The history, hydraulics, and geomorphic impact of outburst floods in the eastern Himalaya

Thesis (Ph.D.)--University of Washington, 2019Outburst floods have shaped many landscapes on Earth and represent a significant geologic hazard, but they are relatively infrequent, and we must rely on the sedimentary record to study the most extreme events that have occurred. The eastern Himalaya has a record of various magnitude outburst flood events, including landslide-dam outburst floods (>10^5 m^3/s) and ancient glacial-outburst megafloods (>10^6 m^3/s) that have done substantial amounts of geomorphic work on the landscape throughout the Quaternary. This dissertation investigates outburst floods in the eastern Himalaya with a combination of fieldwork, remote sensing, numerical flood modeling, and geochronology to study the timing, hydraulics, and net erosional impact of these events. Numerical flood simulations of the year 2000 Yigong River outburst flood help characterize flood hazard in the Siang River valley, India, and are used to examine the relationship between outburst flood hydraulics and geomorphic change observed along the >450 km rugged flood pathway that cuts through the >2 km deep Tsangpo Gorge. Simulated outburst flood hydraulics differ from non-flood flows and show that valley topography exerts a strong control on the distribution of shear stress and the patterns of erosion and deposition produced from the flood. Zircons collected from ancient slackwater flood deposits in the region characterize the age of rocks eroded from flood source terrains in Tibet and from the Tsangpo Gorge. Statistical analyses of these data support the previous hypothesis that megafloods erode more rock from Gorge compared to smaller flows, but also show substantial variability among different megaflood deposit samples. These data suggest that megafloods rework sediments from previous events, which is a result supported by luminescence age data from megaflood deposits that influences the interpretation of detrital outburst flood samples. Radiocarbon and luminescence dating methods constrain the timing of at least 9 megaflood events over the last 42 ka, showing the potential for repeated megaflood events from glacial-lake sources in Tibet. The work presented here advances our knowledge about hydraulics during outburst floods, patterns of preferential erosion, chronology of megafloods, and processes of sediment reworking, altogether improving our understanding of the impact of extreme outburst floods in the region

Data for: Provenance and erosional impact of Quaternary megafloods through the Yarlung-Tsangpo Gorge from zircon U-Pb geochronology of flood deposits, Eastern Himalaya

Reduced zircon data for seven detrital megaflood samples

Data for: Provenance and erosional impact of Quaternary megafloods through the Yarlung-Tsangpo Gorge from zircon U-Pb geochronology of flood deposits, Eastern Himalaya

Reduced zircon data for seven detrital megaflood samples.THIS DATASET IS ARCHIVED AT DANS/EASY, BUT NOT ACCESSIBLE HERE. TO VIEW A LIST OF FILES AND ACCESS THE FILES IN THIS DATASET CLICK ON THE DOI-LINK ABOV

Provenance and erosional impact of Quaternary megafloods through the Yarlung-Tsangpo Gorge from zircon U-Pb geochronology of flood deposits, eastern Himalaya

International audienc

Impact of Tibetan lake outburst floods on erosion, morphology, and sedimentary record of the eastern Himalaya from source to sink.

International audienceThe Tsangpo Gorge region of eastern Tibet is an exceptionally dynamic landscape, where the Yarlung-Siang-Brahmaputra River cuts through the high Himalaya in a zone of extremely localized rapid erosion and rock uplift. Quaternary megafloods (≥106 m3/s) sourced from valley blocking glaciers may have played an important role in the geomorphic evolution of the Gorge and Tibetan plateau margin. We investigate the sedimentary record and hydraulics of outburst floods through the Gorge, with implications for erosion and deposition in the largest source-to-sink sedimentary system on Earth.Within the Himalaya downstream of the Gorge, detrital zircon U-Pb and feldspar luminescence data from slackwater flood deposits suggest megaflood sands came from impoundments of the Yarlung River drainage immediately upstream or west of the Gorge. Megaflood deposits contain a disproportionately large number of zircons eroded from the Gorge compared to active bedload and historical landslide-dam-burst flood deposits from the same drainage. This observation cannot be explained by sediment recycling. Rather, it reflects preferential erosion of the Gorge during megafloods, supporting the hypothesis that megafloods are a primary contributor to rapid exhumation of the region.A numerically simulated flood sourced from a reconstructed 81 km3 glacial lake impoundment immediately upstream of the Gorge reaches peak discharge of 2.6x106 m3/s and flow speeds up to 70 m/s, inundating hillslopes up to 250 m above the modern channel. Tributaries experience 8-60 km of backflow inundation. Simulated megaflood power and energy expenditure is over an order of magnitude higher than that of the modern river, showing the high erosive potential of megafloods in the Gorge. Hydraulic simulations, field observations, and process-based numerical models suggest a lasting impact of megaflood deposits on channel/valley form and processes.Downstream observations show that megafloods have significantly altered landscapes in the sedimentary transfer zone between the headwaters and deepwater Bengal Fan and deposited material >2000 km offshore. Our findings show the profound legacy of these episodic, large magnitude floods on mountain valley processes and on the transmission and preservation of tectonic and climatic signals in the sedimentary record

