Elucidating changes in the degree of tracer dispersion in a subglacial channel

Publisher： International Glaciological Society

ISSN： 1727-5644

Source： Annals of Glaciology, Vol.37, Iss.1, 2003-06, pp. : 275-280

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Abstract

Tracer injections into a subglacial channel at Unteraargletscher, Switzerland, were repeated at intervals of about 2 hours over two diurnal discharge cycles in August and September 2000. Records of dye concentration reveal a pronounced hysteresis in the velocity-dispersion relationship, thereby indicating alterations in the drainage system. Theoretical considerations for Röthlisberger channels suggest an evolution of the conduit cross-section in response to a diurnally varying discharge. We studied the relation between conduit cross-section and tracer dispersion with numerical tracer experiments. The velocity field for steady flow through a given conduit geometry is calculated using a commercial flow solver. Tracer transport is represented by a scalar volume which is advected by the velocity field. Experiments were conducted for several scenarios by varying flow velocity, conduit geometry and conduit roughness. Results show only a weak dependence of dispersion on conduit size. In contrast, changes in roughness of the conduit walls reveal a strong effect on tracer dispersion. Therefore, to explain the observed hysteresis in the velocity-dispersion relationship, we suggest that the evolution of a subglacial flow path might involve changes in roughness.