24 April 2017
The Wellington earthquake landslide problem
The Wellington earthquake landslide problem was the front page story in one of the local papers 10 days ago. The article is online. The crux of the piece is summarised in the first few sentences:
“Up to 10,000 Wellington homes could be damaged by landslides if a major earthquake strikes close to the city. But while landslides may pose the greatest threat to those living in the capital when the big one hits, it could be years before residents know which slopes across the city pose the highest risk.”
The article also highlights a study being undertaken by GNS Science to try to understand the risks associated with the Wellington earthquake landslide problem:
GNS engineering geologist Sally Dellow said earthquake-induced landslides were usually bigger than those caused by rain. A research project was already under way looking at modified slopes in the Wellington region, she said. Homeowners could expect slopes around them to be evaluated, but it would take extra funding of about $5 million for the work to be completed in four years. “GNS Science has been working on this problem for many years, but only recently has the work started to be funded to the level required to deliver the detail needed to give information that can be acted upon.”
A part of the issue here is that the Wellington Fault runs at the foot of a series of steep slopes very close to the city centre. The Leapfrog blog has a rather lovely image that captures this efature well:
As the image above shows, there are many houses built on the slopes around the city. I snapped a couple of pictures before I left on Saturday:
We know from experience elsewhere in New Zealand and further afield that, in general, slopes close to the surface traces of faults perform badly in earthquakes. The Kaikoira earthquake illustrates this quite well. This is a Google Earth image of the terrain close to the fault for example:
Sometimes after a large earthquake, there are large numbers of cracked and deformed slopes close to the fault that have not transitioned to full failure. These deformations are sufficiently large to damage houses very badly though. This is an image I took from a helicpter after the 2005 Pakistan earthquake:
It isn’t possible to say how the slopes will behave in an earthquake in Wellington, and no-one is suggesting that any particular property is at risk. But it is clear that the work that GNS Science and others are doing on the Wellington earthquake landslide problem is both timely and important.
22 April 2017
March for Science: Wellington, New Zealand
Today, my last day in New Zealand for this visit, I was honoured to be able to attend the Wellington March for Science. Its geographic position means that New Zealand has kicked off a global series of events – I think over 600 are planned. The Wellington March for Science was one of the three that happened this morning; Auckland follows this afternoon. The event was well-attended in glorious, fabulous sunshine – Wellington was at its very best. The crowd initially assembled at the Civic Square:
Before marching over to the National Museum at Te Papa:
And then five short speeches, all good natured and well received:
There were some brilliant placards:
Of course different people were there for different reasons 😉
But thank you to everyone who was there for a happy, well-organised, positive and uplifting day. I wish everyone who is marching today my best wishes. This is so very important.
19 April 2017
Losses from landslides in Colombia: the long term picture
News is coming in today of further landslides in Colombia, with at least 14 reported fatalities in Manizales, where it is reported that at least 50 landslides have been triggered by up to 156 mm of rainfall. Some reports suggest that a further 23 people may be missing. This comes on top of the recent Mocoa landslide, in which at least 300 people were killed.
Back in 2012 I posted about landslide losses in Colombia, noting that:-
If we are to reduce the impact of landslides across South America then Colombia needs to be a priority country.
In 2015 Sergio Sepulveda and I analysed landslides in Latin America in a paper that was published in Natural Hazards and Earth System Sciences. The paper is open access, so it is free to download. I posted a summary of the paper at the time. In that paper we also noted that landslides inflict a heavy toll in Colombia.
Of course in the almost five years since my 2012 post, Colombia has gone through considerable change, including the end of the conflict between FARC and the government. This is therefore a timely moment to take another look at landslide losses there.
The graph below shows the cumulative total number of landslides and landslide fatalities in Colombia, based on my long term landslide fatality database. Note that the data only records landslides that cause loss of life. This does not include the landslides in the last 24 hours in Mazinales.
Sadly, the data suggests that there is no sign that losses from landslides in Colombia are reducing, and it is clear that the cost of landslides in Colombia is high. The good news is that they do not seem to be getting worse either, although the recent losses are clearly the worst since I started to collect this data in 2004.
In my post in 2012 I noted that rainfall in Colombia is heavily affected by the ENSO cycle. Interestingly, the large El Nino of 2014-15 did not drive an increase in landslide losses. However, the rainfall events that the country is currently experiencing are being ascribed to a “coastal El Nino” event. This is not a term that I have heard before; it will be interesting to see if this scientifically valid, and if so if it might provide greater insight into the variable triggers of landslides in South America.
