Wednesday, 21 May 2014

Glacier retreat and slope instability, an example from Mount Kazbek, Georgia

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A large landslide close to the Georgia - Russia border this week killed up to eight people, and disrupted construction of a new hydropower diversion tunnel. The landslide was released from approximately 4,100 m on the north-eastern flank of Mount Kazbek, and is estimated to have involved more than 10 mil. m^3 of rock. The site of the landslide release is likely to have been recently deglaciated, and 2010 aerial images from Google Earth indicate there was ongoing instability in the years prior to this failure.  

Aerial image of the landslide release area at 4100 m elevation

In this case the landslide traveled over 10 km downvalley, and the deposit seems to have formed immediately upstream of outlet tunnels for the hydropower diversion. Although this was clearly an energetic event (in particular in terms of the 400 m run-up apparent on the opposite side of the valley), the total fall was almost 3 km, and the travel distance is not unusual for a landslide of this magnitude.

Map of landslide release, travel, and approximate location of the deposits. 
The profile is derived from the white line indicating the path of the rock avalanche. 

While it's terrible to see loss of life as a result of such events, high alpine landslides such as this can provide critical insight into the possible effect of changing climate and glacier retreat in more heavily populated alpine regions. Although minor depressions or crevasses are evident in the glacier surface in the 2010 aerial image, and smaller events have been noted to be regular in the region, such a large event was not predicted. By studying failures such as this one, we can gain important insight into the failure characteristics, and begin to identify additional information required to predict similar events with enough confidence to evacuate communities at risk. As it happens, the diversion tunnel for the Dariali hydropower scheme appears to be well situated to relieve inflows into the lake impounded behind the landslide deposit, and hopefully prevent any further losses downstream.

Interactive map indicating the estimated source (orange), and mapped deposit (grey), as well the aerial photograph of the release area (double click the camera icon to view). The white line denotes the path of the landslide that appears to have descended as a rock avalanche. A .kmz file (including additional aerial photographs) is available as a Google Earth download here

Sunday, 4 May 2014

The Badakshan landslide: A forseeable tragedy?

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A recent rainfall-triggered landslide in Ab Barek (sometimes referred to as Abe Bareek) in the Badakhshan Province of Afghanistan is suspected of taking the lives of up to 2100 villagers. As with so many remote events, details are scarce, although a number of images and videos have appeared on social media websites over the last days. Some images, as that below, indicate the landslide may have crossed a river valley and formed a landslide dam blocking the river running through the town. In a region that has recently experienced heavy rainfall, the formation, and breach of such a dam could have severe consequences to any communities downstream.

Possible landslide dam formation in Ab Barek
Source: Matin Bek/Twitter

As discussed in previous posts (and on a recent EGU presentation), we can use these images in combination with freely available GIS platforms such as Google Earth and QGIS to map the true extents of the landslide source and deposit (a task made easier when the locality is correctly reported). Below is an interactive Google Earth frame with the extents we have been able to map from the imagery to date. The Landslide source is indicated in grey, while the approximate extent of the deposit is in orange. Currently, there are no images of the downstream extent of the deposit, however, the flow of the landslide into a moderately-sized river channel, and upstream is clearly evident both in the photographs and the GIS map.

Interactive map indicating the estimated source (grey), and mapped deposit (orange), as well as four photographs covering the area of the damaged village (double click each image or camera icon to view). The white line denotes the region of the slope that has failed to date. Signs of apparent instability on the slope above this region should be investigated by emergency crews, as further instability behind the steep scarp could be possible. A .kmz file (including additional aerial photographs) is available as a Google Earth download here

The satellite image of the pre-failure hillslope contains a number of arcuate ridges, a clear indication the hillslope has been unstable for a number of years. Some sharper, or more well-defined lineations upslope of the region displaying obvious signs of movement are suggestive of a reactivation of the failure prior to the 2004 DigitalGlobe image. As a natural hazard researcher it's saddening that these signs were not recognised, as such a steep active slope above a populated area should have raised red flags, and at least allowed authorities to educate villagers and encourage the implementation of a monitoring system.

A Google Earth image of the landslide source and visible deposit. Note the lumpy shapes on the hillslope in the region of the landslide source, tell-tale signs that the slope has been moving in the past. Calculated extents of a future landslide-dammed lake are indicated in dark blue (likely) and light blue (possible).

We can begin making a preliminary analysis of hazards posed from the dam formation and breach by importing the .kml file containing source and deposit extents into QGIS, and using freely available geodata to investigate the catchment characteristics and recent rainfall record. Satellite-based terrain data is notoriously poor near the bottom of valleys, however, the upper limit of the deposit appears to be at an elevation of approximately 1810 m above sea level. This is 10 m above the indicated level of the former river at the same location, although photographs indicate the deposit may be more than 20 m deep in places. Using the 1810 m level as a guide we can plot the approximate extent of a landslide-dammed lake (dark blue in the above image), which could possibly cover an area of 0.04 km^2 and contain 200,000 m^3 of water. An upper estimate of the lake volume can be made by following the 1830 m contour (assuming the dam extents to 30 m elevation at it's crest. In this case the lake could cover 0.26 km^2, and hold 3.9 Mil. m^3 of water. Although these numbers are only rough estimates, the apparently fine-grained nature of the sediments forming the dam may aid both initial water retention, and rapid failure by piping through the dam or scour once the dam is overtopped. The catchment upstream of the dam is approximately 18 km^2, and TRMM data indicates it received ~100mm of rainfall during the 7 days leading up to May 4th. Concentrated, this would total 18 Mil. m^3 of water, more than four times the maximum storage capacity of the potential Ab Barek dam. Although much will have run off during the rainstorm events, it seems likely that the water presently stored in the catchment is more than enough to overtop the dam, and resulting flooding may pose a serious risk, both to residents of Ab Barek, and those in towns such as Balas Shemar and Ahen Jalow downstream of the landslide.

7 days accumulated rainfall in Badakhshan Province

Update 05/05/14: An error in the calculation of surface area from a geographic coordinate system (WGS84) in QGIS mean that initial area estimates were too large. These have now been corrected. The landslide source area has been mapped from video footage.