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A rockfall-induced glacial lake outburst flood, Upper Barun Valley, Nepal

A rockfall-induced glacial lake outburst flood, Upper Barun Valley, Nepal Original Paper Landslides (2019) 16:533–549 DOI 10.1007/s10346-018-1079-9 Alton C. Byers I David R. Rounce I Dan H. Shugar I Jonathan M. Lala I Elizabeth A. Byers I Received: 15 March 2018 Dhananjay Regmi Accepted: 9 October 2018 Published online: 22 November 2018 A rockfall-induced glacial lake outburst flood, Upper © The Author(s) 2018 Barun Valley, Nepal Abstract On April 20, 2017, a flood from the Barun River, Makalu- Since the 1980s, a number of field-based studies concerned with Barun National Park, eastern Nepal formed a 2–3-km-long lake at the causes and impacts of contemporary GLOFs have been conduct- its confluence with the Arun River as a result of blockage by ed in Nepal (e.g., Vuichard and Zimmermann 1987; Cenderelli and debris. Although the lake drained spontaneously the next day, it Wohl 2001; Lamsal et al. 2015; Byers et al. 2017). Nearly all have taken caused nationwide concern and triggered emergency responses. place years to decades after the event, and there is often uncertainty We identified the primary flood trigger as a massive rockfall from as to the actual flood triggering mechanisms involved (e.g., Lamsal the northwest face of Saldim Peak (6388 m) which fell approxi- et al. 2015). This comparatively small number of field-based studies is mately 570 m down to the unnamed glacier above Langmale glacial likely related to the expense, difficult working conditions, and re- lake, causing a massive dust cloud and hurricane-force winds. The moteness of the high mountain regions in which Himalayan GLOFs impact also precipitated an avalanche, carrying blocks of rock and have occurred. These studies, supported and enhanced by remote ice up to 5 m in diameter that plummeted a further 630 m down sensing and laboratory analyses, are nevertheless important for into Langmale glacial lake, triggering a glacial lake outburst flood advancing our understanding of the various and complex triggering (GLOF). The flood carved steep canyons, scoured the river’s ripar- mechanisms that can cause a flood and for enabling the development of effective hazard management and risk reduction methods for ian zone free of vegetation, and deposited sediment, debris, and boulders throughout much of the river channel from the settle- downstream communities and infrastructure. The following study ment of Langmale to the settlement of Yangle Kharka about 6.5 km discusses our findings regarding the source, cause, and impact of a 3 −1 s , GLOF that occurred on April 20, 2017 in the remote Barun valley, downstream. Peak discharge was estimated at 4400 ± 1800 m 6 3 and total flood volume was estimated at 1.3 × 10 m of water. This Makalu-Barun National Park, eastern Nepal. study highlights the importance of conducting integrated field studies of recent catastrophic events as soon as possible after they Background occur, in order to best understand the complexity of their trigger- At 4:00 p.m. on April 20, 2017, a flood from the Barun River, ing mechanisms, resultant impacts, and risk reduction manage- Makalu-Barun National Park (Fig. 1) was reported that formed a ment options. 2–3-km-long, 500-m-wide lake at its confluence with the Arun River (Kathmandu Post, April 21, 2017). Debris had dammed the . . . Keywords Glacial lakes Rockfall Avalanche Glacial lake floodwaters directly above the village of Barun Bazaar, which outburst floods Downstream impacts displaced 10 families from their homes, destroyed fields, and threatened to impact at least 80 families living within the imme- diate area in the event that the dam suddenly failed (Shakya 2017). Introduction Thelakealsothreateneddownstreamvillagesthat included Since the early 1960s, hundreds of new glacial lakes have formed in Phaksinda, Diding, Chetabesi, Lumningtar, and other riverside the Nepal Himalaya as a result of warming trends and glacial communities in Bhojpur and Dhankuta districts (MyRepublica retreat. These lakes can be potentially hazardous in the event that 2017), as well as construction activities of the recently approved a glacial lake outburst flood occurs, which suddenly releases the UpperArunhydropowerproject,located 2kmdownstream stored water. Triggering mechanisms, most often ice avalanches (Kathmandu Post 2017a, b; USAID 2014). entering the lake (Emmer and Cochachin 2013; Falatkova 2016; Nie Government response was swift, deploying a team from the et al. 2017), can create a surge wave that breaches the unconsoli- Nepal Army and Nepal Police to assist endangered people and dated terminal moraine dam. Other potential trigger mechanisms drain the lake if necessary. Fortunately, the lake drained sponta- include Bdisplacement waves from rockfalls, moraine failure due neously on April 21, 2017, less than 24 h after it formed. Attempts to dam settlement and/or piping, the degradation of an ice-cored to identify the source of the flood by the Nepal Army were moraine, seismic activity, or the rapid input of water from extreme thwarted when bad weather prevented a helicopter reconnaissance events or from an outburst flood from a glacial lake located of the upper Barun region (MyRepublica 2017). Speculations re- upstream^ (Rounce et al. 2017a). As of 2011, 24 known glacial lake garding the cause and source of the flood ranged from heavy rains, outburst flood (GLOF) events had been recorded for Nepal, the to flooded tributaries of the Barun, to a GLOF from the Lower majority occurring since the 1960s (ICIMOD 2011). At least five Barun glacial lake (Glacier Hub 2017). Beginning May 23, 2017, we additional five GLOFs or glacier-related floods have been reported conducted a 2-week field study in the Barun valley to assess the since that time, i.e., the Seti Kosi (river) flood of May 5, 2012 likely cause and source of the flood. (Kargel et al. 2013), the Langmoche lake flood of April 25, 2015 (Byers et al. 2017), the Lhotse glacier outburst floods of 2015 and Methods 2016 (Rounce et al. 2017b), and the April 20, 2017 Langmale glacial The trigger mechanisms and subsequent flood were re-constructed lake flood discussed in the current paper. through a combination of (1) remote sensing (helicopter flyover, Landslides 16 & (2019) Original Paper Fig. 1 Barun River catchment in eastern Nepal (star on upper right country inset) showing the location of Langmale, Barun, and Lower Barun glacial lakes, Saldim Peak, and kharka (grazing) areas impacted by the flood repeat satellite imagery analysis, repeat oblique photography, comparison with a 2-D numerical model (see BNumerical GLOF hazard/risk assessment); (2) field measurements (survey of high modeling^) and the actual event. water marks and wetted channel cross sections, sampling of air- and flood-deposited particle sizes) followed by flood re- Field measurements and analysis construction using the critical depth method, literature-based ve- Field measurements included surveys of wetted channel cross sections at peak flow (based on high water marks), surveys of locity estimates, and hydrograph constraints; (3) numerical GLOF modeling; and (4) eyewitness reports and video. wetted cross sections of flow in the Barun River 1 month after the flood, and particle size measurements of airborne and flood deposits. Between May 23 and June 6, 2017, six channel cross Remote sensing sections were selected in relatively straight, uniform reaches of A helicopter reconnaissance was used to photograph and video the flood path, where erosional and depositional processes would be minimized. High water marks consisting of new flood deposits observations of the Barun River and prospective GLOF triggering mechanisms on May 3, 2017, 2 weeks after the main flood event. were identified at each cross section. The wetted cross section at These observations were supplemented using repeat satellite im- peak flow was assumed to stretch from the uppermost high water mark on the left bank to the uppermost high water mark on the agery analysis (source: WorldView-2 (DigitalGlobe, Inc.) and PlanetScope (Planet Team 2016)) to identify flood-related features right bank of the river. Cross sections were surveyed using a such as avalanche paths, changes in glacial lake size, and riverbed Forestry Pro 550 laser rangefinder, tape measure, laser level, and staff gage. scouring. Repeat oblique photography of Saldim Peak (6388 m) was used to illustrate the most likely trigger of the GLOF event, i.e., Peak flood discharge and 1 month post-flood stream discharge a rockfall from the mountain’s northwest face. An assessment of were estimated using the critical depth method, which has been the hazard and flood risk associated with Langmale glacial lake used successfully to estimate flows in high-gradient streams where was conducted retroactively to model potential avalanche and Froude numbers approach the value of one (Grant 1997; Jarrett rockfall trajectories and the stability of the terminal moraine 1984, 2008, 2016; Jarrett and England 2002). The discharge equa- following the methods of Rounce et al. (2017a). Avalanche- and tion is as follows: rockfall-prone areas were identified based on slope and land Q ¼ AV classification criteria using the 30 m ASTER GDEM V2 (ASTER GDEM Validation Team 2011), the Randolph Glacier Inventory Version 5.0 (Nuimura et al. 2015;RGIConsortium 2017), and Landsat 7 and 8 images from September 12, 2000 and December where 21, 2016, respectively. The potential downstream impacts of a flood 3 −1 Q Discharge (m s ) from Langmale glacial lake were also modeled using the MC-LCP model (Watson et al. 2015), a geometric flood model, for A 534 Landslides 16 & (2019) 2 Cross-sectional area of flow (m ), with depth corrected for A digital elevation model (DEM) of the area was taken from cosine of thalweg slope NASA’s High Mountain Asia 8-m DEMs Derived from Cross-track −1 V Critical velocity (m s ) Optical Imagery, version 1 (Shean 2017). Data gaps were filled using the mean of the nearest neighbors in each cardinal direction; the and resulting DEM was further hydrologically corrected by filling all pffiffiffiffiffiffi sinks to the elevation of lowest neighbor. The DEM was then V ¼ F Dg converted to a triangulated irregular network (TIN) mesh in QGIS (QGIS Development Team 2016) for use in BASEMENT. Finally, a lake depth of 13.29 m was burned into the TIN mesh to create a lake volume of 1.1 million m . The moraine damming Langmale Lake was approximately 75 m F Froude number (dimensionless ratio of inertial and gravita- tional forces on fluid flow) wide, 100 m long, and 40 m high (Fig. 2). The moraine lacked vegetation, which is common for contemporary moraine-dammed D Mean depth (m) of flow, corrected for cosine of thalweg slope −2 lakes since they are typically young and located at high elevations g Acceleration due to gravity (9.8 m s ) (Costa and Schuster 1998). The moraine was assumed to not have The Froude number associated with the Barun River was as- an ice-core, since no ice was visible after the incision of the GLOF sumed to be 1 based on the criteria for critical flow, i.e., competent and no melt ponds or ice cliffs were observed on the moraine prior high gradient stream (slope > 0.01) with streambed particle size to the event. The lake’s relatively steep moraine and its close ranging from sand to boulders (Grant 1997). The peak flood proximity to the steep glacier front and surrounding rock-walled discharge was characterized as a hyper-concentrated flow, also cirques only increase its potential hazard. Two cross sections of the fitting the criteria for critical flow. It is possible that excessive moraine—one along the moraine’s length and one along its width debris could have created debris flow conditions with supercritical (Fig. 2)—were analyzed to assess the erosion resulting from the flow (F > 1) in some upper reaches, which would result in under- overtopping wave. estimating flood discharge, although this appears unlikely based The avalanche material’s density was interpolated from sev- on the sampled flood deposits (see BField measurements and eral sources. Studies on pure rockslides in the region reported −3 analysis^). Channel slope was high enough (0.06–0.30) that a densities from 1950 kg m (Yigong rock avalanche; Wang et al. −3 cosine correction was applied to avoid overestimating the depth 2017) to 2200 kg m (Langtang rockslide; Kargel et al. 2016), of flow. whereas most ice- and snow-dominated avalanches have densi- −3 Peak discharge velocities of historic GLOFs were drawn from ties around 1000 kg m (Schneider et al. 2014; Somos- previous studies to allow comparison with the peak discharges Valenzuela et al. 2016). A mixed rock (30%) and ice (50%) −3 calculated using the critical depth method. Based on the values avalanche in Alaska had a density of 1500 kg m (Sosio et al. of estimated peak discharge, in combination with flood timing 2012). Based on visual estimates at Langmale, the ratio of rock/ estimates from video and oral testimonies, a hydrograph was debris to snow/ice was estimated to be 3:2, so a density of −3 constructed for the flood above Yangle Kharka. The total flood 1600 kg m was assumed. volume estimated from the hydrograph was further constrained by The mass entry rate of the avalanche into the lake was adapted the low discharge values below Yangle Kharka. from Lala et al. (2018), which simulated potential avalanches and Airborne and flood-deposited sediments were collected for resulting impulse wave-induced GLOFs at the nearby Imja Tsho. textural analysis at five sites, beginning at the source lakebed Because Langmale glacial lake was mostly filled to its previous and ending in the Yangle Kharka floodplain 7.0 km downstream. water level with debris, it was assumed that 1.1 million m entered Grain size distribution for these samples was measured using the the lake, and the inflow of avalanche mass from Imja Tsho was hydrometer method (U.S. Department of Agriculture (USDA) scaled linearly such that its total volume was equal to this 2014). number. BASEMENT only accepts water as inflow; hence, to accurately depict the momentum transfer of the avalanche into Numerical GLOF modeling the lake, the mass entry rate was further scaled by 1.6 to account A simulation of the GLOF was performed using the Basic Sim- for the ratio of avalanche density at Langmale to that of water −3 −3 ulation Environment for Computation of Environmental Flow (1600 kg m versus 1000 kg m ; Fig. 3). To ensure that this and Natural Hazard Simulation (BASEMENT) model, an open- inflow of water was not included in the flood in addition to the access numerical model based on the 2-D shallow water equa- initial volume of lake water, no erosion was permitted at the lake tions (Vetsch et al. 2017). BASEMENT’s inclusion of sediment bed, which allowed the lake to retain most of its initial volume transport makes it particularly suitable for GLOF simulations, and only release ~ 1.3 million m . since it can simulate erosion, scouring, and debris flow in A previous study of moraines in the Nepal Himalaya found addition to water flow (Worni et al. 2014); moreover, it is a 2- that gravel and coarser boulders comprised 80–90% of the D model, which is superior to the geometric and 1-D models particle size distribution (Hambrey et al. 2008), which suggests that have been used for this type of application (Bricker et al. that the finer sediments that were sampled in the field (Table 1) 2017). In addition, characteristics of the overtopping wave were are not representative of the terminal moraine. Therefore, we validated with the Heller-Hager model, which combines analyt- used two grain size distributions from Worni et al. (2013) ical and empirical equations to study wave generation and (Table 2), which were used to model GLOFs using BASEMENT propagation resulting from mass movement into a reservoir in the Indian Himalaya. The use of two distributions also pro- (Heller et al. 2009). vided some quantification of uncertainty associated with the Landslides 16 & (2019) 535 Original Paper Fig. 2 Contour map of Langmale lake showing the geometry of the moraine and cross sections used to analyze erosion. Contour intervals are 10 m. erosion due to the grain size distributions. Density and porosity Eyewitness reports and video were determined from a sample taken at Imja Tsho (Lala et al. Informal, non-structured interviews (Sheftel and Zembrzycki 2013) −3 2018), yielding values of 1800 kg m and 30%, respectively. were conducted with flood eyewitnesses, lodge owners, climbing guides, seasonal Makalu basecamp workers, and one other western scientist who visited the Barun valley on May 6, 2017 with a student group (Carpenter 2017). The oral testimony component Imja original provided valuable information regarding the timing of the Saldim hydrograph Peak rockfall, resultant avalanche, flood, color and content of peak Langmale scaled flood water, geomorphic and infrastructure damage, fatalities, and for volume other attributes of the event that were followed up with further field investigations. Eyewitness video provided real-time footage 60000 Langmale scaled for volume and of the flood in the vicinity of Yangle Kharka while facilitating the density calculation of peak discharge and flood volume totals. Results Remote sensing The helicopter reconnaissance on May 2, 2017 revealed that Lower 0 Barun (27° 49′ 49″ N, 87° 05′ 43″ E; 4552 m) and Barun (27° 50′ 42″ N, 0 102030405060 87° 05′ 01.4″ E; 4843 m) glacial lakes were intact, while Langmale Time (s) glacial lake (27° 48′ 47’ N, 87° 08′ 21″ E; 4843 m) appeared to have recen t l y draine d ( https://www.youtub e .com/ Fig. 3 Inflow hydrographs for BASEMENT simulation, showing the original watch?v=kMOS7Yt45jY&feature=youtube). Satellite imagery hydrograph taken from Imja Tsho (Lala et al. 2018), and its adaptation for the Langmale GLOF model (WorldView-2 and PlanetScope) revealed that between February 19, 2017 and May 8, 2017, the lake area was reduced from 0.083 to 536 Landslides 16 & (2019) 3 -1 Inflow (m s ) Landslides 16 & (2019) 537 Table 1 Texture analysis of five flood-related sediment samples as determined by the hydrometer method No. Depth Sample location Latitude Color % Sand % Silt % Clay Sediment texture Interpretation and distance Longitude from source 1 Top 5 cm Langmale 27° 48′ 48″ N Dark 60.1 31.4 8.5 Sand with Blast deposit glacial lake 87° 08′ 23″ E gray silt- and mixed with bed (center clay-sized lakebed of pre-flood lake) particles sediment 2 Top 5 cm Outlet channel 27° 48′ 33″ N White 92.1 3.4 4.5 Sand Lake drainage bed/lower 87° 08′ 10″ E channel basin (0.5 km deposit from lake) 3 Top 5 cm Outer outlet 27° 48′ 29″ N White 88.1 7.4 4.5 Sand Lake drainage channel 87° 08′ 08″ E channel bed/lower basin (0.7 km deposit from lake) 4 Top 2 cm Mani 27° 47′ 51″ N Light 70.1 21.4 8.5 Sand with Blast deposit wall/Langmale 87° 07′ 35″ E gray silt- and (~ 3.3 km from clay-sized rock/ice impact particles zone) 5 43 cm Yangle Kharka 27° 45′ 33″ N Medium gray 90 9 1 Coarse sand Flood flow deposition area 87° 09′ 58″ E deposit (~ 7.0 km from moraine breach) Original Paper Table 2 Grain size distributions from Worni et al. (2013) used to assess the inherent in reconstructing a flood with massive depositional and uncertainty associated with the moraine erosion due to the inclusion of larger erosional impacts, possible temporary debris-flow characteristics, grain sizes a non-uniform channel, and potential fluctuations between super- Worni et al. Worni et al. critical and subcritical flow. For example, the relatively lower (2013)A (2013)B estimated peak discharges at 2.4 km and 3.6 km below the source Size (mm) Fraction (%) Size (mm) Fraction (%) reflect reaches that respectively shallowed in slope as the flood 428 4 22 reached the valley bottom and spread across an expanded flood- plain in the partly wooded pastures of Nhe Kharka. Both sites 812 11 10 likely experienced net deposition, resulting in underestimates of 22 16 32 21 peak discharge. The relatively higher discharge at 4.9 km below the 64 14 90 22 source reflects a narrowing straight reach with good bedrock control on the right and left banks and possible erosion within 128 20 256 15 the channel bed, which could result in overestimating peak dis- 180 10 720 10 charge. From the helicopter footage, significant superelevation around major bends in the flood path is clearly visible. These bends were avoided in selecting cross-section locations, but even 0.036 km (Supplementary material: Fig. S1), which indicates that at at its most uniform the flood path was not a straight channel, and least part of the flood’s source was from the drainage of Langmale the difference of several meters in the height of high water marks glaciallake(Fig. 4 (B)). Based on empirical volume-area equations between the left and right banks is notable. In addition to super- for glacial lakes (Cook and Quincy 2015), the lake volume was elevation around smaller bends, it is possible that sloshing of the estimated to have decreased from 1.1 million m on February 19, flood from side to side may have contributed to the difference in 2017 to 0.3 million m on May 8, 2017. Footage from the helicopter height of high water marks. showed the terminal moraine was clearly breached and zones below Flood channel cross sections were erratic near the source of the the lake were scoured. Large areas of the flat pastures at Yangle flood and became more uniform as the flood moved downstream Kharka were also destroyed by the scouring and deposition of the (Fig. 6). A number of large remnant boulders upstream from flood (Fig. 4 (C, D)). A short flight down the Barun River valley Yangle Kharka, most likely deposited during a previous flood revealed that the flood became more channelized beyond Yangle event (Chaudhary 2013; Byers et al. 2014; Carpenter 2017), ap- Kharka, scouring the river bed to bedrock. peared to be undisturbed by this event, i.e., they were still capped Repeat oblique photography of the northwest face of Saldim with moss, shrubs, and approximately 40-year-old fir trees. Peak (6388 m) revealed the primary flood trigger, i.e., a large Bedload otherwise became smaller and more uniform in the vi- rockfall from the mountain’s northwest face (Fig. 5). Langmale cinity of Yangle Kharka. glacial lake was previously excluded from the hazard assessment The peak velocities of historic GLOFs in nearby drainages conducted by Rounce et al. (2017a), since it was below the area provide some insight to the range of potentially expected values threshold used in that study. However, the mass movement trajec- for the Langmale flood. Vuichard and Zimmermann (1987) report −1 tories as calculated by the methods described in Rounce et al. peak velocities of 4–5ms for the 1985 Dig Tsho flood. Dwivedi −1 (2017a) showed that Langmale glacial lake was susceptible to both et al. (2000) report a range of 5–10 ms for the 1998 flood from −1 avalanches and rockfalls entering the lake, which included a rock- Tam Pokhari. A range of 4–8ms seems reasonable to use for 6 3 fall with a volume of 0.3 × 10 m from Saldim Peak. The terminal comparison with the critical depth estimates. Peak discharge based 3 −1 moraine did not appear to be ice-cored, but the steep lakefront on these velocities ranged from 1890 to 8651 m s (Table 3). For area angle (Fujita et al. 2012), an indicator of the steepness of the reference, the velocity at normal monsoon flow in the Barun River −1 moraine, of 16° indicated that the lake was susceptible to self- at Yangle Kharka was measured at 3.1 ms on August 13, 2014, 3 −1 destructive failure. The combination of potential mass movement corresponding to a discharge of 35 m s (Byers et al. 2014). Pre- entering the lake and self-destructive failure classified this lake as monsoon discharge in the Barun River was measured 1 month 6 3 3 −1 very high hazard with a potential flood volume of 1.1 × 10 m . The after the flood as follows: May 28, 7.9 m s at the Nhe bridge; 3 −1 3 −1 extent of a potential GLOF was modeled using the MC-LCP model May 29, 7.7 m s above Yangle Kharka; and June 2, 6.7 m s (Watson et al. 2015) and revealed that 33 buildings, 4 bridges, and below Yangle Kharka. 