Glacial Flooding & Disaster Risk Management Knowledge Exchange and Field Training July 11-24, 2013 in Huaraz, Peru HighMountains.org/workshop/peru-2013
Causes and Human Impacts of the Seti River (Nepal) Disaster of 2012 Jeffrey S. Kargel, Lalu Paudel, Gregory Leonard, Dhananjay Regmi, Sharad Joshi, Khagendra Poudel, Bhabana Thapa, Teiji Watanabe, and Monique Fort ABSTRACT: On May 5, 2012, a hyperconcentrated slurry flood in the Seti River suddenly burst forth onto a small village and rural areas in the valley upstream from Pokhara, the second largest city of Nepal. The flood swept away unsuspecting tourists, picnickers, laborers and local residents of Kharapani village. It killed 32 people and left another 40 missing, and it displaced many more. The flood killed livestock, wiped out local livelihoods, destroyed temples, roads, community buildings and vital infrastructure such as suspension bridges, electric poles and drinking water transmission pipes. The disaster, at first seemingly without cause, also took a psychological toll on the survivors in the affected villages and in Pokhara, whose residents wonder if the events could recur and if they could be the next victims. Satellite remote sensing and field investigations support the following scenario. A hazardous condition started by a rockfall blockage, a few weeks prior to the disaster, of the Seti River gorge and then filling of the impoundment reservoir by early spring melting of the snowfields and glaciers. A rock and ice avalanche from Annapurna IV (~7500 m) dislodged the previous rockfall dam when the rock-‐ice avalanche mixture swept into the reservoir. A hyperconcentrated slurry flow then swept down the Seti River. Eyewitness reports leading to and during the disaster and during recovery operations support this sequence. The geologic causes of the disaster pertain to the unique physiographic attributes of the upper Seti Basin as well as the general tectonic environment and lithologic makeup and glacial history of the Himalaya, which together have produced a highly unstable environment of frequent bedrock failures, deep river incision and river damming, deposition of vast amounts of unconsolidated glacigenic sediment, and frequent mass movements and floods involving the sediment, bedrock, ice, and impounded water. We also gathered information about the human root causes of the high death toll and to gather demographic and physiographic data that help to constrain scenarios of potential future disasters of similar types in this area. The toll increased as a result of people inhabiting unsafe places against existing land-‐use/habitation zoning restrictions. Nature and the law, if both had been respected by Seti Valley residents, would not have caused a disaster of this magnitude. However, an even greater disaster could happen any year, as we have identified several possible modes of catastrophic discharge of water and sediment into the Seti River. 1. Introduction On May 5, 2012, just after 9 AM local time, a tourist flight operator—Captain Alexander Maximov—was flying an ultralight Aeroprakt aircraft over the Seti River valley just north of his Avia Club operating base in Pokhara, Nepal. He observed a huge yellow cloud spreading across the upper part of the basin (the Sabche Cirque); according to our interviews of him, the cloud was 0
unlike any meteorological cloud or snow avalanche he had ever seen. He then observed a muddy flash flood racing down the Seti River. During his return to the Pokhara airport, he radioed the first eyewitness report of something devastating in progress, and this news was broadcast by local radio stations. Speculation holds that the timely dissemination of Captain Maximov’s report may have saved many lives in areas farther downstream. The upstream residents and tourists, picnickers, and streambed laborers were caught completely unprepared, with no warning at all, and many perished. A preliminary report prepared by ICIMOD with Kargel’s input offers some details of the disaster and the early development and testing of working hypotheses (REF). Captain Maximov also had wingtip video cameras mounted on his aircraft. The aircraft’s cameras recorded the dust cloud as well as the flood racing at high speed down valley. On the ground, mayhem and death occurred