Free workshop on coastal erosion, 19th March 2010

We are running a free workshop on erosion on 19th March 2010 in North Yorkshire, UK. Details in the flyer below, together with contact details (click on the image for a better view in a new window).

Please do register to come along!

Coastal erosion as art

The Guardian has a slightly bizarre article today. It is entitled "The art of watching your house fall into the sea". It tells the story of artist Kane Cunningham, who has bought a bungalow on the cliff edge at Knipe Point near to Scarborough. This site has been featured here before as recent landsliding has threatened to destroy a number of houses. He says in the article:

"I'm going to turn the moment my studio collapses into an art work: I've set up cameras to film it, and I've commissioned music and poetry to celebrate it. Both our houses punch a hole in what we think of as the value of ­property, and remind us of our moral and ethical responsibility to nature... When my studio disappears, I'll have no sense of loss – that will be its beautiful final act. The sooner it goes, for me, the better."

He has a website about this here. Quite bizarre, but it could be very interesting to watch.

Angler killed in rockfall in NE England

The BBC is reporting that an an angler was killed in a landslide last night in Sandy Bay, near Newbiggin-on-Sea in Northumberland, NE. England. He was reportedly standing at the top of a 15 m high cliff that collapsed, killing him when he fell.

This is Sandy Bay:

Landslides in Art Part 3

This is the latest in my occasional series on the depiction of landslides in art. The previous parts of this series are here:
Part 1
Part 2

Today I turn to a classic painting from the 19th Century, painted by Lord Frederic Leighton, entitled "On the Coast, Isle of Wight":


This oil sketch, which hangs in the National Gallery in London, shows a stretch of the Isle of Wight coastline in southern England. The view is from Ventnor looking towards the west towards Saint Lawrence. The coast here is a famous area of landslides known as The Undercliff - the landslide morphology is clearly visible in the painting.

Landslide in the Lake District (Cumbria) in the recent floods

Thanks to Rhiannon George of Meridian Geoscience for her help with this - the images and most of the information have been provided by her.

The press coverage of the recent (English) all time record rainfall event in Cumbria (in the area most commonly known as the Lake District in NW England) has focused upon the disastrous floods at Cockermouth. However, less well reported is the extensive devastation up in the headwaters of the catchment, and in adjacent areas, where landslides and floods occurred extensively. Rhiannon kindly provided information about two shallow landslides that occurred at Howtown, near to Ullswater. The pictures that Rhiannon has provided are pretty impressive:

This is one of the two slides, which appears to have split into two runout tracks.

This, and the image below, is the other.

These are comparatively small, shallow failures. They appear to have initiated quite low on the slope as shallow slips that have then eroded and entrained the top metre or so of the ground downslope. The cessation of movement appears to be gradient controlled - i.e. the point at which the slope angle decreased to the point that erosion ceased and deposition started.

So why did these slides occur at this point at this time? Well, the rainfall that this area received broke the English record for one day and five day precipitation, so it is not surprising that many floods and slides occurred. The Google Earth image of the site of the second slip, taken prior to the landslides, also helps:

The landslide initiated in the highly disturbed and gullied area on the lower slope. This image suggests to me that the slope was under some distress prior to the failure. In other cases we have found that such gullying and disturbance, which is not seen on the adjacent slopes,
has indicated that for whatever reason, water was flowing into this area underground, maybe through a natural soil pipe network. In the heavy rainfall the flow of water can exceed the capacity of such pipes, causing the local ground to saturate and the the landslide to initiate.

This is an image of the head scarp of the second slide. If the above mechanisms is correct then it might be possible to see pipes in this area, but in the aftermath of the slide they may have become clogged, so some digging around would be needed.

Threat of another landslide at Pennan, the site of the film "Local Hero"

The film Local Hero was released in 1983, having been filmed in the village of Pennan, near Aberdeen in NE Scotland. In my view it is one of the best films of the 1980's - understated, beautiful and genuinely funny, with a very strong environmental message. The village at Pennan has become something of an attraction as a result but, located so far from the beaten track, it has never been a real mecca for visitors. The attraction of the village to the film crew is clear from this photograph (from here):

However, look carefully at the picture above and you will probably see the problem that the village faces. Unfortunately the slope behind the village is unstable - in 2007 the village suffered a mudslide that flowed into homes, the pub and the village hall, causing considerable damage. The pub remained closed until last month, when a new landlord took over. Finally things started to look up for the village again.