0.76 km of agricultural land could be impacted, which included Peak discharge estimates from the critical depth method range 3 −1 the 4 buildings in Yangle Kharka that were impacted by the actual from 2593 to 7640 m s (Table 3), which fall within the estimates event. These downstream impacts were classified as high. The based on plausible peak velocities for the event. We estimate peak 3 −1 combination of the hazard and the downstream impacts retroac- discharge for the event as 4400 ± 1800 m s . At Yangle Kharka, tively suggest that Langmale glacial lake was a very high risk for a the flood water spread out over the relatively flat and wide grazed GLOF. floodplain (300 m wide × 800 m long, 3.5% slope) and was grad- ually released through the bedrock constriction at the lower end of Field measurements and analysis the Yangle Kharka basin (Fig. 6 (map) and profile (G)). The peak Peak flood discharge was estimated at six locations along the discharge below this bedrock constriction is estimated in a stable 3 −1 Barun River (Fig. 6). Cross sections were chosen in relatively bedrock reach as 800 ± 250 m s . straight, stable reaches to avoid areas of significant erosion, depo- The total flood volume is constrained by the maximum flood sition, or superelevation around bends. Measurement of multiple height of 4.8 m in the broad floodplain at Yangle Kharka, consid- cross sections helped to address the large uncertainty that is ered together with the reduced discharge below the bedrock 538 Landslides 16 & (2019) Fig. 4 Satellite imagery of the study area. (A) A Landsat 8 scene of the upper Barun River valley. Boxes show extents of (B), (C), and (D), which are high-resolution (2 m, WorldView-2) multispectral before and after views of key sites along the river. (B) A before and after pair showing the reduction in area of Langmale glacial lake. (C) The channel scour and deposition along the Barun River created by the outburst flood. (D) The flood impacts in the broad floodplain area of Yangle Kharka, followed by the abrupt narrowing of the flood channel below the bedrock constriction at the lower end of Yangle Kharka. Inset (D) shows the location of boulders (4D) deposited near Nhe Khaka (Fig. 12d) (imagery courtesy of the DigitalGlobe Foundation) constriction exiting Yangle Kharka, and the knowledge that a lake be stored in Yangle Kharka, we estimate a total flood volume of no 6 3 did not form in Yangle Kharka. In other words, the flood was small more than 1.3 × 10 m . The minimum total flood volume is enough that it could drain quickly, even with a peak outflow from constrained by the estimated peak flow measurements and the Yangle Kharka that was much less than the peak inflow. Given the shape of the hydrograph, i.e., the duration of the rising limb, peak flow measurements and the potential volume of water that could flow, and falling limb of the event. Landslides 16 & (2019) 539 Original Paper Fig. 6 Cross sections on the Barun River during peak flow based on high water marks between the banks of the river, which show the transition from highly non-uniform wetted channels near the source of the flood to more uniform channels further downstream. Map to the left shows location of the glacier that collapsed above Langmale glacial lake (BX^), cross-section sites, and width of scouring and deposition in the Barun River channel With the assistance of flood eye witness Dorje Sherpa, we on a large boulder on the right bank of the Barun River at Yangle measured the recollected height of the water over time within the Kharka. Combining oral accounts with examination of the flood Yangle floodplain where structures provided reference points, and videos, we were able to reconstruct the approximate shape of the Fig. 5 Before (top) and after (bottom) photographs of the large rock face that collapsed on April 20, 2017 (top photograph: E. Byers, 2016; bottom photograph: A. Byers 2017) 540 Landslides 16 & (2019) −1 Table 3 Cross-section data and peak discharge estimates based on critical depth method (CD) and literature-based peak velocity (v ) estimates of 4, 6, and 8 ms 3 −1 Cross Distance Slope Wetted Mean Peak discharge (m s ) estimates −1 −1 −1 CD v =4 ms v =6 ms v =8 ms section from source x-s area depth p p p (km) (m ) (m) B. 1.5 0.302 740 6.42 5722 2962 5183 7404 Langm- ale C. above 2.4 0.159 565 3.90 3511 2260 3955 5650 Riphuk D. below 3.6 0.149 472 3.07 2593 1890 3307 4725 Riphuk E. Nhe 4.9 0.129 865 7.95 7640 3460 6056 8651 bridge F. above 5.2 0.087 592 8.49 5407 2369 3554 4739 Yangle Average 647 5.97 4975 2588 4411 6234 B-F StdDev 155 2.41 1979 621 1169 1737 B-F G. below 8.3 0.062 138 3.85 849 552 828 1104 Yangle flood hydrograph at Yangle Kharka using upper and lower peak glacial lake (Table 1, samples 1 and 4) indicates that the initial −1 3 −1 velocity estimates of 4 ms (peak discharge of 2369 m s ) and impact of the Saldim Peak rockfall created a massive dust cloud, a −1 3 −1 8ms (peak discharge of 4739 m s ) (Fig. 7). The volume under phenomenon similar to that reported by Kargel et al. (2013) and the hydrograph is set equal to the estimated maximum total flood Kargel (2014) for the Seti Kosi flood in 2012 and by Kargel et al. 6 3 volume of 1.3 × 10 m of water. Experimenting with smaller flood (2015) for the Langtang avalanche following the 2015 Gorkha volumes, we were not able to reproduce a hydrograph with the earthquake. In contrast, flood channel and flow deposits consisted reported shape, i.e., at smaller flood volumes the peak becomes of coarser textures that ranged from sand to coarse sand. almost instantaneous rather than the reported sequence of events with the peak spreading out over 10–15 min, much of the flood subsiding by 30 min and the stream essentially back to normal flow after 1 h. Therefore, we propose that the estimated maximum Vpeak = 4 m/s flood volume is the best estimate of total volume for this event. 6 3 The estimated total flood volume of 1.3 × 10 m is larger than, but Vpeak = 8 m/s 5 3 of the same order of magnitude, as the estimated 7.6 × 10 m of water that was drained from Langmale glacial lake, suggesting that floodwater composition was a combination of lake water, snow and ice meltwater from the unseasonably warm weather as de- scribed by informants, water released from englacial conduits (Benn et al. 2012; Rounce et al. 2017b), water generated by friction created during the rockfall/ice avalanche, and material (trees, boulders, sediment) scoured by the flood as it advanced down- stream. The Langmale flood volume estimate is about a quarter the 6 3 size of the 5 × 10 m estimated for the well-known Langmoche (Dig Tsho) GLOF of 1985 in the Khumbu region (Vuichard and Zimmermann 1987), which destroyed a nearly completed hydro- power station, all bridges for 80 km downstream, and killed at least five people. The Barun valley is much less populated and developed than the Bhote Kosi valley in the Khumbu, which partly accounts for the lack of fatalities and comparatively minor struc- tural damage experienced as a result of the 2017 Langmale flood. 0 102030405060 Texture analysis of freshly deposited sediment supplemented Time since flood onset (min) the reconstruction of the sequence of events by characterizing the differences between airborne and flood deposits (Table 1). The Fig. 7 Estimated flood hydrograph at Yangle Kharka based on the time since the 6 3 covering of whitish sand intermixed with silt- and clay-sized start of the flood. Total flood volume shown is 1.3 × 10 m for upper and lower −1 −1 estimated peak velocities of 4 ms and 8 ms particles upon all surfaces within a 4-km radius of Langmale Landslides 16 & (2019) 541 3 -1 Esmated discharge (m s ) Original Paper Fig. 8 a GLOF modeling results showing the wave amplitude as it traverses the lake from the BASEMENT simulation and the corresponding Heller-Hager amplitude at the point of overtopping and b field observations of the path of the surge wave (red lines) that overtopped the left lateral and terminal moraines of Langmale glacial lake (photograph by D. Sherpa) Numerical GLOF modeling downstream channel cross sections did not vary greatly, and both BASEMENT was used to model the mass movement entering the were within the estimated values shown in Table 3. lake, the propagation of the tsunami-like wave across Langmale Inundation depth from BASEMENT (Fig. 11) also agreed with glacial lake, and the ensuing erosion, scouring, and flow of the observed scouring (Fig. 4 (C)), although the resolution of the DEM GLOF. The wave height at the point of overtopping for both may have contributed to excess spillover onto the western glacier, BASEMENT and the Heller-Hager (see Supplementary material: rather than confining more of the flood to the main eastern Table S1) model agreed, indicating an amplitude of 27.0 m for the channel. This spillover likely accounts for the lower, delayed peak former and 26.8 m for the latter (Fig. 8a). This agrees with ob- discharge at cross section D (Fig. 10) due to the water rejoining the served evidence of the wave, which left a mark on the moraine main channel. indicating a height of at least 25 m (Fig. 8b). The overtopping wave caused significant erosion of the termi- Eyewitness reports and video Oral testimony from a climbing guide (Dendi Sherpa, pers. comm. nal moraine, which was most severe along the lake’s main outlet channel (Fig. 9). The maximum erosion lowered the outlet channel 2017) and lodge owners at the settlement of Langmale (Tashi by approximately 22–25 m depending on the grain size distribu- Sherpa, pers. comm. 2017) confirmed that the flood trigger was a massive rockfall that fell from the southwest face of Saldim Peak tion. This amount of erosion agreed well with field observations (Figs. 8b and 12e) and confirmed that the GLOF was triggered by (6388 m) (Fig. 5) the day of the outburst flood. The rockfall was not the overtopping wave causing a failure of the terminal moraine. actually witnessed by anyone, partly because of the heavy fog that 3 −1 3 −1 covered Saldim Peak that day and also because most of the valley’s Peak discharge from BASEMENT, 5985 m s , 3456 m s , and 3 −1 2152 m s for cross sections B, C, and D, respectively (Fig. 10), seasonal residents were up at the Makalu basecamp closing down also agreed with observed estimates (Table 3). Due to the limited the season. However, all associated sounds (i.e., avalanche, wind, flood) were heard and/or felt by numerous informants, and the spatial extent of the DEM used for the GLOF model, cross sections E, F, and G were not analyzed. Despite these differences of up to fresh scar left behind on the rock wall was clearly visible from the 3 m in erosion at the moraine, the peak discharge at the village (Fig. 5). By comparing and integrating field and laboratory 542 Landslides 16 & (2019) 4745 Inial (masl) Worni et al. (2013) A Worni et al. (2013) B 0 50 100 150 200 250 MA’ MA Distance (m) Inial (masl) Worni et al. (2013) A Worni et al. (2013) B 0 50 100 150 200 250 MB’ MB Distance (m) Fig. 9 Cross sections (see Fig. 2) along the width (MA-MA’) and length (MB-MB’) of the terminal moraine before and after the flood showing the erosion caused by the overtopping wave for two grain size distributions results with these and other first-hand accounts, a reasonable Thefirst slopefailure occurred around 12:30 p.m.and was scenario of the most likely series of events which led to the relatively small, causing only a minor rise in Barun River’swater Langmale GLOF was reconstructed. level when it reached Yangle Kharka. This event was largely Fig. 10 GLOF modeling results showing discharge at cross sections B, C, and D (see Fig. 6) for grain size distributions from Worni et al. (2013) A (top) and Worni et al. (2013) B (bottom) Landslides 16 & (2019) 543 Elevaon (masl) Elevaon (masl) Original Paper Fig. 11 Maximum inundation depth downstream of Langmale glacial lake from BASEMENT simulation ignored by villagers and tourist groups alike in the downstream downstream from a steep section of the Barun river to the vicinity village of Yangle Kharka (Dorje Sherpa, pers. comm. 2017). The of Nhe Kharka (Fig. 12d). second slope failure at 1:30 p.m., however, consisted of a very large Massive new canyons and floodplains were created that volume of rock that fell approximately 570 m down to the un- destroyed dozens of hectares of pasture and forest land (Fig. 13a, named glacier above Langmale glacial lake (BX^ on Fig. 6). The b), reportedly killing at least 24 yaks and dzo (yak-cattle cross- impact precipitated an avalanche, carrying blocks of rock and ice breed). The flood channel cross sections were erratic near the up to 5 m in diameter (Fig. 12a, b) down another 630 m into source of the flood and became more uniform as the flood moved Langmaleglaciallake, triggering theGLOF. Therockfalland downstream (Figs. 6 and 13c). avalanche also created hurricane-force winds and a huge dust Eyewitnesses of the flood at Yangle Kharka reported hearing a and debris cloud of whitish sand-rich deposits that settled over very loud noise sometime in the early afternoon, described vari- shrubs, boulders, lodges, mani walls, and all other surfaces in a ously as Blike an avalanche,^ Bloud roar,^ or Bhelicopter,^ followed fan-like pattern from the impact zone to about 4 km south of by a huge cloud of dust (Dorje Sherpa, pers. comm. 2017; Langmale glacial lake (Table 1,sample4; Fig. 12c). The rockfall left Carpenter 2017). The dust cloud was accompanied by high winds behind a debris fan on top of the glacier, which traced the rock- at least as far as Riphuk, as villagers reported that trees there had fall’s trajectory as it descended to the lake (Supplementary mate- been blown down by hurricane force winds (Carpenter 2017). At rial: Fig. S1). Yangle Kharka, a large flood of water, sediment, and trees then The mass movement that entered Langmale glacial lake caused descended the river at around 2:30 p.m. Although considerable a tsunami-like wave, between 25 m and 30 m in height, that differences exist between informant accounts for the actual times overtopped its terminal and westernmost lateral moraine in the of the two rock avalanches, arrival of the flood at Yangle Kharka, form of a hyper-concentrated slurry of silty sediment (Fig. 12e; and other phenomena, those reported here are considered to be Table 1, sample 2), thereby allowing flood water to enter adjacent reasonably accurate based upon the flood’s