23 December 2016
Jiuhaocha in Pingtung County – more on the impacts of landslides on a mountain village
Yesterday I blogged about the massive impacts of landslides and associated river-borne sediment on the village of Jiuhaocha in Pingtung County, Taiwan. In fact the element that I posted yesterday was only a small part of the story. C-Y Chen has a blog in Mandarin that describes landslide disasters in Taiwan – I hadn’t come across this before but it is very interesting (Google translate does a good enough job to make it readable without difficulty). Back in April he posted an article about the landslide problem in Jiuhaocha, which provides a very interesting description of the long term problem.
The history goes back to 1979 when the village was established as a “model village” on the river terrace. This of course explains the very organised structure of the community, with its grid-like street pattern, as shown in this Panaramio photo from 2009:
Th landslide story starts in 1996, when the area was struck by Typhoon Herb. I was in eastern Taiwan at that time – this event came close to killing me (but that is a story for another day) – but the village was very badly affected by landslides. In particular a slide near to the church (which is the large building with a red roof on the right side of the centre of the image above) crashed into the village, killing four people. C-Y Chen has an image of this landslide:
In 2003 a report by the National Cheng Kung University recognised the landslide dangers in this village, and also highlighted the risk of flooding from the river in extreme rainfall scenarios. By 2005 the river was rapidly aggrading (i.e.the river bed was rising as sediment accumulated) because of the landslides located upstream of the village. This was increasing the risk of a flood related disaster at Jiuhaocha. Heavy rainfall events in 2006 and 2007 saw further landslide activity in the area around the Jiuhaocha, meaning that some of the houses became partially buried in debris. C-Y Chen has this image of the damage:-
And then in 2009 Typhoon Morakot brought 1,900 mm of rainfall to Pingtung County, during which Jiuhaocha was completely buried, leaving only the church standing. By this time the river bed had aggraded by a staggering 40 metres as a result of the landslides upstream. The church is the one building that can be seen in the Google Earth image at that time:-
22 December 2016
Jiuhaocha: the extraordinarily dynamic landscape of Taiwan
I was looking at Google Earth imagery of Taiwan this week. In the early part of my career I spent a great deal of time there – Taiwan is a wonderful country in every respect, but from my perspective the extraordinarily dynamic landscape has always been a fascination. As you are probably aware, in 2009 the southern part of Taiwan was struck by a terrible tropical cyclone – Typhoon Morakot – that triggered many thousands of landslides. One of these, at Shiaolin Village, killed several hundred people. However, I was looking at a different site – the village of Jiuhaocha – a little to the south. This is a satellite image of a small village – I count about 50 houses and other buildings – in the mountains. The image below was taken in 2001. Mountain villages in Taiwan are often the home to comparatively poor, indigenous Taiwanese people and/or the families of ex-servicemen who became farmers when Taiwan was settled after the Maoist uprising in mainland China. Jiuhaocha is a village of the Rukai aboriginal people. The settlement was located within the mountains of Pingtung County in southern Taiwan. The Rukai people are one of the 14 recognized aboriginal tribes of Taiwan:
Jiuhaocha has a long and complex history. But this is an area that was hit very hard by Typhoon Morakot. This storm that caused the following to happen – this is an image of the same location in 2011:
The difference is undeniably shocking. In this case the village has been buried by sediment carried by the river in flood. But the source of this sediment was the multiple landslides triggered by the storm upstream of the village, some of which were very large:
Fortunately the population of Jiuhaocha were evacuated before the typhoon, so no lives were lost. But the risks to populations located in the mountains of a dynamic environment such as Taiwan are very clear.
