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‘Extreme’ sedimentation event associated with Icelandic volcanic eruption

April 16, 2010

The ongoing eruption of the Eyjafjallajokull volcano in Iceland is making big news because of the effect of the ash on flights in northern Europe. My two main sources of information, the Eruptions blog and The Volcanism Blog, both do a great job of not only relaying up-to-date information but providing tons of links to various scientific organizations reacting to and studying this geologic event.

I wanted to highlight this video I came across showing  a jökulhlaup in process. A jökulhlaup is the Icelandic word for a glacial outburst flood that is commonly generated by subglacial melting during a volcanic eruption. I think the term is also used for similar magnitude flood events from glacial lakes when an ice (or debris) dam abruptly fails.

Absolutely amazing.

I also want to point out the train of waves from 0:46-0:57 seconds in the video. I’m not sure — don’t quote me on this — but those appear to be associated with supercritical flow (most are introduced to this concept discussing the formation of antidunes). Perhaps a train of hydraulic jumps or some kind of cyclic step process? A quick search came across this paper interpreting supercritical flow based on the sedimentary deposits from a jökulhlaup in 1918 in Iceland.

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8 Comments leave one →
  1. April 16, 2010 10:30 am

    I’m pretty sure you get standing waves at flows below supercritical, but still quite high sub-critical (0.6< Fr <1). The key to looking for supercritical flow in the jokulhlaup would be to look for places where you can see hydraulic jumps. Would undoubtedly see them in the first 20 seconds of video.

  2. April 16, 2010 11:14 am

    Anne, I thought that the occurrence of a train of waves like that (with quite regular amplitude and wavelength) was indicative of a train of hydraulic jumps and, thus, supercritical flow. There’s about 8 or 9 in a row in that video. This phenomenon can occur in purely subcritical flow? It’s been a while since I’ve thought about this stuff to be honest.

  3. Julien permalink
    April 18, 2010 4:47 pm

    Thanks for this video Brian, that I will immediately forward to some people working on jokulhlaups and turbidity current processes. Absolutely, totally amazing.
    I agree with your observations concerning the waves. If they were a hydraulic jump somewhere, it obviously took place at the begining of the video as Anne noticed – the absolutely beautyfull thickening of the flow associated to increased turbulence (at least visually) is just perfect on the pictures… ! It seems to occur a few hundreds metters upstream of the actual break-of-slope (on the firsts seconds of the video).
    But same as you, I still asking myself about the excat nature of the standing waves downstream…when the flow might be subcritical… unless there are plenty of sub-to-super critical transitions, as suggested in the Fildani’s experiments ? Should send this video to some of these guys, just to know…

  4. Julien permalink
    April 18, 2010 4:52 pm

    Just an idea but on the first seconds we can also observe a somewhat sharp limit between brown, turbulent waters (upstream) and cleaner, calm waters (downstream)… Kind of plunging process of dense water mass… ? (I lunch the word “hyperpycnal”). This would possibly explain the standing waves at a water density front (I’ve read some papers about it a few times ago, Parsons probably, but would need to check the Thierry’s papers as well).

  5. annejefferson permalink
    April 19, 2010 11:12 am

    What I consider the definitive word on those standing waves, from Gordon Grant:

    The standing waves represent what is best referred to as “trans-critical flow”, that is flow that oscillates around Froude No. = 1. We have real-time (though very small scale compared to what you’re seeing in the video) measurements from Grant (1997; available on the WPG website). Basically flow on the downstream portion of the wave is accelerating (Fr >1), while flow on the upstream portion is decelerating. The flow then oscillates between Fr > 1 and Fr < 1, maintaining overall flow at close to Fr ~ 1. Cross-sectionally averaged Froude Number tends to be slightly less, due to drag at the boundary. See paper for details.

    The paper to which he refers is Grant, G.E. 1997. Critical flow constrains flow hydraulics in mobile-bed streams: a new hypothesis. Water Resources Research. 33: 349-358. PDF available here: http://www.fsl.orst.edu/wpg/pubs/criticalflow.pdf

  6. April 19, 2010 12:26 pm

    cool, thanks for the link … another couple to consider:

    Winterwerp, J.C., Bakker, W.T., Mastbergen, D.R. and van Rossum, H.(1992) Hyperconcentrated sand-water mixture flows over erodible bed. J. Hydraul. Eng., 119, 1508–1525

    Parker, G. and Izumi, N. (2000) Purely erosional cyclic and solitary steps created by flow over a cohesive bed. J. Fluid Mech., 419, 203–238.

  7. Andrea Moscariello permalink
    April 26, 2010 1:54 pm

    Hi all,

    nice movie ! Thanks for posting this and making it available to a larger public.

    If you are interested in this process from a sedimentological point of view, look at this book:
    Martini I.P., Baker V.R. and Garzon G. (Eds ) “Flood and Megaflood Deposits: Recent and Ancient” IAS Special Publication n. 32.
    An esteemed colleague Andrew Russell has a article in that book where he describes a very similar event which occurred in similar circumstances in Iceland in the late 90s (if I remember well).

    Nature at its best

    Andrea

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