Article Progress in Physical Geography 2017, Vol. 41(1) 3–28 ª The Author(s) 2016 Recent catastrophic landslide Reprints and permission: sagepub.co.uk/journalsPermissions.nav lake outburst floods in the DOI: 10.1177/0309133316658614 Himalayan mountain range journals.sagepub.com/home/ppg Virginia Ruiz-Villanueva Dendrolab.ch, University of Bern, Switzerland Simon Allen University of Zurich, Zu¨rich, Switzerland; University of Geneva, Geneva, Switzerland Manohar Arora National Institute of Hydrology, Roorkee, Uttarakhand, India Narendra K Goel National Institute of Hydrology, Roorkee, Uttarakhand, India Markus Stoffel Dendrolab.ch, University of Bern, Switzerland; University of Geneva, Geneva, Switzerland Abstract Among the more complex and devastating interactions between climate and hydromorphological pro- cesses in mountain environments are landslide lake outburst floods (LLOFs), resulting from mass move- ments temporarily blocking a drainage system. This work reviews these processes in the Himalayas and highlights the high frequency of this type of phenomenon in the region. In addition, we analyse two recent catastrophic trans-national LLOFs occurring in the Sutlej river basin during 2000 and 2005. Based on high resolution satellite images, Tropical Rainfall Measuring Mission (TRMM), Moderate-Resolution Imaging Spectroradiometer (MODIS) derived evolution of snowline elevation and discharge data we reconstruct the timing and hydrometeorological conditions related to the formation and failure of landslide dams. Results showed that the 2005 flood, originating from the outburst of the Parchu Lake, was not related to heavy precipitation, but was likely enhanced by the rapid and late snowmelt of an unusually deep and widespread snowpack. The flood in 2000 was triggered by the outburst of an unnamed lake located on the Tibetan plateau, identified here for the first time. In this case, the outburst followed intense precipitation in thelakewatershed,whichraisedthelevelofthelakeandthuscausedthebreachingofthedam.Asstream gauges were damaged during the events detailed discharge data is not available, but we estimated the peak discharges ranging between 1100 m3 sÀ1 and 2000 m3 sÀ1 in 2005, and 1024 m3 sÀ1 and 1800 m3 sÀ1 in 2000. These events caused significant geomorphic changes along the river valleys, with observed changes in channel width exceeding 200 m. Results also demonstrate that remotely-sensed data enables valuable large-scale monitoring of lake development and related hydrometeorological conditions, and may thereby Corresponding author: Virginia Ruiz-Villanueva, Dendrolab.ch, Institute of Geological Sciences, University of Bern, Baltzerstrasse 1, Bern 3012, Switzerland. Email: [email protected] 4 Progress in Physical Geography 41(1) inform early warning strategies, and provide a basis for flood risk reduction measures that focus on disaster preparedness and response strategies. Keywords Extreme flood, lake outburst, landslide, precipitation, Himalayas I Introduction off-site impacts include backwater ponding, cat- Flood and landslide disasters are common in the astrophic outburst floods and debris flows, Himalayan region, owing to the unfavourable draw-down induced secondary landslides, or interaction of climate, topography and seismi- reservoir displacement waves (Schuster, city (Nadim et al., 2006). This natural predis- 2000). The propagation of these impacts is asso- position is exacerbated by anthropogenic ciated with long-term effects of channel processes, including land use changes, urbani- instability, avulsions, and river metamorphosis zation, and population growth within landslide due to overloading with landslide-derived sedi- affected areas (Petley, 2010). However, hydro- ment (Korup, 2005). geomorphic processes and related disasters in Most parts of the region between the main the Himalayan mountain system are relatively Himalayan range and the Tibetan Plateau are poorly known compared with many other phys- not very densely populated. However, the high ical environments (Worni et al., 2012, 2013). connectivity between hillslopes and river chan- This is in part due to their inaccessibility, their nels introduces the potential for impact transla- topographic, climatic and geomorphic complex- tion within the drainage network. Therefore, ity, and general lack of long-term monitoring even if landslides occur in remote areas, the activities (Shroder and Bishop, 1998). subsequent flood-related impacts are trans- Among the more complex and devastating ferred very long distances downstream, and interactions between climate and geomorphic potentially become transnational events. More- processes in such mountain environments are over, with development attracting more people catastrophic landslide lake outburst