Detecting the Legacy of Catastrophic Lake Outbursts in the Nepal Himalaya

在尼泊尔-喜马拉雅地区，与冰川以外的自然水坝有关的灾难性湖泊爆发的危害，即主要由山体滑坡或泥石流形成的大坝，几乎是未知的。然而，历史上湖泊爆发的峰值流量超过了水文洪水峰值的两个数量级，地球上一些最大的全新世爆发流量可能起源于造山带。突出事件是否普遍促进了谷底沉积物的冲刷和基岩切割，而不是灾难性的碎屑沉积，也尚不清楚。然而，溃坝的地貌后果破坏了现有的水文气象洪水危害评估，在水文气象洪水中，河道几何形状的变化调节了给定流量的洪水频率。溃决后沉积物通量的增加可能会阻碍水电站水库的维护，并在数年至数十年内威胁水资源，最终加速或推迟喜马拉雅造山带在构造隆升作用下的河流切割。很少有研究从危险的角度来看待喜马拉雅第四纪档案，而目前对冰湖爆发的未来来源和影响的关注受到短期仪器记录的限制。本研究通过对不同年龄的典型沉积物进行定量分析，解决了这一重要的研究缺口。目的是在十年到千年的时间尺度上调查它们的地貌和沉积遗产，以便更好地约束危害评估，并提高我们对喜马拉雅河流从这种极端事件中恢复速度的认识。该方法通过关注三个不同的时间尺度整合了以前的工作范围：(1)受记录良好的历史爆发影响的渠道的十年尺度恢复；(2)百年级溃坝灾后河流恢复；(3)全新世对已筑坝数万年的河流的重新切割。在博卡拉山谷和昆布喜马拉雅的实地工作将辅以对自然溃坝情景的模拟。方法包括利用野外和遥感资料进行地貌和沉积相填图；DGPS测量;地貌分析与建模；宇宙成因核素和放射性碳定年；古洪水水力模拟；以及量化不确定性的贝叶斯数据分析。预期结果包括与近期和史前湖泊爆发有关的沉积物收支和流量特征；关于喜马拉雅地区一些最大的晚第四纪至历史上的天然水坝和湖泊的时间、机制和速率的新见解；以及定制的GIS网络服务，用于定位与百年一遇的水文洪水相比较的假设湖泊爆发的规模。

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.