Thus, it was with great sadness tonight that I read that the residents of the village have been told to prepare for another potential evacuation as another tension crack has opened up on the cliffs above the village. BBC News reports that this crack is 25 m long. Fortunately, this time the potential landslide does not appear to threaten the houses directly, but the road is threatened. The Google Earth image below shows just how critical this road is to the village:

The UK has been suffering heavy rain over the last few days, which is of course the likely cause of this problem. Tomorrow is forecast to be drier in this area, but further rain is forecast for Thursday and, in particular, Friday. It will be a nervous few days for the people of Pennan. I wish them well.

Coastal erosion - the last groyne problem

One of the most emotive topics for local authorities in the UK is that of coastal erosion. Over the last few years in Britain we have increasingly taken a view that coasts should only be defended where absolutely necessary - elsewhere erosion should be allowed to occur at a "natural" rate. In most cases the view has been taken that protection of coasts in which erosion will affect domestic properties (i.e. peoples' houses) does not qualify as necessary. This is particularly harsh for those whose properties are affected by erosion - in the UK domestic insurance does not cover erosion and there is currently no compensation scheme for people whose houses are affected by erosion. Even worse, when the house becomes dangerous the local authority will often take possession of the hose and demolish it. Guess who gets the bill for the demolition...

One of the key reasons for choosing not to protect coasts is the so-called "last groyne" problem. This occurs at the point of the last (down drift) part of the coast that is protected. Here, the coastal protection updrift typically starves the coast of sediment that would normally provide some protection against erosion, leading to greatly increased erosion rates at this point. I was playing with the historic imagery feature on Google Earth this week and came across a very clear example of this problem, from the east coast of Britain.

Here is an image collected in 1999 of the site in question. As ever, click on the image for a better view in a new window:


Here, longshore drift is from the northwest to the southeast, such that the groyne (sea defence) in the centre of the image is the last of the chain. Most of these sea defences have been in place for half a century. What should be immediately obvious is the way that the coast steps back at the point of the last groyne, indicating the much higher erosion rate at this point.

Let's now zoom forward just seven years to an image that was taken in 2006:

The change in this period is quite startling when you look closely. First, you will see that the area previously affected by enhanced erosion has retreated further. The sea defence has broken down and three houses have been lost. It appears that some rock armour has been dumped at this point to try to reduce erosion. The sea has also breached and destroyed the coastal defence just to the NW of this point, and as a result the coast has started to erode. In consequence a whole series of properties have been lost along this frontage.

The town in question is called Happisburgh. The residents have a campaign to try to have sea defences constructed, with an excellent website.

Perhaps most startling is this image, from a sequence of aerial photographs, which shows the same location in 1986:

At this point the groynes were intact all the way down the coast, and the current step back is not present. Therefore, the big step back in the coast that is seen in the first image above occurred in just 13 years.

Cayton Bay / Knipe Point landslide - report on options

A year or so ago I posted on an ongoing landslide at Cayton Bay in North Yorkshire, just an hour or so down the road from my base Durham. This landslide, which is no commonly called the Knipe Point landslide, was threatening 50 or so houses (see image below) - in the end three were demolished, although the rest are still under threat from the slide. Since my post the BGS have created a nice summary website here, from which this picture is taken:

The local council, Scarborough Borough, managed to find from a range of sources about £300,000 to pay for an investigation of the site, which was undertaken by Halcrow. This investigation was completed this week and will be discussed by the Council in a few days time. The Council has put out a press release here, although the report is not available online. The key findings are as follows:

a. The landslide is a deep seated, ancient landslide system. consisting of:
1. A main deep seated failure for which ground movement is controlled by the residual strength of the clay and a deep confined natural groundwater table;
2. Shallow mudslides in the overlying glacial tills for which the ground movement is controlled by an upper natural groundwater table. This slide is highly sensitive to small changes in the groundwater conditions.

I find the above quite surprising as the failures affecting the houses do not move that often - so this apparent high level of sensitivity is a little odd. I will need to read the report (I will try to get a copy).

The press release then discusses stabilisation issues - which is I am sure what the householders are worried about. It briefly mentions the constraints (minimal impact on the environment as the landslide contains a Site of Special Scientific Interest (SSSI), but a 50 year design life).

