This blog provides a commentary on landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings. The blog is written on a personal basis by Dave Petley, who is the Wilson Professor of Hazard and Risk in the Department of Geography at Durham University in the United Kingdom.

This blog is a personal project that does not seek to represent Durham University.

Monday, 31 May 2010

Attabad: flow and erosion are continuing to develop

Flow through the spillway at Attabad has continued to develop during the day.  However, as I suggested in my post this morning, some of the higher discharge values being quoted are without foundation.  At 6 pm the discharge was about 360 cubic feet per second (10.2 cubic metres per second).  Thus, the discharge graph looks like this:


At the moment there appears to be little evidence from these figures that large-scale scour and erosion has been initiated - the increase in flow appears to be the consequence of the increasing lake level, plus perhaps some lateral erosion.

However, erosion at the toe of the slope remains the major concern, with reports suggesting that the gully is continuing to erode back through the channel.  This remains the most likely failure mode of the dam, but this may take a few more days to initiate.  In addition, the increasing channel discharge may cause the flow velocity to increase to the point at which basal erosion starts.  This has not yet happened, but could begin at any stage.

There have been various comments on the blog that the dam is not behaving as expected - i.e. erosion is not initiating - and that as a result measures should be taken to encourage erosion.  I want to stress that this is not correct - from the start we knew that it could be that it could take some time for erosion to initiate.  So far I see nothing surprising in the behaviour of this system, and nothing to suggest that erosion will not allow a lowering of the lake level in the next few days.  At the same time, it is important to emphasise that the probability of a rapid breach has not diminished, and may in fact have slightly increased due to the headward erosion of the lower gully.  

There is no need to panic, but at the same time there is also no justification for lowering the alert status.  We all need to be patient and alert - this has a long way to go yet.   If you are in the area please pay attention to the local alert system.

Attabad - flow on the spillway increasing further


The discharge across the spillway at Attabad is still increasing.  Whilst some reports suggest that it is higher, the Pamir Timeshttp://pamirtimes.net/ suggests that the flow at 2 pm local time was 250 cubic feet per second (7.08 cubic metres per second).  If this is correct, the discharge time graph looks like this:


However, the situation is now somewhat confused as the NDMA report from 6 pm on 30th May (i.e. yesterday) was that total outflow was 900 cusecs (25 cubic metres per second).  Assuming that 250 cusecs is seepage, this would give a spillway flow of 650 cusecs (18.4 cubic metres per second).  This seems to be surprisingly high.  Maybe the NDMA information has the wrong time stamp?

Therefore, at the moment I am a little confused as to what is happening.  I will try to clarify this in the next few hours.

Sunday, 30 May 2010

Retrogressive erosion at Attabad

Flow continues to increase at Attabad.  Inflow is still exceeding outflow and seepage, so the lake level continues to rise, to the detriment of the upstream communities. There has been some confusion about the statistics about outflow - it is now clear that NDMA discussions often use the combined spillway plus seepage figure, but that this is interpreted by the media as being the spillway flow.  According to the NDMA press conference tonight, combined seepage plus spillway is about 400 cubic feet per second (about 11 cubic metres per second), still less than 20% of inflow. The remaining 80% will mostly need to be accommodated by the spillway, so this has a long way to go yet.

Downcutting on the saddle of the spillway, where the gradient (and thus the water velocity) is low, appears to be limited at the moment.  Looking at the following two images, all of the action is on the downstream face.  This picture was taken between 8 am and 9 am local time this morning:




Note the clear erosion (deep downcutting) in the lower part of the channel, which appears to be driving some local instability too.  The following picture was taken just before dark (hence the lower quality), between 6 and 7 pm (local time):


Look carefully at the erosion in the lower part of the channel - the two waterfalls are clearly eroding back through the channel.  This is a common mode of failure for landslide dams - indeed I highlighted it a couple of days ago.  As the discharge through the channel increases we might seethe rate of retrogression of these faces increase.  On the other hand, initiation of downcutting in the channel might eliminate them altogether.

Certainly these features need to be watched very closely over the next few days.  IF (and it is a big if - by no means a certainty) these features started to erode through the saddle then a fairly rapid failure could develop.  However at the moment there is no need to worry unduly - this is just something to watch carefully.