Impact of Tibetan lake outburst floods on erosion, morphology, and sedimentary record of the eastern Himalaya from source to sink.

International audienceThe Tsangpo Gorge region of eastern Tibet is an exceptionally dynamic landscape, where the Yarlung-Siang-Brahmaputra River cuts through the high Himalaya in a zone of extremely localized rapid erosion and rock uplift. Quaternary megafloods (≥106 m3/s) sourced from valley blocking glaciers may have played an important role in the geomorphic evolution of the Gorge and Tibetan plateau margin. We investigate the sedimentary record and hydraulics of outburst floods through the Gorge, with implications for erosion and deposition in the largest source-to-sink sedimentary system on Earth.Within the Himalaya downstream of the Gorge, detrital zircon U-Pb and feldspar luminescence data from slackwater flood deposits suggest megaflood sands came from impoundments of the Yarlung River drainage immediately upstream or west of the Gorge. Megaflood deposits contain a disproportionately large number of zircons eroded from the Gorge compared to active bedload and historical landslide-dam-burst flood deposits from the same drainage. This observation cannot be explained by sediment recycling. Rather, it reflects preferential erosion of the Gorge during megafloods, supporting the hypothesis that megafloods are a primary contributor to rapid exhumation of the region.A numerically simulated flood sourced from a reconstructed 81 km3 glacial lake impoundment immediately upstream of the Gorge reaches peak discharge of 2.6x106 m3/s and flow speeds up to 70 m/s, inundating hillslopes up to 250 m above the modern channel. Tributaries experience 8-60 km of backflow inundation. Simulated megaflood power and energy expenditure is over an order of magnitude higher than that of the modern river, showing the high erosive potential of megafloods in the Gorge. Hydraulic simulations, field observations, and process-based numerical models suggest a lasting impact of megaflood deposits on channel/valley form and processes.Downstream observations show that megafloods have significantly altered landscapes in the sedimentary transfer zone between the headwaters and deepwater Bengal Fan and deposited material >2000 km offshore. Our findings show the profound legacy of these episodic, large magnitude floods on mountain valley processes and on the transmission and preservation of tectonic and climatic signals in the sedimentary record

Quantitative Paleoflood Hydrology

Paleoflood hydrologyKochel and Baker (1982)is the reconstruction of the magnitude and frequency of past floods using geologicalevidence (Baker et al., 2002). Over the last 40 years, paleoflood hydrology has achieved recognition as a new branch of geomor-phology and hydrology (Baker, 2008;Benito and Thorndycraft, 2005;Wilhelm et al., 2019), employing principles of geology,biology, hydrology, and fluid dynamics to infer quantitative and qualitative aspects of unobserved or unmeasured floods on thebasis of physical evidence left in their wake (House et al., 2002). Flood indicators include various types of geologic evidence (flooddeposits and geomorphic features) and flotsam deposits, as well as physical effects on vegetation. Resulting inferences can includetiming, magnitude, and frequency of individual floods at specific sites or for specific rivers, as well as conclusions regarding themagnitude and frequency of channel forming floods. The obvious benefit of paleoflood studies is obtaining information on floodsfrom periods of time or locations lacking direct measurements and observations thereby extending the systematic record especiallywith regards to large floods. Paleoflood studies have been used to support flood hazard assessments as well as understanding of thelinkages between climate, land-use, flood frequency and channel morphology