Detecting the Legacy of Catastrophic Lake Outbursts in the Nepal Himalaya

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In the Nepal Himalaya, the hazard from catastrophic lake outbursts linked to natural dams other than glacial, i.e. mostly those formed by landslides or debris flows, is virtually unknown. Yet peak discharge from historic lake bursts surpass hydrological flood peaks by up to two orders of magnitude, and some of the largest Holocene outburst flows on Earth may have originated in the orogen. Whether outburst events generally promote flushing of valley-floor sediments and enhance bedrock incision instead of catastrophically depositing debris remains unclear also. Yet geomorphic consequences of dam breaks undermine existing hazard appraisals of hydro-meteorological floods, where changes to channel geometry modulate flood frequencies for a given discharge. Elevated post-outburst sediment flux may impede maintenance of hydropower reservoirs and threaten water resources over years to decades, while ultimately accelerate or delay fluvial dissection of the Himalayan orogen in response to tectonic uplift.Very few studies have looked at Himalayan Quaternary archives from a hazard perspective, while the current focus on future sources and impacts of glacial lake outbursts is limited by short instrumental records. This proposal addresses this crucial research gap by quantifying from tell-tale deposits of different ages first-order sediment budgets of, and recovery rates from, selected outburst events. The aim is to investigate their geomorphic and sedimentary legacy on decadal to millennial timescales in order to better constrain hazard assessments, and improve our knowledge on recovery rates of Himalayan rivers from such extreme events. This approach integrates the scope of previous work by focussing on three distinct timescales: (1) Decadal-scale recovery of channels impacted by well-documented historic outbursts; (2) Centennial-scale recovery of rivers catastrophically infilled following dam breaks; and (3) Holocene re-incision of rivers that had been dammed for tens of millennia. Fieldwork in the Pokhara valley and the Khumbu Himal will be complemented by modelling of natural dam-break scenarios. Methods include geomorphic and sedimentary facies mapping from field and remote-sensing data; DGPS surveying; geomorphometric analysis and modelling; cosmogenic nuclide and radiocarbon dating; palaeoflood hydraulic modelling; and Bayesian data analysis for quantifying uncertainties.Expected results include sediment budgets and flow characteristics tied to recent and prehistoric lake bursts; new insights on the timing, mechanisms, and rates linked to some of the largest late Quaternary to historic natural dams and lakes in the Himalaya; and a customised GIS Web Service for locating the magnitudes of hypothetical lake outbursts compared to 1-in-a-100-year hydrological floods.