arrival at Barun Bazaar glacier basins (Table 1, samples 2 and 3). Field investigations at 4:00 p.m. confirmed that when the floodwater overtopped Langmale glacial Fortunately, no one in either the Barun or Arun basins was lake’s western lateral moraine and entered the adjacent debris- killed or injured. Tourist lodges at Langmale, Zak Kharka, Riphuk covered glacier basin, a second and much smaller flood was Kharka, and Tematang escaped major damage. Yangle Kharka, triggered from a large meltwater pond. The floodwater proceeded however, suffered the loss of many hectares of valuable grazing to cascade down a 200-m rock wall into the channel below, where land because of flood deposits of coarse sand and debris (Table 1, the combined torrents (see dual peak in discharge of cross section sample 5). Four structures were also destroyed, including the D; Fig. 10) entrained more material and debris before merging with Makalu-Barun Hotel, an event that was dramatically captured in the Lower Barun glacial lake outlet stream below the settlement of avideo (https://www.youtube.com/watch?v=2VB1PRgb_Ic)(Fig. Riphuk. Boulders, sediment, and uprooted trees were strewn along 13b). Six bridges between Tematang and Langmale were also the length and width of the flood channel from below the settle- washed away, and villagers were deeply concerned that the coming ment of Langmale to Yangle Kharka, with displacement of partic- tourist, pilgrimage, and yartsa gunbu (the highly valuable medic- ularly large boulders (> 10 m in diameter) occurring 0.5 km inal fungi Ophiocordyceps sinensis) harvesting seasons would be 544 Landslides 16 & (2019) Fig. 12 a The rock face that fell from the upper slopes of Saldim Peak and newly deposited boulders in the foreground below the dam breach. b The large ice blocks from the avalanche that were deposited in the remains of Langmale glacial lake. c Sandy deposits from pulverized rock that settled over all surfaces for several kilometers. d The 10+-m-diameter boulders that were deposited by the flood near Nhe Kharka (photograph by C. Carpenter). e The lakebed sediment and debris that covered the basin below the Langmale terminal moraine as well as the basins to the west and east. The arrow in a shows the direction of rockfall from the west face of Saldim Peak; and arrows in b, d, and e show the direction of water flow (all photographs by A. Byers except for photograph d) negatively impacted because of the damaged or destroyed trails, numerical modeling in the assessment of glacier hazards. The bridges, and dramatically altered landscapes. Langmale GLOF experience also demonstrates the need to conduct The eyewitness video showed that the color of the debris-laden field-based GLOF analyses as soon as possible after the event; flood water was dark gray-brown, in contrast to the milky blue- otherwise, valuable evidence may be lost and the complexity of pale tan color from suspended silt observed by the authors during the event misunderstood or misinterpreted. Clues to the sequence the 2014 monsoon high flows (Byers et al. 2014). No reports of of triggering events and impacts, such as high water marks, air- unusual smells were given, such as the Bearthy^ odor reported for borne deposits, blocks of ice within the remaining and drained the Dig Tsho GLOF in 1985 (Vuichard and Zimmermann 1987)or lakebed, flood deposits, and eyewitness recollections, are ephem- Bgunpowder^ smell reported for the Tam Pokhari GLOF in 1998 eral. Even with these observations, reconstructing the entire GLOF (Pasang Sherpa, pers. comm. 2009). process chain is a complex task that has many uncertainties including the size and material composition of the mass entering Discussion the lake, the bathymetry of the lake, and the grain size distribution Remote sensing, field measurements, numerical modeling, oral of the moraine. Future work should consider using two-phase flow testimony, and video footage were used to reconstruct the most models for the mass entering the lake (e.g., Pudasaini 2014; Mergili likely series of events that led to the April 27, 2017 GLOF in the et al. 2017) and perform more detailed investigations of the mo- Barun valley. Methods and results were mutually supportive and raine material. Additionally, further research is needed regarding demonstrate the importance of combining field observations with the role of supplementary flood water from englacial conduits Landslides 16 & (2019) 545 Original Paper Fig. 13 a The flood path from the air. b The destruction at Yangle Kharka, where four buildings were destroyed. c The narrowed flood channel below Yangle Kharka (Benn et al. 2012; Rounce et al. 2017b) and melting caused by extent by restricting building zones to locations out of floodplains friction during the avalanche to overall flood volume. (e.g., Yangle Kharka), well away from torrents debouching from Old flood scars, levees, and even aged stands of fir trees (Abies glacial basins, and beyond other indicators of recent hydrologic/ spectabilis) in the riparian zone suggest that there have been at geomorphic activity. Likewise, based upon assessments of the least two GLOF events in the Barun valley over the past 100 years, timing of climate forcing, and lag times in glacier recession, lake possibly more (Byers et al. 2014; USAID 2014). The recent study of formation and moraine dam failure, it has been suggested that an GLOF risk by Rounce et al. (2017a) found that out of Nepal’s 131 increase in GLOF frequencies can be expected during the next lakes greater than 0.1 km , 11 were classified as very high risk, 31 as decade and into the twenty-second century (Harrison et al. high risk, 84 as moderate risk, and 5 as low risk (Rounce et al. 2018). New adaptive measures will be required in order to mini- 2017a). Risk was classified as the combination of hazard and mize the damage and loss of life that these floods may produce. downstream impacts, which considered mass movement entering Major infrastructure initiatives, such as large hydropower projects, the lake, moraine steepness, and the presence of an ice-cored need to recognize and plan for these events, which to date has been moraine in addition to potential hydropower systems, buildings, absent from most hydropower feasibility studies in Nepal (Butler bridges, and agricultural land that could be impacted by an and Rest 2017; USAID 2014). Likewise, risk awareness, disaster outburst flood. Langmale glacial lake was excluded from this management, and early warning system training will be of critical hazard assessment, since the lake area was less than 0.1 km . The importance to the lives and livelihoods of people living in villages GLOF that occurred at Langmale glacial lake, however, suggests and cities (e.g., Pokhara) located downstream of high mountain that the minimum threshold of 0.1 km used by Rounce et al. glaciated landscapes. (2017a) should be reduced, since smaller lakes can cause a GLOF The impacts of the 2017 Langmale flood on local economies are that has significant downstream impacts. The classification of very uncertain. The flood is of note not only because of its unusual and high hazard and the assessment’s ability to capture the rockfall complex triggering mechanisms (c.f. Kershaw et al. 2005) but also from Saldim Peak suggest that the mass movement trajectories are because it Bimpacted an area of recognized beauty, biological reasonable. The assessment estimated the potential flood volume diversity, and cultural significance^ (Carpenter 2017;see also: 6 3 to be 1.1 × 10 m , which is quite close to the estimated flood HMG 1990) where local economies have been supplemented for 6 3 volume of 1.3 × 10 m . However, a 2-D numerical model like decades by adventure tourists, pilgrims visiting sacred sites, and BASEMENT should be used for any hazard or risk mitigation more recently by yartsa gunbu collectors (Byers et al. 2014). Based efforts. BASEMENT was shown to reproduce the GLOF wave upon observations of the climatically similar Hinku Khola river heights, discharge, and flood path well given reasonable estimates valley below the village of Khote, which experienced a major GLOF of the total mass entering the lake and the morphology of the area. from the Tama Pokhari glacial lake in 1998 (Osti et al. 2011; Lamsal Although floods such as the Langmale GLOF are not predict- et al. 2015), it will take about 15 years for the scoured boulders and able, major damage to infrastructure can be mitigated to some river banks of the Barun’s riparian zone to re-establish pioneer 546 Landslides 16 & (2019) vegetation such as mosses, which in turn will allow for the estab- field studies immediately following a catastrophic event in order lishment of fir, birch, rhododendron, and other seedlings. Trek- to better understand the complexity of their triggering mecha- king tourism may experience temporary adverse impacts while nisms, resultant impacts, and risk reduction management options. lodges, trails, and bridges are rebuilt. Mountaineering will proba- Acknowledgements bly not be impacted, as most expeditions currently helicopter in The authors acknowledge the support of the National Science and out of the Makalu basecamp. Likewise, yartsa gunbu collection Foundation Dynamics of Coupled Natural and Human Systems will likely continue without interruption, as the major harvesting (NSF-CNH) Program (award no. 1516912) for the support of Alton sites are on high alpine ridges well out of the flood path. Recon- Byers and the NASA High Mountain Asia program (award nos. struction of bridges destroyed by the flood had already com- NNX17AB27G and NNX16AQ62G) for the support of David menced in May 2017, and three new bridges were completed in Rounce and Dan Shugar, respectively. The DigitalGlobe Founda- October 2017. However, local people’s enhanced fears of an even tion (www.digitalglobefoundation.org) and Planet Education and more devastating flood from the Lower Barun glacial lake, which Research Program (www.planet.com) are thanked for providing was classified by Rounce et al. (2017a) as a high risk, have clearly the satellite imagery used in the study. Ms. Sabina Devkota, Nepal been heightened. Lower Barun has been growing rapidly and non- Agricultural Research Council, is thanked for conducting the soil linearly in recent decades, reaching 1.8 km in 2017 (Haritashya texture analyses. Himalayan Research Expeditions (P) Ltd. provid- et al. 2018). ed logistical support for the fieldwork. Conclusion The series of events leading to the April 20, 2017 GLOF from Open Access This article is distributed under the terms of the Langmale glacial lake were reconstructed using remote sensing, field measurements, flood modeling, oral testimony, and video Creative Commons Attribution 4.0 International License (http:// creativecommons.org/licenses/by/4.0/), which permits unrestrict- footage. Results collectively suggest that the primary flood trigger ed use, distribution, and reproduction in any medium, provided was a massive rockfall from the northwest face of Saldim Peak (6388 m), which plummeted 1200 m down to Langmale glacial you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and lake. The rockfall caused a massive blast upon contact with the indicate if changes were made. glacier below, which generated a dust cloud and hurricane force winds, and enabled the rockfall to pick up ice blocks during its descent into the lake. The mass movement into the lake, and References resultant tsunami-like surge wave that overtopped the terminal and left lateral moraines, triggered the release of a U.S. Department of Agriculture (USDA) (2014) Soil survey field and laboratory methods hyperconcentrated flood of sediment, trees, and boulders that manual, Soil Survey Investigations report no. 51, Version 2. Natural Resources carved steep canyons and deposited bedload material from below Conservation Service, Lincoln, pp 61–69 https://www.nrcs.usda.gov/Internet/ the settlement of Langmale to the settlement at Yangle Kharka. 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Lala Department of Civil & Environmental Engineering, Electronic supplementary material The online version of this article (https://doi.org/ University of Wisconsin - Madison, 10.1007/s10346-018-1079-9) contains supplementary material, which is available to Madison, WI, USA authorized users. E. A. Byers A. C. Byers ()) West Virginia Department of Environmental Protection, Institute of Arctic and Alpine Research (INSTAAR), Elkins, WV, USA University of Colorado at Boulder, Boulder, CO, USA D. Regmi Email: alton.byers@colorado.edu Himalayan Research Center, Kathmandu, Nepal Landslides 16 & (2019) 549 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Landslides Springer Journals

A rockfall-induced glacial lake outburst flood, Upper Barun Valley, Nepal

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Copyright © 2018 by The Author(s)