21 December 2016
The Folkestone Warren landslide toe weight structure
On the South Coast of England the railway line between Dover (the main ferry port for travel to Europe) and London crosses a very large landslide at Folkestone Warren in Kent. This is a large (3 km wide), deep-seated (the shear surface extends about 40 m below sea level) rotational landslide. The track crosses the entire width of the landslide, located on displaced blocks, as this BGS image shows:
Unsurprisingly this railway line has a very long history of being disrupted by the landslide. The most famous incident occurred on 19 December 1915, when 1.5 million cubic metres of chalk slipped, causing enormous damage to the line. A train was caught in the landslide, but fortunately without any loss of life:
The line was closed until 1919. The slope remained active thereafter – this Google Earth image from 1940 gives a good impression of the magnitude of the problems that British Railways faced at that time:-
In 1948 an enormous toe weight structure, designed to add mass to the toe of the landslide to prevent further rotational failures, was constructed. This was extended between 1951 and 1952. There is a very nice article online (NB this is a PDF) from the Railway Magazine describing these works, and the associated drainage tunnels. This vast, and effective, toe weight can be seen in the modern Google Earth images as a vast platform built at the toe of the landslide:-
The effectiveness of these and the associated landslide management measures is clear from the above image – the slide is much less active now than it has been in the past. But nonetheless this remains a deeply challenging section of railway line for Network Rail to maintain.
19 December 2016
Big French Creek: a high quality new landslide video
KRCRTV featured last week a very nice new landslide video of a collapse that occurred on Highway 299W at Big French Creek in California. The landslide happened last Monday (12th December 2017) at about 1 pm. As you will see from the video the road was closed. The video was captured by Kevin Erwin. It is a great catch in that it records the minute or so leading up to the failure, as well as the movement of the large boulder that is located within the weather rock mass. Kevin loaded the video onto Youtube, so it can be sen below:
The same event was also captured by J2R2J:
Jack Irving kindly highlighted this video to me. He has identified the location as 40.779, -123.311, which looks like this on Google Maps:
The presence of work crews suggests that the highway agency were aware that the slope was problematic, and it is clear that the road had been closed prior to the collapse. Latest news from Caltrans is that the road is still closed:
SR 299 [IN THE NORTHERN CALIFORNIA AREA] IS CLOSED 4.5 MI WEST OF BIG BAR /AT BIG FRENCH CREEK/ (TRINITY CO) 24 HRS A DAY 7 DAYS A WEEK THRU THE WEEK OF 01/08/17 - MOTORISTS ARE ADVISED TO USE AN ALTERNATE ROUTE
Renewed problems at Pacifica
Meanwhile Pasi Jokela has kindly highlighted a renewed problem at Pacifica to the south of San Francisco, where coastal erosion is causing increasing problems for a coastal community. Liveleak has a nice drone video of a “sinkhole” that has appeared on the cliff. This is a fill slope located behind a sea wall – it seems on first inspection that the fill has washed out:
The video also highlights a number of other properties that are at a high level of risk from erosion. NBC Bay Area has a decent report about the problem. I have previously featured the ongoing problems at Pacifica, including another very nice drone video of the site.
15 December 2016
On the difference between prediction and foresight in landslide hazard assessment
In one of the landslide sessions at the AGU Fall Meeting yesterday one of the speakers made an interesting comment about his perception of over-confidence of landslide experts in the prediction of landslide behaviour. The comment was made in the context of landslide runout, which is highly dependent on initial conditions. The point that was made is that, given these conditions are not known in advance, it is impossible to predict with any confidence the behaviour of many landslides. I agree.
However, this does not mean that it is impossible to foresee the behaviour of a potential landslide. For something to be foreseeable does not require exact prediction of its behaviour. Foresight means anticipation of a potential outcome – this is in essence a risk-based approach, and it is entirely possible in almost all cases for landslides.
Let me illustrate my point through a hypothetical example. Let’s imagine a slope in a mountain valley that has undergone several previous phases of movement, none of which have had a long runout but all of which have been substantial. There is a discussion as to whether to allow the construction of houses on the land in the valley below the landslide – should development be permitted or not? There are two elements to the assessment of the hazard posed by the landslide – first, can the landslide reactivate once more? And second, could it run out onto the land in which the houses are being planned?