as the sediment-‐ and log-‐laden slurry flow proceeded in a series of pulses lasting for many hours cumulatively. Each pulse had a peak flow lasting only a few minutes. Many of these pulses, starting with the first one, were recorded on resident and tourist eyewitnesses’ mobile phone video cameras; quite a few of these—many rather tragic— were posted rapidly on Youtube. The eyewitness accounts and videos from the aircraft operator and people on the ground—including media reports as well as our own interviews of survivors— form a cornerstone of observations upon which the geomorphologic process causes and geological underpinning of the disaster could be reconstructed. Other key observations include a seismic signal picked up due to the triggering avalanche, satellite-‐based imaging before and after the disaster, and helicopter-‐borne field reconnaissance and ground-‐based field studies following the disaster. Our analysis of an amateur video taken of the first flood wave as it reached Pokhara indicated a peak discharge of >1000 m3/s. It was further estimated—with wide uncertainty—that the cumulative flow volume emitted during more than twenty flood waves was at least 2 x 106 m3, perhaps more by a factor of several, but not likely more than 107 m3. The Seti River disaster could have been just one more in an unending series of barely noted human tragedies in the Himalaya at the hands of nature if it was not for the modern way that this tragedy was documented. From ubiquitous mobile phone video cameras that almost everybody has these days the event was documented starting within minutes of its avalanche trigger through all the human suffering. Satellite eyes in the sky observed the scene shortly before and shortly after the disaster, aiding in the forensics search for causes. Ubiquitous modern technology—including mobile phones—may be instrumental in development of an SMS text-‐ based warning system. Imprudent habitation and improper usage of the flood plain shared as much blame for much of the disaster as nature carries. Tragic as this disaster was, it could have been much worse according to what our field survey found. 2. Study Area The Seti River, west Nepal, originates from the Annapurna Range (Tethys Himalaya) and flows across the Higher Himalaya and down to the Lesser Himalaya along the Pokhara Valley (Figure 1). It has a very steep profile in the north near the Annapurna Range (Tethys Himalaya and Higher Himalaya) and then flows with a gentle profile in most parts of its length across the Lesser Himalaya, where it flows within terraced clastic sedimentary deposits of the Pokhara Valley. The Pokhara Valley is a result of at least two giant debris-‐flow events in the past (Yamanaka et al., 1982; Fort, 1987). One took place 12,000 ± 1000 B.P., at the end of the last glaciation (Koirala and Rimal, 1996; Koirala et al., 1996), and led to the formation of oldest terrace of the Pokhara Valley
known as the Ghachok Formation. A second event similar size and nature occurred 750 ± 50 B.P. (Koirala et al., 1996; Hanisch and Koirala, 2010) and resulted in the Pokhara Formation (Yamanaka et al. 1982, Fort 1987). Damming of tributaries of the Seti River by the huge amount of sediments along the main channel resulted in the formation of a number of lakes in the Pokhara valley. The sediment source is believed to be the sediments accumulated in the bowl-‐like structure in between the Annapurna IV and Machhapuchre mountains. This bowl-‐like structure has been named as the Sabche Cirque (Yamanaka et al. 1982, Fort 1987). During the Pleistocene late glacial maximum, ice probably occupied the entire cirque basin, whereas today glaciers have retreated to positions along the cirque headwall, where snow avalanches, wind-‐blown snow from the Annapurna peaks, and rock debris from the steep bedrock walls feed them at elevations lower than the terminations of most other glaciers of the eastern and central Himalaya. 3. Methodology Study was started with the working hypothesis such as: i) GLOF; ii) Rockfall-‐impounded lake; iii) karst cave lake; iv) The rock avalanche/landslide trigger; v) All-‐of-‐the-‐above approach with multiple water sources. Every hypothesis was evaluated on the basis of the field observation and mapping and have led us to the conclusions.