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Earth Sciences; Natural Hazards; Geography, general; Agriculture; Civil Engineering
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1612-510X
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10.1007/s10346-018-1079-9
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Abstract

Original Paper Landslides (2019) 16:533–549 DOI 10.1007/s10346-018-1079-9 Alton C. Byers I David R. Rounce I Dan H. Shugar I Jonathan M. Lala I Elizabeth A. Byers I Received: 15 March 2018 Dhananjay Regmi Accepted: 9 October 2018 Published online: 22 November 2018 A rockfall-induced glacial lake outburst flood, Upper © The Author(s) 2018 Barun Valley, Nepal Abstract On April 20, 2017, a flood from the Barun River, Makalu- Since the 1980s, a number of field-based studies concerned with Barun National Park, eastern Nepal formed a 2–3-km-long lake at the causes and impacts of contemporary GLOFs have been conduct- its confluence with the Arun River as a result of blockage by ed in Nepal (e.g., Vuichard and Zimmermann 1987; Cenderelli and debris. Although the lake drained spontaneously the next day, it Wohl 2001; Lamsal et al. 2015; Byers et al. 2017). Nearly all have taken caused nationwide concern and triggered emergency responses. place years to decades after the event, and there is often uncertainty We identified the primary flood trigger as a massive rockfall from as to the actual flood triggering mechanisms involved (e.g., Lamsal the northwest face of Saldim Peak (6388 m) which fell approxi- et al. 2015). This comparatively small number of field-based studies is mately 570 m down to the unnamed glacier above Langmale glacial likely related to the expense, difficult working conditions, and re- lake, causing a massive dust cloud and hurricane-force winds. The moteness of the high mountain regions in which Himalayan GLOFs impact also precipitated an avalanche, carrying blocks of rock and have occurred. These studies, supported and enhanced by remote ice up to 5 m in diameter that plummeted a further 630 m down sensing and laboratory analyses, are nevertheless important for into Langmale glacial lake, triggering a glacial lake outburst flood advancing our understanding of the various and complex triggering (GLOF). The flood carved steep canyons, scoured the river’s ripar- mechanisms that can cause a flood and for enabling the development of effective hazard management and risk reduction methods for ian zone free of vegetation, and deposited sediment, debris, and boulders throughout much of the river channel from the settle- downstream communities and infrastructure. The following study ment of Langmale to the settlement of Yangle Kharka about 6.5 km discusses our findings regarding the source, cause, and impact of a 3 −1 s , GLOF that occurred on April 20, 2017 in the remote Barun valley, downstream. Peak discharge was estimated at 4400 ± 1800 m 6 3 and total flood volume was estimated at 1.3 × 10 m of water. This Makalu-Barun National Park, eastern Nepal. study highlights the importance of conducting integrated field studies of recent catastrophic events as soon as possible after they Background occur, in order to best understand the complexity of their trigger- At 4:00 p.m. on April 20, 2017, a flood from the Barun River, ing mechanisms, resultant impacts, and risk reduction manage- Makalu-Barun National Park (Fig. 1) was reported that formed a ment options. 2–3-km-long, 500-m-wide lake at its confluence with the Arun River (Kathmandu Post, April 21, 2017). Debris had dammed the . . . Keywords Glacial lakes Rockfall Avalanche Glacial lake floodwaters directly above the village of Barun Bazaar, which outburst floods Downstream impacts displaced 10 families from their homes, destroyed fields, and threatened to impact at least 80 families living within the imme- diate area in the event that the dam suddenly failed (Shakya 2017). Introduction Thelakealsothreateneddownstreamvillagesthat included Since the early 1960s, hundreds of new glacial lakes have formed in Phaksinda, Diding, Chetabesi, Lumningtar, and other riverside the Nepal Himalaya as a result of warming trends and glacial communities in Bhojpur and Dhankuta districts (MyRepublica retreat. These lakes can be potentially hazardous in the event that 2017), as well as construction activities of the recently approved a glacial lake outburst flood occurs, which suddenly releases the UpperArunhydropowerproject,located 2kmdownstream stored water. Triggering mechanisms, most often ice avalanches (Kathmandu Post 2017a, b; USAID 2014). entering the lake (Emmer and Cochachin 2013; Falatkova 2016; Nie Government response was swift, deploying a team from the et al. 2017), can create a surge wave that breaches the unconsoli- Nepal Army and Nepal Police to assist endangered people and dated terminal moraine dam. Other potential trigger mechanisms drain the lake if necessary. Fortunately, the lake drained sponta- include Bdisplacement waves from rockfalls, moraine failure due neously on April 21, 2017, less than 24 h after it formed. Attempts to dam settlement and/or piping, the degradation of an ice-cored to identify the source of the flood by the Nepal Army were moraine, seismic activity, or the rapid input of water from extreme thwarted when bad weather prevented a helicopter reconnaissance events or from an outburst flood from a glacial lake located of the upper Barun region (MyRepublica 2017). Speculations re- upstream^ (Rounce et al. 2017a). As of 2011, 24 known glacial lake garding the cause and source of the flood ranged from heavy rains, outburst flood (GLOF) events had been recorded for Nepal, the to flooded tributaries of the Barun, to a GLOF from the Lower majority occurring since the 1960s (ICIMOD 2011). At least five Barun glacial lake (Glacier Hub 2017). Beginning May 23, 2017, we additional five GLOFs or glacier-related floods have been reported conducted a 2-week field study in the Barun valley to assess the since that time, i.e., the Seti Kosi (river) flood of May 5, 2012 likely cause and source of the flood. (Kargel et al. 2013), the Langmoche lake flood of April 25, 2015 (Byers et al. 2017), the Lhotse glacier outburst floods of 2015 and Methods 2016 (Rounce et al. 2017b), and the April 20, 2017 Langmale glacial The trigger mechanisms and subsequent flood were re-constructed lake flood discussed in the current paper. through a combination of (1) remote sensing (helicopter flyover, Landslides 16 & (2019) Original Paper Fig. 1 Barun River catchment in eastern Nepal (star on upper right country inset) showing the location of Langmale, Barun, and Lower Barun glacial lakes, Saldim Peak, and kharka (grazing) areas impacted by the flood repeat satellite imagery analysis, repeat oblique photography, comparison with a 2-D numerical model (see BNumerical GLOF hazard/risk assessment); (2) field measurements (survey of high modeling^) and the actual event. water marks and wetted channel cross sections, sampling of air- and flood-deposited particle sizes) followed by flood re- Field measurements and analysis construction using the critical depth method, literature-based ve- Field measurements included surveys of wetted channel cross sections at peak flow (based on high water marks), surveys of locity estimates, and hydrograph constraints; (3) numerical GLOF modeling; and (4) eyewitness reports and video. wetted cross sections of flow in the Barun River 1 month after the flood, and particle size measurements of airborne and flood deposits. Between May 23 and June 6, 2017, six channel cross Remote sensing sections were selected in relatively straight, uniform reaches of A helicopter reconnaissance was used to photograph and video the flood path, where erosional and depositional processes would be minimized. High water marks consisting of new flood deposits observations of the Barun River and prospective GLOF triggering mechanisms on May 3, 2017, 2 weeks after the main flood event. were identified at each cross section. The wetted cross section at These observations were supplemented using repeat satellite im- peak flow was assumed to stretch from the uppermost high water mark on the left bank to the uppermost high water mark on the agery analysis (source: WorldView-2 (DigitalGlobe, Inc.) and PlanetScope (Planet Team 2016)) to identify flood-related features right bank of the river. Cross sections were surveyed using a such as avalanche paths, changes in glacial lake size, and riverbed Forestry Pro 550 laser rangefinder, tape measure, laser level, and staff gage. scouring. Repeat oblique photography of Saldim Peak (6388 m) was used to illustrate the most likely trigger of the GLOF event, i.e., Peak flood discharge and 1 month post-flood stream discharge a rockfall from the mountain’s northwest face. An assessment of were estimated using the critical depth method, which has been the hazard and flood risk associated with Langmale glacial lake used successfully to estimate flows in high-gradient streams where was conducted retroactively to model potential avalanche and Froude numbers approach the value of one (Grant 1997; Jarrett rockfall trajectories and the stability of the terminal moraine 1984, 2008, 2016; Jarrett and England 2002). The discharge equa- following the methods of Rounce et al. (2017a). Avalanche- and tion is as follows: rockfall-prone areas were identified based on slope and land Q ¼ AV classification criteria using the 30 m ASTER GDEM V2 (ASTER GDEM Validation Team 2011), the Randolph Glacier Inventory Version 5.0 (Nuimura et al. 2015;RGIConsortium 2017), and Landsat 7 and 8 images from September 12, 2000 and December where 21, 2016, respectively. The potential downstream impacts of a flood 3 −1 Q Discharge (m s ) from Langmale glacial lake were also modeled using the MC-LCP model (Watson et al. 2015), a geometric flood model, for A 534 Landslides 16 & (2019) 2 Cross-sectional area of flow (m ), with depth corrected for A digital elevation model (DEM) of the area was taken from cosine of thalweg slope NASA’s High Mountain Asia 8-m DEMs Derived from Cross-track −1 V Critical velocity (m s ) Optical Imagery, version 1 (Shean 2017). Data gaps were filled using the mean of the nearest neighbors in each cardinal direction; the and resulting DEM was further hydrologically corrected by filling all pffiffiffiffiffiffi sinks to the elevation of lowest neighbor. The DEM was then V ¼ F Dg converted to a triangulated irregular network (TIN) mesh in QGIS (QGIS Development Team 2016) for use in BASEMENT. Finally, a lake depth of 13.29 m was burned into the TIN mesh to create a lake volume of 1.1 million m . The moraine damming Langmale Lake was approximately 75 m F Froude number (dimensionless ratio of inertial and gravita- tional forces on fluid flow) wide, 100 m long, and 40 m high (Fig. 2). The moraine lacked vegetation, which is common for contemporary moraine-dammed D Mean depth (m) of flow, corrected for cosine of thalweg slope −2 lakes since they are typically young and located at high elevations g Acceleration due to gravity (9.8 m s ) (Costa and Schuster 1998). The moraine was assumed to not have The Froude number associated with the Barun River was as- an ice-core, since no ice was visible after the incision of the GLOF sumed to be 1 based on the criteria for critical flow, i.e., competent and no melt ponds or ice cliffs were observed on the moraine prior high gradient stream (slope > 0.01) with streambed particle size to the event. The lake’s relatively steep moraine and its close ranging from sand to boulders (Grant 1997). The peak flood proximity to the steep glacier front and surrounding rock-walled discharge was characterized as a hyper-concentrated flow, also cirques only increase its potential hazard. Two cross sections of the fitting the criteria for critical flow. It is possible that excessive moraine—one along the moraine’s length and one along its width debris could have created debris flow conditions with supercritical (Fig. 2)—were analyzed to assess the erosion resulting from the flow (F > 1) in some upper reaches, which would result in under- overtopping wave. estimating flood discharge, although this appears unlikely based The avalanche material’s density was interpolated from sev- on the sampled flood deposits (see BField measurements and eral sources. Studies on pure rockslides in the region reported −3 analysis^). Channel slope was high enough (0.06–0.30) that a densities from 1950 kg m (Yigong rock avalanche; Wang et al. −3 cosine correction was applied to avoid overestimating the depth 2017) to 2200 kg m (Langtang rockslide; Kargel et al. 2016), of flow. whereas most ice- and snow-dominated avalanches have densi- −3 Peak discharge velocities of historic GLOFs were drawn from ties around 1000 kg m (Schneider et al. 2014; Somos- previous studies to allow comparison with the peak discharges Valenzuela et al. 2016). A mixed rock (30%) and