HR: 0800h
AN: C41B-0479    [Abstracts]
TI: Estimating the Basal Sampling Area of a Borehole During the Summer-Winter Transition
AU: * Tschetter, T J
EM: tschettj@uwyo.edu
AF: Dept. of Geology and Geophysics, Univ. of Wyoming, Dept. 3006; 1000 University Ave., Laramie, WY 82071, United States
AU: Humphrey, N F
EM: neil@uwyo.edu
AF: Dept. of Geology and Geophysics, Univ. of Wyoming, Dept. 3006; 1000 University Ave., Laramie, WY 82071, United States
AU: Harper, J T
EM: joel@mso.umt.edu
AF: Dept. of Geosciences, Univ. of Montana, 32 Campus Dr. #1296, Missoula, MT 59812, United States
AU: Pfeffer, W T
EM: pfeffer@tintin.colorado.edu
AF: Institute of Arctic and Alpine Research, Univ. of Colorado, Campus Box 450, Boulder, CO 80309, United States
AB: We present a simple numerical model that generates the water level changes observed during the summer- winter transition of a temperate valley glacier. The model allows us to approximate the basal area sampled by a borehole during this transition. For a period spanning three years, borehole water levels were recorded continuously in 43 different boreholes drilled to the bed of the Bench Glacier, Alaska. Summer melt season water levels generally consist of large diurnal variations; winter water levels show little to no diurnal variation and are generally quite high (70-100% of ice overburden pressure). An observed style of transition from summer to winter borehole water level behavior is a repeating series of asymptotic water level rises that are abruptly terminated by rapid water level decreases. We first present a conceptual model to generate this behavior. Once a borehole becomes disconnected from the 'fast' hydrologic system, the water level rises due to ice deformation closure of the borehole and the basal zone connected to the borehole (now referred to as the borehole system); the water in the borehole system is displaced up the borehole. This rise in water level reduces the effective pressure in the borehole system, and consequently reduces the rate of deformation. The abrupt water level fall results from locally catastrophic releases of water from the borehole. Due to similar water levels at the initiation of each discharge event, and the similar water level at the termination of each discharge event, we believe the water from the basal system repeatedly escapes through the same orifice. Rates of discharge up to 10^-3 m³ s^-1 are observed, and the total volume discharged is as great as 0.5 m³. Inflow to the system is negligible in the late season. Our numerical model uses the Glen flow law and known parameters to generate the theoretical curves of rising water level for varying sizes of borehole systems due to ice closure. Curve fitting the observed water level rises allows for approximation the volume of water in the borehole system. This volume can then be used to approximate the basal area sampled by the borehole.
DE: 0720 Glaciers
DE: 0776 Glaciology (1621, 1827, 1863)
SC: Cryosphere [C]
MN: 2007 Fall Meeting