VELOCITY-DISCHARGE RELATIONSHIPS DERIVED FROM DYE TRACER EXPERIMENTS IN GLACIAL MELTWATERS - IMPLICATIONS FOR SUBGLACIAL FLOW CONDITIONS

Repeated dye tracer tests were undertaken from individual moulins at H aut Glacier d'Arolla, Switzerland, over a number of diurnal discharge cycles during the summers of 1989-1991. It was hoped to use the concep ts of at-a-station hydraulic geometry to infer flow conditions in subg lacial channels from the form of the velocity-discharge relationships derived from these tests. The results, however, displayed both clockwi se and anticlockwise velocity-discharge hysteresis, in addition to the simple power function relationship assumed in the hydraulic geometry approach. Clockwise hysteresis seems to indicate that a moulin drains into a small tributary channel rather than directly into an arterial c hannel, and that discharges in the two channels vary out of phase with each other. Anticlockwise hysteresis is accompanied by strong diurnal variations in the value of dispersivity derived from the dye breakthr ough curve, and is best explained by hydraulic damming of moulins or s ub/englacial passageways. Despite the complex velocity-discharge relat ionships observed, some indication of subglacial flow conditions may b e obtained if tributary channels comprise only a small fraction of the drainage path and power function velocity-discharge relationships are derived from dye injections conducted during periods when the supragl acial discharge entering the moulin and the bulk discharge vary in pha se. Analyses based on this premise suggest that both open and closed c hannel flow occur beneath Haut Glacier d'Arolla, and that flow conditi ons are highly variable at and between sites.