Figure 1: Google map showing the location of Annapurna IV and the Seti river valley The field work mainly consists of the geological mapping at 1:25000 scale. Basic mappable lithological units were identified first and lithological boundaries were traced on the topographic
base map. Geological traverses were made mainly on the roads, rivers and foot trails. Dilute HCl was used to identify the carbonates. Structural data (strike and dip of beds and foliation) were measured with the help of geological compass and plotted on the map. A cross-‐section was prepared across the mapped area. Representative samples were taken from the bed rock, sediments of the Annapurna Formation, air fall dust, and sediment of the Seti River flood plain for laboratory analysis. Flood inundation map was prepared using the existing top sheets and the detail topographical survey at 1 m contour level and by using software like HEC-‐RAS and Arc GIS. 4. Results 4.1. Bedrock geology of the Seti River Basin A regional geological map and cross-‐section of the Seti River Basin covering the area north of Pokhara was prepared in the field (Figs. 2 and 3). The lithology of the area was separated into several mappable units based on distinctions of lithology, presence or absence of fossils, sediment consolidation, and position within the overall rock sequence. The lithological boundaries observed along the accessible routes were extended to the inaccessible areas based on orientation of the beds and foliation (strike and dip). The bedrock of the area can be divided into three tectonic units, i.e., the Lesser Himalaya, Higher Himalaya and the Tethys Himalaya separated by major regional faults namely the Main Central Thrust (MCT) and Annapurna Detachment (AD). Unconsolidated materials are much younger and include the Annapurna formation (calcareous silts, sands, and gravels) and Recent glaciers, moraines, debris flows, and alluvial gravels. As we describe below, the 2012 disaster has a strong involvement from the deep Seti River gorge and the high, steep peaks of the Annapurna Range; these are erosional features developed in the rocks of the Tethys Himalaya and Higher Himalaya. The disaster also involved sediment derived from the Quaternary age Annapurna formation. Hence, the disaster relates very strongly to the geology of the Sabche Cirque as well as to both ancient and extant glaciers and glacial processes. A detailed report of the geology and geomorphology of the Seti Basin, due for submission to a peer-‐reviewed journal, is in preparation. 4.2. Geology implications for the Seti Flood Disaster of May 5, 2012 and future hazards The main objectives of the present study were to evaluate which of the proposed working hypotheses is supported by geological data and to access future hazard in the Pokhara valley. 4.2.1. Implication for the karst formation Karst topography is formed in easily soluble rocks such as carbonates (limestone and dolomites) and evaporates (gypsum and salt). It is evident from the present geological mapping that about 7 km stretch of the Seti river flows across carbonate rocks (marbles, calc-‐schists and calc-‐gneisses) (See Fig. 2). These rocks are mainly composed of calcite (CaCO3). Karst topography is very common in the Pokhara valley in the carbonate cemented terraces. Mahendra Cave, Chamere Cave and Gupteshowr Cave are some examples. Therefore, it is quite possible that underground channels and caves are present also in the Seti River gorge. This would increase the volume
available to store water when the gorge is dammed and hence increase the potential flood volumes when the dam is broken. 4.2.2. Implications for the sediment source of the 2012 hyperconcentrated slurry flow Sedimentological analysis shows that the samples from the Annapurna Formation, airfall dust and Seti River flood plain deposits (recent sediment and ancient terraces) have similar sedimentological and
Fig. 2. Geological map of the Seti River basin north of Pokhara. A-‐B line of cross section in Fig. 4.
Fig. 3. Simplified, schematic geological cross-‐section across the Pokhara valley. The line of cross-‐ section is shown in Fig. 2. MCT: Main Central Thrust, AD: Annapurna Detachment (=South Tibetan Detachment System). compositional characteristics. All the samples contain rock fragments composed of marble, limestone, calc-‐gneiss and calc-‐schists. It indicates that they have come from the same source (provenance). Our interpretation is that the fine sediment contained in the slurry flow was derived from the Annapurna formation, which in turn represents glacial deposits laid down during the Pleistocene, when a huge glacial ice mass occupied the Sabche Cirque. Extensive glacial erosion took place, enlarging and giving form to the Sabche Cirque and also generating abundant rock debris, which accumulated in moraines and a thick debris cover on the ice. During a climatic amelioration, probably around 13,000 years ago, supraglacial lakes formed on the debris-‐covered glaciers, and these enlarged and merged into an enormous lake at least 5 km across and possibly over 1000 m deep. The lake was dammed at the downstream side by a thick accumulation of ice-‐cored debris. Glaciers continued to flow into the lake and transported boulders, sand, and silt, and moraines collapsed into the lake as ice gradually retreated, causing huge debris flows and landslide deposits in the lake. This ancient mass of lake silt beds, moraines, debris flow deposits, and landslides—mainly of glacial and glaciolacustrine origin—are what now comprises the Annapurna formation. Outbursts from the lake and/or the Annapurna sediments produced the terraced deposits of the Pokhara Valley. These outbursts as well as steadier glacier meltwater discharge also eroded the deep gorge near the exit from the Sabche Cirque. Erosion of the gorge was both due to dissolution of carbonates and mechanical/hydraulic erosion due to the Seti River and its tributaries.
