RESEARCH
◥ resolution. The seismic records for each near- REPORT river station contain two distinct pulses of high- amplitude noise (Fig. 1C). The first is the flood front, which propagates between stations at 8.7 m/s. GEOMORPHOLOGY Theabruptriseofseismicpowerateachstation suggests that the maximum flow depth was reached within 2 min. At Chaku station, this cor- Glacial lake outburst floods responded to a calculated maximum discharge of 1500 to 2100 m3/s (Fig. 2 and table S1). The sec- as drivers of fluvial erosion ond pulse is of higher magnitude but travels slower, at 5 m/s. The total duration of the flood, in the Himalaya including both pulses, at each of the stations was less than an hour. We interpret the first pulse as a water wave and the second as a package of Kristen L. Cook1*, Christoff Andermann1, Florent Gimbert1,2, coarse sediment, on the basis of the following Basanta Raj Adhikari3, Niels Hovius1,4 reasoning: If both pulses were associated with propagating water waves, then the more ener- Himalayan rivers are frequently hit by catastrophic floods that are caused by the failure getic second wave should have a greater flow of glacial lake and landslide dams; however, the dynamics and long-term impacts of depth and travel faster (17). The prediction from such floods remain poorly understood. We present a comprehensive set of observations theory is that near-river stations should be espe- that capture the July 2016 glacial lake outburst flood (GLOF) in the Bhotekoshi/Sunkoshi cially sensitive to coarse sediment, whereas far- Downloaded from River of Nepal. Seismic records of the flood provide new insights into GLOF mechanics river stations should be predominantly sensitive and their ability to mobilize large boulders that otherwise prevent channel erosion. to water flow (17, 18) (fig. S1). The second wave Because of this boulder mobilization, GLOF impacts far exceed those of the annual was prominent at near-river stations but much summer monsoon, and GLOFs may dominate fluvial erosion and channel-hillslope weaker at farther seismic stations, consistent with coupling many tens of kilometers downstream of glaciated areas. Long-term valley the theory (fig. S2). Theory also predicts that coarse-
evolution in these regions may therefore be driven by GLOF frequency and magnitude, sediment transport generates seismic noise at http://science.sciencemag.org/ rather than by precipitation. higher frequencies compared to turbulent flow (17, 18) (fig. S1), and power spectra from the two pulses indicate an increase in power at higher ake outburst floods (LOFs) have long been portance of glacial lake outburst floods (GLOFs) frequencies during the second pulse (fig. S3). recognized as both a hazard and major in driving fluvial erosion by examining the Theratioofthepulsevelocitiesis0.6,match- agent of geomorphic change in the Himalaya Bhotekoshi/Sunkoshi River, where we compare ing bedload/water velocity ratios observed exper- L (1–5). These floods originate from lakes that monsoon floods to a GLOF that occurred in July imentally and in small-river settings (20). This have formed behind a landslide dam or in 2016. In addition to documenting relative impacts velocity difference ensured that the flood front association with a glacier, dammed by a frontal and discharges, we use seismic observations to outpaced any entrained sediment and therefore moraine or glacial ice. Such lakes can drain cat- gain insight into GLOF dynamics, and we explore remained depleted in bedload.
astrophically for several reasons, including mass theroleofboulder-sizedsedimentinpromoting Field- and satellite-based observations show on December 29, 2019 movements or avalanches into the lake, seismic GLOF-driven erosion. that the GLOF affected the river channel over a activity, piping within the dam, overtopping of Onthenightof5July2016,Gongbatongshacuo ~40-km stretch between the confluence with the the dam, or degradation of blocking ice (4, 6). The Lake, a 1.7 × 104–m2 moraine-dammed lake in the Zhangzangbo River and Barabise town (Fig. 1). resulting floods can have short-lived discharges Tibet Autonomous Region, China, drained cata- The flood impact extended into the adjacent up to several orders of magnitude higher than strophically, releasing approximately 1.1 × 105 m3 hillslopes through undercutting and destabiliza- background discharges in the receiving rivers (7). of water (14). The cause of the breach is unknown, tion of the river banks, leading to bank collapses, Because of their magnitude and unpredictability, but fresh deposits above the lake suggest that it slumps, and landslides. The extensive flood- LOFscanbehighlydestructiveandcompromise may have been associated with a debris flow event, induced damage to local infrastructure was almost local infrastructure such as roads, buildings, and possiblyincreasingthevolumeoftheflood(Fig.1B). exclusively the result of bank erosion and mass hydropower facilities (3, 8–10). The flood proceeded down the Zhangzangbo River wasting, rather than inundation (fig. S4). Although large LOFs have been recognized as into the Poiqu/Bhotekoshi/Sunkoshi River and We quantified the magnitude of channel, bank, strongly affecting river morphology and dynamics caused severe damage in the Bhotekoshi valley, and hillslope change with repeat terrestrial lidar (4, 11–13), they are often treated as one-off events. destroyingtheintakedamofahydropowerproj- surveys from October 2015, March 2016, and The potential impact of repeated LOFs, partic- ect, the Araniko highway, and numerous build- November 2016 in nine locations that together ularly the less dramatic small-to-medium mag- ings in the towns of Kodari and Tatopani. The zone covered 20% of the channel length between nitude floods, on the longer-term behavior of the of damage