Three landslide-induced railway incidents in a single day

News today of three different incidents from around the world:
1. New Zealand
NZ Herald reports that a milk train hit a landslide in Manawatu Gorge.  Fortunately there were no injuries, or even a need to cry over spilt milk...



2. India
Bangalore Mirror reports that three coaches of the Thiruvananthapuram-Mangalore Express train were buried by a landslide in a narrow cutting at Mulunthurthy.  Three people were injured, plus there were some minor injuries during the evacuation.

3. Canada
Various Canadian newspapers report that there was a serious landslide-induced derailment of a freight train at St-Lazare in western Quebec.  The accident trapped the injured train crew in their cab, requiring that they were rescued by firefighters.  Heavy rainfall was reported to be the trigger.  The images of the site, from the Montreal Gazette, are impressive:

 

 Landslides represent  an important risk to railways in upland areas, and where extensive earthworks have been used.  Railway companies expend huge amounts of resource mitigating the threat, usually with success.  Incidents are quite rare, but previous examples described on this site include:
May 2010: 19 killed when a train struck a landslide in China;
April 2010: Nine people killed when a train was struck by a landslide in northern Italy;
July 2009: Four people killed when a train struck a landslide from a cutting in China;
Dec 2008: A train was struck by a flowslide formed from power station ash in Tennessee, USA;

Earthquake damage in Christchurch - an ironic billboard

This billboard, which predates the Darfield earthquake, was attached to a building in the Central Business District of Christchurch in New Zealand:

Do you think this is what they had in mind when they described the "open plan office with balcony access" and "cool funky office environment"?:

Images of the Darfield (Canterbury) earthquake fault rupture

Yesterday I was exceptionally fortunate to be able to spend the morning looking at the surface expression of the fault responsible for the Darfield earthquake, which hit the Canterbury plains area of New Zealand a fortnight ago.  I was kindly guided around by Russ Van Dissen of GNS Science, and the visit was organised by Chris Massey, also of GNS - so many thanks to them.  This GNS map shows the surface expression of the fault across the Canterbury Plain:

These pictures will work from east to west.

The east-most expression of the fault occurs close to the town of Rolleston, where a railway track crossed the fault.  The rails have been repaired, but the kink in the formerly straight railway track is fairly obvious.

Just around the corner a road crosses the fault as well.  Here the damage, again to the formerly straight road, is unrepaired as the fault is expressed as a broad deformation zone accommodating about 60 cm of movement:

If you are struggling to see the feature, remember that the road stretching into the distance (not the bit at the very bottom of the picture) was straight before the earthquake.  You should be able to see that from the middle of the image the road has been shifted to the right.  In this area there is little evident vertical change, bearing in mind that the surface was not perfectly flat before the earthquake.

A few kilometres to the west there is another formerly-straight road crossing the fault.  Here we see a little more lateral movement - actually about a metre.  Again, there is no vertical movement.  The movement is evident in this image - look at the alignment of the edge of the road:

Lets now jump a few more kilometres to the west, where things start to get really interesting!  Another formerly straight road is our indicator of strain - but now it is becoming difficult to believe that this happened in a single 30 second event (I can assure you it did):

Note here the highway people have resurfaced the road, so the original cracks are no longer visible.  There is a ditch running down the side of the road that also shows the deformation rather clearly:

The movement here is about 3 metres or so.  Again there is little evidence of vertical deformation.

The farmer at this site very kindly allowed us to enter his field (please do not do this without permission), where the array of cracks, and associated deformation, is astonishing.  GNS have an aerial view of the field here; below is the ground view:

The movement of the fault is evident in the power lines that cross the fault here.  The movement of the fault has put the cables under tension, meaning that the insulators (the pieces that join the cable to the pylon) are no longer hanging vertically:

A few more kilometres to the west and we are into the maximum displacement area.  Here we see over 3 metres of horizontal deformation and about 1.5 metre vertically.  This is the view from the hanging wall side - the drop down onto the footwall, and the lateral motion should be evident:

The picture below was taken on the footwall side looking back towards the fault - note the horizontal motion (the road has been patched up) and the vertical change.  Remember that this was a straight road across a flat plain before the earthquake:

The maximum deformation is recorded a little to the west again, where a road is offset by almost its entire width, indicating movement of about 4 metres.  There is some vertical deformation too, but this appears to be more of a ground roll than a true vertical movement of the hanging wall block:

Our last site is at the western end of the fault trace.  Here the surface expression of the fault is reducing, leaving a small step in the road that is quite hard to see.  The best evidence is from the power cables, some of which are now very tight (those on the upper right), whilst others are very slack (those linking to the pole on the far right).  Note how the tension cable supporting the pole on the left has also gone loose - this would have been taut before the earthquake:

It is at this point, almost perfectly on the fault, that the highest accelerations were measured.