Finally, failures continue to occur on the main scarp above the dam.  This one happened early this evening:


It did not result in a wave.

Reported increases in spillway flow at Attabad

If you live in the Hunza and Indus valleys, please do not rely upon this blog to provide warnings and up to date information.

Using only reliable reports in the media (mostly information from the Pamir Times) and those from NDMA, the following graph shows the reported flow through the spillway at Attabad.  There is a huge caveat here regarding the reliability of the reports.  However, the most recent figure is the NDMA reported value.  Times are local to Pakistan (UT +5 hours)


It is clear that flow is now increasing quite rapidly.  However, do not be panicked by this - it is as we would expect and does not indicate anything in particular at this stage.  Meanwhile of course the water level in the lake is still rising at about 2.5 cm per hour, which will be increasing the impact on the upstream communities.  We must not forget them is this crisis.

If you live in the Hunza and Indus valleys, please do not rely upon this blog to provide warnings and up to date information.

Attabad - flow on the spillway continues to develop slowly

Flow through the spillway continues to increase slowly.  So far there are no signs of the development of a breach, but the amount of water passing through the spillway remains much less than the inflow.  This image, taken yesterday morning, shows water passing through the channel:


The difference between the flow in the channel and the amount of water coming through the dam as seepage (the large channel bottom right) at this time is very clear.  The channel flow at this time was in the order of 20 cubic feet per second; the seepage is over 100 cubic feet per second.  Since then channel flow has increased (I'll blog on this shortly).

It is important to stress that the threat has not reduced.  There was always a good chance that flow would develop slowly and that erosion would take time to initiate.  Flow needs to reach >2,500 cubic feet per second to equilibriate the lake level even at the current rate of inflow.  The river discharge will more than double over the next month, putting further stress on the spillway.  The best image illustrating the latent threat at this site is this one, also taken yesterday:


At the moment this is very reminiscent of the early stages of flow at Tangjiashan:

Attabad overtopping - first image

The Daily Times has published the first picture of the spillway in operation:

Meanwhile, the Pamir Times reports that: "The volume of water passing through the spillway has reportedly increased to around 50 Cusecs".  50 Cusecs is 1.4 cubic metres per second.  This is still a fraction of the total inflow, so we have a long way to go. 

Saturday, 29 May 2010

Attabad: initial flow established

Please note that it is hard for me to keep up to date with this evolving situation.  If you are potentially-affected by this hazard please use the formal channels of communication set up by the government.  Do not rely on this blog to provide warnings.

Flow through the spillway is now properly established and is increasing with time.  The latest NDMA report suggests that flow is 300 cubic feet per second (8.5 cumecs), which sounds surprisingly high - indeed I am sceptical of this figure.  UPDATE: The Pamir Times is reporting 25 cubic feet per second (0.71 cumecs).  this figure sounds more sensible.   Reports suggest that so far the flow is stable.

So where does this leave us? Well, as I suggested this morning, erosion is likely to take a while to develop, so we are far from out of the woods as yet.  Some lateral erosion at this stage might be helpful in widening the channel, increasing its capacity.  However, as the flow increases the chances of the channel becoming blocked will slowly reduce, but again it is too early to eliminate this possibility.  As flow increases the potential for erosion also rises, so for me the critical period is from now until the point at which outflow = inflow.  This point is some days away.

It is clear that the authorities are taking this very seriously now, with helicopter surveillance in addition to the land-based monitoring.  This is good to hear.

Attabad: overtopping has begun

The media are reporting that water has now "entered the spillway" - i.e. that overtopping has begun.   The report, which is from the Chinese Agency Xinhua, carries a rather strange quote from an official from NDMA:

"The next 12 hours could be crucial as the water flow would reach its maximum intensity," commented an official of the National Disaster Management Authority (NDMA), the organization mainly responsible for the overall rescue and damage control operation.

Maybe this is a misquote?  The next twelve hours are crucial of course, but there is a good chance that a breach won't develop in this time.  The water level will continue to rise until outflow (seepage + spillway flow) = inflow.  As some of the water is being released, this may well take much more than 12 hours to develop.  In addition, a breach will only start when erosion of the base occurs at the highest point of the saddle.  This may well take time to occur.