18 April 2017
The Almaluu-Bulak landslide in Kyrgyzstan: amazing new Google Earth imagery
Possibly the most astonishing landslide video of 2016 showed the Almaluu-Bulak landslide in Kyrgyzstan. I wrote about it at the time, and also managed to pin down the location. The video is worth seeing a second time:
This was the second, larger, event in the sequence, both of which occurred on 27th April 2016. One person was killed.
Last month, the Google Earth Blog wrote about this landslide. The prompt was new imagery in the tool that captures the before and after situation of this landslide, and the adjoining area. And wow, this is worth a look. The easiest way (assuming you have Google Earth) to view the images is via this KML file. They have even mapped some of the landslides, though a quick inspection shows that there are many, many more. Indeed this is a landscape that is dominated by landslide processes.
But, in terms of the Almaluu-Bulak landslide itself, the imagery is fantastic. This is the image collected in November 2016:-
It is a most beautiful example of a flowslide. The deposit at the toe is very similar to the deposits seen at the toe of some flume experiments when the flume toe is planar, unrestricted surface, although others will be able to better comment on this.
A really interesting aspect is the before and after comparison of the source area. This is an image after the event:
Note the cracks in the hillslope in the foreground, and behind the crest of the landslide scar. But look at this image, taken two months before the failure event, from the same perspective:
Note the multiple arrays of cracks in the source area. This was a landslide that was developing over a long period. Interestingly, these cracks do not correspond to the current back scar. But we know that two failure events occurred in close succession. I would hypothesise that these crack define to source of the first landslide, whilst the second occurred because of toe release by the first.
The large cracks behind the scar suggest that there is a risk of a third, possibly even larger, event at this site, but that can only be ascertained with a more detailed analysis that involves fieldwork.
17 April 2017
The Meethotamulla garbage landslide: an avoidable tragedy
The number of victims recovered from the Meethotamulla garbage dump landslide in Sri Lanka is now 28, with at least 30, and possibly as many as 100, people still missing. Rescue operations continue, although the reality is that it is unlikely that anyone will be recovered alive now. Meanwhile, controversy has developed over the blame for the tragedy. This Reuters report says:
The police said they were investigating whether the landslide was natural or man-made.
This is not in question. There is nothing natural about this landslide in any way. The question is not whether it was natural, but whether it was foreseeable (note not this is not the same as asking whether it was predictable, as I noted in a post in December). To me the answer is hinted at by this image, published in the Sri Lanka Daily Mirror in 2014, showing the dump at that time, but an investigation is needed to answer the question properly:
Compare this image with this one, published by LankaeNews, showing the aftermath of the failure:
There is a very interesting article on Groundviews website about the garbage dump, its impact on local residents, and of course the landslide. On the Meethotamulla garbage dump itself, it says of the local residents:
One thing they all had in common was their distaste for the nearby dump. It was clearly visible from most of the lanes – swarming with flies, with the occasional pig rooting through the garbage. Even if you couldn’t see it, you could certainly smell it. The people I interviewed said the stench was particularly unbearable when it was hot, forcing many to close their windows and doors to avoid the smell.
Worse, respiratory diseases and skin rashes were quite common thanks to the proximity of the dump.
But the article also quotes a 2010 piece in the Sunday Leader that is deeply worrying:
[CMC Commissioner Badrani Jayawardena] claimed that this site is the only one of its kind to be maintained. “No where in Sri Lanka is it maintained like this,” she said. “Everyday we put a layer of soil on top of the garbage to seal it. We do it step by step but within five days we get the area covered. The reason it looks like such a huge dump is because of the soil as well.”
Putting layers of soil on the garbage way well have been wise in terms of containing the odours, but may have been dangerous in terms of the stability of the dump. This sounds to me to be a recipe for creating planes of weakness, increasing the likelihood of failure. The investigation, if done properly, will need to look into this as well.
In the short term, in addition to completing the recovery operation, there is an urgent need to start to make the dump stable. The peak summer rainy season in Colombo is in May, with an average of almost 400 mm rainfall over the month.
16 April 2017
The Meethotamulla garbage dump disaster in Sri Lanka
At the time of writing, the loss of life from the landslide at the Meethotamulla garbage dump in Colombo, Sri Lanka is 26 people, with reports varying as to how many more victims have yet to be recovered. A press release from the Ministry of Defence in Sri Lanka, which is leading the response, indicates that 145 houses have been buried. This is of course the second major garbage dump landslide this year.