I hope this gives a useful overview of the fault.  I am happy to answer any questions, though Geonet and GNS Science are the experts. 

Comments welcome

Ruapehu lahar information

One of the many highlights of the splendid IAEG Congress in Auckland this week was a talk by GNS geologist Chris Massey on the 18th March 2007 lahar at Mount Ruapehu on North Island.  The lahar occurred as a result of the failure of a tephra wall holding back the crater lake at the summit, and is shown by this NASA image:

The potential for a lahar had been anticipated and the site was intensively monitored with real time instruments such as water level sensors and geophones; with  two web cams; and with periodic surveys using a terrestrial laser scanner.  An emergency plan was in place and worked well.  The need for caution was undeniable - on 24th December 1954 a lahar from the same site demolished a railway bridge at Tangiwai, killing 151 people on a train that tried to cross a bridge that had been destroyed by the lahar.

The event has been written up in a paper (Massey et al. 2010), and there is a spectacular set of images of the event captured by the web cam available here (NB it took me a while to get my eye into these images).   

An interesting aspect of this event is that one can examine just how good the natural hazard science community is at assessing hazard.  There is a New Zealand Civil Defence report, written in 2002, about the threat of a lahar at Ruapehu online here. 


Reference
Massey, C., Manville, V., Hancox, G., Keys, H., Lawrence, C., and McSaveney, M. 2010.  Out-burst flood (lahar) triggered by retrogressive landsliding, 18 March 2007 at Mt Ruapehu, New Zealand—a successful early warning.  Landslides 7 (3),303-315, DOI: 10.1007/s10346-009-0180-5.

Updated: The New Zealand earthquake

Bizarrely, I am currently sitting in the QANTAS lounge at Sydney Airport waiting for a flight to New Zealand, where the IAEG Congress starts on Monday.  The theme of the conference is "Geologically Active"...

So, what do we know about the earthquake so far.  The best source of information is the Geonet site - http://www.geonet.org.nz/ - which puts seismic data online in real time.  They are reporting that the earthquake occurred at 4:35 am local time 30 km west of Christchurch at a depth of (now updated to) 10 km.  The reported magnitude is (now updated to) 7.1 - USGS is reporting 7.0, but this is probably a moment magnitude.  This is the isoseismal map from Geonet:

This suggests that some damage in the Christchurch area is likely, which seems to be confirmed by the initial news reports.  Hopefully the timing of the earthquake, and the high level of preparation for earthquakes in New Zealand, will mean that casualties will be light and damage manageable.  Perhaps the most interesting data at this stage is the map of locations in which people have reported they felt the earthquake.  This looks like this at the time of writing.  The dark orange dots represent an intensity sufficiently large to cause significant damage:

GEONET shake map

	There are no reports of landslides as yet, but given the proximity of the Southern Alps some landslides are likely:
	More later.

Real time landslide monitoring in New Zealand - the Utiku landslide

I have posted previously about the near real time landslide monitoring project that is being undertaken on the Taihape landslide by the GEONET team in New Zealand. The most amazing aspect of this project is that the monitoring data is streamed onto a web server and can then be viewed using a rather neat online graphing package (available here).

The great news is that the team have now instrumented another landslide on North Island, this time on a slightly more active slide at Utiku, which is not far from Taihape. Once again the data is being put online in near real time, and once again it can be viewed using the graphing package. This is available here.

This is how Geonet describes Utiku:
"The Utiku landslide has been classified as a deep-seated translational block-slide earth-flow. This classification refers to the characteristics of the landslide. Deep-seated refers to the depth of movement (depth to the landslide slip-plane); the slip plane of the Utiku landslide has been recorded at 20 m below ground level at the toe (bottom) and increasing to 65 m towards the back scarp (top). The term translational refers to the movement style of the landslide, where it moves as a relatively intact mass (raft) of material, along a planar zone of weakness. In this case, the zone of weakness corresponds to a thin clay layer thought to represent a bedding plane within the local sandstone . The term block-slide and earth-flow describe the landslide structure and movement mechanisms."

