbett ausgeräumt wurden. Hierfür benutzte Marker sind unter Anderem volumetrische Berechnungen, Erosionsund Einschneideraten der Flüsse, Bäume in ihrer Wachstumsposition zur Altersdatierung und Stufen im Längsgerinneprofil der Seitenflüsse. Das bis heute abgetragene Volumen ergibt Sedimentaustragsraten von bis zu 4200 t km−2 yr−1 am Fuß des Fächers. Die lithologische Zusammensetzung aktiver Flussbänke in Seitentälern zeigt, dass Material der Formation gegenüber lokalem Grundgestein immer noch dominiert. Dieses lithologische Ungleichgewicht verdeutlicht, wie schnell Sedimentmassen in drei Ereignissen über 70 km talabwärts und bis zu 8 km flussaufwärts (in die Seitentäler) abgelagert wurden. Lokale Einschneideraten in die Pokhara Formation liegen zwischen 0.16-0.22 m yr−1 und weisen auf einen sich schnell verändernden Flussverlauf hin. Um die Landschaftsentwicklung nach solch massiven Sedimentablagerungen analysieren zu können, müssen die Sedimentologie, die geomorphologischen Prozesse, der Ursprung und die Mechanismen der Talverfüllung verstanden und als Basiswissen vorausgesetzt werden. Aus den gewonnen Resultaten schließen wir, dass das Pokhara Tal von mehreren katastrophal aufeinanderfolgenden Naturereignissen in einem Zeitraum von ~200 Jahren seit dem 12. Jahrhundert heimgesucht wurde. Nirgendwo im Himalaya finden wir vergleichbare Talverfüllungen, weder in ihrer Größe, noch in dieser detailliert aufgenommenen geomorphologischen Geschichte und sedimentologischen Aufzeichnungen. Das Pokhara Tals ist damit eine der am besten datierten Talverfüllungen des gesamten Himalaya. Diese Arbeit zeigt, dass in sedimentären Archiven - unabhängig von der Paleoseismologie - historische Starkbeben datiert und erkannt werden können. So hilft das Wissen über den zeitlichen Verlauf der Talverfüllung, die Entwicklung des Fächers und die Anpassung des Flusssystems in Zukunft Entscheidungen zu treffen, die das geomorphologische Risiko für die Stadt Pokhara vermindern und gleichzeitig bauliche Maßnahmen besser an die lokalen Risikofaktoren angepasst werden können. KW - geomorphology KW - geohazards KW - Himalaya KW - radiocarbon age dating KW - Geomorphologie KW - Naturgefahren KW - Himalaya KW - Radiokarbondatierung Y1 - 2018 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-413341 ER - TY - JOUR A1 - Stolle, Amelie A1 - Langer, Maria A1 - Blöthe, Jan Henrik A1 - Korup, Oliver T1 - On predicting debris flows in arid mountain belts JF - Global and planetary change N2 - The use of topographic metrics for estimating the susceptibility to, and reconstructing the characteristics of, debris flows has a long research tradition, although largely devoted to humid mountainous terrain. The exceptional 2010 monsoonal rainstorms in the high-altitude mountain desert of Ladakh and Zanskar, NW India, were a painful reminder of how susceptible arid regions are to rainfall-triggered flash floods, landslides, and debris flows. The rainstorms of August 4-6 triggered numerous debris flows, killing 182 people, devastating 607 houses, and more than 10 bridges around Ladakh's capital of Leh. The lessons from this disaster motivated us to revisit methods of predicting (a) flow parameters such as peak discharge and maximum velocity from field and remote sensing data, and (b) the susceptibility to debris flows from catchment morphometry. We focus on quantifying uncertainties tied to these approaches. Comparison of high-resolution satellite images pre- and post-dating the 2010 rainstorm reveals the extent of damage and catastrophic channel widening. Computations based on these geomorphic markers indicate maximum flow velocities of 1.6-6.7 m s(-1) with runout of up to similar to 10 km on several alluvial fans that sustain most of the region's settlements. We estimate median peak discharges of 310-610 m(3) s(-1), which are largely consistent with previous estimates. Monte Carlo-based error propagation for a single given flow-reconstruction method returns a variance in discharge similar to one derived from juxtaposing several different flow reconstruction methods. We further compare discriminant analysis, classification tree modelling, and Bayesian logistic regression to predict debris-flow susceptibility from morphometric variables of 171 catchments in the Ladakh Range. These methods distinguish between fluvial and debris flow-prone catchments at similar success rates, but Bayesian logistic regression allows quantifying uncertainties and relationships between potential predictors. We conclude that, in order to be robust and reliable, morphometric reconstruction of debris-flow properties and susceptibility requires careful assessment and reporting of errors and uncertainties. (C) 2015 Elsevier B.V. All rights reserved. KW - debris flow KW - peak discharge KW - channel geometry KW - geomorphometry KW - Bayesian logistic regression KW - Transhimalaya Y1 - 2015 U6 - https://doi.org/10.1016/j.gloplacha.2014.12.005 SN - 0921-8181 SN - 1872-6364 VL - 126 SP - 1 EP - 13 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Schwanghart, Wolfgang A1 - Bernhardt, Anne A1 - Stolle, Amelie A1 - Hoelzmann, Philipp A1 - Adhikari, Basanta R. A1 - Andermann, Christoff A1 - Tofelde, Stefanie A1 - Merchel, Silke A1 - Rugel, Georg A1 - Fort, Monique A1 - Korup, Oliver T1 - Repeated catastrophic valley infill following medieval earthquakes in the Nepal Himalaya JF - Science N2 - Geomorphic footprints of past large Himalayan earthquakes are elusive, although they are urgently needed for gauging and predicting recovery times of seismically perturbed mountain landscapes. We present evidence of catastrophic valley infill following at least three medieval earthquakes in the Nepal Himalaya. Radiocarbon dates from peat beds, plant macrofossils, and humic silts in fine-grained tributary sediments near Pokhara, Nepal’s second-largest city, match the timing of nearby M > 8 earthquakes in ~1100, 1255, and 1344 C.E. The upstream dip of tributary valley fills and x-ray fluorescence spectrometry of their provenance rule out local sources. Instead, geomorphic and sedimentary evidence is consistent with catastrophic fluvial aggradation and debris flows that had plugged several tributaries with tens of meters of calcareous sediment from a Higher Himalayan source >60 kilometers away. Y1 - 2016 U6 - https://doi.org/10.1126/science.aac9865 SN - 0036-8075 SN - 1095-9203 VL - 351 SP - 147 EP - 150 PB - American Assoc. for the Advancement of Science CY - Washington ER - TY - JOUR A1 - Stolle, Amelie A1 - Bernhardt, Anne A1 - Schwanghart, Wolfgang A1 - Hoelzmann, Philipp A1 - Adhikari, Basanta R. A1 - Fort, Monique A1 - Korup, Oliver T1 - Catastrophic valley fills record large Himalayan earthquakes, Pokhara, Nepal JF - Quaternary science reviews : the international multidisciplinary research and review journal KW - Catastrophic valley infill KW - Great Himalayan earthquakes KW - Radiocarbon age dating KW - Provenance analysis KW - Paleoseismology KW - Nepal Y1 - 2017 U6 - https://doi.org/10.1016/j.quascirev.2017.10.015 SN - 0277-3791 VL - 177 SP - 88 EP - 103 PB - Elsevier CY - Oxford ER -