Detecting the Legacy of Catastrophic Lake Outbursts in the Nepal Himalaya
Final Report Abstract
Nepal’s second largest and rapidly growing city Pokhara is largely built on fluvial gravels and debris-flow deposits laid down catastrophically in medieval times. We compiled one of the largest and most detailed radiocarbon chronologies for a major prehistoric to historic valley fill in the Himalayas. Three peaks in the age distribution coincide, within error, with the dates of documented M~8 earthquakes in ~1100, 1255, and 1344 CE; however, the 1223 earthquake—an event only briefly mentioned in written sources—is also a strong contender. Catastrophic valley infill may have been co- or post-seismic and occurred in at least three different pulses, judging from the distribution of dates and thin beds of local sediments indicating an intermittent dominance of input from the hillslopes of the Pokhara basin. Our results highlight the value of studying large valley fills as a means to independently test and support fault trenching in palaeoseismology. Studying valley fills further allows capturing the geomorphic impacts of strong seismic shaking in an unprecedented manner. Detailed sedimentological analyses necessitate a reinterpretation of these geomorphic impacts: instead of having simply dammed several tributaries, the largely massive sediments of the Pokhara Formation rushed upstream into tributaries for several kilometres, dumping distinct beds of slackwater deposits from a Higher Himalayan source up to 70 km away. None of the dozens of studies concerned with immediate earthquake impacts on landscapes have reported catastrophic valley-floor sedimentation of this size. Two mechanisms are physically plausible to move several cubic kilometres of Higher Himalayan rocks from a very limited source area downstream over 70 km within a few years to decades: catastrophic rock-ice avalanches detaching from the Annapurna Massif and transforming into highly mobile, long-runout debris flows, or the catastrophic release of large volumes of water from one or several naturally dammed lakes in the Sabche Cirque. Both mechanisms have historic, though much smaller, analogues in this cirque, the Himalayas, and elsewhere, and could have also acted in concert to produce the Pokhara Formation. Several independent lines of evidence of ongoing river response to medieval co- or postseismic sedimentation challenge the notion that mountain rivers recover speedily from earthquakes within years to decades. The valley fills around Pokhara show that even highly erosive and dynamic Himalayan rivers may need more than several centuries to adjust to catastrophic perturbations. In this context, we point out that large terrace staircases in the Himalayas need not necessarily be linked to glacial/interglacial cycles or Holocene changes in the intensity of the South Asian monsoon. Many contemporary problems of channel erosion, bank collapse, and sinkhole formation in Pokhara and its vicinity are a direct legacy of the medieval catastrophic sedimentation; such problems require sound engineering solutions informed by how rivers are likely to respond in future years to decades. From the planning perspective of decades and beyond, our results motivate some rethinking of post-seismic hazard appraisals and infrastructural planning in active mountain regions.
Publications
- 2016. Repeated catastrophic valley infill following medieval earthquakes in the Nepal Himalaya. Science 351, 147–150
Schwanghart, W., Bernhardt, A., Stolle, A., Hoelzmann, P., Adhikari, B.R., Andermann, C., Tofelde, S., Merchel, S., Rugel, G., Fort, M., Korup, O.
(See online at https://doi.org/10.1126/science.aac9865) - 2017. Catastrophic valley fills record large Himalayan earthquakes, Pokhara, Nepal. Quaternary Science Reviews 177, 88–103
Stolle, A., Bernhardt, A., Schwanghart, W., Hoelzmann, P., Adhikari, B.R., Fort, M., Korup, O.
(See online at https://doi.org/10.1016/j.quascirev.2017.10.015)