The most informative images that I have seen of Meethotamulla landslide were tweeted by Azzam Ameen (a BBC reporter) on Saturday. They were apparently collected by the Sri Lankan Air Force. This image shows an overview of the Meethotamulla site, with the failed side of the dump clearly shown by the freshly exposed garbage:-
According to Google Earth the dump is about 380 metres long. From the image it appears that the landslide is a pseudo-rotational slip that has formed a very steep back scar (further work is going to be needed to make this site safe before the rainy season begins). The slide appears to have engulfed houses along a large section of the toe. The interaction with the buildings is complex, as this image (from the same source) shows:
It appears that the debris has over-ridden some buildings, but bulldozed others to incorporate them into the landslide:
This disaster did not come out of the blue. This article, from August last year, highlights grave concerns about the stability of the Meethotamulla site. It includes this image, which clearly shows that mass movements were occurring in the garbage:
It is undeniable that this site was unsafe. The garbage mound is clearly too high and too steep, inviting a rotational failure. With houses so close to the toe of the slope the hazards were severe. I am no expert on garbage dump landslides, but it seems likely that the decay of the waste will cause it to weaken with time, increasing the risk. In addition, in a humid monsoonal climate, the pore pressures are also likely to increase with time (as is well-documented in embankments, for example). Thus, the stability of this slope was probably decreasing even without the addition of further garbage.
This is another case in which we know and understand the hazards, but fail to manage them. The results are once again tragic.
13 April 2017
Shimla landslide video
An excellent video has been posted on Youtube of a landslide that occurred at Shimla in Himachal Pradesh in India on 7th April. The landslide has occurred on a slope that appears to have been cut as part of a construction project that is in the foreground:
The apparent lack of slope support at this site seems bizarre given the materials from which this slope is formed:
I have to admire the coolness of the construction workers, who seem remarkably unconcerned about this developing slope failure. Amazingly, the main failure event was captured from a second angle as well:
It appears the the highway is the road that links Shimla and Chandigarh. The construction site may be the base station for a ropeway and a shopping mall.
A gentle landslide from Idaho
Meanwhile, the Weather Channel has posted a most elegant video of a smaller landslide that occurred above a road in Idaho:
I have rarely seen such a gentle, almost balletic, landslide. Amazingly, it did not even cause the light pole at the toe of the landslide to topple as it was gently moved across the road. The Idaho State Journal has an account of this landslide:
ITD crews were on scene with cameras rolling as a hillside broke free and covered a section of U.S. 95 south of Bonners Ferry on Friday, April 7. In the video, you can see tons of mud, rocks, and trees slide down, pushing two layers of concrete barriers off the road. Overall, 800 cubic yards of debris slid onto the road. Thanks to quick action of ITD crews on scene, no one was injured by the slide. ITD Land Surveyor Mathew Wilson, who took the video, was assessing the hillside stability. He heard popping and cracking and knew another slide was imminent. Thinking quickly, the flaggers on scene stopped traffic and just three minutes later the hillside gave way. ITD crews cleared the debris from the road Friday evening. Additionally, crews dug out an extra 600 cubic yards of dirt beside the road to create a channel for water to run.
9 April 2017
Papamoa: a brilliant video illustrating the mobility of landslides
One of the hardest things to explain about landslides is the ways in which changes in the structure of soils can cause dramatic reductions of strength, and thus increases in mobility. A brilliant video has been posted to Facebook by Emma Kapua that illustrates this point as never before, I think. This landslide occurred in the Papamoa Hills of New Zealand, one of many landslides triggered by the remnants of Tropical Cyclone Debbie as it passed over the country last week:
The force of nature! Papamoa Hills Regional Park 6th April 2017 after heavy rain from ex. TC Debbie.
Posted by Emma Kapua on Thursday, 6 April 2017
You will see that this starts as a classic, small rotational failure. It is clearly formed from intact soil at this point:-
As the landslide goes around the circular shear surface it fragments rapidly, and changes phase:-
Thereafter the landslide behaves as a fluidised mass, and thus accelerates rapidly down the steep part of the slope:-
Understanding the processes that allow some soils to respond in this way has been a key aspect of landslide research, and of course it is essential to appreciate these mechanisms if the likely impact of future landslides is to be determined. It is the failure to understand that landslides can show this behaviour that can lead to an under-appreciation of the hazard that some landslides can pose. Clearly once a slide is moving in this way then the destructive potential is very high.
