anecdotal direct observation of supgraglacial lake drainage events

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anecdotal direct observation of supgraglacial lake drainage events

Josh Carmichael-2

Colleagues,

This year's AGU produced some first-hand direct observations of lake drainage events. The community of Western Greenland workers has grown so that I am unaware of who has field experience in supraglacial lakes projects.

 

I am interested in descriptions of direct, first-hand observations of supraglacial lake drainage events. I must emphasize that I am requesting visual or other sensory observations that are difficult to quantify otherwise. My objective is to assemble a consistent and objective-as-possible documentation of these events. I will then provide that information to the cryolist after I have collected and organized it, for your usage.

 

These questions afford observers convenience in responding to prescribed questions:

 

I have questions with Example Answers to illustrate what I am hoping to acquire. You can simply delete my invented answers and fill in your account.

 

Geographical data:

Date: 06/13/2008

Time: 2:00 pm?  Early afternoon.

Location: 68.402N, 49.756W

 

1.    Apparent initiation of drainage

While retrieving a pressure data logger, I heard loud a rumble, and surface of ice shook violently

 

2.    Distance from lake shore during initially observed event:

100 meters during apparent event initiation for about 5 seconds, then 500 meters during remaining drainage

 

3.    Did you observe apparent recession of water at lake shore

Uncertain. There was an apparent drop in water level (less than a meter), and then I ran away from the shore. After I felt safe at a distance of 500 meters, I could easily see wet, reflective ice exposed, indicative of water recession.

 

4.    Duration of observation:

About 4 hours of audible activity, 1 hour of visual activity total

 

5.    Coincident visual Noises:

I observed the motion of floating ice ‘swirling’ around at 500 meters distance, like rubber ducks in a draining bathtub. I also observed large ice blocks rotate and flip toward the end of drainage. Water in the near ice/slush surface seemed to drain too.

 

6.    Coincident audible noises:

I observed an initial loud rumble, followed by successive cracking noises, like timbers or wood breaking. At 500 meters, I could hear additional cracking, and a low rumble similar to that from a waterfall. The intensity of the rumble was loudest after most water was visually gone. I could also hear an apparent “sucking” noise from the entire ice surface around me, as standing water in the surface disappeared.

 

7.    Coincident shaking of ice surface:

The initiation event had a coincident “jolt” that caused me to lose my balance. As I ran, I could feel additional jolts that felt analogous to walking across cracking pond or lake ice. I perceived a tilt in the ice surface as well. The most violent apparent shaking occurred half through the visual part of the drainage as a continuous tremor, similar to the feeling of a train going by.

 

8.    Post-drainage observations near “ground zero”

We found 2 moulins, and fracture ice everywhere. Block sizes were as large as trailers (20m x 5m x 5m). We did not approach the moulins too closely (about 20 meters) for safety. The primary conduit was about 20 meters in diameter. Another conduit was about 10 meters in diameter. From a helicopter, we could see a large radial fracture network around the primary conduit rim that joined the secondary conduit.

 

9.    Additional comments:

I was most surprised that the water dropped around my feet at 500 meters from the lake surface. It seemed as though the water table dropped in the ice subsurface. I was also shocked at how loud the initial boom from the lake was, it sounded like a nearby rifle shot—I would describe it as disorienting. During our post drainage hike the only water signs were in small pothole like puddles. We have a short video that I have attached, with some high-resolution jpegs.


--
Josh
--------------------------------
Joshua D Carmichael
Glaciology & Seismology
University of Washington/
Applied Physics Lab
Seattle WA
http://earthweb.ess.washington.edu/~joshuadc


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