Evan S. Miles

Geomorphic and geologic controls of geohazards induced by Nepal’s 2015 Gorkha earthquake

Nepals quake-driven landslide hazards Large earthquakes can trigger dangerous landslides across a wide geographic region. The 2015 Mw 7.8 Gorhka earthquake near Kathmandu, Nepal, was no exception. Kargal et al. used remote observations to compile a massive catalog of triggered debris flows. The satellite-based observations came from a rapid response team assisting the disaster relief effort. Schwanghart et al. show that Kathmandu escaped the historically catastrophic landslides associated with earthquakes in 1100, 1255, and 1344 C.E. near Nepals second largest city, Pokhara. These two studies underscore the importance of determining slope stability in mountainous, earthquake-prone regions. Science, this issue p. 10.1126/science.aac8353; see also p. 147 Satellite imaging isolated hazard potential for earthquake-triggered landslides after the 2015 Gorkha earthquake in Nepal. INTRODUCTION On 25 April 2015, the Gorkha earthquake [magnitude (M) 7.8] struck Nepal, followed by five aftershocks of ≥M 6.0 until 10 June 2015. The earthquakes killed ~9000 people and severely damaged a 550 by 200 km region in Nepal and neighboring countries. Some mountain villages were completely destroyed, and the remote locations, blocked roads, and landslide-dammed rivers prevented ground access to many areas. RATIONALE Our “Volunteer Group” of scientists from nine nations, motivated by humanitarian needs, focused on satellite-based systematic mapping and analysis of earthquake-induced geohazards. We provided information to relief and recovery officials as emergency operations were occurring, while supported by one of the largest-ever NASA-led campaigns of responsive satellite data acquisitions over a vast disaster zone. Our analysis of geohazards distribution allowed evaluation of geomorphic, tectonic, and lithologic controls on earthquake-induced landsliding, process mechanisms, and hazard process chains, particularly where they affected local populations. RESULTS We mapped 4312 coseismic and postseismic landslides. Their distribution shows positive associations with slope and shaking intensity. The highest areal densities of landslides are developed on the downdropped northern tectonic block, which is likely explained by momentary reduction of the normal stress along planes of weakness during downward acceleration. The two largest shocks bracket the high-density landslide distribution, the largest magnitudes of the surface displacement field, and highest peak ground accelerations (PGAs). Landslides are heavily concentrated where PGA was >0.6g and slope is >30°. Additional controls on landslide occurrence are indicated by their clustering near earthquake epicenters and within specific lithologic units. The product of PGA and the sine of surface slope (defined as the landslide susceptibility index) is a good indicator of where most landslides occurred. A tail of the statistical distributions of landslides extends to low values of the landslide susceptibility index. Slight earthquake shaking affected vulnerable materials hanging on steep slopes—such as ice, snow, and glacial debris—and moderate to strong shaking affected poorly consolidated sediments deposited in low-sloping river valleys, which were already poised near a failure threshold. In the remote Langtang Valley, some of the most concentrated destruction and losses of life outside the Kathmandu Valley were directly due to earthquake-induced landslides and air blasts. Complex seismic wave interactions and wave focusing may have caused ridgetop shattering and landslides near Langtang but reduced direct shaking damage on valley floors and at glacial lakes. CONCLUSION The Gorkha earthquake took a tremendous, tragic toll on human lives and culture. However, fortunately no damaging earthquake-caused glacier lake outburst floods were observed by our satellite analysis. The total number of landslides was far fewer than those generated by comparable earthquakes elsewhere, probably because of a lack of surface ruptures, the concentration of deformation along the subsurface thrust fault at 10 to 15 km depth, and the regional dominance of competent high-grade metamorphic and intrusive igneous rock types. Landslide distribution and effects of a huge landslide. (A) Landslides (purple dots) are concentrated mostly north of the tectonic hinge-line. Also shown are the epicenters of the main shock and largest aftershock. Displacements are from the JAXA ALOS-2 ScanSAR interferogram (21 Feb and 2 May 2015 acquisitions). (B and C) Before-and-after photographs obtained by D. Breashears in Langtang Valley showing complete destruction of a large part of Langtang village by a huge landslide. The Gorkha earthquake (magnitude 7.8) on 25 April 2015 and later aftershocks struck South Asia, killing ~9000 people and damaging a large region. Supported by a large campaign of responsive satellite data acquisitions over the earthquake disaster zone, our team undertook a satellite image survey of the earthquakes’ induced geohazards in Nepal and China and an assessment of the geomorphic, tectonic, and lithologic controls on quake-induced landslides. Timely analysis and communication aided response and recovery and informed decision-makers. We mapped 4312 coseismic and postseismic landslides. We also surveyed 491 glacier lakes for earthquake damage but found only nine landslide-impacted lakes and no visible satellite evidence of outbursts. Landslide densities correlate with slope, peak ground acceleration, surface downdrop, and specific metamorphic lithologies and large plutonic intrusions.

