22 February 2017
The Sanxicun landslide in Sichuan Province, China
On 10th July 2013 the catastrophic Sanxicun landslide, located at 30.917, 103.565, occurred during heavy rainfall in the Dujiangyan area of Suchuan Province in China. This was a large landslide – the estimated volume is 1.9 million cubic metres – and parts of the landslide travelled as a highly mobile flow for about 1000 metres. The landslide struck 11 buildings in a tourist resort, killing 166 people. I described this landslide on this blog at the time, but it is interesting that there was considerable uncertainty at the time as to the number of victims. The image below was posted by Xinhua the day after the disaster:
It is good therefore to see a paper, Gao et al. (2017), published in the journal Landslides that provides a proper description for this event. The site of the landslide is easy to see on Google Earth imagery, even though the scar rapidly revegetated:
This is an interesting landslide. It is clear that it was triggered by very heavy rainfall – Gao et al. (2017) note that recorded precipitation at one nearby weather station was 1059 mm, the heaviest rainfall recorded in this area since the start of records in 1954. But, this area had also been heavily affected by the 2008 Sichuan earthquake, which is thought to have contributed to the instability of the site. The main body of the landslide moved only 50-80 metres, but about 300,000 cubic metres turned into a mobile flow with two distinct flow paths (which can be seen above). The main body of these flows consisted of about 275,000 cubic metres that transitioned into a channelised debris flow. It is this portion that struck a tourist area, leading to the very high loss of life.
Interestingly, Gao et al. (2017) have modelled the landslide using the DAN3D system. They conclude from both these models and the field evidence that the maximum velocity of the landslide was about 47 metres per second, which is around 170 km/h (or around 105 mph). Thus, this was an exceptionally mobile landslide, explaing the high levels of damage. This means that the entire duration of the landslide was about 70 seconds.
I have noted before that one of the most insidious impacts of earthquakes in mountain chains is the increase in landslide hazard in subsequent rainfall events. This event is a dreadful illustration of that effect. Sadly, this hazard remains poorly appreciated.
Gao, Y., Yin, Y., Li, B. et al. 2017. Characteristics and numerical runout modeling of the heavy rainfall-induced catastrophic landslide–debris flow at Sanxicun, Dujiangyan, China, following the Wenchuan Ms 8.0 earthquake. Landslides doi:10.1007/s10346-016-0793-4
21 February 2017
Mega-landslides as a vehicle for colonisation of distant islands
One of the great challenges of biology is to explain colonisation of distant islands, and indeed continents, by animals that are too heavy to be carried aloft. Various theories have been proposed, such as rafts of vegetation carried by floods, but none seem to answer the question adequately. In a paper just published in the Journal of Biogeography, Garcia-Olivares et al. (2017) have come up with a novel solution to the colonisation problem – mega-landslides. Using a study of weevils in various of the Canary Islands, they hypothesise that very large landslides (such as those from volcanic flank collapse) create large rafts of floating organic material that can carry not just single individuals but whole populations to distant islands, where they can establish a new colony of the animal. The authors even include a nice image of the sequence of events from a landslide perspective:
The appealing thing about this theory is that it creates a testable hypothesis in terms of the population genetics. The authors have used DNA testing to demonstrate that for the weevils, the population and individual level genetics fit the hypothesis. They suggest therefore that the weevils that they examined on the island of La Palma are a part of the population located on on the island of Tenerife who were incorporated into the 500 cubic kilometre mega-landslide in La Orotrava, which occurred about 500,000 years ago. The weevils would then have been transported on rafts carried by the Canary Current between the two islands, before landing on La Palma to establish the new colony. Thus, argue the authors:
“mega-landslides are expected to promote the ocean deposition and rafting of significantly more organic material than that associated with the landslide itself, although the altitude of secondary deposition will be a function of the tsunami height. In the context of island biogeographical theory, for which colonization is a fundamental component, mega-landslides may be an important driver of colonization, mediated by ocean currents and archipelago geomorphology.”