sits within the area affected by strong Khukundol and Barabise (Fig. 1 and table S2). fluvial system has received little attention. The ground motion and landsliding induced by the Eight of the scanned reaches experienced down- impact of individual LOF events must be con- 2015 moment magnitude 7.8 Gorkha earthquake, cutting varying from 1 to 10 m, whereas one had sideredalongwithLOFfrequencyandmeasured which had an estimated return time of a few no bed-elevation change (Fig. 3). against the accumulated effect of annual mon- hundred years (15, 16). We observed bank erosion in numerous sec- soon floods of variable size. We evaluate the im- The 2016 GLOF passed through an array of six tions of the channel, and all scanned reaches broadband seismometers installed in 2015 along contained segments with at least 2 m of later- the Bhotekoshi River valley, 28 to 35 km down- al erosion (Fig. 3 and fig. S5). Bank erosion included 1GFZ German Research Centre for Geosciences, Potsdam, Germany. 2University of Grenoble Alpes, CNRS, IRD, Institut stream of the Zhangzangbo confluence (Fig. 1). parallel retreat of steep banks and erosion and des Géosciences de l’Environnement (IGE), Grenoble, France. Because both turbulent flow (17) and bedload undercutting at the base of slopes. Analysis of 5-m- 3Department of Civil Engineering, Pulchowk Campus, transport (18) in rivers can generate detectable resolution RapidEye imagery (table S3) indicates Institute of Engineering, Tribhuvan University, Nepal. seismic ground motion (19), the seismic record that the 2016 GLOF caused the mean width of 4Institute of Earth and Environmental Science, Potsdam University, Potsdam, Germany. of the GLOF can be used to probe the flood and theactivechannelbetween the Zhangzangbo con- *Corresponding author. Email: [email protected] sediment dynamics at high temporal and spatial fluence and Barabise to increase from 29.5 ± 3 m
Cook et al., Science 362,53–57 (2018) 5 October 2018 1of5 RESEARCH | REPORT
85°55'E 86°0'E
85°E 86°E glacial lakes gauging station 50 Zhangzangbo River GLOF source A glaciers Sunkoshi 1300
2500 28°5'N 28°5'N Nepal border 3800 5000 50 km (m) elevation N Zhangzangbo Lake
28°N Hydropower
28°N B Oct. 01, 2015 Barabise
Pachuwarghat
Khurkot 27°N 27°N
85°E 86°E 87°E 88°E
Lidar scan area 200 meters 28°0'N 28°0'N 2016 outburst-induced landslide Oct. 29, 2016 Downloaded from 2010-2015 river-induced landslide Poiqu/Bhotekoshi River Kodari Nepal/China border Friendship Bridge Arniko Highway largest landslide location of interest Tatopani seismic station http://science.sciencemag.org/
5 kilometers Upper Bhotekoshi hydropower dam Phulpin Bridge 200 meters Chakam 1 C Khukundol 1 - 90 Hz 21 Hindi
27°55'N 2 27°55'N
8.7 m/s Hindi 5 m/s 23 3 Chaku Listi 4 25 5 on December 29, 2019 Chaku Tyanthali 27 KK 6 Tyantali 7 29
Distance along stream (km) along stream Distance KK
8 14:00 14:30 15:00 15:30 16:00 16:30 17:00 Borderlands Time (UTC) D
Zhangzangbo River confluence 27°50'N 27°50'N
Friendship bridge hydropower Phulpin bridge
elevation (m) elevation Chaku Khukundol Barabise Barabise camera, Pachuwarghat Borderlands Khurkot cross section Sunkoshi dam
9 0 1000 2000 3000 4000 0 50 100 150 200 250 Barabise gauge distance (km)
85°55'E 86°0'E
Fig. 1. Map of the Bhotekoshi study region. Scanned reaches, mapped before the bursting event in October 2015 (top) and after in October 2016 landslides, seismic stations, and locations discussed in the text are shown. (bottom). (C) Seismic record of GLOF propagation. Normalized ground velocity White numbers 1 to 9 indicate each scan location, with reference to table S1. time series from four stations at different distances downstream of the (A) The inset provides the regional context, with glaciers and glacial Zhangzangbo confluence. Dots indicate the manually picked pulse arrivals. lakes shown (9). The shading and black outlines indicate drainage basins Straight lines correspond to linear fits of distance versus time and yield the upstream of Khurkot, Pachuwarghat, Barabise, and the Upper Bhotekoshi pulse velocities. UTC, universal time coordinated. (D) Longitudinal profile from hydropower dam. (B) Google Earth and RapidEYE imagery showing a the Advanced Land Observing Satellite 12.5-m digital elevation model of the magnified view of the lake that was the source of the outburst flood, both Poiqu/Bhotekoshi/Sunkoshi River, with locations of interest marked.
Cook et al., Science 362,53–57 (2018) 5 October 2018 2of5 RESEARCH | REPORT in 2015 to 41.3 ± 3 m in 2016, with highly variable the river channel were minimal and were as- earthquake (15, 16), the channel underwent widening throughout the mapped area (Fig. 3). sociated with external influences such as an- minimal modification during the 2015 monsoon The lateral changes in 2016 contrast with the thropogenic modification, local landsliding, and (Fig. 3 and fig. S6). stability of the river between 2010 and 2015. tributary input. Despite the large amount of Lateral erosion of the channel by the GLOF During these six monsoon seasons, changes to landslide debris produced during the 2015 Gorkha led to the activation of landslides that propagated
10000 Fig. 2. Discharge with distance downstream 1981 GLOF discharge of the Zhangzangbo confluence. Data from 5 10 15 20 5030 75 2016 GLOF discharge 11 years table S4. Estimated discharges for the 1981 ( ) and 2016 GLOFs and the calculated discharges Return period discharges for floods of varying return periods (5 to 75
/s) years). The blue line connects the 30-year- 3 return discharges. The violin plots show the full 1000 distribution of monsoon discharges (July and August) for each hydrological station (hydropower dam, Barabise, Pachuwarghat, discharge (m discharge and Khurkot). Error bars indicate estimated uncertainty (for details, see materials