Discharges of Turbid Water during Mini-Surges of Variegated Glacier, Alaska, U.S.A.

Abstract

Discharges of water, sediment, and dissolved impurities from Variegated Glacier, Alaska, were monitored in the early summers of 1980 and 1981 during the occurrence of mini-surges. Seasonal trends, weather-related events, and diurnal variations similar to behavior of other temperate glacier streams were found. The principal effect in the stream associated with mini-surge occurrence was a brief discharge of extremely turbid water. The turbidity is assumed to be introduced into the basal hydraulic system by initiation of the fast motion of a mini-surge at a time and location on the upper glacier known from other measurements. The mean water velocity in the hydraulic system over the intervening distance is thereby determined (0.3 ms−1). The mean water velocity, together with the water discharge (≈16 m3 s−1 at the terminus), places constraints on the distribution of water velocity u and total cross-sectional area AT of the flow paths along the glacier base. This leads to the conclusion that within the zone of mini-surge occurrence in its unperturbed state: u is about 0.1 ms−1 or possibly less; AT is about 102 m 2 or possibly more, and it must be divided into a very large number of small passageways, be blocked by constrictions, or both. The total water cross-section corresponds to a layer 0.1–0.2 m thick when spread uniformly over the glacier width. The water velocity is close to or less than the propagation velocity of the mini-surges. Between the zone affected by mini-surges and the stream, a dynamically less active lower section of the glacier is probably underlain by a small number of conduits, in which the water velocity may be very high (≥ 2 m s−1). Water discharge following the mini-surges puts an upper limit on water-storage changes associated with the anomalous ice motion.

Waves of Accelerated Motion in a Glacier Approaching Surge: the Mini-Surges of Variegated Glacier, Alaska, U.S.A.

“…The foregoing observations, together with the observations of Raymond and Malone (1986) and Humphrey and others (1986), serve to define a repeating, reproducible glacier-flow phenomenon that has significance both as a flow phenomenon in its own right and in relation to the mechanisms of glacier surging.…”

“…The strong turbidity peak seen in the outflow stream in connection with each mini-surge (Humphrey and others, 1986) can be explained as the consequence of enhanced basal cavitation giving basal water temporary access to rock debris over an increased area of the glacier bed. Cavitation that either increases the size of existing water conduits or creates new conduits through which water can flow fast enough to entrain suspendable sediment (glacial flour) will contribute to increased turbidity in water moving through the basal water system toward the outflow stream portal.…”

Waves of Accelerated Motion in a Glacier Approaching Surge: the Mini-Surges of Variegated Glacier, Alaska, U.S.A.

“…Superimposed on the summer-time "normal" are high peaks of daily motion in the range 80-1 40 cm/d; these are the mini-surges, and they are accompanied by the several characteristics described below, and by Raymond and Malone (1986) and Humphrey and others (1986). Records of seismicity and of daily motion from automatic camera photographs indicate that outside the time windows of Figures 2 and 3 there was one additional mini-surge in 1980, on 16 August, and none in 1979.…”

“…The high outflow turbidity generated by each mini surge (Humphrey and others, 1986) has a notable counterpart in the abnormally high turbidity of the outflow stream water during the main surge (Kamb altd others, 1985, p. 476). In both phenomena it is attributed to enlarged access, via cavitation, of the basal water system to glacial flour at the bed (section 10.2; Kamb and others, 1985, p. 478).…”

“…The present paper deals with water-pressure measurements in relation to short-term measurements of glacier motion. A second paper (Raymond and Malone, 1986) deals with strain -meter and seismicity measurements, and a third paper (Humphrey and others, 1986) deals with observations of water flow and turbidity in the terminal outlet stream. A fourth paper (in preparation by Kamb and others) will interpret the observations in terms of a theoretical model of mini-surge propagation, and a fifth paper will concentrate on observations in 1981 related to initiation of the mini surges.…”