ice (50%) −3 calculated using the critical depth method. Based on the values avalanche in Alaska had a density of 1500 kg m (Sosio et al. of estimated peak discharge, in combination with flood timing 2012). Based on visual estimates at Langmale, the ratio of rock/ estimates from video and oral testimonies, a hydrograph was debris to snow/ice was estimated to be 3:2, so a density of −3 constructed for the flood above Yangle Kharka. The total flood 1600 kg m was assumed. volume estimated from the hydrograph was further constrained by The mass entry rate of the avalanche into the lake was adapted the low discharge values below Yangle Kharka. from Lala et al. (2018), which simulated potential avalanches and Airborne and flood-deposited sediments were collected for resulting impulse wave-induced GLOFs at the nearby Imja Tsho. textural analysis at five sites, beginning at the source lakebed Because Langmale glacial lake was mostly filled to its previous and ending in the Yangle Kharka floodplain 7.0 km downstream. water level with debris, it was assumed that 1.1 million m entered Grain size distribution for these samples was measured using the the lake, and the inflow of avalanche mass from Imja Tsho was hydrometer method (U.S. Department of Agriculture (USDA) scaled linearly such that its total volume was equal to this 2014). number. BASEMENT only accepts water as inflow; hence, to accurately depict the momentum transfer of the avalanche into Numerical GLOF modeling the lake, the mass entry rate was further scaled by 1.6 to account A simulation of the GLOF was performed using the Basic Sim- for the ratio of avalanche density at Langmale to that of water −3 −3 ulation Environment for Computation of Environmental Flow (1600 kg m versus 1000 kg m ; Fig. 3). To ensure that this and Natural Hazard Simulation (BASEMENT) model, an open- inflow of water was not included in the flood in addition to the access numerical model based on the 2-D shallow water equa- initial volume of lake water, no erosion was permitted at the lake tions (Vetsch et al. 2017). BASEMENT’s inclusion of sediment bed, which allowed the lake to retain most of its initial volume transport makes it particularly suitable for GLOF simulations, and only release ~ 1.3 million m . since it can simulate erosion, scouring, and debris flow in A previous study of moraines in the Nepal Himalaya found addition to water flow (Worni et al. 2014); moreover, it is a 2- that gravel and coarser boulders comprised 80–90% of the D model, which is superior to the geometric and 1-D models particle size distribution (Hambrey et al. 2008), which suggests that have been used for this type of application (Bricker et al. that the finer sediments that were sampled in the field (Table 1) 2017). In addition, characteristics of the overtopping wave were are not representative of the terminal moraine. Therefore, we validated with the Heller-Hager model, which combines analyt- used two grain size distributions from Worni et al. (2013) ical and empirical equations to study wave generation and (Table 2), which were used to model GLOFs using BASEMENT propagation resulting from mass movement into a reservoir in the Indian Himalaya. The use of two distributions also pro- (Heller et al. 2009). vided some quantification of uncertainty associated with the Landslides 16 & (2019) 535 Original Paper Fig. 2 Contour map of Langmale lake showing the geometry of the moraine and cross sections used to analyze erosion. Contour intervals are 10 m. erosion due to the grain size distributions. Density and porosity Eyewitness reports and video were determined from a sample taken at Imja Tsho (Lala et al. Informal, non-structured interviews (Sheftel and Zembrzycki 2013) −3 2018), yielding values of 1800 kg m and 30%, respectively. were conducted with flood eyewitnesses, lodge owners, climbing guides, seasonal Makalu basecamp workers, and one other western scientist who visited the Barun valley on May 6, 2017 with a student group (Carpenter 2017). The oral testimony component Imja original provided valuable information regarding the timing of the Saldim hydrograph Peak rockfall, resultant avalanche, flood, color and content of peak Langmale scaled flood water, geomorphic and infrastructure damage, fatalities, and for volume other attributes of the event that were followed up with further field investigations. Eyewitness video provided real-time footage 60000 Langmale scaled for volume and of the flood in the vicinity of Yangle Kharka while facilitating the density calculation of peak discharge and flood volume totals. Results Remote sensing The helicopter reconnaissance on May 2, 2017 revealed that Lower 0 Barun (27° 49′ 49″ N, 87° 05′ 43″ E; 4552 m) and Barun (27° 50′ 42″ N, 0 102030405060 87° 05′ 01.4″ E; 4843 m) glacial lakes were intact, while Langmale Time (s) glacial lake (27° 48′ 47’ N, 87° 08′ 21″ E; 4843 m) appeared to have recen t l y draine d ( https://www.youtub e .com/ Fig. 3 Inflow hydrographs for BASEMENT simulation, showing the original watch?v=kMOS7Yt45jY&feature=youtube). Satellite imagery hydrograph taken from Imja Tsho (Lala et al. 2018), and its adaptation for the Langmale GLOF model (WorldView-2 and PlanetScope) revealed that between February 19, 2017 and May 8, 2017, the lake area was reduced from 0.083 to 536 Landslides 16 & (2019) 3 -1 Inflow (m s ) Landslides 16 & (2019) 537 Table 1 Texture analysis of five flood-related sediment samples as determined by the hydrometer method No. Depth Sample location Latitude Color % Sand % Silt % Clay Sediment texture Interpretation and distance Longitude from source 1 Top 5 cm Langmale 27° 48′ 48″ N Dark 60.1 31.4 8.5 Sand with Blast deposit glacial lake 87° 08′ 23″ E gray silt- and mixed with bed (center clay-sized lakebed of pre-flood lake) particles sediment 2 Top 5 cm Outlet channel 27° 48′ 33″ N White 92.1 3.4 4.5 Sand Lake drainage bed/lower 87° 08′ 10″ E channel basin (0.5 km deposit from lake) 3 Top 5 cm Outer outlet 27° 48′ 29″ N White 88.1 7.4 4.5 Sand Lake drainage channel 87° 08′ 08″ E channel bed/lower basin (0.7 km deposit from lake) 4 Top 2 cm Mani 27° 47′ 51″ N Light 70.1 21.4 8.5 Sand with Blast deposit wall/Langmale 87° 07′ 35″ E gray silt- and (~ 3.3 km from clay-sized rock/ice impact particles zone) 5 43 cm Yangle Kharka 27° 45′ 33″ N Medium gray 90 9 1 Coarse sand Flood flow deposition area 87° 09′ 58″ E deposit (~ 7.0 km from moraine breach) Original Paper Table 2 Grain size distributions from Worni et al. (2013) used to assess the inherent in reconstructing a flood with massive depositional and uncertainty associated with the moraine erosion due to the inclusion of larger erosional impacts, possible temporary debris-flow characteristics, grain sizes a non-uniform channel, and potential fluctuations between super- Worni et al. Worni et al. critical and subcritical flow. For example, the relatively lower (2013)A (2013)B estimated peak discharges at 2.4 km and 3.6 km below the source Size (mm) Fraction (%) Size (mm) Fraction (%) reflect reaches that respectively shallowed in slope as the flood 428 4 22 reached the valley bottom and spread across an expanded flood- plain in the partly wooded pastures of Nhe Kharka. Both sites 812 11 10 likely experienced net deposition, resulting in underestimates of 22 16 32 21 peak discharge. The relatively higher discharge at 4.9 km below the 64 14 90 22 source reflects a narrowing straight reach with good bedrock control on the right and left banks and possible erosion within 128 20 256 15 the channel bed, which could result in overestimating peak dis- 180 10 720 10 charge. From the helicopter footage, significant superelevation around major bends in the flood path is clearly visible. These bends were avoided in selecting cross-section locations, but even 0.036 km (Supplementary material: Fig. S1), which indicates that at at its most uniform the flood path was not a straight channel, and least part of the flood’s source was from the drainage of Langmale the difference of several meters in the height of high water marks glaciallake(Fig. 4 (B)). Based on empirical volume-area equations between the left and right banks is notable. In addition to super- for glacial lakes (Cook and Quincy 2015), the lake volume was elevation around smaller bends, it is possible that sloshing of the estimated to have decreased from 1.1 million m on February 19, flood from side to side may have contributed to the difference in 2017 to 0.3 million m on May 8, 2017. Footage from the helicopter height of high water marks. showed the terminal moraine was clearly breached and zones below Flood channel cross sections were erratic near the source of the the lake were scoured. Large areas of the flat pastures at Yangle flood and became more uniform as the flood moved downstream Kharka were also destroyed by the scouring and deposition of the (Fig. 6). A number of large remnant boulders upstream from flood (Fig. 4 (C, D)). A short flight down the Barun River valley Yangle Kharka, most likely deposited during a previous flood revealed that the flood became more channelized beyond Yangle event (Chaudhary 2013; Byers et al. 2014; Carpenter 2017), ap- Kharka, scouring the river bed to bedrock. peared to be undisturbed by this event, i.e., they were still capped Repeat oblique photography of the northwest face of Saldim with moss, shrubs, and approximately 40-year-old fir trees. Peak (6388 m) revealed the primary flood trigger, i.e., a large Bedload otherwise became smaller and more uniform in the vi- rockfall from the mountain’s northwest face (Fig. 5). Langmale cinity of Yangle Kharka. glacial lake was previously excluded from the hazard assessment The peak velocities of historic GLOFs in nearby drainages conducted by Rounce et al. (2017a), since it was below the area provide some insight to the range of potentially expected values threshold used in that study. However, the mass movement trajec- for the Langmale flood. Vuichard and Zimmermann (1987) report −1 tories as calculated by the methods described in Rounce et al. peak velocities of 4–5ms for the 1985 Dig Tsho flood. Dwivedi −1 (2017a) showed that Langmale glacial lake was susceptible to both et al. (2000) report a range of 5–10 ms for the 1998 flood from −1 avalanches and rockfalls entering the lake, which included a rock- Tam Pokhari. A range of 4–8ms seems reasonable to use for 6 3 fall with a volume of 0.3 × 10 m from Saldim Peak. The terminal comparison with the critical depth estimates. Peak discharge based 3 −1 moraine did not appear to be ice-cored, but the steep lakefront on these velocities ranged from 1890 to 8651 m s (Table 3). For area angle (Fujita et al. 2012), an indicator of the steepness of the reference, the velocity at normal monsoon flow in the Barun River −1 moraine, of 16° indicated that the lake was susceptible to self- at Yangle Kharka was measured at 3.1 ms on August 13, 2014, 3 −1 destructive failure. The combination of potential mass movement corresponding to a discharge of 35 m s (Byers et al. 2014). Pre- entering the lake and self-destructive failure classified this lake as monsoon discharge in the Barun River was measured 1 month 6 3 3 −1 very high hazard with a potential flood volume of 1.1 × 10 m . The after the flood as follows: May 28, 7.9 m s at the Nhe bridge; 3 −1 3 −1 extent of a potential GLOF was modeled using the MC-LCP model May 29, 7.7 m s above Yangle Kharka; and June 2, 6.7 m s (Watson et al. 2015) and revealed that 33 buildings, 4 bridges, and below Yangle Kharka. 