Ecosystem Processes and Human Influences Regulate Streamflow Response to Climate Change at Long-Term Ecological Research Sites

Analyses of long-term records at 35 headwater basins in the United States and Canada indicate that climate change effects on streamflow are not as clear as might be expected, perhaps because of ecosystem processes and human influences. Evapotranspiration was higher than was predicted by temperature in water-surplus ecosystems and lower than was predicted in water-deficit ecosystems. Streamflow was correlated with climate variability indices (e.g., the El Niño—Southern Oscillation, the Pacific Decadal Oscillation, the North Atlantic Oscillation), especially in seasons when vegetation influences are limited. Air temperature increased significantly at 17 of the 19 sites with 20- to 60-year records, but streamflow trends were directly related to climate trends (through changes in ice and snow) at only 7 sites. Past and present human and natural disturbance, vegetation succession, and human water use can mimic, exacerbate, counteract, or mask the effects of climate change on streamflow, even in reference basins. Long-term ecological research sites are ideal places to disentangle these processes.

Refined energy-balance modelling of a supraglacial pond, Langtang Khola, Nepal

Abstract Supraglacial ponds on debris-covered glaciers present a mechanism of atmosphere/glacier energy transfer that is poorly studied, and only conceptually included in mass-balance studies of debris-covered glaciers. This research advances previous efforts to develop a model of mass and energy balance for supraglacial ponds by applying a free-convection approach to account for energy exchanges at the subaqueous bare-ice surfaces. We develop the model using field data from a pond on Lirung Glacier, Nepal, that was monitored during the 2013 and 2014 monsoon periods. Sensitivity testing is performed for several key parameters, and alternative melt algorithms are compared with the model. The pond acts as a significant recipient of energy for the glacier system, and actively participates in the glacier’s hydrologic system during the monsoon. Melt rates are 2-4 cm d-1 (total of 98.5 m3 over the study period) for bare ice in contact with the pond, and <1 mmd-1 (total of 10.6m3) for the saturated debris zone. The majority of absorbed atmospheric energy leaves the pond system through englacial conduits, delivering sufficient energy to melt 2612 m3 additional ice over the study period (38.4 m3 d-1). Such melting might be expected to lead to subsidence of the glacier surface. Supraglacial ponds efficiently convey atmospheric energy to the glacier’s interior and rapidly promote the downwasting process.

A grid-based model of backwasting of supraglacial ice cliffs on debris-covered glaciers

Abstract Ice cliffs might be partly responsible for the high mass losses of debris-covered glaciers in the Hindu Kush-Karakoram-Himalaya region. The few existing models of cliff backwasting are point-scale models applied at few locations or assume cliffs to be planes with constant slope and aspect, a major simplification given the complex surfaces of most cliffs. We develop the first grid-based model of cliff backwasting for two cliffs on debris-covered Lirung Glacier, Nepal. The model includes an improved representation of shortwave and longwave radiation, and their interplay with the glacier topography. Shortwave radiation varies considerably across the two cliffs, mostly due to direct radiation. Diffuse radiation is the major shortwave component, as the direct component is strongly reduced by the cliffs’ aspect and slope through self-shading. Incoming longwave radiation is higher than the total incoming shortwave flux, due to radiation emitted by the surrounding terrain, which is 25% of the incoming flux. Melt is highly variable in space, suggesting that simple models provide inaccurate estimates of total melt volumes. Although only representing 0.09% of the glacier tongue area, the total melt at the two cliffs over the measurement period is 2313 and 8282 m3, 1.23% of the total melt simulated by a glacio-hydrological model for the glacier’s tongue.

Air temperature variability in a high-elevation Himalayan catchment

Abstract Air temperature is a key control of processes affecting snow and glaciers in high-elevation catchments, including melt, snowfall and sublimation. It is therefore a key input variable to models of land–surface–atmosphere interaction. Despite this importance, its spatial variability is poorly understood and simple assumptions are made to extrapolate it from point observations to the catchment scale. We use a dataset of 2.75 years of air temperature measurements (from May 2012 to November 2014) at a network of up to 27 locations in the Langtang River, Nepal, catchment to investigate air temperature seasonality and consistency between years. We use observations from high elevations and from the easternmost section of the basin to corroborate previous findings of shallow lapse rates. Seasonal variability is strong, with shallowest lapse rates during the monsoon season. Diurnal variability is also strong and should be taken into account since processes such as melt have a pronounced diurnal variability. Use of seasonal lapse rates seems crucial for glacio-hydrological modelling, but seasonal lapse rates seem stable over the 2–3 years investigated. Lateral variability at transects across valley is high and dominated by aspect, with south-facing sites being warmer than north-facing sites and deviations from the fitted lapse rates of up to several degrees. Local factors (e.g. topographic shading) can reduce or enhance this effect. The interplay of radiation, aspect and elevation should be further investigated with high-elevation transects.

