If re-post then plz delete it....
Daily updates on Situation of Hunza artificial lake.
NDMA freeboard information for Attabad
In the absence of any other information on the state of play at Attabad, I have plotted up the daily updates on freeboard from NDMA. The reliability of these is somewhat unclear as there are often contradictions between the indicated water level rise and the daily difference between freeboard measurements. There are also sometimes errors - e.g. the data for 19th May suggests that freeboard increased rather than declined overnight, but I have corrected that. So the graph for the last 19 days or so looks like this (the last datapoint is for 19th May at 18:00 local time):
Over the last four days the water level has risen at an average of 1.1 m per day, which means that the remaining freeboard should be lost in 3 or 4 days from 19th May, even allowing for the apparent slight reduction in rate in the last couple of days. On this basis, overtopping might be expected tomorrow (22nd May) or on Saturday 23rd May, but happen earlier.
Of course all of this is dependent upon the NDMA values being correct, but for the last two weeks the Focus and NDMA freeboard figures have effectively been the same. This does also mean that some of the media reported values of lake rise due to the GLOF earlier in the week were wildly incorrect.
The final spillway at the Attabad landslide
The GLOF into the lake behind the Attabad landslide yesterday appears to have raised the water level by something over two metres, in addition to the increase of about a metre from the normal inflow (please see the data on my monitoring site). The upshot is that the freeboard is likely be 4 metres or less. Inflow has increased substantially as well to about 93 cubic metres per second. It has not been possible to measure seepage in the last 24 hours as the site is now too dangerous.
All work on the spillway has ceased and the machinery has been withdrawn. The final form of the spillway is shown in this Focus image:
It pains me to say this, but I simply cannot see how this will resist the peak flow in the Hunza River. However, it has at least served the purpose of reducing the total volume of water involved in any flood that should occur.
The boat service has also now ceased as it would clearly be too dangerous should a breach start.
Water flow past the blockage may start through one of three key processes:
1. Overtopping and water flow through the spillway. In Tangjiashan the initiation of erosion started some hours after initial flow; this may or may not be the case here. Erosion may start on the downstream face or at the crest of the spillway, or there may be slope failures on the downstream face. It is also possible that the sides of the channel may fail, creating a new blockage, stopping water flow for a while.
2. A seepage induced failure. This is less likely than 1., but remains a distinct possibility;
3. Overtopping initiated by a landslide into the lake.
None of these can be ruled out at this stage, but 1. is the most likely. If so, the size of the resultant flood will depend upon the rate of erosion, if any.
I hope above all hopes that erosion happens slowly. My thoughts are with the people of Hunza.
Even more seepage at Attabad
The rate of seepage at the Attabad landslide is now increasing at a rapid rate, and a new seepage point emerged in the last 24 hours. This graph shows the seepage rate since the start of monitoring:
The current rate of increase does pose the risk of a seepage-induced failure. This is not inevitable, but the risk is increasing with time. The authorities need to be prepared for the possibility that seepage could develop very rapidly indeed. The most recent Focus update provided an image of the two main seepage points:
The one below the spillway is the one that developed at the end of last week - yesterday this was discharging about 1 cubic metre per second. To the right is the main seepage point - yesterday this was producing 2.8 cubic metres per second. Note how much erosion these seepage points have generated in the channels they have cut. This does not bode well.
Attabad - the spillway
The Pamir Times has today published a photo of the final spillway at Attabad. All work has now apparently ceased as the overtopping approaches:
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.
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.
All of this means that a flood is now likely, hopefully of just a moderate size, but possibly larger. Once again I urge, as I have throughout this crisis, that the authorities assume that the flood will be large and thus ensure that everyone within 50 metres of the river between Attabad and Tarbela is protected.
Finally, I just cannot understand why the authorities are not getting expert advice to help them to deal with this. Help is being offered - why are they not accepting it?
Developments at Attabad
I have today revamped the monitoring website of the Attabad landslide in Hunza, Pakistan to make the display of information more straightforward as we enter the end game of this situation. In the last day or so there have been three critical developments:
1. Seepage is increasing rapidly
Today's seepage figure is 2.53 cubic metres per second - that is a substantial increase on yesterday, as the graph below shows:
2. 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.
3. 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,”
I also note that he stated that "international experts’ help is also being taken on the issue". I wonder who is providing this help, as they haven't spoken to me. I am delighted that they are getting some expert input though.
I'll post again on this tomorrow. The current probable window of overtopping is 22nd to 26th May, though this could easily change to an earlier date.
New satellite image of the Attabad landslide, Pakistan
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.
Meanwhile, the seepage rate continues to increase as the lake fills, and a new seepage point emerged yesterday. The graph of measured seepage against time shows the continuing increasing trend:
Whilst the seepage is not yet at worrying levels, the rate of internal erosion from this water flow could increase rapidly as the flow rate rises. Therefore, we need to continue to be mindful of the threat that this seepage potentially poses, especially in the last few days. We cannot rule out the possibility of a piping-induced failure.
The rate of increase of water level has slowed a little over the last few days, probably because the surface area of the lake has dramatically increased. Based on current rate of filling this has pushed the estimated date of water flow through the spillway back a little to 25th-28th May. This may well come forward again if the rate of inflow into the lake increases, which is what we would expect at this time of year. As of yesterday, the freeboard was just 12 metres, with the rate of rise of the water level being about 80 cm per day. Downward excavation of the spillway has now apparently ceased at a total depth of 14 metres (against the planned 30 m); the focus is now on widening the channel. As far as I am aware there is no armouring of the structure to prevent erosion.
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