Three key actions are recommended:

1. "Maintain and continue monitoring of the surface and subsurface ground movement, groundwater and weather station network."
2. "Liase with stakeholders to review the findings of the report and discuss the way forward for managing the cliff instability risk at the site in the short and long term."
3. "Review funding options for promotion of the preferred engineering stabilisation options, and prepare an application for funding under the relevant and most appropriate legislation."

The preferred stabilisation option is unfortunately large and complex:

• Installation of deep drainage to reduce and control groundwater levels in the deeper water table.
• Construction of bored piles at Knipe Point to isolate the lower Cayton Cliff landslide system from the land above the cliff top.

The press suggest that this would cost £12 million (some reports suggest £20 million!). I would think that finding this sum of money is going to be tricky given the limited number of houses involved, although maybe the threat to the main road will help here.

British Geological Survey landslide web page

The British Geological Survey (BGS) appear to have been quite active of late in overhauling their web pages, reflecting what seems to be an improved ethos of delivery of information to end users. The landslide pages seem to have been subjected to this upgrade of late, which has led to the creation of a really good set of case study pages, backed up with photos, reports and maps. These pages are available below - they are a little slow on my system, but that might be my wireless router:

http://www.bgs.ac.uk/science/landUseAndDevelopment/landslides/caseStudies.html

A good example of this resource is the 1993 Holbeck Hall landslide (see BGS page here), which includes this image:


Perhaps most excitingly, parts of the UK national landslide database are going to be made available online for free through the new Geoindex tool. A Beta version of this tool is already available - it looks to be a really exciting tool.

Great stuff!

Cave rockfall in Shropshire, England

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

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

Future British seasonal precipitation extremes - implications for landslides

One of the great questions of the age is of course the ways in which climate change will affect the weather patterns that we are likely to see in the future. In the case of landslides the key issue is the ways in which precipitation patterns will alter, especially the most intensive rainfall events that are responsible for many of the most damaging landslides. One of the most significant steps forward over the last few years has been the ability of global climate models to handle these extreme events, meaning that at last we are starting to develop some capability.

This week an important paper has been published by Fowler and Ekstrom (2009), which seeks to look at the likely changes to very intense rainfall events in the UK. Helen Fowler, is based just up the road from me at Newcastle (the city with the chronically under-performing football team), and her co-author have used modelling ensembles to examine how UK precipitation regimes are likely to change in the time period 2070 to 2100 under the SRES A2 emissions scenario, which is currently effectively our best estimate as to how carbon dioxide emissions will change with time (Fig. 1).

Fig 1: SRES Emissions Scenarios. A2, as used in this study, is shown in Fig. (b). Source: http://www.grida.no/publications/other/ipcc_sr/?src=/Climate/ipcc/emission/014.htm

Ensemble modelling looks at the results of a series of different climate models to examine the range of outputs. Each model operates in a slightly different way, meaning that there will always be a range of results. Therefore, papers presenting ensemble model outcomes always present a range. One of the key issues of interest is whether there is some consistency between them. In this study. Of course the results of such modelling runs are highly complex - in this paper the authors have looked at the 1 day and 10 day precipitation events with a current return period of 25 years. The 1 day event can be thought of as the impact of an intense storm; the 10 day probably simulates a series of low pressure systems tracking across the country, as has happened several times in the last few of years. In landslide terms the 1 day storms might trigger the catastrophic debris flow and sallow failure events, whilst the 10 day events might trigger deeper seated and large slope failures.

First the model is run for the a control period (1961-1990) to check that they can realistically simulate observed conditions. They can. The models are then run to look at what would happen in the period between 2070 and 2100, and the results are then pooled using a fairly interesting approach. Well, the first thing to say is that the Global Climate Models (GCMs) do predict a much warmer climate - global mean temperatures are predicted to be 3.1 to 3.56 degrees warmer than at present. Interestingly though the occurrence of these intense rainfall events also greatly increases for three of the four seasons:
Winter: Increases in occurrence of extreme precipitation of 5 to 30%
Spring: Increases in occurrence of extreme precipitation of 10 to 25%
Summer: Very varied results, with some models suggesting decreases and other increases. More work is needed
Autumn (Fall): Increases in occurrence of extreme precipitation of 5 to 25%

A few of the models do predict larger (and smaller) increases - look at the paper for the full detail. Overall, the authors conclude that "Nevertheless, importantly for policy makers, the multi-model ensembles of change project increases in extreme precipitation for most UK regions in winter, spring and autumn. This change is physically consistent with warmer air in the future climate being able to hold more moisture. The use of multi-day extremes and return periods also showed that short-duration extreme precipitation is projected to increase more than longer-duration extreme precipitation, where the latter is associated with narrower uncertainty ranges."