Assessment of the likelihood of reactivation is quite straightforward, and based on the previous history of failures is likely to conclude that this is possible. In our imaginary example all parties agree that a further failure could occur. Analysis of the potential runout is much harder due to the problems outlined above. One approach is to model, but that is difficult given the dependence on initial conditions, the possible volume of the mobilised mass, etc. But there is an alternative, which is to look for similar landslides in similar materials, and to determine how they have behaved. This is particularly useful if they are located in the vicinity of the landslide in question. If they show long runout then there is the potential that the landslide in question might also show a high mobility, and thus pose a risk to the houses.
In our hypothetical example a brief examination of slopes in the local area suggests that some have displayed high mobility.
Thus, in this case long runout / high mobility behaviour is not predictable but it is foreseeable. Given the vulnerability of the people in the houses, a reasonable approach would be to prevent development even if the likelihood of high mobility of the landslide was comparatively low. This is a risk-based approach; it is common across the spectrum of natural hazards and it does not require that exact predictions of behaviour are made.
14 December 2016
Progressive Rock Failure – a meeting in Switzerland in June 2017
One of the most interesting types of landslide are those driven by progressive rock failure. These landslides tend to show long periods of creep deformation that, in some cases, transitions into a paid failure event. Not all landslides go through this transition though, which means that hazard analysis for these failures is a problem. It is also clear that these failures may occur (and indeed often do occur) without a trigger, meaning that their behaviour can be quite hard to manage. However, there is also some evidence that at least some show characteristic patterns of acceleration before final collapse, which can be used to predict failure. Opinions differ as to the drivers for these processes. There are also uncertainties as to what underpins the changes in behaviour that these landslides exhibit – is there for example a critical strain at which failure begins? and if so, what determines this parameter?
These issues are important not just for landslides. Progressive rock failure may be a key issue in tunnels, boreholes and mines. Thus there is a great deal to learn from getting together a diverse range of people interested in progressive rock failure, from both the academic and the applied field.
ETH Zurich has decided to arrange such a meeting, which will be held from 5th to 8th June 2017. Pre-registration for this meeting is currently open. Details of this meeting are available at the Symposium Website. In my view this is likely to be a great meeting – highly focused with a group of really top people in the field. This sort of meeting is the perfect complement to the mighty AGU Fall Meeting. I fully intend to be there.
Oh, and the setting looks rather cool too!
13 December 2016
Landslides caused by the 1929 M=7.8 Murchison earthquake in New Zealand
Whilst there is a strong focus in New Zealand at the moment on the multiple landslides triggered by the Kaikoura earthquake last month, GNS Science have released a new report about the landslides triggered by the 1929 M=7.9 Murchison earthquake on the other side of South island. This was a major event that probably triggered even more landslides than the most recent earthquake. The report details the collation and analysis of a full inventory of the landslides triggered by the earthquake, based upon mapping from aerial photographs, supplemented with other sources of data. Because the primary source of information is a set of images collected in 1968, the inventory is only for larger landslides (those with a surface area grater than 2500 square metres). Nonetheless there are 5356 landslides in the inventory extending over an area of about 16,000 square kilometres.
Some of the statistics are impressive – the earthquake triggered two landslides with a volume of more than 100 million cubic metres, and there are 42 valley blocking landslides (landslide dams) in the dataset. Several of these landslide dams breached catastrophically over the weeks following the earthquake. As a result there was extensive flooding and damage in the towns of Seddonville, Little Wanganui and Karamea. In total 14 people were killed by landslides, and two miners died in mine collapse events triggered by the shaking.
To give an idea of the extent of the landsliding triggered by the Murchison Earthquake, the report includes this twin oblique and vertical pair of images of an area of landslides in the Karamea Valley:
Some of the large landslides are truly impressive. This is the 18 million cubic metre Lake Stanley rock avalanche, some 85 km from the epicentre for example:
At the first AGU landslides session tomorrow (which will be streamed live), one of the authors, Brenda Rosser, will be describing the New Zealand landslide dataset compiled by GNS Science.
