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

Abstract

Outburst 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.