Glacier characteristics and retreat between 1991 and 2014 in the Ladakh Range, Jammu and Kashmir

ABSTRACT Glaciers in the Ladakh Range lie between the Himalaya, a monsoon-forced region of glacier retreat, and the Karakoram, a region of anomalous glacier stability driven by mid-latitude westerlies. Given this context, glaciers might be expected to display behaviour intermediate to the two adjacent ranges. However, no glacier change data have been compiled for the Ladakh Range itself. Here, we examine 864 glaciers in the central section of the Ladakh Range, covering a number of smaller glaciers not included in alternative glacier inventories. Glaciers in the range are small (median 0.25 km2; maximum 6.58 km2) and largely distributed 5000–6000 m above sea level (a.s.l.). 657 glaciers are available for multitemporal analysis between 1991 to 2014 using data from Landsat multispectral sensors. We find glaciers to have lost 12.8% of their area over the period. Glacier changes are consistent with observations in the Western Himalaya (to the south) and in sharp contrast with the Karakoram (to the north) in spite of its proximity to the latter. We suggest this sharp transition must be explained at least in part by non-climatic mechanisms (such as debris covering or hypsometry), or that the climatic factors responsible for the Karakoram behaviour are extremely localised.

Pond Dynamics and Supraglacial-Englacial Connectivity on Debris-Covered Lirung Glacier, Nepal

The hydrological systems of heavily-downwasted debris-covered glaciers differ from clean-ice glaciers due to the hummocky surface and debris mantle of such glaciers, leading to a relatively limited understanding of drainage pathways. Supraglacial ponds represent sinks within the discontinuous supraglacial drainage system, and have been documented to sporadically drain englacially. To assess pond dynamics, pond water level measurements were made on Lirung Glacier during May and October of 2013 and 2014. The four field seasons coincided with aerial, satellite, and terrestrial orthomosaic images and digital elevation models, which provided snapshots of the ponds and their surroundings. We analysed the glaciers closed surface catchments to identify surface drainage pathways and englacial drainage points, and compared this to field observations of surface discahrge. The ponded area was higher in the pre-monsoon than post-monsoon, with individual ponds filling and draining seasonally associated with the surface exposure of englacial conduit segments. We recorded four pond drainage events, all of which occurred gradually (duration of weeks), observed diurnal fluctuations indicative of varying supply and discharge, and we documented instances of interaction between distant ponds. The DEM drainage analysis identified numerous sinks >3m across the glacier surface, few of which exhibited ponds (23%), while the field survey highlighted surface discharge only explicable via englacial routes. Taken together our observations provide evidence for widespread supraglacial-englacial connectivity for meltwater drainage paths. Results suggest that progressive englacial conduit collapse events, themselves likely driven by supraglacial pond drainage, enable the glacier surface to evolve into a configuration following relict englacial conduit systems. Within this system, ponds form in depressions of reduced drainage efficiency and link the supraglacial and englacial drainage networks.

Highly variable aerodynamic roughness length (z0) for a hummocky debris‐covered glacier

The aerodynamic roughness length (z0) is an essential parameter in surface energy balance studies, but few literature values exist for debris-covered glaciers. We use microtopographic and aerodynamic methods to assess the spatial variability of z0 for Lirung Glacier, Nepal. We apply Structure-from-Motion to produce digital elevation models for three nested domains: five 1 m2 plots, a 21,300 m2 surface depression, and lower 550,000 m2 of the debris-mantled tongue. Wind and temperature sensor towers were installed in the vicinity of the plots within the surface depression in October 2014. We calculate z0 according to a variety of transect-based microtopographic parameterisations for each plot, then develop a grid version of the algorithms by aggregating data from all transects. This grid approach is applied to the surface depression DEM to characterize z0 spatial variability. The algorithms reproduce the same variability among transects and plots, but z0 estimates vary by an order of magnitude between algorithms. Across the study depression, results from different algorithms are strongly correlated. Using Monin-Obukov similarity theory we derive z0 values from the meteorological data. Using different stability criteria we derive median values of z0 between 0.03 m and 0.05 m, but with considerable uncertainty due to the glaciers complex topography. Considering estimates from these algorithms, results suggest that z0 varies across Lirung Glacier between ∼0.005 m (gravels) to ∼0.5 m (boulders). Future efforts should assess the importance of such variable z0 values in a distributed energy-balance model.