Hancox, G.T., Ries, W.F., Parker, R.N., Rosser, B., 2015. Landslides caused by the MS 7.8 Murchison earthquake of 17 June 1929 in northwest South Island, New Zealand. GNS Science Report 2015/42. pp127 (2 Volumes)
12 December 2016
AGU 16: live stream of the first landslide session on Wednesday
This week is AGU 16 – the Fall Meeting of the American Geophysical Union in San Francisco. As usual it is an amazing, stimulating and slightly intimidating event. This is science on a grand scale both in terms of the projects themselves and the number of attendees. Last year the meeting attracted almost 24,000 attendees. This year feels just as busy, perhaps more so.
The main landslide event occurs on Wednesday and Thursday, with two oral sessions on the former and an epic poster session on the latter. The AGU have decided to live stream the first of the two landslide sessions. This is at 8 am local time (which is 4 pm in the UK) and runs for two hours. I have the honour of kicking the session off – many thanks to the convenors for the very kind invitation to do this. Below are the details of the session and the list of talks:
Wednesday, 14 December 2016, 08:00 – 10:00 am
Moscone South – 102
Primary Convener: Dalia Kirschbaum
Conveners: Paola Reichenbach and Hiroshi Fukuoka
08:00: NH31B-01 The global landslide distribution in the 2015-16 El Nino event (Invited). David N Petley and Melanie Froude
08:15: NH31B-02 New Zealand’s National Landslide Database. Brenda Rosser, Sally Dellow, Soren Haubrook and Phil Glassey
08:30: NH31B-03 Hydrological Evaluation of Precipitation Intensity-Duration Thresholds for Regional Landslide Hazard Assessment. Thom Bogaard and Roberto Greco
08:45: NH31B-04 Extreme Events in the tropics – Hurricane Manuel and La Pintada Landslide. M. Teresa Ramirez-Herrera and Krzysztof Gaidzik
09:00: NH31B-05 Landslide Susceptibility Mapping of Tegucigalpa, Honduras Using Artificial Neural Network, Bayesian Network and Decision Trees. Elias Leonardo Garcia Urquia, Anika Braun, and Hiromitsu Yamagishi.
09:15: NH31B-06 Regional Landslide Hazard Assessment Considering Potential Climate Change. Susana Almeida, Elizabeth Holcombe, Francesca Pianosi and Thorsten Wagener
09:30: NH31B-07 Spatiotemporal Patterns and Risks of Rainfall-Triggered Landslides across Conterminous United States Inferred from Satellite Observations and Model Simulations. Ke Zhang, Yang Hong and Jonathan J Gourley
09:45: NH31B-08 Considerations about the mechanism and mitigation plan of rainfall-induced landslide after The 2016 Kumamoto Earthquake (Invited). Naoki Sakai.
There are some excellent topics there, so it promises to be a most-interesting session. You can join it for free by registering for AGU On Demand. There isn’t an option to ask questions remotely, but if anyone is interested we could discuss the session using #AGU16 and #NH31B in real time (though you will have to start without me for obvious reasons!
9 December 2016
The landslide distribution from the M=7.8 Gorkha Earthquake in Nepal
In the aftermath of the M=7.8 Gorkha Earthquake in Nepal last May, there has been a sudden and very welcome interest in landslides in that country. A number of groups undertook mapping of the landslide distribution, and the publications are now starting to appear. In a paper just published in Landslides (Martha et al. 2016), a group from the National Remote Sensing Centre and India have analysed an inventory of coseismic landslides triggered by the earthquake. The results are interesting. They have used a range of high resolution satellite instruments to generate a high quality landslide map. India has some wonderful satellites that represent ideal tools for this purpose, although the team have also used a range of other instruments as well. The upshot is probably the best landslide inventory published to date.
The raw statistics are important. Martha et al. (2016) have mapped 15,551 landslides triggered by the M=7.8 Gorkha Earthquake in total, including 213 landslides triggered by the large aftershock in Dolakha. The total volume of the landslides is about 620 million cubic metres. Whilst this number of landslides sounds high, it is probably much lower than we had anticipated for an earthquake of this magnitude in the highly unstable mountains of Nepal. The reasons for this remain unclear.