The implications for landslides are stark. Increases on this level of the occurrence of extreme precipitation events will inevitably increase the occurrence of slope failures. Therefore, we should expect to see an increase in the occurrence of slope failures. Unfortunately, as landslides are triggered by just a small proportion of our existing rainstorm events, increases in this range are likely to have a disproportionate impact.

Of course the next thing to do will be to build the outputs of these models into slope stability models. This will be a fascinating exercise.

Reference:
H. J. Fowler, M. Ekström (2009). Multi-model ensemble estimates of climate change impacts on UK seasonal precipitation extremes International Journal of Climatology DOI: 10.1002/joc.1827

The Dorset landslide revisited

It is now sensible to revisit the 7th May Dorset landslide, upon which I blogged here. Now, eight days on, it is clear that there was a slight media over-reaction to this event, probably spurred by reported comments that this was the largest failure for a century. It wasn't.

The image below from the BBC, clearly shows what actually happened. A section of cliff has collapse from the Blue Lias at the front of the landslide complex. The failure is wide but does not propagate very far inland. This is much smaller than other recent movements on this landslide complex, albeit visually dramatic from the beach. It is true that if there had been people on the beach at the time the consequences could have been serious. However, there weren't, meaning that this is not a really significant event.


John Lock has very kindly provided two images from the Lyme Regis side, which I reproduce below. The first shows the site of the landslide before the failure, taken from the Lyme Regis side, the second shows the same site afterwards.

Of course, the interesting factors here are:
1. The way in which the media provide misleading impressions of what actually happened; and
2. The potential implications for stability upslope given the loss of the toe of this part of the complex. I am sure that the local authority will now be pretty active in assessing the latter.

Black Ven landslide in Dorset

NOTE: see follow-up blog here, outlining why this landslide is not as large or significant as was initially reported.

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Update: ITV have a low res video of the landslide site here. This confirms the general location and shows that the failure is indeed large.
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Update 2: The BBC now have some good aerial footage of the landslide here. It is slightly intriguing as it appears that the slide is wide but not very long. From a preliminary view there should be real concern about what might happen here in the next heavy rainfall event.
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Update 3: Based upon the BBC footage, the failure area appears to be the lower part of the slope highlighted by the red box on Fig. 3 below. The landslide appears to be a 400 m wide section of this lower cliff, extending about 100 m inland. This is a really intriguing development. It will be interesting to see how the sections upslope now behave as they have lost their toe support. This will be a fascinating landslide to watch over the next few months. I suspect that we have not seen the last movement at this site as yet.
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Original text:
Timed to perfection to coincide with the launch of the Durham University Institute of Hazard and Risk Research tomorrow, news reports indicate that there as another major landslide in the UK overnight. This time the failure occurred on the well-known Black Ven landslide system in Dorset, the site of a great deal of landslide research over the last few years.

Black Ven is shown by the arrow on the Google Earth image to the left, located on the south coast of England, just east of the town of Lyme Regis in Dorset. It is a big, active landslide complex consisting of an upper section in a glauconitic sandstone known as Foxmould. The lower part consists mainly of a series of shales and marls with thin limestone bands that form benches in the landscape. This is quite well shown in the cross section in Fig. 1 (Koh 1992). NB Click on any image for a better view.

Figure 1: Indicative cross section of Black Ven landslide in Dorset, showing the permeable sandstones of the Foxmould overlying the impermeable shales and marls (Koh 1992). NB Click on any image for a better view.

The landslide is clearly shown in the picture from the 1980's shown in Fig. 2 (Brunsden et al. 1988). The yellow rocks at the top is the Foxmould, and the grey rocks lower down are the shales. The landslide usually activates because of a deep seated rotational failure in the Foxmould. The rotated blocks then break-down over time to form flows that cross the benches below, eventually flowing out into the sea.

Figure 2: Photograph of Black Ven landslide in Dorset, showing the permeable yellow sandstones of the Foxmould overlying the impermeable shales and marls (Brunsden et al. 1988). Rotational failures in the Foxmold transition to flows that run across the lower benches. NB Click on any image for a better view.