García-Olivares, V., López, H., Patiño, J., Alvarez, N., Machado, A., Carracedo, J. C., Soler, V. and Emerson, B. C. (2017). Evidence for mega-landslides as drivers of island colonization. Journal of Biogeography. doi:10.1111/jbi.12961
20 February 2017
San Bernadino National Forest landslide video
One of a number of new landslide videos that have been published in the last few days comes from San Bernadino National Forest in Southern California, triggered by the latest in a procession of intense rainfall events in recent weeks. This is a very impressive video of a large landslide in the Forest Falls area. In many ways the most interesting aspect is the high energy mobilisation of the flow in the valley below the landslide, which was just caught by the person making the film:-
Another, I think earlier, movement event of this landslide in San Bernardino National Forest was captured from a helicopter by Fox11. They report that this area is known as Slide Canyon:-
A still from this video captures rather well the extent of the landslide, which presumably mobilised in the later event:-
A major landslide in La Paz, Bolivia
Meanwhile, the Wall Street Journal has published amazing footage of a major landslide that struck the outskirts of La Paz in Bolivia last week:
The still in the video above shows the aftermath of the landslide, which appears to have occurred in a succession of failures rather than one major collapse.
A landslide under the Pfeiffer Canyon Bridge in the Big Sur, California
Meanwhile, back in the USA this news report shows the effects of a landslide beneath the Pfeiffer Canyon Bridge in the Big Sur, California.
The damage to the bridge is spectacular:
It is hard to imagine that a structure with such serious damage can be saved, but Caltrans is undertaking inspections to determine the way forward. More rain is forecast.
14 February 2017
The scale of the erosion problem at the Oroville Dam site
As the fairly desperate attempt continue to shore up the spillways at the Oroville Dam site, and to lower the water level ahead of the next rainstorm, better images are emerging of the scale of the problems on both spillways. If we start with the main spillway, which suffered the original erosion events last week, it is hard to appreciate the serious nature of the problems. However, this image (via the Sacramento Bee) from the weekend shows workers in the hole eroded in the main channel:-
Of course the high flows down the spillway since then are likely to have exacerbated the problem significantly. Meanwhile, the headward erosion problem in the emergency spillway is now garnering most of the attention, primarily because of the potential for a major collapse. This image, from Hamodia, shows the nature of the erosion that has developed at multiple locations immediately down slope of the emergency spillway at the Oroville dam site:
The most serious problem appears to be a gully towards the bottom of the image, but there is also substantial amounts of erosion occurring on the other side too. The danger is of course that the these gullies will suffer headward erosion until they undermine the spillway lip. whereupon collapse may occur. One challenge is that the quality of the rock does not appear to be high, which accounts for the rapid erosion in both cases. This image, via Twitter, shows the nature of the bedrock with which the crews are dealing:-
The challenges are substantial. Fortunately, as before, there is no threat to the dam itself.
13 February 2017
An update on the Oroville Dam spillways – a new phase of erosion leads to evacuation of 130,000 people
An update on the Oroville Dam spillways
Somewhat unexpectedly the crisis on the Oroville Dam spillways deepened yesterday. As was widely reported, the emergency spillway was called into action as the water level within the lake exceeded the inflow. All seemed well – the quantities of water passing the crest were not considered to be particularly high, whist the erosion of the main spillway seemed to have eased despite the vast quantities of water flowing through the damaged structure. However, as CaDWR noted in a press release yesterday, erosion started to develop at the edge of the emergency spillway as well. Of course in this case there is no mechanism to control the situation – unlike in the primary spillway there is no sluice gate – meaning that there was the potential for a catastrophic release that would exceed the capacity of the downstream channels:
The concern is that erosion at the head of the auxiliary spillway threatens to undermine the concrete weir and allow large, uncontrolled releases of water from Lake Oroville.
Rich Briggs (@rangefront) tweeted these two images, taken from news reports, that show the problem. The first highlights the location of the headward erosion:
Whilst this one shows the magnitude of flows over the emergency spillway, and on the unprotected land to the side:
It is clear that the flow has exceeded the emergency spillway and has flowed over unprotected ground adjacent to it, which inevitably was vulnerable to erosion. To me this seems quite extraordinary – how did the design allow that to occur – but that is a question for another day. The response of the authorities was first to almost double the flow down the main spillway (from 55,000 to 100,000 cubic feet per second), which will have accumulated more damage as a result, and second to start the evacuation of 130,000 people downstream.
Reports suggest that the water level in the reservoir has now dropped sufficiently for the flow over the emergency spillway to have ceased. The authorities now have until Wednesday to both lower the water level and to patch up the spillways before the next storm arrives.
Fortunately the main dam remains undamaged and safe. The likelihood of a catastrophic failure of the Lake Oroville spillways should now be low, but there is a huge amount of work ahead.
I will be doing the Reddit AMA (Ask me Anything) today (Monday) at midday EST (5 pm GMT). Join in if you would like to discuss this event (or any other).
