Glacial Flooding & Disaster Risk Management Knowledge Exchange and Field Training July 11-24, 2013 in Huaraz, Peru HighMountains.org/workshop/peru-2013
Rates of Change on Spillway Lake, Ngozumpa Glacier, Nepal Ulyana N. Horodyskyj University of Colorado, Boulder Introduction: Through a combination of modern satellite imagery, repeat photography from the 1950s, and on-‐going field measurements, we know that glaciers of the Mt. Everest and Cho Oyu regions in Nepal have undergone and continue to undergo major losses of ice volume (e.g., Thompson et al., 2012; Bolch et al., 2008). This volume loss occurs not so much by loss of area, but more from a reduction in the thickness of the glacier at average rates that locally can exceed 1m/year. The formation of supraglacial (surface) lakes appears to be the catalyst for this loss; once formed, these lake basins can grow and migrate by back wasting and calving of surrounding ice walls (e.g., Byers 2007; Sakai et al. 2002; Benn et al. 2001; Benn et al. 2000). Fill and drain events in these basins throughout the summer, triggered either by monsoonal precipitation and/or englacial conduit flooding, can accelerate the melt as more ice, previously covered up by debris, is “cleaned off” by the drainage of water, thus exposing more ice to solar radiation (Horodyskyj et al., in progress). An important concern is whether lake formation itself is accelerating. To this end, Spillway Lake, a large supraglacial base-‐level lake on Ngozumpa, one of Nepal’s longest glaciers, (Fig. 1), has been profiled by researchers at the University of St. Andrews and Swansea University (2001/2009). Interpolated point bathymetry maps, made with a simple depth sounder (Fig. 2) revealed where parts of the lake were deepening, sometimes upwards of 13 m/yr (e.g., Thompson et al. 2012). The lake may eventually pose a flooding hazard to Sherpa villages down-‐glacier, hence the importance in quantifying the physics responsible for deepening and growth.
Figure 1. Location map for Ngozumpa glacier and the terminal Spillway Lake. source: BBC (http://www.bbc.co.uk/news/science-‐environment-‐16317090) Project Objectives: As supraglacial lakes deepen, they may eventually melt their way through to rock, making the transition to a proglacial lake with an active calving ice front, leading to even more rapid ice loss (e.g., Imja Lake, near Mt. Everest). In certain cases, however, debris (rocks and sediment) covers the lake floor, insulating the underlying ice and slowing or perhaps reversing the process, leading to an equilibrium lake that may be considered “dead.” The bottom of Spillway Lake is deepening in some places but may be partly debris covered elsewhere, providing a unique opportunity to study the physics crucial to glacial demise as influenced by lake formation. Thus, research objectives for 2012/2013 included (1) establishing a new bathymetric map for Spillway through point interpolation (November 2012) and (2) doing open water transects with a side-‐scan sonar system, to fill in any gaps as well as provide imagery (e.g., hard vs. soft bottoms; boulders; ice) of the lake (May 2013). In order to quantify the physics of deepening, four locations (named Northwest, Northeast, Main, and Southwest) in Spillway were chosen for long-‐term temperature monitoring (starting end of May 2013) of the surface and bottom. Preliminary Results: Spillway Depth and Area: A survey by Thompson et al. (published 2012; survey work completed in 2009) revealed the deepest parts of Spillway to be close to 27 meters. Areas of deepening seem to be associated with ice walls, mainly south-‐ and west-‐ facing, which calve and collapse through the melt season (Fig. 2). A new (2012) areal analysis reveals that the southern part of the basin has remained mostly stable, with not much expansion. The northern basins have lost area, especially in the northeast. What was once open water is now debris consisting of large boulders and sediment, due to a partial drainage event (~4 meter water level drop) sometime between the last survey work and November 2012. South-‐facing walls responsible for calving
and deepening in 2009/2010 have since been covered up by debris in 2012. However, deepening hotspots remain, particularly in the main basin of the lake.