The reports suggest that the landslide occurred in the area known as the Spittles, although I have yet to see any pictures to confirm this. Koh et al. (1992) indicated the location of the Spittles on the sketch map of the landslide shown to the left. This is just to the left of the landslides shown in the photograph in Fig. 2. This is an area that has been showing very considerable slope distress for many years, with lots of evidence of deformation as the toe of the landslide slips away. This area is quite clear on the Google Earth Imagery in Fig. 3 - I have put a faint red box around the general area usually known as the Spittles. Zooming into the upper part of the area delineated in red, the Google Earth image very clearly shows the active deformation that has characterised this zone for a while, with abundant evidence of tension cracks and small scarps (Fig. 4). These have been developing over a long period, so failure should not be a surprise. The landslide is large - the BBC report that it is about 400 m, which appears to be the width. If so this is a notable failure.

Figure 3: Google Earth image of the Black Ven landslide. The red area highlights the zone known as the Spittles, which appears to be the area that has failed in early May 2008. Update: The BBC film footage suggests that the failure has occurred in the lower left side (i.e. SE corner) of this box. NB Click on any image for a better view.

Figure 4: Google Earth image of the Black Ven landslide zoomed in on the upper part of the Spittles. Note the multiple tension cracks and scarps indicating that failure was likely. NB Click on any image for a better view.

The timing of the failure is quite interesting - it appears that the UK experiencing an exceptional level of landslide activity at the moment (e.g. Cayton Bay). Interestingly, amateur weather stations do not suggest that the weather of late has been exceptionally wet in this area - for example, an amateur station just along the coast at Dawlish suggests that the rainfall total in April was 64.6 mm, which is lower than the totals for both 2006 and 2007. The first few days of May have been essentially dry. The only notable rainfall event of late occurred on 29th April, when 18.5 mm of rain fell. Although high, this is not really exceptional. Rainfall in 2007 was recorded as 916 mm against a long term average of 850 mm (i.e. about 8% above the mean value), but this was below average for the last four months of the year. This pattern continued in January and February 2008. What is interesting is that March was unusually wet (103.5 mm against an average value of 68.1 mm - i.e. 93% above the long term average). I assume that the movement we are seeing is a delayed response to this rainfall period.

I would really welcome any comments, especially from anyone near to the site. Please email me or leave a comment below.

References:
Koh, A. 1992. Black Ven. In Allison, R.J. The Coastal Landforms of West Dorset. Geologists Association, Essex, 67-79.
Brunsden, D., Gardner, R., Goudie, A. and Jones, D. 1988. Land Shapes. Channel 4.

Cayton Bay Landslide

It is not often that I get to write about a landslide that is (practically) in my own backyard, but this is one of those rare occasions. In fact for the last fortnight or so this landslide has been generating quite a lot of local news - see for example a report in the Daily Telegraph.

The landslide in question is in Cayton Bay in North Yorkshire. Here, a set of cottages have been built close to the edge of a coastal cliff. Over this winter the cliff has suffered a set of landslides that have caused a series of reasonable large slips that have allowed the cliff to retreat.

This Google Earth image shows the situation quite well.

Google Earth image of the Cayton Bay site

The threatened cottages are in the north-west corner of the images. The landslide is the heavily wooded area between the cottages and the sea. Unfortunately as the image from the Daily Telegraph shows, since this image has been collected the cliff has retreated rapidly due to reactivation of the landslide - estimates are that the cliff top has moved back 7 m this winter - and now at least some of the houses are seriously threatened. Reports indicate that at least two have had to be demolished to date. Given that insurance rarely covers landslide hazards, this must be heart-breaking for the residents.