10 February 2017
Oroville Dam: extraordinary erosion, and a crisis, on the spillway
The Oroville Dam in California is the tallest dam in the USA. with a height of 230 m, this is an earthfill embankment dam built between 1961 and 1968 for the purposes of water supply, hydroelectric power generation and flood control. After years of drought, California is suffering a series of huge rainstorms – so-called atmospheric rivers – that have rapidly raised the water level in the dam. To allow flood control, the dam has been undergoing controlled discharges of water through the spillway. This structure can be clearly seen to the left of the dam in the Google Earth image below:
On Tuesday, after such a release water, major damage was noted on the spillway, apparently caused by the failure of the concrete base and then erosion of the underlying substrate. The California Department of Water Resources tweeted this image of the damage on 8th February:
Over the next few days further discharges of water have been undertaken to test the spillway and to control the water level in the lake. The CaDWR tweeted this image of one such test earlier today:
Unsurprisingly, the condition of the spillway has dramatically deteriorated:
So now the Department of Water Resources are left in a bind. KQED is providing excellent coverage of this event, with an update yesterday as follows:-
With the spillway mostly out of commission since major releases were curtailed, Lake Oroville has been rising at the rate of about half a foot [about 15 cm] an hour since midday Tuesday. Its level has increased 30 feet [about 10 m] since then, with the reservoir’s surface now 20 feet [6 m] below an emergency spillway.
The emergency spillway, which would release water down a steep slope adjacent to the spillway, has never been used in the dam’s half-century of operation. DWR officials and others say water flowing down the slope will likely result in a large volume of debris being dumped into the Feather River, which flows through the city of Oroville on its way to the Sacramento Valley.
That’s one reason dam managers are willing to risk the destruction of the concrete spillway, calculating that would be preferable to the unknowns involved in an uncontrolled emergency spillover.
“It’s going to be rocks, trees, mud — liquid concrete — going down that river,” retired DWR engineer Jerry Antonetti told Sacramento’s KCRA as he watched the spillway Wednesday night. “I’d open ‘er up, sacrifice the bottom of that thing — it’s going to go in the river — clean it out next year and build a new spillway.”
The emergency spillway can be seen to the left of the main structure in the image below:
It is not difficult to anticipate the damage that the use of this structure will cause. Fortunately the integrity of the Oroville Dam itself is not in question, but managing the spillway and associated damage is a massive challenge.
9 February 2017
Review of a paper: the Dongla Landslide in Sichuan, China
In a paper just published in the journal Landslides (Luo et al. 2017), my colleague at the University of Sheffield Lis Bowman and her co-authors from China examine the fascinating Dongla Landslide in Sichuan, China. This is an ancient landslide mass located in Muli County (at 28.361, 100.629 if you want to take a look on Google Earth). The Google Earth image below, taken in 2015, shows the location:
On the left side of the image, close to the place marked Lanman, there is a new 500 kV electricity substation. To provide access to this, and to improve road links through the area, a 134 m bridge was constructed across the Shuilou River at this location in 2011. Unfortunately it was not appreciated that this site is a part of a very large, ancient landslide mass. Later in 2011 a part of the Dongla landslide reactivated. Involving 6 million cubic metres, the progressive development of this landslide deformed the bridge. This area is clearly shown in the 2013 Google Earth image, although note that the apparent deformation of the bridge is due to the terrain matching algorithm used by Google, not the landslide!
The paper describes a fascinating history of this landslide. Between 1990 and 2000 the area was subjected to extensive small-scale illegal gold mining that has left the terrain and vegetation scarred and damaged. These pits have provided pathways for rainwater to enter the landslide. But excavation of the toe of the slope for the bridge and road appears to have reactivated a portion of the landslide; for example in December 2012 the mass moved up to five metres. Luo et al. (2017) provide some fascinating images of the development of damage in the bridge as a consequence, including this one:
Monitoring of the landslide showed a highly seasonal movement pattern, with enhanced periods of motion being associated with increased rainfall. The movement occurred on shear surfaces located at about 20 m depth. In order to provide a short term reduction in movement the slope toe was reinforced with a large toe weight, consisting of 80,000 cubic metres of soil, which reduced the rate of movement, allowing access to the substation by heavy machinery. However, this was not a permanent fix, and by February 2014 the rate of movement was once again increasing. In 2015 the bridge was removed with explosives, and a new site was identified. The landslide continues to pose a threat though, requiring monitoring, with the major hazard being the potential to create a landslide dam.
This is a fascinating study that emphasises the importance of proper engineering geological investigation. These ancient landslide bodies occur widely in high mountain environments, and repeatedly cause major impacts to infrastructure projects. Identifying them requires real skill – and the use of engineering geomorphology – but the time and effort is well-spent. As the study by Luo et al. (2017) shows, failing to identify them can be both extremely expensive and cause long-term impacts. It is best not to unleash the dragon. Once it is out of the cage it can be very hard to contain it.