floods and bringing increasing wealth and infrastruc- (LLOFs), resulting from landslides or other ture into exposed downstream regions, the mass movements (e.g. rock falls) temporarily LLOF risk is likely to increase in the Himalayas blocking or damming a river valley (Hewitt, (Schwanghart et al., 2016b). However, LLOF 2006a; Jakob et al., 2016). These dams can form risk assessment is hampered by difficulties in lakes which can breach at timescales varying slope stability analyses, and lack of information from days to years after their formation (Li about the occurrence and triggering mechanism et al., 1986), producing some of the largest of past landslides and related lake formation and known catastrophic floods (O’Connor et al., failure (Hewitt, 2006a). 2013). The resulting dam geometry and the The aim of this work is to provide a first hydrogeomorphic characteristics of the comprehensive review of LLOF studies both upstream catchment area are primary factors in the broader Hindu Kush–Karakoram–Hima- determining the stability and longevity of land- layan (HKH) region and more specifically at our slide dammed lakes, with some lakes becoming study site, the Upper Sutlej River basin, so as to permanent new landscape features (Korup, explore the frequency of this type of events in 2004). However, landslide dams may threaten the past. We also focus specifically on two locations at significant distances up- or down- important trans-national LLOFs occurring in stream of the original emplacement site. Such the Sino-Indian Sutlej river basin during 2000 Ruiz-Villanueva et al. 5 Figure 1. (a) Catastrophic LLOFs in the Himalayan Range extracted from bibliography and (b) observed dams and the two LLOF analysed in this work in the Upper Sutlej River Basin. and 2005. These events were analysed by Gupta 1998; Shroder et al., 1993; Vuichard and Zim- and Sah (2008) who reported the first data and merman, 1986; Yamada, 1998). More recently, impacts of the floods in terms of economic dam- in the early 1980s, several catastrophic rock age. Here we extend their work by reconstruct- slope failures were recognised in the Karakoram ing the hydrometeorological conditions leading (Hewitt, 1988) and in the Nanga Parbat range to the formation and failure of the landslide (Shroder et al., 1989). The works presented by dams and by assessing some of the geomorphic Hewitt (1982, 1998) provided an extensive imprints relating to these flood events. In addi- record of natural river damming in the Kara- tion we report here the source of the flood in koram region, with thirty-five outburst floods 2000 (an unnamed lake), which was previously recorded in the past two hundred years. Among unidentified. the earliest studies reporting mass movements in the HKH region, many of them damming valleys, are those by Costa and Schuster II LLOFs in the Hindu (1987, 1991) and Shroder and Bishop (1998). Kush–Karakoram–Himalayas Costa and Schuster (1987) were also the first Numerous catastrophic outburst floods have to classify the type of dams and fans formed occurred during the Holocene in many areas after landslides. Throughout the 1990s, surveys of the Tibet-Himalayan orogeny (Figure 1; pointed to the existence of more than 200 land- Cenderelli and Wohl, 2003; Hewitt, 1988, slides in the Upper Indus basin (Hewitt, 2002b). 1998; Richardson and Reynolds, 2000; Shroder, In a subsequent study, Hewitt (2006b) described 6 Progress in Physical Geography 41(1) 161 mass movements damming rivers in the higher risk) torrential systems even during Indus River Basin and defined the concept of minor events. Although they illustrated these ‘naturally fragmented rivers’. Over the last issues analysing a catastrophic flood in the decade, many other studies reported LLOFs in French Alps, their results can be extrapolated the HKH region (Dunning et al., 2007; Mitchell to any other mountain region. et al., 2001; 2007; Petley, 2010; 2011; Sangode The geomorphic consequences of dam break- et al., 2011; Schneider et al., 2013; Seong et al., ing are massive erosion, sediment transport, and 2008; Walsh et al., 2012; Weidinger et al., the formation of flood terraces downvalley 2002), highlighting the importance of this type (Morche and Schmidt, 2012). The fluvial of cascading process in the region. response might be marked enough to be recog- The causes and triggers of landslides and nised in the river profile for long time periods dam formation vary significantly (Schneider (Korup et al., 2006), possibly even over thou- et al., 2013) and in some cases are still widely sands of years (Hewitt et al., 2011). Prehistoric debated (Hewitt, 2001). Weidinger (1998a), for rockslide dams offer insights into such fluvial instance, studied modern lake-damming land- responses over a range