So, I would like to make two points:
1. The development of a breach in the next 12 hours is very possible; the failure of a breach to develop does not mean that one won't occur.  At Tangjiashan the breach took several days to start, but was then rapid;
2. It is also possible that an initial small flood will happen, but that this will then cease.  For example, this could be the case if the sides of the channel collapse, blocking the flow.  We must be clear that if there is a small flood that then declines then the situation is certainly not over.  People must not go back into the danger zone until we know that the threat has passed.  

Of course what we are hoping for now is slow erosion and the release of the water over the next few days.  Such a scenario is certainly not beyond the bounds of possibility.

I will post again as more information becomes available, but don't be surprised if this takes a few days to develop.

Friday, 28 May 2010

Attabad: freeboard report 28th May

The media in Pakistan are reporting that the freeboard is now 2 feet (61 cm).  Assuming that this was taken at the normal measuring time (i.e. 3 pm local time) the freeboard graph looks like this:


Jean Schneider, who is on site and flew over the spillway today, reports that overtopping should begin overnight.

New images and maps of Attabad

Whilst we wait for news from Pakistan, here are two new resources that readers may find useful.  First, NASA have released a new Aster image of the lake site, collected in the last few days.  This is available here, with a link to my blog (thanks to them for this).  The image shows the manner in which the lake has extended in the last few weeks - NASA have helpfully provided an image with the previous extents shown:


The unannotated image, this time orientated north-south, is also rather fine:


Alonside this, UNOSAT have released another map of the areas inundated by the flood.  This is available here, and looks like this:

We may see some landslides along the banks of the lake as the water level is drawn down by the breach, so this imagery may prove to be very useful.

Attabad: Overtopping is now likely to be imminent

It is unfortunate timing that the likely day of overtopping has fallen on a Friday, the holy day in Pakistan, meaning that flow of information is impeded.  As a result I have not yet had an update for today on the lake level.  Jean Schneider of BOKU visited the site yesterday by helicopter and measured the freeboard at 2.25 feet, which is about 70 cm.  I am not sure what time this measurement was made.  Based upon the rate of rise of water level, that should put the time of overtopping at some time today.  However, this will depend upon the continued rise of the water at a similar rate, no further heave of the floor of the spillway and, perhaps most importantly, no further collapse of the walls of the spillway.

Meanwhile, the continued mixed messages from the Pakistan administration has been illustrated by an article in The News today:

"Zameer Abbas, Assistant Commissioner and in charge of relief camps in Hunza Nagar, told this correspondent that the Karakoram Highway had been closed for every kind of vehicular traffic as the lake water was expected to reach spillway today (Friday) after which the overflow would begin, paving the way for controlled release of water.  Dismissing the chances of water outburst, he said the landslide blocked some one-and-a-half portion of the Hunza River and it was apparently out of question that water would force out of this huge amount of mud and debris."

I will post again when more information becomes available.  Please can you email me at: d.n.petley@durham.ac.uk, and/or add a comment to this post, if you have news on the situation.

Thursday, 27 May 2010

Attabad: the freeboard is now reportedly about 1,3 metres

The latest NDMA figures suggest that the freeboard is down to about 4.3 feet (1.31 metres) as of 3 pm today.  That is a reduction of about 70 cm on the day, meaning that we are probably less than two days from the point of overtopping:



 I am reasonably confident of these values as the Focus geologists took this picture yesterday of the spillway:


The circled boulder is the crest of the spillway, so water flow will start when this point is reached.  Of course the one potential problem is a slope failure on the spillway walls, which could block the channel.  This is a real possibility, but we won't know until tomorrow.

Close up the channel at this point looks like this:


Unfortunately the large boulder will resist lateral cutting, initially at least, which is likely to increase the rate of down-cutting.  This is unfortunate.  Once the water reaches the top it will initially flow down this channel, crossing the summit of the dam:


And then will flow down here:


The disturbance here is from collapse of the banks induced by the seepage.  Finally, it will flow down the main river channel:

The terrace in the centre left of the image is at 60 metres above the river level.  Previous (comparatively recent) floods have not reached this level, which is half the height of the dam.  This is the level that we believe is comparatively safe from the flood.