Rock debris eroded and transported by small residual glaciers, rockfalls, and avalanches—such as the rock/ice avalanche of May 5, 2012—continue to add masses of sediment within the Sabche Cirque. Remnants of the Annapurna formation as well as the younger glacial deposits and mass movement materials still tower in unstable relief and remain geomorphologically very active, producing many small debris flows and floods. Most of these are contained within the Sabche Cirque and have no consequences to people downstream. The largest of the floods and mass movements could travel as far as Pokhara and thus have a potential to produce tragic consequences in the Seti Valley below, as residents unfortunately discovered on May 5, 2012. 4.2.3. Implications for the water source of the 2012 hyperconcentrated slurry flow Large single events of anomalous monsoon rainfall could produce up to 107 m3 or even 2x107 m3 of water runoff, which could be discharged in as little as a day from the Sabche Cirque and may produce peak flows of 100-‐400 m3/s. The May 5, 2012 event was not a monsoonal event and not due to any kind of precipitation event, and anyway it produced brief but highly peaked discharges exceeding 1000 m3/s. The behavior of the hyperconcentrated slurry flood is such that sudden release from a natural water reservoir must have taken place. We have considered four candidates for a reservoir: (1) in or on the glacier ice, (2) inside caves in the Annapurna formation, (3) inside caves within the high-‐grade metamorphic rocks of the Tethyan Group or Higher Himalayas, or (4) inside the gorge. We have found evidence that all of these types of spaces may exist in the Sabche Cirque and its outlet gorge, any of which could remain a factor for future impoundments of water and potential outburst floods. However, six clues point to the gorge as the main reservoir and hence indicate a blockage and then a sudden unblockage of the gorge as the primary cause of the outburst. Next we summarize these clues and what they imply about the sequence of events that led to the disaster. 1. Water flow in the Seti River was virtually cut off to Pokhara and the upstream villages in the days and weeks before the disaster, according to many eyewitness reports. This implies a nearly complete stream blockage. The blockage must have been below the point where the major tributaries within the Sabche Cirque join but above the upstream villages where the diminished (almost zero) flow was observed. This constrains the point of blockage to a short segment of the Seti River. Had the blockage been above the point where the two major tributaries (the north branch and west branch) join in the gorge area, perhaps half (more or less) of the water would have been blocked but much flow would have continued. This first clue tends to rule out the glacier ice and the Annapurna formation as hosts of the reservoir, but it allows the gorge and also could allow any karstic cavernous spaces connected to the gorge that could have been filled due to damming of the gorge. 2. When the Seti River was blocked, a trickle continued but changed color from the usual white caused by abundant suspended rock flour. (“Seti” means “white,” so it is the White River) Hence, water draining from the glaciers and from the Annapurna formation was blocked, leaving only small amounts of water runoff from points below the glaciers and the Annapurna formation. This again points to a blockage in the gorge area below the Annapurna formation and below where the two major tributaries join. 3. The ice/rock avalanche from Annapurna IV triggered the outburst, and to have done so there should be a direct pathway from the avalanche to the reservoir. The main glacial lakes and drained ice basins we have observed (Fig. Y) are west of where the avalanche impacted and traversed, and so a glacier lake outburst flood appears improbable as the cause of the 2012 6
disaster. We have traced the route of the avalanche to the gorge, and so again the gorge is implicated as the most likely reservoir. 4. Satellite observations have definitively identified several discrete erosional events along the walls of the gorge. Our observations via satellite and helicopter have identified the biggest of these as a site of recurrent rockfalls into the gorge. 5. Helicopter-‐borne observations show a white sediment staining or covering of the walls of the gorge consistent with it having contained a sediment-‐laden reservoir in the gorge. 6. Our observations have indicated that the gorge volume is far greater than we had initially believed, and so it could have contained enough water and sediment volume to have explained the slurry flood disaster. The gorge is both wider in some sections and far deeper (exceeding 500 m) than we had suspected at first. The gorge geometry is such that a contained volume of more than 107 m3 is possible, i.e., enough to explain the 2012 outburst flood volume. Furthermore, there remains some speculation with some limited supporting observations that the gorge geometry may widen at the bottom with karst cave-‐like structures, which may add to the present estimations of contained volume. Consequently, our earlier assessment that the flood water volume required multiple sources is no longer a requirement. The gorge alone—with or without additional water-‐filled karstic caverns—might be sufficient to explain the flood volume. The rockfall-‐dammed gorge hypothesis is thus the simplest and likeliest explanation. 