The most interesting aspect of the paper though is that the spatial correlation between the landslides triggered by the earthquake and the peak ground acceleration is weak. There is a strong correlation with slope angle (this is always the case), but the landslides occurred in a zone to the north of the earthquake affected area that does not correspond with the high peak ground accelerations. This diagram, from the paper, shows this rather nicely:-
Whilst it might be tempting to explain this by the distribution of steep slopes, I do not think that this alone explains the result. Thus, in the case of the M=7.8 Gorkha Earthquake, the landslide distribution is more complex than one might have expected. Martha et al. (2016) make the point that the landslide distribution seems to have been strongly controlled by the behaviour of the fault rupture, and that the spatial termination of the landslides towards the east seems to be controlled by the termination of the rupture event. I think this is spot on. Below is an INSAR map of the tectonic ground deformation caused by the earthquake. You can get an idea of the location in relation to the map above using the location of the epicentre of the mainshock to the west and the M=7.3 Dolakha aftershock to the east (with the cluster of aftershocks that followed that event:
And this is a map of the aftershocks, published in an open access paper by Ichiyanagi et al. (2015). Note that this is from an array in Kathmandu, so the distribution towards the west may not be as complete:
In general the high density of aftershocks lies in the northern area of the fault rupture, as does the landslides. These aftershocks did not trigger the landslides, but the similarity in extent suggests that the dynamics of the behaviour of the fault plays a strong role in controlling the landslide distribution.
The authors of the paper in landslides have made the inventory data available via the Bhuvan tool.
Ichiyanagi, M., Takai, N., Shigefuji, M., Bijukchhen, S., Sasatani, T., Rajaure, S., Dhital, M.R. and Takahashi, H. 2015. Aftershock activity of the 2015 Gorkha, Nepal, earthquake determined using the Kathmandu strong motion seismographic array. Earth, Planets and Space, 68, 25. DOI: 10.1186/s40623-016-0402-8.
Martha, T.R., Roy, P., Mazumdar, R. et al. (2016). Spatial characteristics of landslides triggered by the 2015 Mw 7.8 (Gorkha) and Mw 7.3 (Dolakha) earthquakes in Nepal. Landslides. doi:10.1007/s10346-016-0763-x
8 December 2016
Civita di Bagnoregio: the worlds first landslide museum?
The village of Civita di Bagnoregio in the Province of Viterbo of Central Italy is located about 120 km from Rome. The settlement was founded about 2500 years ago, but was essentially abandoned in at the end of the 17th Century as a result of a major earthquake on 11th June 1695, which triggered a major landslide below the town. The vulnerability of this site to landslides in evident from photographs of the village:
The town is located on a cliff 200 m high formed from a c.20 m thick layer of jointed Quaternary ignimbrite (a deposit formed from pumice fragments deposited by pyroclastic flows) sitting over a layered pyroclastic deposit. Sitting below this is a weak Tertiary clay formation. This is a classic setting for landslides, of which Civita di Bagnoregio has a long history. Indeed, in a new paper in Landslides, Margottini and Di Buduo (2016) document over 150 landslide events dating back to 1373 AD, but noting that this might well be an underestimate. A quick look at the Google Earth imagery explains why:
In recent years the town has gone through something of a revival due to tourism, perhaps unsurprisingly. To capitalise, the local government (Administration of Bagnoregio) has established a Geological and Landslide Museum in the Palazzo Alemanni in the centre of the village. The museum opened in April 2012. It cover three floors, dedicated to providing information about research on landslides. Unsurprisingly there is a strong focus on the nature and impact of landslides in the Civita di Bagnoregio locality. The museum also organises conferences, temporary exhibitions and educational activities.
I am not aware of any other museum dedicated to landslides. I have yet to visit – I think a trip to Italy might be in order!
Margottini, C. & Di Buduo, G. 2016. The Geological and Landslides Museum of Civita di Bagnoregio (Central Italy). Landslides doi:10.1007/s10346-016-0778-3