Attabad: update for 27th May

Filling of the lake behind the slide at Attabad continues, with the reported freeboard being 1.98 metres as of yesterday afternoon.  I have not yet received a value for today - I will update the figures when this comes through.  For now the freeboard time graph looks like this:


This would suggest a likely date of overtopping of 28th to 29th May, but once again caveats apply.  It is now clear that the spillway floor is deforming, and in particular uplifting.  This means that the rises in lake level are not automatically translating into a reduction of freeboard.  There are two possible explanations for this uplifting behaviour.  First, it could be that they clay is swelling as it becomes wet.  Second, it is more likely that the clay is deforming due to the stockpile of excavated material on the sides (see my post yesterday) pushing down on the clay, and squeezing it out, much like a toothpaste tube.

Either way, the upshot is that the freeboard is a moving target.  Through into the mix the ongoing failures on the flanks of the spillway, which could block the channel, and the overtopping date becomes more uncertain.  Meanwhile the lake level continues to rise; estimates from satellite imagery suggest that the volume may have now exceeded 500 million cubic metres.  This is three times the volume that the Chair of NDMA was quoted as saying in the Pamir Times yesterday That is a huge amount of water, most of which is likely to pass through the Hunza valley in the next few days.  If a large flood does occur then the water will entrain debris from the dam and from the channel downstream, increasing its size.  Thus, I maintain the view that the 60 metre level is the minimum safe height down to Tarbela, although I also note that this assumed that there was an effective spillway.  Of course, if the flood induces slope failures on the banks then people at higher elevations could also be at risk. The reported available volume at Tarbela is much greater than this flood, however I have no means to validate this.

Finally, at a meeting I attended last night on disaster risk reduction there was a discussion about preparedness for disasters.  The comment was made that as a crisis develops a government should prepare for the worst and hope for the best.  This would seem to be a sensible approach at Attabad.

Wednesday, 26 May 2010

Attabad: update report for 26th May

The combined monitoring effort appears now to be providing consistent measurements of the remaining freeboard, and these are now being used in the media, which is helpful.  As of late last night the reported freeboard was 8.8 feet, which is 2.56 metres.  This provides a freeboard - time graph that looks like this:


This suggests that we are still on course for overtopping on about 27th or 28th May, although clearly this remains a moving target for all the reasons previously outlined.

Meanwhile, the media in Pakistan have now picked up on the inconsistencies in the measurements reported by NDMA, and in particular their incorrect measurements of freeboard.  This article is strongly critical of the error by the government agencies:

"However, during a tour of the dam site on Monday the NDMA chairman, Nadeem Ahmed, discovered significant differences in measurements recorded by the Frontier Works Organisation (FWO) and the Gilgit-Baltistan region’s public works department.  “He was furious when he noticed the discrepancies and realised the NDMA had been reporting inaccurate data,” a participating official said.  Mr Ahmed immediately ordered a dam-site demonstration of the methods used by the two government agencies, before declaring that the public works department’s “freeboard auto-level gauge” system, based on instrumentation manufactured by Nikon of Japan, should in future be the sole official data source

Again the question has to be asked as to why NDMA did not use the expert advice that was freely offered?  This decision continues to mystify me.

The state of the spillway is also a concern now within Pakistan.  From the same article:

The engineers said the 25-metre-deep spillway was too narrow to accommodate outflows of more than 2,400 cusecs from the lake, which held an estimated 1.2 million cusecs on May 17. A cusec is a unit of water flow equal to one cubit foot per second. "There is a serious possibility that the entry of water into the spillway could trigger a collapse of debris that would block it altogether,” an engineer said.

Several people have asked why the slopes on the sides of the spillway are failing given that the weather is dry.  The answer I think lies in the material - which dries on the surface but remains wet and very weak just below - and the loading from the excavated materials on the slopes above the spillway:


Finally, the Pamir Times have published a photo of the dam from the upstream side.  The watermark is a shame (but understandable).  What is notable here is that the upstream face of the dam has undergone a slope failure (the scar is quite obvious on the left side above the lake).  This does not threaten the integrity of the dam in any way, but does indicate that the materials from which is it formed are comparatively weak.