Inevitably, questions are being asked as to what is going on at this site. Inevitably there is a great deal of speculation, including suggestions that the site has been affected by the building of a new bypass or that the construction of extensions to the bungalows has triggered failure. In my experience such causes are unlikely. The cause is probably rather more local. It is clear that the the coastal slope here is clearly a part of a large, rotational landslide complex which has shown activity before. This complex has been documented and indeed mapped well-before the most recent failures - the BBC for example notes that observations of instability were made by highway engineers in the late 1960's. Importantly, in May 2004 Jon Carey, Paul Fish and Roger Moore from Halcrow gave a presentation at a meeting of the Yorkshire Geological Society entitled "LANDSLIDE GEOMORPHOLOGY OF CAYTON BAY, NORTH YORKSHIRE". The abstract of the paper says:
This paper describes the geomorphology of a large coastal landslide complex at Cayton Bay, North Yorkshire. The area inland of the landslide is occupied by a strategically important road and a number of properties, and knowledge of current landslide behaviour and possible future scenarios is therefore important for future planning and risk. Instability at the site is associated with a series of faults which bring argillaceous Upper Jurassic rocks to sea-level. These soft rocks are overlain by more resistant sandstones. The sequence is capped by a thick and variable series of glacial sediments, that comprise tills with inter-bedded sand and gravel lenses, deposited during the Dimlington Stadial of the Late Devensian. In connection with the development of a future coastal strategy for Cayton Bay, detailed geomorphological field mapping was conducted which identified two major landslide systems. These include a periodically active mudslide complex at Cayton Cliff, recognised by a series of shallow scarps and benches with occasional back-tilted blocks, and an area of dormant deep-seated landslides at Tenants’ Cliffs, that includes a series of graben and horst structures. The origins of the landslides are unclear, but probably involved a variety of processes that led to a reduction in material shear strength or increases in pore water pressures. The timing of original failure may relate to deglaciation following the Dimlington Stadial, or periods of wet climate in the Holocene. Since sea-levels were not higher than present in the Holocene along this stretch of the coast, coastal erosion is not thought to have been a factor. The causes of the contemporary instability are likely to be due to the combined effects of coastal erosion and of groundwater, both of which are predicted to increase in future years due to the impacts of climate change. The implications include increasing risks to coastal assets and a need to manage and mitigate such risks.

This team from Halcrow are pretty competent, so I would trust their interpretation. Their view that activity of the landslide is probably associated with high pore pressures is probably correct, and it is notable that the weather has been cool and wet for the last 18 months or so in this area. The key question that needs resolving is where the water is coming from - is it just that pore pressures are higher than usual (this is very possible) or could there be some other source, such as leaking pipes or changed drainage? If the cause is just naturally high ground water then the long term implications for the cottages is potentially serious. The site is very large, which means that the cost of stabilisation are very high. The land is a Site of Special Scientific Interest, which means that it is protected, and it is owned by the National Trust, who are keen to protect the environment.

Strange landslide in the UK

Farmers Weekly Interactive carries a rather strange report and some excellent photos of a landslide that occurred near to Widnes in NW England on 29th February:

"Tractor Driver James Fletcher who works for Chester-based contractor Mike Harley, had a lucky esacpe when the field he was ploughing gave way beneath him. He was on the final run of a sloping field near Alvaney, Widnes, when the bank he was on collapsed, sending an estimated 15,000 tonnes of soil sliding down the slope. Mr Fletcher's tractor and plough was carried a quarter of a mile, through 360 degrees, crossing two fields and ended up being half buried in soil. Amazingly, the tractor started again once dug out, but had to be taken away on a low-loader."

IC Cheshire Online carries a rather more detailed report:

A wave of mud and rocks carried a tractor through three fields with its driver trapped helplessly inside. James Fletcher was ploughing a potato field at Teuthill Farm, Alvanley, at around 12.30pm last Friday when the ground gave way around him. He was trapped at the wheel as his tractor was dragged on its side through three fields and only escaped when the tractor hit two trees, smashing the windows so he could jump free. James, 45, of Bates Lane, Helsby, said: “I got there at about 9am and this was my last run up the field. “The next thing I knew, I was still moving forward but I was going backwards as well. The tractor was still ploughing and I just thought I was getting stuck. “Then the ground opened up and spun the tractor around. It smashed me into an oak tree which knocked the tractor over onto its side.“I was trying to open the door but it was too high above me. I was frightened to death.“I hit another oak tree on the other side which righted the tractor.“At the last minute I saw the house and I thought: ‘It’s time to get off now’. “If I hadn’t got out that would have been it.”

From what one can work out, the key facts are:

1. The landslide was triggered by a tractor with a large plough;
2. About 15,000 tonnes of material failed, travelling for about 500 m down a fairly gentle slope;
3. The rate of motion was sufficiently fast to prevent the driver from escaping (the driver was very lucky by the way - if the tractor had rolled completely over, rather than righting itself, he might well have been killed.

This landslide is intriguing because of its exceptional mobility and speed of motion. In many ways it appears to be similar to a quick clay failure, but this is unlikely. Data from an amateur weather station at nearby Widnes suggests that there was not an exceptional of rainfall that day (2 mm), or indeed for the preceding few days. Thus, the nature and cause of this landslide is somewhat perplexing.

I am trying to find out more about it.

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.