The rising cost of landslides in the Three Gorges dam area

One of the great concerns about the Three Gorges Dam in China has always been the potential for large-scale landslides. As the project nears completion, inevitably very over-budget, the level of the lake is rising and the cost of these landslides in becoming apparent. Today AFP and Xinhua have runs stories that raise concerns still further. These stories report that since September there have been 166 landslide and debris flow events on the banks of the reservoir, forcing the relocation of 28,600 people. The economic losses associated with these slides are approximately US$79 million.

Unfortunately these events have occurred before the onset of this year's rainy season and before the planned further increase in the water level later this year.

Large landslides in Peru and Kyrgyzstan, the Afghanistan earthquake plus heavy rain expected in the Wenchuan area

Each year in mid-April we move into the global "landslide season", when the development of the Northern Hemisphere summer, and the associated weather patterns elsewhere, means that the number of landslides starts to increase dramatically. This is all too clear from the range of landslide events in the last few days, plus the threat of heavy rainfall in the earthquake affected areas of China:

1. Major landslide in Peru
A range of news agencies (for example AFP and CRI) are reporting that there was another large landslide in Peru, again in La Libertad Province (the second major landslide this week in that province). Although details are sketchy, this time the landslide appears to have been very large (one report suggests 1 km long), hitting two villages (Chamanacucho and Aricapampa). Reports suggest that about 30 people were killed. The coordinates of Aricapampa are (-7.80583, -77.7172), which yields the following Google Earth images:


It appears that the landslide is still active, which is hampering the recovery operation substantially.

2. Major landslide in Kyrgyzstan
According to RIAN there was also a large landslide in Kyrgyzstan yesterday. The landslide appears to have hit Raikomol village Jalalabad province in in south Kyrgyzstan, killing 16 people and a large number of cattle. All of the victims, 11 of whom are apparently children, have been recovered. The ENG24 website has posted this rather grainy, but very helpful, image of the slide:

The reports suggest that it is about 300 m long. It appears to be a massive earthflow. Unfortunately the source zone is not in the image - I would be very interested to see how and where this started.

3. The Afghanistan earthquakes
The two moderately-sized (USGS Mw=5.5 and 5.1) but shallow (USGS depth = 5.7 and 3,.2 km) earthquakes in Afghanistan this morning appear to have caused damage in at least some villages, with about 20 reported fatalities at the moment. A Google Earth image of the area affected suggests that it really is a very remote zone:

Given the remoteness of the area and the rugged terrain the number of reported fatalities might well rise during the day. Earthquakes of this size would not normally cause much damage, but the early indications are that these two events really are exceptionally shallow. I would anticipate that there will have been at least some landslides in the upland areas, but probably over quite a limited area.

4. Heavy rain forecast for the earthquake affected areas of China
Xinhua is forecasting that heavy rain will hit the areas affected by the Wenchuan earthquake over the next few days. Up to 100 mm is expected to fall. This will be the first heavy rainfall of this years rainy season. Given the amount of mobile sediment on the hillsides, and the occurrence of debris flows in heavy rainfall last September, some further problems might be expected if this heavy rainfall does occur:

Updated: Large landslide in Peru

Various Peruvian news agencies, such as Adonde.com, are carrying reports of a large rainfall-triggered landslide on Saturday night / Sunday morning in Peru. This slide, which appears to have occurred in the town of Retamas in Parcoy District of Pataz Province, has reportedly buried a number of houses, killing 13 people. The following perspective Google Earth image of the location suggests that landslides may well be a substantial problem in this area (the location is -8.0197, -77.4783 if you want to take a look for yourself):


Helpfully, the following image of Retamas is available on Panaramio at this page:


I guess it is not hard to understand how a landslide can kill 13 people in this landscape. As an aside, given that this is an area of high seismic hazard, those ridge line dwellings look very poorly located.

Update: the Latin American Herald Tribune has an article with further information here. The aricle also has an image:


The slide appears to have occurred on a steep slope above the town. The article notes that the town is “is situated on the side of a mountain and whose residents mine gold and silver...The informal mining operations in the town have affected the stability of the soil in the area", which explains the large scarps on the Google Earth image.

A round-up of some recent landslide events

I thought that it was time to provide a round-up of some landslide events from around the world:

1. A narrow escape in Australia
Thanks to Remke van Dam for bringing this one to my attention. This Australian family had a pretty lucky escape last week when a pair of large rocks struck their car (image from here):


2. A road-blocking rockfall in Alaska
Thanks to John Fritz for this one. In Alaska, a pretty large rockfall has blocked the Whittier tunnel (see news report here):


The landslide occurred on Saturday; the road is expected to be blocked until Wednesday.