I'll try to provide another update later, but have to travel to London to attend the launch of the UCL Institute for Risk and Disaster Reduction this evening.

Tuesday, 25 May 2010

Attabad - a bizarre race between two processes

Yesterdays "coalition" resurvey of the spillway and water level now appears to provide a definitive freeboard value.  As of 8 am this morning the freeboard was 3.027 metres, representing an increase of water level of 1.09 metres in the last 24 hours.  This elevated rate of water level rise probably reflects the warmer weather in the area in the last few days, which will be driving snowmelt. 

I have recalculated the freeboard values assuming that the official numbers released today are correct, meaning the the early figures under-estimated the true freeboard figure.  The resulting graph is as follows:


At the current rate of increase the freeboard will be lost in two to three days, but as before we are reliant upon these figures being correct.  This episode of incorrect freeboard measurement does not reflect well on NDMA, given the importance of these numbers.  Of course, the fact that the saddle is located at a higher elevation means that the lake volume will be larger - i.e. it increases the size of the potential flood.

Of great concern to me though is the continued problem of closure of the spillway.  It is clear that the banks of the spillway are failing in multiple locations.  These failures are progressively closing the spillway, as this Focus image, taken of the spillway banks yesterday morning, shows:


The concern would be that the spillway closes completely before the water level reaches this level, which would effectively increase the freeboard once again, albeit probably briefly.  In effect we are in a high stakes  race between the rate of water level rise and the rate of closure of the spillway.  The strength of the clay material forming the spillway banks is clearly low, as we believed from the start.  Either way, the spillway is clearly completely incapable of handling the flow, so erosion is inevitable.  The one bright spot is that the failed banks may erode preferentially, allowing the channel to widen more rapidly than it down-cuts, as happened at Tangjiashan. 

Finally, lets not forget the hardship that this crisis is imposing on the local people.  The Pamir Times today puiblished this image of a camp for Internally Displaced People (IDPs):


I need say no more. 

Monday, 24 May 2010

A further update on Attabad

Important update: a combined Focus and FWO team have now resurveyed the freeboard and have agreed that there correct freeboard figure was 11.5 feet (3.5 metres) this morning.  This means that overtopping is likely to be a few more days away, although failure could still occur at any time.

NDMA have now released their daily report for yesterday, so I have replotted the freeboard graph (see note above):


You will see that there has been further divergence between the Focus and the NDMA data, with NDMA still reporting 2.01 metres of freeboard, and representing a decrease of freeboard of just 43 cm in 24 hours.  On that basis, the overtopping may still be a few days away.  It should be noted here that the NDMA freeboard was physically measured, whilst the Focus figure was estimated from a distance.  On that basis it may well be that the NDMA data are more reliable.

NDMA have now suspended freeboard measurements, quite rightly, so we are all in the dark from here-on.

24th May update on Attabad (post now complete)

FOCUS have kindly provided an update on the state of affairs at Attabad, and NDMA have also posted online their statistics for 22nd May (note this is now two days out of date).  The freeboard graph looks like this:


Key points:
1. The Focus data (which has consistently been the most reliable) indicates a freeboard of 1.15 metres on 23rd, with the water level increasing at about 80 cm per day, giving a likely overtopping date of today or tomorrow.
2. However, the NDMA data gives a freeboard 60 cm higher than Focus on 22nd May, and a lower rate of water level rise (48 cm in 24 hours).  If this is correct, overtopping will occur later in the week.

Seepage continues to increase, but a seepage-induced failure now looks unlikely in the context of loss of freeboard.  Focus have provided some images of the situation taken from their monitoring point.  Before I show these, I do want to make the point that the Focus team have provided an exceptional service to the people of Hunza, and indeed of Pakistan, as well as to readers of this blog.  They have done so iin extraordinarily difficult conditions.  This is the text and monitoring point in which they have been living for the last two months:


The mast is the CCTV station.  We have all benefited from the information they have provided - I hope that comments on this post will reflect this extraordinary effort and contribution, and that in due course the people of Pakistan will recognise what they have achieved.

