31 May 2010
Attabad – flow on the spillway increasing further
Posted by Dave Petley
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.
Dave Petley is the Vice-Chancellor of the University of Hull in the United Kingdom. His blog provides commentary and analysis of landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings.
Hi Dave,Retrogressive erosion from the downstream face does seem to be a likely scenario. Have you any idea of the height difference between the high point of the spillway and the bottom of the waterfalls?A decision to open the KKH from Gilgit to Hunza tomorrow (as reported on Pamir Times) seems premature if it was originally closed because of concerns over a breach. Provision for emergency road closures over low points / unstable points of the road would be necessary, coupled with clear communication between Attabad and these points.
The latest news coming from there reports total outflow through spillway and seepage to be 800 cusecs, 400 cusecs each.
any new pictures from the site? how far is tarbela dam from the landslide site?
Although not an expert on channel flow, my expectation about what flow would be now is the opposite of yours: I would have expected it to be even higher, increasing more than it increased over the prior interval.Flow is proportional to the pressure head, which increases as the lake level rises, and also the conductance of the channel, which is also increasing due to the elevated water level (the x-sectional profile of the spillway at the choke point grows as the water level rises). In a circular pipe, the conductance is proportional to the radius to the 4th power, so even modest increases in the height of the water channel can have substantial effects.
I found some close up pictures of spillway. Click Here for pictures as of Sunday.
Thanks a lot Raheel… great find!
I agree with Divalent, the cross-sectional area of water flowing in a V-notch channel is proportional to the square of the depth of water and the velocity of water in a V-notch channel according to Manning's formula will increase with the depth of water to the power of two-thirds. So discharge should increase proportional to the power of 8/3 of the depth of water (Obviously this is approximate as the channel is not a perfect V-notch shape). Consequently the discharge should increase at an accelerating rate with time until the steady state is nearly reached. Obviously an increased rate of water flow in the channel brings with it an increased likelihood of erosion of the bed of the spillway and consequent downcutting.Erosion of the spillway channel banks should increase the flow for no increase in lake head above the low point.I am not sure how the discharge through the spillway is being measured, but without a proper gauging station in a stable channel geometry, assessment of flows will be highly subjective and liable to significant error.Seepage through the landslide is unlikely to increase significantly as there is little increase in driving head, unless seepage erosion is an issue, which itself would give cause for concern.
This may be a Long Walk HomeThis was expected that under the present plan the subject spillway/ barrier of Hunza River at Attabad will be eroded and major water out flow of the lake may start soon but unfortunately this is not happening as expected. Keeping in view the long length of blockage of the barrier and nature of the surrounding hills, that by attempting any mechanical blasting to reduce the barrier; it is feared that hills prone to triggering further land sliding will start refilling the blocked mouth.Reportedly, the fresh water pouring in the lake is much higher than the small drain at spill way point. The expected erosion of 4 to 5 seepage point is also not working; thus the water in the lake keep on rising; engulfing more and more of upstream houses and villages.But it is most important that mechanical measures be under taken to induce the drainage of the water to at least purge the increase of fresh water level pouring in from the melting glaciers.Blasting of the barrier may be out of the question as it may trigger the further landslides and result in adding in the blockage. As suggested earlier artificial siphoning through large hoses may be the only solution to remain in control of the situation. At the same time effort for a secure excavation of barrier as well as enhancing the seepage point to facilitate the main drain of lake may be carried out to achieve the required results.Regards
Landslides from Agatha, "one of the top ten deadliest Eastern Pacific storms on record:""So far, flooding and landslides have killed at least 83 people in Guatemala, 13 in neighboring El Salvador, and one in Honduras. Guatemala is also suffering from the Pacaya volcano in Guatemala, which began erupting four days ago. At least three people have been killed by the volcano, located about 25 miles south of the capital, Guatemala City. The volcano has destroyed 800 homes with lava and brought moderate ash falls to the capital."http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=1496
mr khokar.. which madrassa did you attend? I hope not the Binori masjid one !! large hoses ? why is it unfortunate that major erosion is not happening soon? though agree with you that blasting in mountains is not advisable ever.
Mr. Khokar,Which planet do you live in.For God's sake you again come up with a non workable solution: "artificial siphoning through large hoses". Large hose; do you have any idea of the length of the barrier: close to 2 KM,do you hv any idea about the height of th e barrier: aprox.90 meters ; the volume of water in lake would be aprox. 450 million cubic meters and who will procure and carry such large quantity of hose to the area.!Dr Petley in the earlier message of your gave a logical reply that the solution was not workable.Have you ever visited Hunza. Dr Petley made a site visit and he knows what he is talking about.So please let us get serious, over 20,000 people have been evacuated and are living in camps.
To achieve a spillage—How ToThe water out flow from the spill way is not being achieved as expected.In light of the present scenario that erosions of spill way is planned to achieve a good flow assisted by its erosion from the barrier. Is there not any possibility to create a iron curtain door dropped next to the edge of the barrier of 3 to 5 meter deep and with the help of mechanical excavators the V shape spill way be dug toward the iron curtain door and be opened when ready. We may lose the door but in a way will sure achieve a good spillage. Try it and we may save the lives and property of thousands.
The spillway has a sufficient size to eventually discharge the quantity of water entering the lake, it will just take some more days and probably a couple more metres of lake rise to achieve this steady state.Ideally, what needed to be done was to excavate a channel from the crest down the downstream face of the landslide and line it with large boulders. The size of the boulders would need to be such that they would not be eroded/dislodged under the maximum spillway discharge. Unfortunately this would logically be the peak river flow which is I assume several hundred cubic metres/second. This would also mean that upstream of the landslide would remain flooded for the foreseeable future.Creating such a lined channel would involve huge resources and take time, probably more time than was available between the landslide and now. Engineering a controlled erosion of a river channel through the landslide does not seem feasible. Downcutting is dependent upon both the size of particles in the channel bed and the velocity of the water. As soon as downcutting starts, the discharge from the lake is increased, raising the flow rate and logically increasing the rate of downcutting. This would seem to be a self-perpetuating and accelerating phenomenon unless the channel encounters significantly coarser material (such as large rock blocks) or the channel widens faster than it can downcut. There are too many variables to engineer this process.
Dear anonymous,I respect and admire Dr Petly and his kind contribution to the core of my heart. He is doing a great job and we all are thankful to him.I understand that to carry out siphoning over such a long area may not be feasible. But don’t you think that with the passage of time the 2 km long barrier may be drying up fast and getting more strong and compact in nature.It may look another wild and way out, weird suggestion; but what about creating some pits along the initial length of the barrier up till a distance of a good steep and use the hose from one pit to another. At least it will keep the barrier wet; and may help in triggering a flow and erosion. You know my people are getting much up set there as they are losing their houses and orchard fast. A heritage is sinking. Sorry; If I sound too outlandish; please ignore. Regards
One method to obtain a reasonably accurate flow rate would be a monitoring station a safe distance downstream of the dam with the subtraction of any small tributaries between.
my dear mr anonymous, there is no safe distance downstream….