This is quite an old slide (at least 1,800 years, and possibly much older), but it remains active. The road that crosses the slide shows some minor signs of deformation that has required patching:

It will be very interesting to see how this landslide behaves as groundwater levels increase.

Landslides from the Fiordland earthquake, New Zealand

GNS Science in New Zealand have released details of the mapped landslide distribution from the Fiordland earthquake on 15th July. As I noted a day later, the number of landslides appears to be surprisingly low:

The earthquake produced only 187 landslides, most of which were shallow and comparatively small. The area affected by landslides was 5600 square kilometres. Comparison with the graph below (see my original posting) suggests that this is at the bottom end of the expected number for a Magnitude 7.8 earthquake.

According to Graham Hancox the reasons for this lower than expected level of landslides are thought to be:

1. The ground motions were smaller than would normally be expected for a magnitude 7.8 earthquake;

2. The dominant fault rupture motion was away from land;

3. Lower than average rainfall occurred in the two months prior to the 2009 quake.

The map suggests that there is a very strong relationship between the ground motion and the occurrence of landslides, with most of the mass movements being concentrated in the vicinity of the epicentre. This is consistent with, for example, the Chi-Chi earthquake in Taiwan.

30 years ago - the Abbotsford landslide, New Zealand

As well as being something of a landmark birthday for my wife Kirstie, Saturday will also be the 30th anniversary of one of the best known and most interesting landslides in New Zealand - the Abbotsford landslide of 8th August 1978. This was a large (5 million cubic metre) slide that caused the loss of 69 houses, with an overall cost of about NZ $10 million. Fortunately, no lives were lost. The landslide was recently described in a paper by Graham Hancox (Hancox 2007) of GNS Science, from which I have gained most of the information here.

The landslide occurred in a suburb of the South Island of Dunedin. The remarkable picture below, from the Hancox paper, shows the state of the site three days before failure:


On the left side of the image (the east) lies a quarry from which about 300,000 cubic metres of sand had been removed. Running across the centre right of the image is a large array of cracks that were opening up as the landslide moved. These cracks clearly extend into the suburb, although they become less easy to discern in this area. In fact cracking was first noted in some of the houses in 1972 or even before. Through time the cracks grew until in 1978 they defined the rear scarp of the landslide as seen above. The cracks caused several water main breakages in late 1978 and early 1979.

The slide was extensively monitored in the period before final failure. The data suggests that the rate of movement was steadily increasing through time, with rates as high as 10-15 cm/day being noted in the week before failure!

Final failure began at about 9 pm on 8th August. It lasted about 30 mins, during which time a large block moved forward by about 50 m, leaving a graben structure behind that was about 16 m deep. This is very clear in the image below. The slide was a deep-seated translational block slide that covered an area of about 18 hectares. It was about 800 m, 400 m long and up to 40 m deep. The average movement was about  × 400 m, up to 40 m thick). According to Hancox (2007) it slid down a 7°-dip slope at an average speed of about 1.7 m/min. The rate of movement was sufficiently slow that no-one was killed, although many people needed to be rescued and, of course, lost their homes.

(source: Teara)

(Source: http://www.kvc.school.nz/Kaikoraistream/Intro_Folder/Geology.htm)

So why did the slope fail? Well, the first key factor is that the site was susceptible to failure under natural conditions. The materials were dipping in the same direction as the slope and were weak and susceptible to sliding. There were ancient landslide deposits on the site that point to previous instabilities, well before humans could have played a major role. Second, the removal of the sand from the quarry removed support from the slope, making it far more likely to fail. In fact, Hancox suggests that the slope only needed groundwater to increase by 0.5 m for movement to start.

The slide was probably triggered by increased water levels in the slope. This is likely to have come from two sources. First, the few years before the landslide were wetter than had been the previous 20 years or so. Second, Hancox (2007) suggests that there was a leaking water main that may have been allowing 5 million litres to enter the slope each year.

