MULTI-YEAR FIELD OBSERVATIONS AND HEAT TRANSFER EFFECTS ON TWO SUPRAGLACIAL STREAMS, JUNEAU ICEFIELD, ALASKA
The two supraglacial streams of this work are located at the confluence of the Gilkey and Vaughan Lewis Glaciers of the Juneau Icefield. Both supraglacial streams empty into a “sinkhole” or moulin. Deep canyons are cut with nearly vertical side walls as the stream reaches the moulin. The canyon side walls are usually not smooth, and have flutings or grooves which run parallel to the stream bed. As thermal energy is required to melt the stream bed and side walls, it is essential to investigate the fluid mechanics and heat transfer processes that create the canyon.
METHODS
Reference stakes were placed in the ice to monitor the water level of a small lake or reference pool. Readings of the water levels and the air temperatures were usually taken at one-half hour intervals around the clock for multiple consecutive days. Cross sectional profiles of the stream bed and detailed measurements of the grooves in the canyon wall were taken with the aid of a vertical plumb-bob and a tape measure. Development of the grooves (with time) was directly observed with respect to a reference piton driven into the canyon wall. These data established the concept of an “equilibrium channel width” for a steady stream flow.
SUMMARY RESULTS
In a stream section of uniform flow, the velocity, temperature and cross section of the stream channel remain unchanged with time and the net rate of heat transfer to the stream water surface is utilized in downcutting the stream channel. The components of the heat transfer at the stream surface are respectively due to: heat convection, short wave radiation to the surface, long wave radiation from the stream and heat losses due to vaporization at the stream surface. Only short wave radiation, convection and long wave back radiation are significant. The ratio of the viscous dissipation to the heat transfer required by the rate of downcutting can be related to total stream flow, stream slope and rate of stream downcutting. For most stream sections viscous dissipation is negligible. The only exception is near a moulin where the stream slope may increase to larger (≥ 1.0) values. Minimal water superheat in the range of 32.005° to 32.01°F is required for the downcutting observed between the pool and moulin. Cross correlation calculations show a clear 5 hour time lag between peak diurnal air temperatures and stream flow (pool level).