Figure 2. Bathymetry (depth) map of Spillway Lake, from 2009 survey work (Thompson et al. 2012). 2010 GeoEye imagery is in the background, for comparison. Most, but not all, deep areas (between 22-‐27 meters) correlate with large ice walls. New bathymetry and GeoEye imagery (2012), for comparison, to be shown in July 14th presentation. Spillway Basin Temperatures: Four lake basins within Spillway were targeted for longer-‐term surface and bottom temperature monitoring: Northwest (NW), with a 10-‐m buoy; Northeast (NE), with a 15-‐m buoy; Main, with a 20-‐m buoy; and Southwest (SW, near the outflow), with a 5-‐m buoy. Figure 3 shows results from post-‐thaw (according to local sources: May 21, 2013) and pre-‐monsoon. The SW and NE basins have the warmest surface temperatures (Fig 3a); over the past two years, they are the basins that have gotten shallower, rather than deeper. The NW basin is influenced by nearby inflow channels of glacial meltwater, resulting in overall colder surface temperatures. The Main basin, which has been deepening significantly in recent years, is also one of the colder basins. A cool-‐down of surface temperatures at the end of May is due to a snowfall event; all basins except SW were affected by this event; in the NE basin, bottom temperatures were higher than surface ones due to this. Significance is that even if surface temperatures drop, the bottom can retain heat for melting any exposed ice. Overall, bottom temperatures (Fig 3b) reveal that the SW basin is the warmest, consistent with its shallow depth and sediment-‐covered bottom. The Main basin is second warmest, at least for a few days post-‐thaw, before dropping to the coldest temperatures and remaining mostly isothermal. This sudden drop may be indicative of a subaqueous calving event. An independent temperature probe was sent to the bottom of all the basins. Unlike in the other basins, where the probe got stuck in the
sediment at the bottom, in the Main basin, the probe came back up with ice forming around it and a bottom temperature of 0.1 deg C. Significance of this lies in there being a relatively unprotected ice base, subject to more rapid melting and, hence, deepening. Of all the basins, this one shows the most potential for significant deepening. Two time-‐lapse cameras are focused on the region, to further distinguish sub-‐aerial from subaqueous calving events.
Figure 3. (a) Surface temperature data for the Main, SW, NE and NW basins of Spillway Lake, Ngozumpa glacier. (b) Bottom temperature data for the same regions. Data collection: May 23 – June 5, 2013.
Project Scope: The importance of this research is two-‐fold. First, the work done here can be applied to other glaciers across the Himalaya making the transition from small supraglacial “ponds,” to large coalesced supraglacial lakes at their termini. As glaciers continue to adjust to current climatic conditions, these kinds of lakes will become more commonplace. Ngozumpa is still in the initial stages of forming a large supraglacial lake, unlike Imja (east of Ngozumpa) and Tsho Rolpa (west of Ngozumpa), which have grown lakes with long fetches and active ice calving fronts, leading to further glacier demise. Ngozumpa presents an opportunity to observe this transition, as it happens, and quantify the physics behind it. Second, this work matters to the locals that live in the vicinity of the glacier and down-‐valley from it. Should Spillway Lake ever overtop its moraine dam, it could lead to a catastrophic outburst flood. Though most villages are perched higher than the river water level, the potential exists for significant land erosion from a large flood event. An outburst in 1985, from Dig Tsho (Langmoche glacier) resulted in loss of life and infrastructure in down-‐valley settlements: five people perished and a $1.5 million hydroelectric power plant near Namche, a large Sherpa town, was destroyed (Bajracharya et el. 2007). Acknowledgements: The author would like to thank USAID for the individual climber-‐scientist grant and the High Mountain Glacial Watershed Program for providing funds for two field seasons and scientific equipment, without which this project would not be possible. References: Benn. D.I., Wiseman, S., and C.R. Warren (2000), Rapid growth of a supraglacial lake, Ngozumpa Glacier, Khumbu Himal, Nepal, IAHS Publ. 264, 177-‐185. Benn, D.I., Wiseman, S., and K.A. Hands (2001), Growth and drainage of supraglacial lakes on the debris-‐mantled Ngozumpa Glacier, Khumbu Himal, Nepal, Journal of Glaciology, 47, 626-‐638. Bolch, T., Buchroithner, M., Pieczonka, T. and A. Kunert (2008), Planimetric and volumetric glacier changes in the Khumbu Himal, Nepal, since 1962 using Corona, Landsat TM and ASTER data, Journal of Glaciology, 54, 187, 592-‐600. Byers, A. (2007), An assessment of contemporary glacier fluctuations in Nepal’s Khumbu Himal using repeat photography, Himalayan Journal of Sciences, 4(6), 21-‐ 26. Horodyskyj, U.N., Breashears, D., Bilham, R. (in progress), Supraglacial Lakes Changes, Through the Lens of a Camera. Sakai, A., Nakawo, M., and K. Fujita (2002), Distribution, characteristics and energy balance of ice cliffs on debris-‐covered glaciers, Nepal Himalaya, Arctic and Antarctic Alpine Research, 34, 12-‐19.
Thompson, S., Benn, D.I., Dennis, K., and A. Luckman (2012), A rapidly growing moraine-‐dammed glacial lake on Ngozumpa Glacier, Nepal, Geomorphology, 10.1016/j.geomorph.2011.08.015.
Published on Sep 25, 2013
Through a combination of modern satellite imagery, repeat photography from the 1950s, and on-going field measurements, we know that glaciers...