4.3. Implication for future hazards 4.3.1. Landslide, rock fall, and rock avalanche The bedrock in the upper Seti Basin are quite unstable due to the steep slopes and high relief and the known history of large and small mass movements. Huge rock sliding along the bedding plane (plane failure) is quite common in the area. Freeze and thaw action of water is playing significant role in the failure of slopes, including failure according to knickpoint theory (REF). Large rock sliding along the gorge wall of the Seti River may cause frequent damming of the Seti River. Similar flash floods are thus possible in the future by the failure of the landslide dam. 4.3.2. Ice avalanche Ice from a cornice was apparently involved in the May 5, 2012 avalanche. Hanging glaciers are also present high on the walls of the Sabche Cirque. Collapsing ice could impart enough energy to the unconsolidated sediment of the Annapurna formation to generate a large sediment mass movement. An ice avalanche into a glacial lake could generate a GLOF. 4.3.3. Debris flow hazard by liquefaction The Annapurna Formation is loose sediment of about 500-‐600 m thick (more in some areas) covered in the upper part by glaciers and kettle lakes. In the spring and summer, huge amounts of ice and snow are melted and this melt water saturates the sediments, potentially weakening the cohesion of the sediment. Strong monsoon rains could likewise saturate the sediment. Many debris flows are observed in the Annapurna formation, so we know that sediment flow is a frequent occurrence. The recent debris flows we have observed are relatively small, and events of that magnitude do not pose a threat. However, if a strong earthquake occurs at this situation, or if a large discrete monsoonal rain event adds onto already-‐saturated conditions, the sediment
may liquefy from a large area and flow downstream and potentially cause a huge disaster in the Pokhara valley. 4.3.4. Glacier lake outburst flood We have discovered a substantial ice basin which could impound several million cubic meters of water if not for the fact that the basin is breached already. We also see indications of rapid drainage downstream from it in recent years. It would appear that glacier lake outburst floods (GLOFs) have occurred from the glaciers in the Sabche Cirque, but their hydrograph peak discharge magnitudes were apparently too small to be noticed downstream. We have some concern about these as possible future hazards if the ice basin should become closed and lakes should grow and a GLOF of larger magnitude should occur or if sediment from the Annapurna formation should be ingested into a GLOF and a large, fluid debris flow should form. Hence, there should be satellite-‐based or aircraft-‐based monitoring of glacial lakes in the Sabche Cirque. 5. Socioeconomic/demographic survey of the disaster’s impacts To investigate the socio-‐economic status in flood-‐affected area of Seti river basin in Pokhara we carried out a socio-‐economic field survey in the downstream affected areas of the Seti river basin from the end of the December 2012 to first week of January 2013. Similarly, flood inundation mapping of Seti River was also conducted in this period. The results are summarized here. The most of the riverbank dwellers are migrants and have low ability and low propensity to purchase safer lands for settlements in urban area. Since labor was the main occupation of majority of the affected population, their rent paying capacity would be lower. Therefore, they settled along the marginal public lands without caring about the risk of flood havoc or in a calculated gamble knowing that there is some risk. More than 90 percent of households had prior knowledge about probable risks in the settlement. They had seen mud in the river water without heavy rain. However, they were busy in eking out their livelihoods and became less careful about the risk and stayed at their own dwellings. The river mapping focuses on producing a flood inundation map, overlaying the map prepared from the survey data so, that the scenario of the Seti River and its periphery can be visualized, and possible future risks to nearby areas can be better assessed. Ultimately, the hazardous and vulnerable zones by the river are depicted. Furthermore, we have suggested precautions and remedies that could be undertaken to mitigate future floods, hyperconcentrated slurry flows, and debris flows in this and nearby valleys. We have identified several villages that are exceptionally prone to being swept away by floods. We have withheld naming them pending detailed study so as to avoid a possible panic response when our findings are still preliminary and based on a quick assessment. In one village more than fifty houses are clustered together and about fifteen houses on the banks in clear and immediate danger of being swept away by possible floods. In another village a school along with the school children and teachers as well as residents are in immediate jeopardy of a flood due to a GLOF or a monsoon-‐driven flood or a landslide; in June 2012 already a landslide buried part of that village but amazingly nobody was killed. Residents in these and other villages can be swept away at any time; therefore there is an urgent need for developing and implementing suitable tools and procedures for forecasting and real-‐time warning of flash floods and debris flows.
