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Alaska Section, American Water Resources Association Richard Kemnitz, Alaska Section AWRA Northern-Region Director I would like to invite you to the January 2001 Brown-Bag presentation by Dennis Trabant, USGS, University of Alaska Fairbanks. Please note this meeting is the second Wednesday in January. We look forward to seeing you, please feel free to bring a colleague to the meeting. January 10, 2001 "Ice-dam Kinematics Before, During, and After an Outburst Flood" Dennis Trabant Abstract Hidden Creek Lake is a glacier-dammed lake in a tributary valley that is about 15 kilometers
above the terminus of Kennicott Glacier, Alaska. Surface displacements of Kennicott Glacier
ice between the lake and 1,200 meters from the calving face were measured before, during, and
after the July 2000 outburst of Hidden Creek Lake in an attempt to locate the effective ice
dam and evaluate the volume of water stored under the glacier prior to the outburst.
Three-dimensional displacements at 22 sites were measured about 125 times during the 24-day
period beginning 20 days before the lake level started to fall and ending the day after the
lake was empty. The ice surface rose and fell in response to the changing lake level, but the
ice-surface fluctuation lagged the water-surface fluctuation by tens of hours. The time lag
increased with distance from the lake; the lag varied from 3.8* hours at a distance of 430
meters to 39.9* hours at a distance of 1,300 meters. The maximum measured lowering (21.5* meters)
of the ice surface occurred at a site that was originally 430 meters from the lake. If the lowering
is a response to lost buoyant support, the inferred ice thickness at this site is about 185 meters,
which is a reasonable value. Rapid lowering of the ice surface at sites approximately 800 meters
from the lake may be explained by the loss of buoyant and longitudinal support. At these sites,
about two-thirds of the ice overburden was supported buoyantly, and the remaining one-third, about
60 meters in thickness, was borne by longitudinal stress transmitted by the floating ice closer to
the lake. Measured horizontal ice movement was generally toward the lake except at two sites that
were more than a kilometer from the lake. At these two sites, the movement was down the Kennicott
Glacier. On the ice dam, horizontal ice speeds increased tenfold near the lake and by a factor of
about five at sites approximately 800 meters from the lake during the outburst. Timing of the speed
increases lagged the lowering of both the lake and the ice surface, and the lag increased with
distance from the lake. Horizontal speeds increased from a few decimeters per day to several meters
per day, but the increases were short lived, and speeds returned to pproximately pre-outburst rates
within hours after the end of the outburst. |