New Zealand is of course a landslide prone environment, but there can be little doubt that many lessons have been learnt from this failure. By modern standards it seems amazing that the houses were still inhabited when cracks as large as those shown in the first photograph were developing, and movement rates of 10 cm per day were being recorded. The authorities in New Zealand are clearly using the anniversary of the landslide to remind people of the need to remain vigilant.

Reference
Hancox, G.T. 2008. The 1979 Abbotsford Landslide, Dunedin, New Zealand: a retrospective look at its nature and causes. Landslides 5: 177-188. Journal page.

Landslide-induced train accident in China

The landslide misery in China continues, despite the strange lack of typhoons to date this summer. The latest incident occurred in Liuzhou City of Guangxi Province when a train struck a landslide and derailed. According to Xinhua four people were killed and 50 others were injured, at least ten of whom were seriously hurt. Xinhua has published a photograph of the aftermath of the incident:

Thus it appears that the landslide was comparatively small and occurred on a slope associated with a railway cutting.

This week there was also a (non-fatal) train derailment caused by a landslide near to Wellington in New Zealand (image from 3 News):

Earthworks failures on railway systems are surprisingly common. For example, Glendinning et al. (2009) reported the following for the UK rail network:

"Between January 2000 and March 2002, 91 earthwork slips occurred, each causing more that 750 minutes of train delays on Network Rail. While it is possible that the rate of failure on railway slopes may have reached equilibrium, this seems unlikely as the number of recorded earthworks failures rose from 47 in 2003/04 to 107 in 2007/08."

Fortunately of course the vast majority of slope failures on the railway network cause no more than disruption, but the potential for greater impact is always present.

Reference

S. Glendinning, J. Hall and L. Manning 2009. Asset-management strategies for infrastructure embankments. Proceedings of the Institution of Civil Engineers Engineering Sustainability 162, 111-20.

Earthquake induced landslides

Every so often an event occurs to shake up out of our complacency. So often this is a disaster that is mind-bogglingly destructive, such as the Wenchuan Earthquake landslides last year. However, just occasionally something that happens that is far less damaging than would be expected. This of course is easier to ignore, but in fact can be just as informative as the big events.

Yesterday is just such a case. Sitting in a meeting I received a GDACS alert to say that there had been a Magnitude 8.2 earthquake in the far south of New Zealand. The earthquake was shallow - less than 20 km - and the south of New Zealand is highly mountainous, suggesting that it was likely to have induced a large number of landslides. The last big event in this area, the Fiordland earthquake of 21st August 2003, which was"only" Mw=7.2, triggered lots of slides (see the excellent Geonet report on this event), so the assumption that this large event would do the same was quite reasonable. Since the event the earthquake has been downgraded by the USGS to Mw=7.6 at a depth of 12 km - although smaller I would expect that this would still be a massive landslide-inducing event.

I'm wrong. In fact report coming from fly-over surveys of the epicentral area suggest that there were very few landslides triggered by the event. This is really surprising. Over the last 25 years, since the pioneering work by David Keefer at the USGS, a lot of work has been done examining the relationship between landslides and earthquakes. The map below shows the distribution of well-documented studies of earthquake-induced landsliding apologies if I missed out your particular study - please let me know!) - each yellow dot is an earthquake event for which extensive landslides have been documented. The background image is the GSHAP earthquake hazard map - dark areas have a high level of hazard:


The colours indicate the size of the earthquake and the dots are located at the epicentre of the earthquake (which is why some are obviously offshore). You will see that there is a pretty good coverage of areas that are obviously both seismically hazardous and have high relief, with some areas of high concentration because of proximity to research teams (Italy and California for example). You will also see that New Zealand, thanks to the efforts of GNS and Mike Crozier at Victoria University in Wellington, is pretty well covered.

For a substantial proportion of these earthquakes the area affected by landslides has been measured. Unsurprisingly, there is a pretty strong relationship between the area affected by landslides and the earthquake magnitude:

So when an earthquake occurs in a mountainous area we have a pretty good idea of the area that we would expect to be affected by landslides. Note that there is quite a large range for any given earthquake magnitude - this is the influence of earthquake depth, topography, prior weather conditions (i.e. has it been wet, in which case the ground is likely to be less stable), vegetation, humans, etc.