0.76 km of agricultural land could be impacted, which included Peak discharge estimates from the critical depth method range 3 −1 the 4 buildings in Yangle Kharka that were impacted by the actual from 2593 to 7640 m s (Table 3), which fall within the estimates event. These downstream impacts were classified as high. The based on plausible peak velocities for the event. We estimate peak 3 −1 combination of the hazard and the downstream impacts retroac- discharge for the event as 4400 ± 1800 m s . At Yangle Kharka, tively suggest that Langmale glacial lake was a very high risk for a the flood water spread out over the relatively flat and wide grazed GLOF. floodplain (300 m wide × 800 m long, 3.5% slope) and was grad- ually released through the bedrock constriction at the lower end of Field measurements and analysis the Yangle Kharka basin (Fig. 6 (map) and profile (G)). The peak Peak flood discharge was estimated at six locations along the discharge below this bedrock constriction is estimated in a stable 3 −1 Barun River (Fig. 6). Cross sections were chosen in relatively bedrock reach as 800 ± 250 m s . straight, stable reaches to avoid areas of significant erosion, depo- The total flood volume is constrained by the maximum flood sition, or superelevation around bends. Measurement of multiple height of 4.8 m in the broad floodplain at Yangle Kharka, consid- cross sections helped to address the large uncertainty that is ered together with the reduced discharge below the bedrock 538 Landslides 16 & (2019) Fig. 4 Satellite imagery of the study area. (A) A Landsat 8 scene of the upper Barun River valley. Boxes show extents of (B), (C), and (D), which are high-resolution (2 m, WorldView-2) multispectral before and after views of key sites along the river. (B) A before and after pair showing the reduction in area of Langmale glacial lake. (C) The channel scour and deposition along the Barun River created by the outburst flood. (D) The flood impacts in the broad floodplain area of Yangle Kharka, followed by the abrupt narrowing of the flood channel below the bedrock constriction at the lower end of Yangle Kharka. Inset (D) shows the location of boulders (4D) deposited near Nhe Khaka (Fig. 12d) (imagery courtesy of the DigitalGlobe Foundation) constriction exiting Yangle Kharka, and the knowledge that a lake be stored in Yangle Kharka, we estimate a total flood volume of no 6 3 did not form in Yangle Kharka. In other words, the flood was small more than 1.3 × 10 m . The minimum total flood volume is enough that it could drain quickly, even with a peak outflow from constrained by the estimated peak flow measurements and the Yangle Kharka that was much less than the peak inflow. Given the shape of the hydrograph, i.e., the duration of the rising limb, peak flow measurements and the potential volume of water that could flow, and falling limb of the event. Landslides 16 & (2019) 539 Original Paper Fig. 6 Cross sections on the Barun River during peak flow based on high water marks between the banks of the river, which show the transition from highly non-uniform wetted channels near the source of the flood to more uniform channels further downstream. Map to the left shows location of the glacier that collapsed above Langmale glacial lake (BX^), cross-section sites, and width of scouring and deposition in the Barun River channel With the assistance of flood eye witness Dorje Sherpa, we on a large boulder on the right bank of the Barun River at Yangle measured the recollected height of the water over time within the Kharka. Combining oral accounts with examination of the flood Yangle floodplain where structures provided reference points, and videos, we were able to reconstruct the approximate shape of the Fig. 5 Before (top) and after (bottom) photographs of the large rock face that collapsed on April 20, 2017 (top photograph: E. Byers, 2016; bottom photograph: A. Byers 2017) 540 Landslides 16 & (2019) −1 Table 3 Cross-section data and peak discharge estimates based on critical depth method (CD) and literature-based peak velocity (v ) estimates of 4, 6, and 8 ms 3 −1 Cross Distance Slope Wetted Mean Peak discharge (m s ) estimates −1 −1 −1 CD v =4 ms v =6 ms v =8 ms section from source x-s area depth p p p (km) (m ) (m) B. 1.5 0.302 740 6.42 5722 2962 5183 7404 Langm- ale C. above 2.4 0.159 565 3.90 3511 2260 3955 5650 Riphuk D. below 3.6 0.149 472 3.07 2593 1890 3307 4725 Riphuk E. Nhe 4.9 0.129 865 7.95 7640 3460 6056 8651 bridge F. above 5.2 0.087 592 8.49 5407 2369 3554 4739 Yangle Average 647 5.97 4975 2588 4411 6234 B-F StdDev 155 2.41 1979 621 1169 1737 B-F G. below 8.3 0.062 138 3.85 849 552 828 1104 Yangle flood hydrograph at Yangle Kharka using upper and lower peak glacial lake (Table 1, samples 1 and 4) indicates that the initial −1 3 −1 velocity estimates of 4 ms (peak discharge of 2369 m s ) and impact of the Saldim Peak rockfall created a massive dust cloud, a −1 3 −1 8ms (peak discharge of 4739 m s ) (Fig. 7). The volume under phenomenon similar to that reported by Kargel et al. (2013) and the hydrograph is set equal to the estimated maximum total flood Kargel (2014) for the Seti Kosi flood in 2012 and by Kargel et al. 6 3 volume of 1.3 × 10 m of water. Experimenting with smaller flood (2015) for the Langtang avalanche following the 2015 Gorkha volumes, we were not able to reproduce a hydrograph with the earthquake. In contrast, flood channel and flow deposits consisted reported shape, i.e., at smaller flood volumes the peak becomes of coarser textures that ranged from sand to coarse sand. almost instantaneous rather than the reported sequence of events with the peak spreading out over 10–15 min, much of the flood subsiding by 30 min and the stream essentially back to normal flow after 1 h. Therefore, we propose that the estimated maximum Vpeak = 4 m/s flood volume is the best estimate of total volume for this event. 6 3 The estimated total flood volume of 1.3 × 10 m is larger than, but Vpeak = 8 m/s 5 3 of the same order of magnitude, as the estimated 7.6 × 10 m of water that was drained from Langmale glacial lake, suggesting that floodwater composition was a combination of lake water, snow and ice meltwater from the unseasonably warm weather as de- scribed by informants, water released from englacial conduits (Benn et al. 2012; Rounce et al. 2017b), water generated by friction created during the rockfall/ice avalanche, and material (trees, boulders, sediment) scoured by the flood as it advanced down- stream. The Langmale flood volume estimate is about a quarter the 6 3 size of the 5 × 10 m estimated for the well-known Langmoche (Dig Tsho) GLOF of 1985 in the Khumbu region (Vuichard and Zimmermann 1987), which destroyed a nearly completed hydro- power station, all bridges for 80 km downstream, and killed at least five people. The Barun valley is much less populated and developed than the Bhote Kosi valley in the Khumbu, which partly accounts for the lack of fatalities and comparatively minor struc- tural damage experienced as a result of the 2017 Langmale flood. 0 102030405060 Texture analysis of freshly deposited sediment supplemented Time since flood onset (min) the reconstruction of the sequence of events by characterizing the differences between airborne and flood deposits (Table 1). The Fig. 7 Estimated flood hydrograph at Yangle Kharka based on the time since the 6 3 covering of whitish sand intermixed with silt- and clay-sized start of the flood. Total flood volume shown is 1.3 × 10 m for upper and lower −1 −1 estimated peak velocities of 4 ms and 8 ms particles upon all surfaces within a 4-km radius of Langmale Landslides 16 & (2019) 541 3 -1 Esmated discharge (m s ) Original Paper Fig. 8 a GLOF modeling results showing the wave amplitude as it traverses the lake from the BASEMENT simulation and the corresponding Heller-Hager amplitude at the point of overtopping and b field observations of the path of the surge wave (red lines) that overtopped the left lateral and terminal moraines of Langmale glacial lake (photograph by D. Sherpa) Numerical GLOF modeling downstream channel cross sections did not vary greatly, and both BASEMENT was used to model the mass movement entering the were within the estimated values shown in Table 3. lake, the propagation of the tsunami-like wave across Langmale Inundation depth from BASEMENT (Fig. 11) also agreed with glacial lake, and the ensuing erosion, scouring, and flow of the observed scouring (Fig. 4 (C)), although the resolution of the DEM GLOF. The wave height at the point of overtopping for both may have contributed to excess spillover onto the western glacier, BASEMENT and the Heller-Hager (see Supplementary material: rather than confining more of the flood to the main eastern Table S1) model agreed, indicating an amplitude of 27.0 m for the channel. This spillover likely accounts for the lower, delayed peak former and 26.8 m for the latter (Fig. 8a). This agrees with ob- discharge at cross section D (Fig. 10) due to the water rejoining the served evidence of the wave, which left a mark on the moraine main channel. indicating a height of at least 25 m (Fig. 8b). The overtopping wave caused significant erosion of the termi- Eyewitness reports and video Oral testimony from a climbing guide (Dendi Sherpa, pers. comm. nal moraine, which was most severe along the lake’s main outlet channel (Fig. 9). The maximum erosion lowered the outlet channel 2017) and lodge owners at the settlement of Langmale (Tashi by approximately 22–25 m depending on the grain size distribu- Sherpa, pers. comm. 2017) confirmed that the flood trigger was a massive rockfall that fell from the southwest face of Saldim Peak tion. This amount of erosion agreed well with field observations (Figs. 8b and 12e) and confirmed that the GLOF was triggered by (6388 m) (Fig. 5) the day of the outburst flood. The rockfall was not the overtopping wave causing a failure of the terminal moraine. actually witnessed by anyone, partly because of the heavy fog that 3 −1 3 −1 covered Saldim Peak that day and also because most of the valley’s Peak discharge from BASEMENT, 5985 m s , 3456 m s , and 3 −1 2152 m s for cross sections B, C, and D, respectively (Fig. 10), seasonal residents were up at the Makalu basecamp closing down also agreed with observed estimates (Table 3). Due to the limited the season. However, all associated sounds (i.e., avalanche, wind, flood) were heard and/or felt by numerous informants, and the spatial extent of the DEM used for the GLOF model, cross sections E, F, and G were not analyzed. Despite these differences of up to fresh scar left behind on the rock wall was clearly visible from the 3 m in erosion at the moraine, the peak discharge at the village (Fig. 5). By comparing and integrating field and laboratory 542 Landslides 16 & (2019) 4745 Inial (masl) Worni et al. (2013) A Worni et al. (2013) B 0 50 100 150 200 250 MA’ MA Distance (m) Inial (masl) Worni et al. (2013) A Worni et al. (2013) B 0 50 100 150 200 250 MB’ MB Distance (m) Fig. 9 Cross sections (see Fig. 2) along the width (MA-MA’) and length (MB-MB’) of the terminal moraine before and after the flood showing the erosion caused by the overtopping wave for two grain size distributions results with these and other first-hand accounts, a reasonable Thefirst slopefailure occurred around 12:30 p.m.and was scenario of the most likely series of events which led to the relatively small, causing only a minor rise in Barun River’swater Langmale GLOF was reconstructed. level when it reached Yangle Kharka. This event was largely Fig. 10 GLOF modeling results showing discharge at cross sections B, C, and D (see Fig. 6) for grain size distributions from Worni et al. (2013) A (top) and Worni et al. (2013) B (bottom) Landslides 16 & (2019) 543 Elevaon (masl) Elevaon (masl) Original Paper Fig. 11 Maximum inundation depth downstream of Langmale glacial lake from BASEMENT simulation ignored by villagers and tourist groups alike in the downstream downstream from a steep section of the Barun river to the vicinity village of Yangle Kharka (Dorje Sherpa, pers. comm. 2017). The of Nhe Kharka (Fig. 12d). second slope failure at 1:30 p.m., however, consisted of a very large Massive new canyons and floodplains were created that volume of rock that fell approximately 570 m down to the un- destroyed dozens of hectares of pasture and forest land (Fig. 13a, named glacier above Langmale glacial lake (BX^ on Fig. 6). The b), reportedly killing at least 24 yaks and dzo (yak-cattle cross- impact precipitated an avalanche, carrying blocks of rock and ice breed). The flood channel cross sections were erratic near the up to 5 m in diameter (Fig. 12a, b) down another 630 m into source of the flood and became more uniform as the flood moved Langmaleglaciallake, triggering theGLOF. Therockfalland downstream (Figs. 6 and 13c). avalanche also created hurricane-force winds and a huge dust Eyewitnesses of the flood at Yangle Kharka reported hearing a and debris cloud of whitish sand-rich deposits that settled over very loud noise sometime in the early afternoon, described vari- shrubs, boulders, lodges, mani walls, and all other surfaces in a ously as Blike an avalanche,^ Bloud roar,^ or Bhelicopter,^ followed fan-like pattern from the impact zone to about 4 km south of by a huge cloud of dust (Dorje Sherpa, pers. comm. 2017; Langmale glacial lake (Table 1,sample4; Fig. 12c). The rockfall left Carpenter 2017). The dust cloud was accompanied by high winds behind a debris fan on top of the glacier, which traced the rock- at least as far as Riphuk, as villagers reported that trees there had fall’s trajectory as it descended to the lake (Supplementary mate- been blown down by hurricane force winds (Carpenter 2017). At rial: Fig. S1). Yangle Kharka, a large flood of water, sediment, and trees then The mass movement that entered Langmale glacial lake caused descended the river at around 2:30 p.m. Although considerable a tsunami-like wave, between 25 m and 30 m in height, that differences exist between informant accounts for the actual times overtopped its terminal and westernmost lateral moraine in the of the two rock avalanches, arrival of the flood at Yangle Kharka, form of a hyper-concentrated slurry of silty sediment (Fig. 12e; and other phenomena, those reported here are