Saturday, 6 September 2014

Randa rockfall update: One of those rare cases

Image describing the upward-propagation of failure during the first minute of the 29/08/2014 event. The background image is a pre-failure photo taken two weeks prior (12/08/14, credit: J. Beutel).

One of the fortunate things about working in the European Alps is that many valleys are densely populated, and few large events go unnoticed. The Randa rock slope in the Matter Valley is a particularly well-studied location, and nearby scientific installations are currently used to monitor climate, permafrost, debris flows, and other alpine hazards.

In the case of the 29/08/14 failure, members of the ETH Zurich had captured images of the rock slope two weeks prior to the event, and as the SLF was working in the region at the time, images immediately (approx. 30 min) after the event are also available. This is a rare case for such a large alpine rockfall, and in addition to the video I posted earlier could offer opportunities to investigate the driving mechanism and failure process with more detail than is usually possible. Such data can be very useful for scientists and engineers seeking to better understand rock slope behaviour, and may help us more accurately predict future activity of similar rock slopes from pre-failure observations.

Although there is no substitute for on-site inspection, by comparing pre- and post-failure images we can make a first-pass at delineating the approximate region of failure. And by comparing blocks outlined by large fractures to stages of failure in the video we can also make a reasonable estimation of the sequence of failure (see above).

Randa rock slope immediately after the 20/08/14 failure (credit: M. Phillips). The region that failed during the 29/08/14 video is highlighted in dark red, while the region highlighted in orange corresponds to a potential instability noted in my previous post. dashed grey lines marked on the grass above the scarp delineate regions of exposed soil and possibly disturbed vegetation, suggesting relatively recent movement. Colored circles can be used to reference pre- and post-failure photographs. Click here to view the original image.

The dark silty soil generated during the event makes it difficult to correlate features on the rock face beneath the failure, however, it seems like it initiated at a site just above the soil-covered region of the rock slope (i.e. above the light green marker). The slope crest was then the last to collapse. Often such an upward-propagation is associated with removal of a key block that has been weakened by high tensile or shear stresses at the toe of a creeping rock slope. This would be consistent with excellent work by members of the Engineering Geology group at ETH Zurich, which identifies slow creep in the upper section of the slope, and correlates this to bedrock structure and seasonal thermomechanical effects.

As in the lower image, the failed region appears to lie immediately above a long undulating discontinuity (black lines in inset) that slopes downhill at an angle slightly greater than that of the upper grass-covered surface. White arrows indicate where the discontinuity seems to have opened up as a result of the upper section creeping downslope relative to the lower rock mass. This kind of movement would be consistent with the initiation of failure immediately above a step in the sliding surface. The current activity could therefore be the result of slow creep that has been ongoing since the Randa rockslides in 1991. The wet summer may have accelerated this process, and although not strictly a 'trigger' (as the failure occurred on the first sunny day in several weeks), increased pore pressure and weathering on the sliding surface may have bought the slope to failure more rapidly, and therefore increased the probability of a collapse on any given day.

If this event is the result of deeper slope movement (as opposed to an isolated collapse of surficial material), then possible soil disturbance 10's of meters back from the active cliff face (see image above) may be an indicator of much larger failures in the future (although no-where near the volume of the 1991 events). Irrespective of the driver or mechanisms, this apparent re-activation of the slope is interesting, and further observation or investigation could be warranted.

An alternative view of the Randa rock slope prior to failure (12/08/14, credit: J. Beutel). Two apparently open fractures are indicated by white arrows. The inset provides an interpretation of creep that may have contributed to the opening of steeper sections of the undulating discontinuities. As in the previous image, the region that failed during the 29/08/14 video is highlighted in dark red. Colored circles can be used to reference pre- and post-failure photographs. Click here to view the original image, additional annotated and un-annotated pre-failure images are also available.

Monday, 1 September 2014

Ongoing instability of the Randa rock slope (Switzerland)

Ongoing rockfall activity from the Randa rock slope at 1 pm on the 29th of August 2014

Two very large rockfalls in 1991 left an almost 1,000 m high cliff on the western side of the Matter Valley in Southern Switzerland. The valley is one of the most travelled in the Alps as it serves as access to the town of Zermatt and the skifields immediately adjacent to the Matterhorn.

Research into the cause of the 1991 failures and current state of stability has been ongoing for more than 20 years, and the site is now a classic example of alpine rock slope failure. Although regular rockfall is common from the remaining scarp, the event captured on video is one of the larger failures since 1991. Rockfall was ongoing throughout the day, and the event in the video occurred just after midday.

Large failure from near the crest of the remaining rock slope

The video was captured during a scientific workshop with members of the Chair of Landslide Research from the Technische Universität München, and the Geomorphological and Environmental Research Group at the Universität Bonn.

Randa rock slope in early August (05/08/2014)

Close up of the region of current activity (this is estimated to be perhaps 60 m high). Dark red indicates the approximate region of the present failure. Orange indicates a region of rock that may have been destabilised as a result of the activity.

A pre-failure photo taken two weeks prior to the event (12/08/14). The photo was taken by members of ETH Zurich as they descended by helicopter from the Randa in situ rock laboratory (credit: J. Beutel). Inset indicates the approximate failure location, as well as apparently open cracks on adjacent failure planes.

Wednesday, 21 May 2014

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

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?

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.