Short-term velocity measurements at Columbia Glacier, Alaska: August-September 1984

During the summers of 1984 and 1985, laser measurements were made of the distance from a reference location to markers on the surface of the lower reach of Columbia Glacier, Alaska. The data have numerous gaps, mostly because of inclement weather. The laser measurements were corrected for variations in atmospheric temperature and pressure and for the trajectory of the marker. The marker speed was calculated by using a combination of cubic splines and digital filters to arrive at a data set with 1-hour time intervals. The speed varies from 7 to 15 m/d and has three noteworthy components: (1) a low-frequency perturbation in speed with a time scale of days related to increased precipitation, (2) semidiurnal and diurnal variations related to sea tides, and (3) diurnal variations related to glacier surface melt. The low frequency and tidal period variations are separated by low-pass filtering and subtracting the filtered data from the original data. The variations from melt runoff and tides have the same frequency range and are examined using harmonic analysis and multiple regression with the various forcing functions. The high-frequency portion of the ice speed signal is dominated by tidal influences and to a lesser extent by meltwater influences. The low-frequency portion of the signal is dominated by the effects of precipitation. Kamb et al., 1985] and calving rates. As a result, an intensive field program in August and September 1984 collected data on ice calving, ice velocity, terminus position, meteorological variables, tide stage, and runoff from a marginal river (as a surrogate for subglacial discharge). This program is described in detail by Vaughn et al. [1985]. The purpose of this report is to analyze the ice speed data and relate its variations with variations in the various forcing functions. The primary focus is the ice speed derived from electronic distance meter (EDM) measurements (EDM, Figure 1); temperature, insolation, and precip- Recent glaciological research has indicated a need for the better understanding of the relation between ice flow and subglacial water flow [

Section: Data Setmentioning

confidence: 99%

Analysis of time series of glacier speed: Columbia Glacier, Alaska

Walters¹,

Dunlap²

1987

“…Displacements of specific points, over time intervals ranging from 10 min to 108 days have been measured on the lower glacier. Measurements as frequent as every 10 or 15 min were made using an automated laser EDM (electronic distance measurement) system and are limited to a small number of markers for less than 30 days in each year 1984-1986 [Vaughn et al, 1985]. The displacement of up to 30 points near the terminus was measured from oblique photographs taken daily with automated cameras.…”

Section: Velocity Of the Lower Glaciermentioning

confidence: 99%

“…In each measurement sequence the distance of the target (a reflector) along a trajectory at a given time was measured using an automated EDM. The measured distance had an error of about 1 cm [Vaughn et al, 1985]. For each of the time-distance data sets the velocity was determined using the derivative of a smoothing cubic spline [Reinsch, 1967].…”

Section: Velocity Of the Lower Glaciermentioning

confidence: 99%

Columbia Glacier, Alaska: Changes in velocity 1977–1986

Krimmel¹,

Vaughn²

1987

The Columbia Glacier, a grounded, iceberg-calving tidewater glacier near Valdez, Alaska, began to retreat about 1977. Drastic retreat occurred in 1984, and by early 1986, retreat amounted to 2 km. The glacier has thinned more than 100 m since 1974 at a point 4 km behind the 1974 terminus position. Between 1977 and 1985 the lower glacier ice velocity increased from 3-8 m/d to 10-15 m/d. Ice velocity in the region 0.5 km above the terminus was highest near the time the glacier was most receded (late fall), and lowest near the time of maximum length (early summer), for years 1977-1982. Velocity in the region 52-57 from the head of the glacier was highest in mid-spring, and lowest in early fall from 1977 to 1985. Through the years 1983-1985, the dates of maximum and minimum velocities within 0.5 km of the receding terminus tended toward the dates of the 52-57 km maximum and minimums. This occurred because as the terminus receded, it was no longer strongly influenced by the reverse slope of the terminal moraine shoal. Velocities near the terminus fluctuated by 2-3 m/d during summer and fall, when liquid water input was variable, and were relatively constant during winter. Hourly variations in ice velocities are controlled by liquid water input to the glacier hydraulic system and tide stage. Velocity increases near periods of high surface water input and decreases during periods of high tide as a result of hydrostatic back pressure.

“…During August 1984 the number of seismically recorded calving events varied from 5 to 50 per day (Figure 7). During this time, ice velocity near the glacier terminus [Vaughn et al, 1985;Walters and Dunlap, 1987] also varied by as much as 50%. The velocity variations seem closely associated with precipitation as recorded on a rain gauge located on Heather Island.…”

mentioning

confidence: 90%

Calving icebergs: A source of low‐frequency seismic signals from Columbia Glacier, Alaska

Qamar¹

1988

Low‐frequency icequakes produced by Columbia Glacier, Alaska, are caused by large icebergs which break off the glacier terminus into Prince William Sound. Occasionally, many ice seracs fall in rapid succession, producing complex, low‐frequency icequake signals with durations sometimes exceeding 10 min. An empirical relationship V = 4000 T between the volume V of an iceberg in cubic meters and the duration T of the seismic signal in seconds, can be used to estimate the calving rate of Columbia Glacier from seismological data alone. During 1984 and 1985, a period of rapid retreat of the glacier, the calculated number of icequakes per month showed seasonal fluctuations that corresponded to variations in calving rate determined by photogrammetry. It is likely that most previously reported low‐frequency icequakes in southern Alaska are produced by calving, tidewater glaciers.