3. A fatal landslide in Idaho
In contrast to the first item above, a very unlucky couple were killed by a landslide in Idaho last week. According to this report, the couple's 4x4 (SUV) was hit by a tree that had toppled onto the road as a result of a small landslide near to Orofino. As the image below (from here) shows, the tree hit the cab, killing the occupants:

Cave rockfall in Shropshire, England

The British newspapers are reporting that overnight a rockfall occurred in Hermitage Cave near to Bridgnorth in Shropshire, England. This is news because at the time there was a group of teenagers camping in the cave. Sadly, one of them was killed and another was badly injured. These caves are old dwellings cut into the Bridgnorth Sandstone (see image below from here):

The caves used to be much larger but rockfalls over time have progressively reduced their extent. There are reports that the ill-fated group lit a fire in the cave. I must stress that this is unconfirmed. Sometimes however falls like this are triggered by campfires lit in the cave, which induce thermal stresses in the ceiling that trigger the collapse. I am not sure if this is the case here - it could well be that the group were just very unlucky indeed - but fires in caves with low ceilings can be a quite significant hazard. Sadly, it is very unlikely that the group would have known about that particular danger.

A first take on the Italy earthquake

You will probably be aware that an apparently quite destructive earthquake struck central Italy overnight. I thought I'd try to give a first take on the likely impact of this event. First, as ever, there is some pretty good information about the earthquake on the USGS Earthquake program website. They have provided the following helpful maps:

Earthquake location:
Shaking intensity:

Exposed population (PAGER):


At this point (c. 8:00 UT) the USGS is estimating that there are about 68,000 people living in areas that have suffered an earthquake intensity of VIII (severe shaking) or above, mostly in the town of L'Aquila. This is an area with a mix of old and new buildings built in a hilly area, as this image (from here) shows:

Although the earthquake is not huge (USGS estimates are Mw=6.3), the shallow depth (10 km) and fairly vulnerable buildings means that the impact could be quite substantial, albeit in a fairly limited area. Italy is well-prepared for earthquake response, which will help.

So, what of landslides? Well, an earthquake of this size should be capable of triggering a fair number of slides. A good starting point is the Keefer (1984) relationship between earthquake magnitude and area affected by landslides:

This gives an area affected by landslides as about 2000 km2 (give or take quite a lot, though). The area is certainly landslide prone, as this Google Earth perspective view shows (I have marked the epicentre location as per the USGS. The town in the foreground is L'Aquila):

Incidentally, the mountain in the background is Gran Sasso, which houses an important particle physics laboratory in a deep tunnel. I would be interested to know how the experiments have fared during the earthquake.

The Casita landslide revisited

One of the most deadly hurricanes of modern times was Hurricane Mitch, which tracked across Central America in late October 1998. Many of the tens of thousands of victims were killed by landslides. Perhaps the most notable event was a lahar (a volcanic landslide) that swept down from near the summit of Casita volcano in Nicaragua, killing about 2500 people over the course of its 6 km path (and some more in the hyper-concentrated flow (debris rich flood) events that travelled a further 10 or so kilometres from the toe of the slide. Unfortunately, despite the magnitude of this event the amount of published literature about it has remained quite limited. It is therefore terrific that a paper has just been published by Graziola Devoli and her colleagues (Devoli et al. 2009) that seeks to summarise the published and unpublished reports about this remarkable landslide.

Wikipedia has a very decent image of the upper track of the landslide, which gives a pretty good idea of the scale of this event:

Whilst MDA have a great overview image of the source, track and runout zone:

Complex landslides such as this are poorly understood. In particular, as in the landslides that I highlighted in Sichuan, the mechanisms of initiation and movement are quite intricate. Devoli et al. (2009) have used a range of geological, geotechnical and analytical techniques to get a better idea of what happened.

The landslide was triggered by very heavy rainfall - they suggest that about 750 mm (that's about a years worth for where I live) of rain fell in a little over 80 hours. Interestingly, they conclude that the landslide can be divided into three key phases:

1. Failure started in a fractured and altered volcanic breccia in the northern area of the scarp which released a volume of about 260,000 cubic metres. The flow that developed from this failure swept downslope and entrained colluvium deposits at the toe of the slope in the southern part in less than about 40 seconds.
   
2. The rapid removal of the colluvium on the slope triggered a second failure. This also originated in the scarp shown on the image above. In this phase about 640,000 cubic metres of volcanic breccia slipped over a unit of clay-rich pyroclastic deposits. It is unclear as to whether this flow joined the first one or occurred separately. Either way, blocks in this flow travelled 9 km or more downslope.
   
3. The third and final stage consisted of a as a sudden debris / rock avalanche that originated in the uppermost section of what is how the landslide scar. This failure, with a volume of 690,000 cubic metres, appears to have occurred very soon after the first two events.

In the aftermath there has been considerable concern raised about the likelihood of failure of the entire flank of Casita Volcano (indeed, even I have published on this theme - see van Wyck de Vries et al. 2000). The paper concludes that under conditions of high groundwater instability of these flanks can occur, although as we understand these very large failures so poorly I would be cautious in the interpretation of this particular result. This is a slope that needs an active monitoring programme for sure.

References
Devoli, G., Cepeda, J. and Kerle, N. 2009. The 1998 Casita volcano flank failure revisited — New insights into geological setting and failure mechanisms. Engineering Geology, 105, 65-83.

van Wyk de Vries, B., Kerle, N., Petley, D., 2000. A sector collapse forming at Casita
volcano, Nicaragua. Geology 28, 167–170.