Monitoring the Hunza Landslide lake at Attabad
Report Compiled by A Khokar May 24, 2010
Several people have asked for an update on the situation at Attabad, and a forecast of what will happen, so here it 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.
This picture shows the spillway from the lake side
Second, this image shows the spillway looking upstream towards the lake. I was somewhat shocked when I saw this image:
The Expected Erosion
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:
New Channel has appeared Below Seepage point
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 Land slide on the Jian River, China event, which took place some two years ago owing to the 2008 Sichuan earth quake. The water flow through the spillway, which was narrow but not steep, but was designed to initiate erosion.
( Tangjiashan example pic)
To this (rapid failure of the landslide mass):
Attabad lake may resemble the flow of Tagiiahan
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.
Source: Dave Petley, who is the Wilson Professor in the Department of Geography at Durham University in England.
1. Attabad – a video of a new landslide at the dam site
There was a big new rockfall at the Attabad landslide site today, caught on camera and posted on Youtube. You should be able to view it below:
The implications of this include:
1. There is a chance that a failure like this could block the spillway;
2. There is a chance that a landslide could send a wave over the top of the dam;
3. If the slides are entering the lake then the water level will increase more quickly;
4. These slides of course represent a real danger to anyone in the area.
2.Land Slide Dam Hunza–Graph Sheets Data, Photo Slide of Attabad
3. Construction of spillway
4. The likelihood of a flood from the Attabad landslide
The water level at Attabad is now within ten metres of the base of the spillway at the saddle, and continues to rise at about 90 to 100 cm per day. Thus, water flow through the spillway is now about ten days away, or possibly less.
So what will happen when water flow starts? Regular readers will know that I have been continually pressing the authorities to assume that a large flood will occur, even if this was an outside possibility. My recommendations were, and still are, to evacuate everyone downstream of the dam located within 60 metres of the river level. Past experience suggests that the flood could be large all the way to Tarbela – i.e. that standard attenuation models don’t apply here.
Throughout this crisis I have maintained that a catastrophic flood was possible but not probable. This was very much based upon the plans for the spillway, which we were assured would be 30 metres deep, up to 40 m wide, and with a low longitudinal gradient. We were even assured that the base would be armoured to prevent erosion.
The reality is rather different. A couple of days ago the Pamir Times published this image of the spillway (the annotation is from them – I would like to formally thank them for highlighting the monitoring site I run):
Those building the spillway have admitted that it is just 5 metres wide and 14 metres deep. None of us should be under any illusions – this is unlikely to be large enough to cope with the flows that the river experiences in the summer floods. Furthermore, even though the flow from seepage that has developed below the spillway over the last few days is a fraction of the current inflow (seepage = 510 litres per second; inflow = 77,750 litres per second), the water flowing from the seepage point has been able to erode the dam materials. This does not bode well in terms of the landslide dam resisting erosion.
5. Development of Seepage Points
The increase in seepage rate remains a source of concern too, both in terms of the potential for an earlier water release and the likelihood of internal weakening of the dam structure.
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:
landslide dam, pakistan
6- A new seepage point has developed
Part of the reason for the increase in seepage is the development of a new seepage point below the spillway. This is not unexpected, though the rate of seepage is increasing quite quickly at this point. The location is shown in this image, provided by Focus:
Note two things here. First, the seepage is probably allowing water to flow beneath the spillway, which will be reducing the resistance to erosion. Second, and more importantly, even though the water flow here is just 0.23 cubic metres per second, a substantial amount of scour and erosion has already been initiated. Readers will not need me to describe the implications of this in terms of the likely performance of the spillway when water flow commences.
7. The authorities are now admitting that the dam will probably breach
For the first time, government agencies are admitting that the dam is likely to erode away. For example, Onlinenews is reporting that Lt General Shahid Niaz stated today that the water will be flushed out within 2 to 3 days. This is a good development. Indeed, he is also reported to have said “”Such types of lakes have to ultimately breach and there is no chance for its preservation as a dam or a permanent water reservoir,”
Note the two large, very recent tarmac repairs extending right across the road. The nearer of the two is very suspicious given its shape. There are also other hairline cracks in the road – right in the foreground for example. This one may have a small amount of displacement across it (the line is slightly offset) although this may well be just an artifact of the image. However, compare the two repairs with the Montreal Gazette aerial view of the slide – in this case looking from the north towards the south (i.e. in the opposite direction to the Google Earth image:
The larger of the two repairs is clearly visible in the image, which means that the lateral margin (edge) of the landslide is almost exactly where the other repair is located. It may well be that slide was creeping prior to final collapse, although this is clearly no more than speculation at this time.
8.NASA have released a new ASTER image of the lake that continues to build behind the landslide at Attabad in Hunza, N. Pakistan. Included in this image is the outline of the lake as it was in their image in March, showing the increase in surface area:
A couple of things to note on this image:
1. The red areas indicate vegetation. Given that this area is very close to being a high altitude desert, this indicates the location of cultivated areas. The image clearly illustrates the way in which cropland is now being inundated;
2. The cultivated area on the southern side of the image is Shiskat, which is built on a fan. The image dramatically shows how the recent growth of the extent of the lake has left this area completely isolated. If the lake were to last through the summer then this area is likely to face serious challenges next winter;
3. The danger posed by a further landslide into the lake creating a wave that could rapidly overtop the dam is dramatically illustrated here. There is no shortage of potentially unstable slopes. This is a source of very real worry to me now.
Source: Monitoring the Hunza Landslide lake at Attabad
Hunza Disaster, Progress at spillway
Lake formation in G0jal Upper Hunza
25th May update on Attabad
Latest data received from FOCUS and National Disaster Management Authority of Pakistan NDMA is given below:
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.
3. 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. They have done so in extraordinarily difficult conditions from the FOCUS monitoring point in which they have been living for the last two months.
As on May 25th
FOCUS monitoring point looking down onto the dam site. Image taken yesterday by Focus.