6 April 2017
A tragedy foretold? The Mocoa landslide was anticipated in 2014
In Colombia, the authorities are now shifting from rescue to recovery in the aftermath of the Mocoa landslide. The government has ordered a formal inquiry into the landslide, and the events that led to it, amongst claim and counter-claim about the importance of deforestation and cultivation in the destabilisation of land. A good article in New Scientist covers many of these aspects. This draws attention in particular to an article that was published in a local newspaper in 2014, written by Jorge Kuaran from the National College of Journalists. This article, which is in Spanish, appears to have been accessed so many time in the last 24 hours that the site has exceeded its bandwidth, but an archive is available on the Wayback Machine. It is a really fascinating piece that highlights the dangers of the gully that runs through Mocoa, in the aftermath of a false alarm in the town. This is a corrected Google Translation from the original Spanish:
Stories like the one of Armero, could be repeated in Mocoa, capital of Putumayo; But this time it would not be the fault of the nature, but of the lack of responsibility of the people who hold the authority in the municipality and the department. Nature, after all, has been warning and announcing a tragedy for more than two years.
The article provides additional detail about the concerns, which centre around landslides that have both blocked the channel and accumulated debris that can be mobilised into a debris flow:
Thanks to the evidence obtained by www.notifronteras.com , through the same officials of the municipal administration, it can be observed that on the Quebrada La Taruca located to the north-west of the urban area of Mocoa, and that borders with the basin of the ravine El Conejo, there has been a slide of stone, plant material, soil and other materials, about 150 meters wide by 300 meters high, which is on the channel of the ravine La Taruca … This implies by simple reasoning that the area where the great amount of material of the slip is located, has highly unstable ground by the groundwater that runs and that at any moment can generate an avalanche of proportions unimaginable given the topography of the land, that has a quite steep slope, since the head of the ravine starts more or less to a height of the 1,700 m and arrives at Mocoa that is located to a height of about 600 m with a distance of the head of the ravine to the urban area of about 7 kilometers, which means that when an avalanche is formed at the site of damming and sliding, by the force that the mass movement takes, it can reach Mocoa in less than 20 minutes, no Giving time to organize any massive evacuation of the families bordering the basin of the Taruca creek and the Sangoyaco river.
The article includes images and even videos of the debris choking the channel down which the Mocoa landslide flowed:
This is a chilling prediction of what appears to have happened. Jorge Kuaran is clear about the risk to the people of Mocao:
A tragedy of the proportions that we imagine has at risk the life of a large number of [people from Mocoa], since there are many neighborhoods or urban population living on the banks of the Sangoyaco river…I see a nonexistent emergency plan nor measures to overcome a catastrophe of considerable magnitude, it is necessary for the authorities of the municipality and the department of Putumayo to take serious steps in this regard.
Meanwhile, Mongabay has a good article (also in Spanish) that examines the causes of the Mocoa landslide disaster. This includes the following image of the catchment upstream of Mocoa, which supports the idea that this disaster was caused by shallow landslides that became a channelised debris flow:
The scale of this disaster is now becoming clear – the number of bodies recovered is now known to be 293, with 314 people believed to be missing. This would lead to a total of 607 fatalities.
5 April 2017
Mocoa debris flow: analyses of the event
The loss of life from the Mocoa debris flow is now known to have exceeded 290 people, of whom 144 are reported to be children, whilst the search continues for those who are missing. NASA Earth has now published an analysis of the rainfall event that triggered the landslide, noting that:
IMERG data were used to estimate the amount of rain that fell near Mocoa, Colombia, during a seven-day period from March 26 to April 2, 2017. IMERG indicated that area rainfall totals during the week were frequently greater than 80 mm (3.1 inches). The analysis also showed the locations of heavy rainfall that extended from east of Mocoa into the high mountains that surround the city.
Meanwhile the ESA has released a slightly weird interpretation of “land movement”, based on an analysis of Sentinel 1 data byI. Parcharidis, of the Harokopio University of Athens :-
The red area cannot be a single landslide, and thus is not a simple explanation for the Mocoa debris flow. I wonder if this is area something to do with canopy wetness, or soil moisture, and thus indicates where a cloudburst type of rainfall event might have occurred? Others, who know more about radar data, may be able to comment. Meanwhile IBTimes has an excellent if heartbreaking gallery of images of the aftermath of the debris flow, which includes this image of the boulders transported by the debris flow:
Exactly what happened upstream of Mocoa remains unclear, but this image appears to support my hypothesis that the Mocoa debris flow might have started as one or more shallow landslides that transitioned into a channelised debris flow.
It is hard to imagine that ploughing through the town. Unbelievably dreadful.