Luo, G., Hu, X., Bowman, E.T. and Liang, J. 2017. Stability evaluation and prediction of the Dongla reactivated ancient landslide as well as emergency mitigation for the Dongla Bridge. Landslides. doi:10.1007/s10346-017-0796-9
8 February 2017
Further movement of the Eaglepointe landslide in Utah
Over the last few days further movement has been recorded on the Eaglepointe landslide in North Salt Lake, Utah. This is the second period of reactivation this year. KSL notes the following:
North Salt Lake city leaders and homeowners are once again concerned a landslide after a portion of a hillside shifted late Monday night or early Tuesday morning. “You see it. You see the slough, you see the fall off, and it’s just scary,” North Salt Lake resident Terry Rasch said. He lives a few houses up and couldn’t believe it when he woke up Tuesday and saw his neighbor shoveling mud, and then he noticed why. A large chuck of land slid down a few feet, closer and closer to his neighbor’s home. City workers put hay bales and sandbags into place to try to keep any more movement and mud from flowing into the house below. This is the same area where a home at 739 Parkway Drive was destroyed in a substantial slide on Aug. 5, 2014.
Google Earth has a nice sequence of images showing the development of this landslide in recent years. This is an image from 2013:-
There is also a good repository of reports into the site, including assessments of the location prior to the first failure, on a webpage hosted by the City of North Salt Lake. It appears from the news reports that further movement is expected at this site in response to precipitation events.
6 February 2017
Bova Marina – an interesting retaining wall failure video
According to Wikipedia, Bova Marina is “is a comune (municipality) in the Metropolitan City of Reggio Calabria in the Italian region Calabria, located about 120 kilometres (75 mi) southwest of Catanzaro and about 30 kilometres southeast of Reggio Calabria”. A video appeared on Youtube yesterday showing a rather spectacular retaining wall failure, which appears to be driven by a landslide occurring on the slope behind:
There is very little detail about this event on the internet at present. An article on strill.it, in Italian, suggests that significant distress was observed in this wall back in September, and that the final collapse occurred at about 10 am on 4th February. The road, SS106, was closed off before the collapse at Bova Marina occurred.
I am unsure as to the exact location of this event, but the site shown in Google Earth below looks like the most probably candidate. Can anyone confirm?
3 February 2017
Helicopter sluicing of the Kaikoura landslides
The latest update on the works to mitigate the impact of the Kaikoura earthquake in New Zealand provides some interesting insight into how the authorities are going about dealing with the landslides on the roads to the north and south of Kaikoura itself. This is the main north – south highway (SH1) that links the port of Picton, where the ferries to North Island dock, and the main population centre on the island around Christchurch. The biggest issues lie to the north of Kaikoura. The work is being coordinate by NCTIR – North Canterbury Transport Infrastructure Recovery – an alliance representing NZ Transport Agency and KiwiRail, on behalf of Government. This is the update:-
- Work continued this week making safe the slips to the north of Kaikoura ready for removal.
- Helicopter sluicing has continued on four of the nine large slips to the north of Kaikoura.
- Up to nine helicopters are dropping more than one million litres of sea water on these slips each day to wash loose rock and material down the slip face and into the sea.
- This work is critical to make it safe for our crews and machinery to move on site and begin removal. It can only happen once the design work is nearing completion so the crews can make rapid progress on the site.
- Geotechnical engineers are now assessing secondary slips along this section of the route. These are smaller slips but still need to be stabilised before vehicles will be allowed back on SH1.
- NCTIR continues to meet with the community to inform them of progress and what lays ahead in this massive task.
The aspect that caught my attention here is the concept of helicopter sluicing, in which water is being dumped onto the landslides to remove debris. This is a novel approach that is also pragmatic, but I have rarely heard of it before. The NZ Civil Aircraft blog has some nice images of the helicopters undertaking this work, whilst an article on Stuff from December includes an image of a machine undertaking helicopter sluicing on a landslide:
Some of the machines undertaking this helicopter sluicing are those normally used for whale watching. South Pacific Helicopters have put a video on their Facebook page of these sluicing operations. These operations show both the challenges faced by these landslides and the ingenuity of the geologist and engineers in responding to them.
And this is of course an excuse to show the amazing video, one of my all time favourites, from 2009 of the use of helicopters to remove loose rock from a steep slope in Norway, which I featured in two posts at that time (post 1 and post 2):-