considered to be Table 1, sample 2), thereby allowing flood water to enter adjacent reasonably accurate based upon the flood’s arrival at Barun Bazaar glacier basins (Table 1, samples 2 and 3). Field investigations at 4:00 p.m. confirmed that when the floodwater overtopped Langmale glacial Fortunately, no one in either the Barun or Arun basins was lake’s western lateral moraine and entered the adjacent debris- killed or injured. Tourist lodges at Langmale, Zak Kharka, Riphuk covered glacier basin, a second and much smaller flood was Kharka, and Tematang escaped major damage. Yangle Kharka, triggered from a large meltwater pond. The floodwater proceeded however, suffered the loss of many hectares of valuable grazing to cascade down a 200-m rock wall into the channel below, where land because of flood deposits of coarse sand and debris (Table 1, the combined torrents (see dual peak in discharge of cross section sample 5). Four structures were also destroyed, including the D; Fig. 10) entrained more material and debris before merging with Makalu-Barun Hotel, an event that was dramatically captured in the Lower Barun glacial lake outlet stream below the settlement of avideo (https://www.youtube.com/watch?v=2VB1PRgb_Ic)(Fig. Riphuk. Boulders, sediment, and uprooted trees were strewn along 13b). Six bridges between Tematang and Langmale were also the length and width of the flood channel from below the settle- washed away, and villagers were deeply concerned that the coming ment of Langmale to Yangle Kharka, with displacement of partic- tourist, pilgrimage, and yartsa gunbu (the highly valuable medic- ularly large boulders (> 10 m in diameter) occurring 0.5 km inal fungi Ophiocordyceps sinensis) harvesting seasons would be 544 Landslides 16 & (2019) Fig. 12 a The rock face that fell from the upper slopes of Saldim Peak and newly deposited boulders in the foreground below the dam breach. b The large ice blocks from the avalanche that were deposited in the remains of Langmale glacial lake. c Sandy deposits from pulverized rock that settled over all surfaces for several kilometers. d The 10+-m-diameter boulders that were deposited by the flood near Nhe Kharka (photograph by C. Carpenter). e The lakebed sediment and debris that covered the basin below the Langmale terminal moraine as well as the basins to the west and east. The arrow in a shows the direction of rockfall from the west face of Saldim Peak; and arrows in b, d, and e show the direction of water flow (all photographs by A. Byers except for photograph d) negatively impacted because of the damaged or destroyed trails, numerical modeling in the assessment of glacier hazards. The bridges, and dramatically altered landscapes. Langmale GLOF experience also demonstrates the need to conduct The eyewitness video showed that the color of the debris-laden field-based GLOF analyses as soon as possible after the event; flood water was dark gray-brown, in contrast to the milky blue- otherwise, valuable evidence may be lost and the complexity of pale tan color from suspended silt observed by the authors during the event misunderstood or misinterpreted. Clues to the sequence the 2014 monsoon high flows (Byers et al. 2014). No reports of of triggering events and impacts, such as high water marks, air- unusual smells were given, such as the Bearthy^ odor reported for borne deposits, blocks of ice within the remaining and drained the Dig Tsho GLOF in 1985 (Vuichard and Zimmermann 1987)or lakebed, flood deposits, and eyewitness recollections, are ephem- Bgunpowder^ smell reported for the Tam Pokhari GLOF in 1998 eral. Even with these observations, reconstructing the entire GLOF (Pasang Sherpa, pers. comm. 2009). process chain is a complex task that has many uncertainties including the size and material composition of the mass entering Discussion the lake, the bathymetry of the lake, and the grain size distribution Remote sensing, field measurements, numerical modeling, oral of the moraine. Future work should consider using two-phase flow testimony, and video footage were used to reconstruct the most models for the mass entering the lake (e.g., Pudasaini 2014; Mergili likely series of events that led to the April 27, 2017 GLOF in the et al. 2017) and perform more detailed investigations of the mo- Barun valley. Methods and results were mutually supportive and raine material. Additionally, further research is needed regarding demonstrate the importance of combining field observations with the role of supplementary flood water from englacial conduits Landslides 16 & (2019) 545 Original Paper Fig. 13 a The flood path from the air. b The destruction at Yangle Kharka, where four buildings were destroyed. c The narrowed flood channel below Yangle Kharka (Benn et al. 2012; Rounce et al. 2017b) and melting caused by extent by restricting building zones to locations out of floodplains friction during the avalanche to overall flood volume. (e.g., Yangle Kharka), well away from torrents debouching from Old flood scars, levees, and even aged stands of fir trees (Abies glacial basins, and beyond other indicators of recent hydrologic/ spectabilis) in the riparian zone suggest that there have been at geomorphic activity. Likewise, based upon assessments of the least two GLOF events in the Barun valley over the past 100 years, timing of climate forcing, and lag times in glacier recession, lake possibly more (Byers et al. 2014; USAID 2014). The recent study of formation and moraine dam failure, it has been suggested that an GLOF risk by Rounce et al. (2017a) found that out of Nepal’s 131 increase in GLOF frequencies can be expected during the next lakes greater than 0.1 km , 11 were classified as very high risk, 31 as decade and into the twenty-second century (Harrison et al. high risk, 84 as moderate risk, and 5 as low risk (Rounce et al. 2018). New adaptive measures will be required in order to mini- 2017a). Risk was classified as the combination of hazard and mize the damage and loss of life that these floods may produce. downstream impacts, which considered mass movement entering Major infrastructure initiatives, such as large hydropower projects, the lake, moraine steepness, and the presence of an ice-cored need to recognize and plan for these events, which to date has been moraine in addition to potential hydropower systems, buildings, absent from most hydropower feasibility studies in Nepal (Butler bridges, and agricultural land that could be impacted by an and Rest 2017; USAID 2014). Likewise, risk awareness, disaster outburst flood. Langmale glacial lake was excluded from this management, and early warning system training will be of critical hazard assessment, since the lake area was less than 0.1 km . The importance to the lives and livelihoods of people living in villages GLOF that occurred at Langmale glacial lake, however, suggests and cities (e.g., Pokhara) located downstream of high mountain that the minimum threshold of 0.1 km used by Rounce et al. glaciated landscapes. (2017a) should be reduced, since smaller lakes can cause a GLOF The impacts of the 2017 Langmale flood on local economies are that has significant downstream impacts. The classification of very uncertain. The flood is of note not only because of its unusual and high hazard and the assessment’s ability to capture the rockfall complex triggering mechanisms (c.f. Kershaw et al. 2005) but also from Saldim Peak suggest that the mass movement trajectories are because it Bimpacted an area of recognized beauty, biological reasonable. The assessment estimated the potential flood volume diversity, and cultural significance^ (Carpenter 2017;see also: 6 3 to be 1.1 × 10 m , which is quite close to the estimated flood HMG 1990) where local economies have been supplemented for 6 3 volume of 1.3 × 10 m . However, a 2-D numerical model like decades by adventure tourists, pilgrims visiting sacred sites, and BASEMENT should be used for any hazard or risk mitigation more recently by yartsa gunbu collectors (Byers et al. 2014). Based efforts. BASEMENT was shown to reproduce the GLOF wave upon observations of the climatically similar Hinku Khola river heights, discharge, and flood path well given reasonable estimates valley below the village of Khote, which experienced a major GLOF of the total mass entering the lake and the morphology of the area. from the Tama Pokhari glacial lake in 1998 (Osti et al. 2011; Lamsal Although floods such as the Langmale GLOF are not predict- et al. 2015), it will take about 15 years for the scoured boulders and able, major damage to infrastructure can be mitigated to some river banks of the Barun’s riparian zone to re-establish pioneer 546 Landslides 16 & (2019) vegetation such as mosses, which in turn will allow for the estab- field studies immediately following a catastrophic event in order lishment of fir, birch, rhododendron, and other seedlings. Trek- to better understand the complexity of their triggering mecha- king tourism may experience temporary adverse impacts while nisms, resultant impacts, and risk reduction management options. lodges, trails, and bridges are rebuilt. Mountaineering will proba- Acknowledgements bly not be impacted, as most expeditions currently helicopter in The authors acknowledge the support of the National Science and out of the Makalu basecamp. Likewise, yartsa gunbu collection Foundation Dynamics of Coupled Natural and Human Systems will likely continue without interruption, as the major harvesting (NSF-CNH) Program (award no. 1516912) for the support of Alton sites are on high alpine ridges well out of the flood path. Recon- Byers and the NASA High Mountain Asia program (award nos. struction of bridges destroyed by the flood had already com- NNX17AB27G and NNX16AQ62G) for the support of David menced in May 2017, and three new bridges were completed in Rounce and Dan Shugar, respectively. The DigitalGlobe Founda- October 2017. However, local people’s enhanced fears of an even tion (www.digitalglobefoundation.org) and Planet Education and more devastating flood from the Lower Barun glacial lake, which Research Program (www.planet.com) are thanked for providing was classified by Rounce et al. (2017a) as a high risk, have clearly the satellite imagery used in the study. Ms. Sabina Devkota, Nepal been heightened. Lower Barun has been growing rapidly and non- Agricultural Research Council, is thanked for conducting the soil linearly in recent decades, reaching 1.8 km in 2017 (Haritashya texture analyses. Himalayan Research Expeditions (P) Ltd. provid- et al. 2018). ed logistical support for the fieldwork. Conclusion The series of events leading to the April 20, 2017 GLOF from Open Access This article is distributed under the terms of the Langmale glacial lake were reconstructed using remote sensing, field measurements, flood modeling, oral testimony, and video Creative Commons Attribution 4.0 International License (http:// creativecommons.org/licenses/by/4.0/), which permits unrestrict- footage. Results collectively suggest that the primary flood trigger ed use, distribution, and reproduction in any medium, provided was a massive rockfall from the northwest face of Saldim Peak (6388 m), which plummeted 1200 m down to Langmale glacial you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and lake. The rockfall caused a massive blast upon contact with the indicate if changes were made. glacier below, which generated a dust cloud and hurricane force winds, and enabled the rockfall to pick up ice blocks during its descent into the lake. The mass movement into the lake, and References resultant tsunami-like surge wave that overtopped the terminal and left lateral moraines, triggered the release of a U.S. Department of Agriculture (USDA) (2014) Soil survey field and laboratory methods hyperconcentrated flood of sediment, trees, and boulders that manual, Soil Survey Investigations report no. 51, Version 2. Natural Resources carved steep canyons and deposited bedload material from below Conservation Service, Lincoln, pp 61–69 https://www.nrcs.usda.gov/Internet/ the settlement of Langmale to the settlement at Yangle Kharka. 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Lala Department of Civil & Environmental Engineering, Electronic supplementary material The online version of this article (https://doi.org/ University of Wisconsin - Madison, 10.1007/s10346-018-1079-9) contains supplementary material, which is available to Madison, WI, USA authorized users. E. A. Byers A. C. Byers ()) West Virginia Department of Environmental Protection, Institute of Arctic and Alpine Research (INSTAAR), Elkins, WV, USA University of Colorado at Boulder, Boulder, CO, USA D. Regmi Email: alton.byers@colorado.edu Himalayan Research Center, Kathmandu, Nepal Landslides 16 & (2019) 549

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Published: Nov 22, 2018

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