This map suggests that we should expect to see many landslides for the event yesterday. This is clearly not the case - which is something of a surprise. It will be interesting to see what happened here - for that we will have to wait for more detailed studies over the next few months. I wonder if the dynamics of this event might be slightly unusual, accounting for the vast difference in initial estimates of magnitude between the USGS and Geonet? Perhaps this translated into much lower ground accelerations than might be expected - and hence the low number of landslides.

A few items of interest

You may have noticed that the number of posts on this blog has dipped of late. My apologies for this - we learnt just before Christmas that my seven year old son Adam needed open heart surgery, which was undertaken earlier this month. I am glad to say that it was a complete success and he has today returned to school, so normal service will hopefully be returned.

So, I thought I would start off with a short post highlighting some interesting stories that have emerged over the last few weeks:

The aftermath of the Gonaives mudslides in Haiti
There is a short but very interesting (great for teaching) video on the World Focus website that describes the legacy of the mudslides last summer in Gonaives in Haiti. An earlier post on this problem is available here.

Wenchuan (Sichuan) Earthquake: 18,000 households to be relocated to avoid geological disasters
China Daily reported a couple of weeks ago that 18,000 households are having to be relocated over the next three years to avoid geologically-related (i.e. landslide and sediment) disasters, at a cost of $166 million. The article also gives some interesting data on the scale of the problem., noting that the province "will establish a monitoring network for some 3,695 at-risk sites". They note that "8,061 sites with potential geological disasters were found in 39 county-level areas in Sichuan after the quake, threatening the safety of 158,000 households with a population of 640,000 people, and assets worth 30 billion yuan." Finally, the article notes that "the effects of potential geological disasters triggered by the quake will last for ten years".

What happens when a small rockfall hits a car
Meanwhile, the Manawatu Standard in New Zealand (I am sure that you never miss a copy....) carried an interesting story about a car being hit by a rock in Manawatu Gorge on North Island, with the following picture of the damage:

It is fortunate that there were no passengers in the car at the time.

Increasing rates of erosion in northern India
There is also quite a nice article on an Indian news site, Samay Live, about increased rates of erosion in Himachal Pradesh in N. India as a consequence of landslides. The article notes that:
"Expressing concern over large-scale soil erosion in Himachal Pradesh mainly due to landslides, Chief Minister Prem Kumar Dhumal said today said the government has initiated several measures for soil treatment. Dhumal said according to scientific surveys, the soil of the state has the capacity to bear erosion of only 10 tonnes of soil per hectare but this erosion has unfortunately risen to between 16 tonnes to 40 tonnes per hectare at present...The state's about 53.80 per cent soil is affected by erosion, out of which the impact on 34 per cent soil is so immense that it was on the verge of losing its fertility."

Young River Landslide, New Zealand

Interesting news from New Zealand, where for the last few months there has been some concern about a landslide dam on South Island. The landslide itself occurred in a fairly remote area of the Southern Alps at 4:40 am on 29th August 2007. The landslide, which is well-described in a GNS Science poster here (there are some great images and some good data on that poster), blocked the river valley to a depth of about 100 metres. This was a big debris slide, with a volume of about 11 million cubic metres and a runout distance of about 1.8 km. Interestingly it does not appear to have created an air blast as large slides of this type often do. This may mean that the mass moved as a series of events closely spaced in time, rather than one big, instantaneous failure.

Over time a lake built up behind it, finally overtopping on 5th October 2007. Understandably, there was concern about the possibility of an outburst flood. In this case there are few human assets at risk downstream, but this is a popular hiking area, potentially putting people on the trails at risk. As a result, the trails were closed by the Department of Conservation whilst the landslide was monitored in real time by GNS Science and Otago Regional Council. Unfortunately, such a closure was not good for the local community as it reduced the number of tourists.

The images below show the landslide from the downslope side. I have annotated it below to show the key features. Note that the deposit has banked up on the right side (from this view). The natural spillway that has formed is just on the lee of this banked up deposit. This deposit is coarse-grained (bouldery), which may well be the reason why the channel has not eroded downwards to release the water.

The image below is from the landslide dam itself looking across the lake. Note the size of the boulders - these are part of the landslide deposit. This does show that if the landslide dam is stable then the resultant lake can in some circumstances become an asset.