The team have provided this image of the site looking down from this monitoring point:


This is a view from the monitoring point looking down onto the dam site, taken yesterday by Focus:


My concern about the state of the spillway is increasing.  This image shows the spillway from the same point:


The spillway looks in a fairly poor state in the image above.  However, the image posted today on the Pamir Times is even worse:


It is clear that the sides of the spillway have closed up over the last few weeks, leaving a narrow channel that is almost totally inadequate.  The weakness of the materials also bodes ill for events when over-topping occurs. 

Sunday, 23 May 2010

Meanwhile, elsewhere

Although all eyes, and increasingly those of the media (which is showing considerable interest in this blog), are on Attabad, other landslides are happening around the world as we enter the rainy season in many landslide prone areas of the Northern Hemisphere.  In the last few days a number of significant landslide events have occurred:


1. A landslide-induced rail crash in China this morning.
Xinhua reports that an 8000 cubic metre, rainfall-induced landslide derailed a train on the line between Shanghai and Guilin in Dongxiang county in Guangxi Zhuang Autonomous Region.  It is reported that 19 people were killed and a further 71 people were injured.  Xinhua has two images of the landslide, which appears to have occurred on a modified slope in deep, highly weathered soils:


Thus one may be the most telling, as it is clear that just above the top of the slide, on the right side, the slope has been regraded, and indeed there is clearly some fill in the exposed slope section, suggesting that the two of the slide was in a part of the slope that had been modified:






 2. The first serious rainfall event of the year has happened in Taiwan
In Taiwan there is deep concern about the likely impact of landfalling typhoons this year, given the devastating impact of Typhoon Morakot last August.  Those fears will have been heightened today as the first real rainfall event of the year occurred.  The Central Weather Bureau has published an image of the rainfall totals for the day:






Of course in most places such rainfall totals (>200 mm in a day) would represent an exceptional event.  In Taiwan this qualifies as a light shower, but nonetheless there has been some landsliding in the mountains, with the resort town of Tungpu being cut off as a result of rockfalls.  Taiwan is going to be very interesting to watch again this year; with continued inadequate means to manage slopes nationwide I fear for what might occur.


3. Sri Lanka is suffering its May landslide problem
Sri Lanka often suffers heavy rains in May as the SW monsoon starts up.  This year is no different, with a series of storms associated with Cyclone Laila triggering widespread flooding and landslides.  The Disaster Management Centre is reported to have said that 20 people have been killed, and 500,000 people have been affected.

I will post an update on Attabad tomorrow morning at about 7:30 am UT.

Attabad - less than two metres to go before water flow

Several people have asked for an update on the situation at Attabad, and a forecast of what will happen, so here goes.

First, the situation as I understand it is as follows.  In the last few days the availability of data on the situation on the ground has become difficult for reasons that are very understandable.  However, Focus have continued to take measurements of lake depth when possible, and NDMA have also posted information online.  Thus, a graph of the freeboard of the two measurements looks like this:


Whilst there is close agreement between the two datasets over the last ten days or so, and generally good agreement overall, the difficulties of forecasting the final point of overtopping are clear when just the last seven days of measurements are shown:


The next NDMA update will help - however my experience through the whole of this has been that the Focus data is the most reliable.  With a Fovcus measured increase of water level of 85 cm in the 24 hours ending yesterday, and a freeboard of 1.95 metres at lunchtime (local time) yesterday, the most likely time of overtopping is late tomorrow (24th) or early on the 25th.  This situation is not going to be any easier to manage should overtopping start at night.  However, the most recent NDMA data might suggest a slightly later date, and of course there is always the possibility of an earlier failure as discussed previously.

So, what is going to happen at the time of overtopping?  It is impossible to say with any certainty, but the most recent images from FOCUS, provided late on Friday, do help.  First, this overview image shows the dam, spillway etc, taken from the rock spur that is the location of the early warning team.  It is pleasing to see that there is no-one on the dam itself, which has been evacuated.  Note the seepage points.