A fuller report on these aspects of our work is in preparation and will be submitted to a peer-‐ reviewed journal. A PhD dissertation on this topic has been initiated. 6. Recommendations The 2012 disaster is not likely to be replicated exactly, but something similar is likely. To protect the Pokhara residents from future geological hazards comparable to the 2012 event, or potentially worse, the following are recommended. (1) Landslide, rockfall, and debris flow mapping of the Seti River basin. (2) Mapping of the landslide and rockfall hazards (potential for future events) (3) Studies of the monsoon precipitation regime and extreme weather (temperature and precipitation) with effects on runoff. (4) Studies of the earthquake or precipitation-‐driven liquefaction potential assessment of the Annapurna formation. (5) Modeling of flow peak discharge/flood level/inundation for floods, hyperconcentrated slurries, and debris flows arising from the Sabche Cirque. (6) Design of a warning system perhaps involving SMS text messaging. The residents and officials receiving any warnings should be trained in how to respond if an anomalous situation is observed. Residents will have to be a part of the system, so it will be important to engage with the residents by consulting with them during the system design and implementation and in educating them on the nature of the hazard environment and training them in the use iof the warning system. (7) Discussion with city, district, and national officials about land-‐use and demographics in relationship to the 2012 disaster and remaining hazards. 7. Acknowledgements This work was supported by the USAID Climate Change Resilient Development (CCRD) Project (Grant Number CCRDCS0009) and by the NASA/USAID Science of Terra and Aqua Program. 8. References Fort, M., 1987, Sporadic morphogenesis in a continental subduction setting: an example from the Annapurna Range, Nepal Himalaya.Zeitschr. Geomorphology, Suppl. V. 63, pp. 9-‐36. Koirala, A. and Rimal, L.N., 1996, Geological hazards in Pokhara Valley, western Nepal.-‐J. Nep. Geol. Soc., 13: 99-‐108. Koirala, A., Rimal, L.N., Sikrikar, S. M., Pradhananga, U. B. and Pradhan, P. M., Hanisch, J., Jäger, S., Kerntke, M., 1996: The engineering and environmental geological map of the Pokhara Valley 1:50.000, DMG/BGR-‐Project, Dept.Mines,Geology, Kathmandu. Harris, N. And Whalley, J., 2001. Mountain building. Block 4. The Open University, UK, 165p. Jackson, M., and R. Bilham, 1994. Constraints on Himalayan Deformation inferred from Vertical Velocity Fields in Nepal and Tibet, J. Geophys. Res., 99(B7), 13897-‐13912. Mattauer, M., 1989. Monts et Merveilles, Beautes et Richesse de la Geologie. Hermann Editeurs des Sciences et des Art, Paris, 267p.
Shrestha, A.B., P. Mool, J. Kargel, R.B. Shrestha, Samjwal Bajracharya, Sagar Bajracharya, and D. Tandukar, 2012, Quest to unravel the cause of the Seti flash flood, 5 May 2012, online report posted by ICIMOD. http://www.icimod.org/?q=7377. Yamanaka, H. and Iwata, S., 1982, River terraces along the middle Kali Gandaki and MarsyandiKhola, Central Nepal. J. Nepal Geol. Soc., v. 2, pp. 95-‐111.
On May 5, 2012, a hyperconcentrated slurry flood in the Seti River suddenly burst forth onto a small village and rural areas in the valley u...
Published on Sep 25, 2013
On May 5, 2012, a hyperconcentrated slurry flood in the Seti River suddenly burst forth onto a small village and rural areas in the valley u...