This week it was reported here that a decision has now been taken to reopen the trails downstream of the landslide from 1st November (i.e. just before the start of the summer). Unsurprisingly, the local people are rather pleased: "Makarora Residents Association deputy chairman Devon Miller said the closed valley had affected some of the tourist operators in the township, so the decision to re-open was a welcome one."It's a good positive announcement and the community is happy," he said." (Otago Daily Times).

The decision to reopen the trails but to maintain some restrictions in heavy rain, using a new warning system, appears to be sensible. So often landslide management is about balancing risks - i.e. what is the risk of a collapse of the dam affecting someone on a trail compared to the risk to the local communities associated with the loss of tourism, etc. In this case the stability of the dam suggests that this risk has now dropped to close to the residual level. Note that this does not mean that there is no risk - there are hazards associated with spending time in remote mountains. The risk from the landslide dam is no greater, and may be substantially less, than those other risks.

All-in-all the approach taken by the parties involved in New Zealand has been exemplary, in terms of picking up the event in the first place, in terms of the ways that they have monitored it and in terms of the decision-making process to minimise risk.

Presentation from New Zealand

This week I am giving three presentations in New Zealand - first in Christchurch today, then Wellington tomorrow and Auckland on Thursday. The title of the presentation is Earthquake Induced Landslides - Lessons from Taiwan and Pakistan.

The talk is to the various branches of the New Zealand Geotechnical Society and the New Zealand Society of Earthquake Engineering.

I have uploaded the presentation to authorstream so that it can be downloaded (note that I have now given up using slideshare as I have found it too unreliable. The presentation can be found (and can be downloaded as a Powerpoint file from) here and should be viewable below:


Uploaded on authorSTREAM by Dr_Dave

Abstract:
Earthquake induced landslides – lessons from Taiwan and Pakistan

The recent Wenchuan earthquake in Sichuan Province of China has highlighted the impact of seismically-triggered landslides in mountain environments. In China, it is now estimated that approximately 28,000 fatalities, representing about 35% of the deaths in the earthquake, occurred as a result of burial by mass movements. In addition, the blockage of all of the mountain roads by landslides meant that rescue of buried victims in the so-called “golden” 24 hours was seriously impeded, and more than 150 rescuers were subsequent killed by slides triggered by aftershocks. This presentation seeks to examine the occurrence of landslides in two other major seismic events, namely the 1999 Chi-Chi earthquake in Taiwan and the 2005 Kashmir earthquake in Pakistan. Using field data from both sites, the occurrence of landslides induced by the earthquake is examined in both space and time. It is shown that the distributions of landslides are rather different in the two cases, even though they earthquakes themselves were of similar magnitudes. The reasons for this difference are explored, in particular in relation to topographic controls on the magnitude of earthquake accelerations on slopes.

In the second part of the presentation, the occurrence of landslides in the years following the earthquakes is explored. It is shown that there is a notable increase in the occurrence of landslides in the aftermath of the seismic event, and that these landslides release large volumes of sediment into the river system. This increase in landslide activity can have a major impact on the rehabilitation of earthquake-affected areas for long periods of time. Thus, there is a need to improve preparation for earthquakes in landslide-prone areas and to recognise that the impacts of such events are prolonged, rendering the recovery phase rather difficult from an engineering perspective.

Shotover rockfall in New Zealand

Reports today suggest that an interesting landslide problem in New Zealand has finally come to a close. The location of the problem was the Shotover River in Otago, which is a little to the north of Queenstown on South Island. Shotover is famous as an extreme sports location - in particular jet-boating on the Shotover River, white water rafting and a canyon swing (a sort of bungee jump I think) in the gorge, plus walking, pony trekking, etc.

The problem that arose was the occurrence on 12th July of a rockfall on the canyon wall above the river in an area upstream of that used by the jetboats. The Google Earth image of this area is quite good (Fig. 1), illustrating well the steep topography, narrow valley and the location of the jetboat station.

Fig. 1: Google Earth image of the Shotover River site.

The rockfall was pretty intriguing - a large pillar of schist had detached from the rock mass and was creeping downwards (Fig 2). The mass was pretty large - about 30,000 tonnes and as the image shows it was quite unstable. Fortunately, the authorities in New Zealand are pretty well set up to deal with such things, so quickly swung into action. Perhaps unsurprisingly, the gorge had to be closed to rafters and other users, although the jetboats were essentially unaffected. A monitoring programme was put into place whilst a strategy was formulated.