Second, this image shows the spillway looking upstream towards the lake.  I was somewhat shocked when I saw this image:


The spillway is really very narrow indeed - only 7 to 8 metres deep at the base and 15 metres at the top, and about 14 m deep.  As such it seems too small to be able to handle even the current inflow rate (about 80 cubic metres per second), let alone the summer flood.  The channel is unlined, the walls are unprotected, it is not straight and it is too steep, all shown by this image inside the spillway:


So erosion and a release of a substantial volume of the lake water is I think very likely.  However, this may well not start immediately.  Indeed, at a rate of water level rise of 80 cm per day, which will reduce as outflow commences, it may take a couple of days for the spillway to carry the full inflow.  Thus, I would not be surprised if the spillway does not erode when water flow starts, but that it takes a few hours to a few days to initiate this process.  Once it begins, erosion will probably happen in three ways:
1. I would expect to see basal erosion, Once erosion starts along the whole of the base of the channel the process is likely to accelerate quite quickly.
2. I think we will certainly see erosion of the sidewalls, probably causing collapse of the banks, probably starting fairly soon after flow starts.  This can be very bad news indeed (temporary blockages can allow the water level to build up and then permit a rapid release) or it can be quite helpful in terms of widening the channel, increasing the capacity (see below).
3. We may see erosion on the downstream face as soon as water flow starts, which then could eat back into the dam, steepening the channel.  We could even see new slope failures on the downstream face, but the lower gradient here might help. 

That the materials on the downstream face are erodible is shown by the newly-formed channels below the seepage points.  Note also just how much seepage there is now - and remember that this is just one of several points:


It is also worth bearing in mind that this is about 2 cubic metres of flow per second; the current inflow is about 70 cubic metres per second, and the summer flow rate is probably  at least five times as great again.


The most likely scenario therefore is that water flow across the spillway is stable for a while, whilst eroding the sidewalls, then as the water volume increases the flow rate increases and basal erosion starts.  This will accelerate as more water is released, and the breach develops.  I still hope that this will happen reasonably slowly (i.e. over a few hours), but I believe that it could be rapid, especially in view of the inadequate spillway form.  Of course a very rapid failure could still be initiated by another landslide, or by sudden development of the seepage.

If you want information about what a collapse of a landslide barrier actually looks like, can I refer you to the Tangjiashan event, which I monitored two years ago.  This went from this (water flow through the spillway, which here was narrow but not steep, but was designed to initiate erosion):



To this (rapid failure of the landslide mass):


Peak flow occurred about 5 hours after erosion started.  In this case though the dam was broader and more resistance to erosion (the materials were more coarse-grained), which meant that the channel widened rather than deepening rapidly. Peak flow was about 9000 cubic metres per second.

I hope that we are equally as fortunate with Attabad.

Finally, I have stopped updating the monitoring site  for now as the monitoring data flow is too slow.  Updates will happen through this site.

Friday, 21 May 2010

Seepage at the Attabad landslide

Focus have today kindly provided a new set of measurements of the state of the Attabad landslide.  I will update the monitoring site shortly, but in the meantime here is the graph of the development of seepage.  The plot of seepage against time suggests that the trend in the rate of development of seepage changed on the last few days.  Note how the most recent point does not fit on the curve that one would extrapolate from the previous days data::


However, when the same data are plotted against lake depth this discontinuity in the trend disappears:


This might suggest that the seepage is being drive primarily by the water depth, and that the little disconnect in the current trend suggests that seepage is likely to increase substantially in the final stages before over-topping.  This is probably supported by the suggestion that sinkholes are opening up on the dam by the Pamir Times yesterday, although the image that they provided to illustrate this does not really seem to show sinkholes:


Thursday, 20 May 2010

Surprising landslides from the Amazonas earthquake in Peru

On Tuesday evening at 11:45 pm Peru suffered a Mw=6.0 earthquake 125 km (80 miles) NNW of Moyobamba in Amazonas.  This is the USGS location on a Google Earth image:


The USGS record suggests that this earthquake was very deep - 132 km - which means that such an event would not normally be associated with landslides.  However, the local paper La Republica is reporting that a large section of a road between Bagua and Tarapoto was severely damaged.  They have several images that appear to show very serious damage.  Some of that damage may well be due to liquefaction, but this image undoubtedly shows very large-scale landslides:


The Google Earth image below shows the location of the two towns and the earthquake epicentre.  I am unsure where on the road between the two this landslide event occurred:


This image is also intriguing.  I can think of several different explanations for how this came about.  I wonder what the true story is:


  That such large landslides and other damage should occur in an earthquake of this magnitude and so deep is very surprising.  I wonder what is going on here.