Water Flow at the Base of a Surging Glacier

Abstract

Water tracing experiments were successfully conducted over a distance of ten kilometers along the base of Variegated Glacier for the purpose of characterizing the water drainage system of the glacier in the surging as compared to the non-surging state. Three tracing experiments were conducted, and fluorescent dyes, Rhodamine WT and Tinopal AMS, were injected into boreholes at separate locations. The two Rhodamine WT experiments were conducted over a 10 km distance, both during the most rapid surging motion of the glacier, and after its cessation. In each experiment, the terminus streams were monitored for stream discharge, sediment content and tracer concentration. Rhodamine WT tracer was significantly adsorbed on the suspended sediment, particularly during the surge. The adsorption behavior followed the Langmuir model, and calculated distribution coefficients of Kd = 100 to 1000 ml/g were measured for during the glacier surge. The Kd values measured after the surge were lower than during the surge by a factor of 10 to 1000. The much higher Kd values in the surging as compared to non-surging glacier states can be best explained by a factor of 10 to 1000 decrease in the modal and/or mean grain-size of the suspended sediment. The abundance of fine-grained sediment during the surge is probably due to increased grinding of rock material at the glacier bed. Theoretical models of tracer dispersion in a single tunnel, were compared to models of dispersion in linked-cavity systems to infer the details of water flow at the glacier bed. The broad, roughly symmetrical, dye-return curve measured during the glacier surge conforms to diffusive dispersion theory, and differs sharply from the highly asymmetrical dispersion curve measured after the surge. Results indicate the dispersion behavior, and calculated Manning roughness, of the post-surge Variegated Glacier is similar to those of glaciers that do not surge. The drainage system of the Variegated Glacier in the surging state is consistent with a model of tracer dispersion in an interconnecting network of conduits and cavities, and is strikingly different from the tunnel system indicated for the non-surging state.