Fig 2: The detached rock block early soon after the problem first arose. Picture from the Otago Daily Times

Clearly the block needed to be brought down so that rafting etc could be restarted, so a fire hose was used to try to induce failure (this is sometimes called water jacking). Unfortunately, the weather over the last month has been very cold, which has meant that this approach has been slower than had been anticipated. Nonetheless the block continued to move. The main block finally detached at 6:30 am yesterday after temperatures rose and there was a few hours of heavy rainfall (Fig. 3). The block has broken up and has not blocked the river, meaning that the crisis is finally over. The local council hopes to reopen the river next week.

Fig 3: The site of the detached rock block early yesterday after the block finally collapse. Picture from the Otago Daily Times

The Taihape Landslide monitoring project

Last summer I was lucky enough to spend a couple of days with my friends from GNS Science who are undertaking a monitoring project on the Taihape landslide in New Zealand (see Google Earth image right). Taihape, which is located as -39.68°, 175.80°, is described on Wikipedia as "a small, picturesque town near the middle of the North Island . It services a large rural community and lies on the main north-south route through the centre of the North Island." It has a population of about 2000 people and is in general a very nice little place. Unfortunately, the area around the town is somewhat landslide prone (actually, mapping landslides here would probably involve trying to find those few areas that have not been affected by mass movements!).

Perhaps not surprisingly, part of the town is built on a landslide, which moves (slowly) during periods of wet weather. The landslide is certainly not new - dating suggests that it first moved over 1,800 years ago. Nonetheless, the level of recent movement, although slow and gentle, can cause some damage to buildings and roads in the affected area of the town. The landslide is a translational block slide with a shear surface at about 25 - 35 m depth. It's pretty big - over 200 houses and a school are sited on it. Movement appears to be associated primarily with rainfall. Details of the landslide are available here.

Hence for the last year or so the hugely impressive chaps from Geonet have been working with GNS Science to monitor the movement of the landslide. In my view this is about the most impressive monitoring project I have seen to date. The data are collected by an automated laser monitoring system that sights onto prisms located on the landslide. This is supplemented by two rain gauges and four borehole piezometers. Excitingly, the data are available in graph form online in real time, so that anyone can see how the landslide is behaving. The data are available here, which uses a Java graphing tool. It takes a few minutes to load, but stick with it as it is fantastic. The results are great fun. You can plot side by side the movement recorded by different prisms and relate this to the groundwater level and the amount of rainfall. Although there hasn't been much rainfall since the monitoring started, it is already possible to see the link between rainfall, groundwater and movement. It is really worth playing with the graphing function.

The strength of this project is in its thoroughness and its transparency. The monitoring system being used is about as good as is possible using current technology, and the team are taking great care to quantify and deal with the potential errors. The fact that the results are online is hugely impressive. I can only wish that more landslide movement data was available in this way.

Rock avalanche in New Zealand and Sodbury landslip in the UK

In the last week or so there have been two contrasting examples of landslides that demonstrate the poor relationship between size and impact.

First, there was this rather interesting and quite large rock avalanche in Aoraki-Mount Cook National Park, New Zealand:

The 13th January 2008 Aoraki-Mt Cook landslide

Picture from: New Zealand Herald

There is a really nice set of photos in Flash form on the New Zealand Herald website here and a good video overview of the landslide here. This is quite a large slide, with a scar (source) size of about 120 metres high, 80 m wide and 30 m deep, giving a volume of about 300,000 metres cubed by my calculation (or about 1 million metres cubed by some reports). It ran out over a glaciaer for a distance of about 2 km, probably because the slopes are steep and the debris has flowed across ice. However, no-one was killed or injured as this happened in an uninhabited area. Interestringly, there was no obvious trigger to the event.

On the other hand, rail services in SW England have been seriously disrupted by this landslide:

The 24th January 2008 Chipping Sodbury landslide
Picture from: Sky News

In comparison with the Aoraki-Mount Cook this landslide is tiny - probably jest a few hundred cubic metres. However, the landslide, which occurred as a result of the prolonged heavy rain that the UK is suffering at the moment, occurred on the SW mainline of the UK rail network. Although it didn't reach the tracks, it has damaged signal equipment. As a result, train services were cancelled for a day and will be heavily delayed and disrupted for a while. I expect that the whole of the embankment will now need to be inspected.