HYDRAULIC PROCESSES IN SUBGLACIAL CONDUITS

The hydraulics of subglacial conduits are unusual compared to most natural hydraulic situations because the flow conditions significantly influence conduit geometry which in turn influence flow conditions. The feedback between flow conditions and conduit geometry complicates our understanding of past conditions/geometry inferred from geologic studies of esker deposits. For transient conditions (hours) of rapidly rising water discharge and increasing pressure gradients, flow in an ice-walled conduit replicates classic pipe flow. Under these conditions, sediment and larger cobbles mobilize and are transported downstream. For durations longer than a few hours, melting of the ice walls becomes important and increases the conduit cross-sectional area and reduces the water pressure and flow velocity. Consequently, conditions for sediment mobilization and deposition change although the discharge may be constant or still increasing. The enlarged conduit relaxes over a period of days to weeks, depending on glacier thickness, and shrinks through ice creep until the conduit reaches a radius in equilibrium with 'normal' flow conditions. During this period of relaxation, sediment deposition probably occurs. During all flow conditions, except for extremely high pressures, ice creep transports subglacial sediment of all sizes to the conduit margins. In addition to melt enlargement of the ice-walls, under some conditions, the sediment floor may erode forming a subglacial 'canal'. The persistence of a 'canal' depends in part on the competence of the canal walls and the erosional/depositional conditions in the 'canal'. This presentation will review the hydraulics of water flow through subglacial conduits with respect to sediment transport and the formation of eskers.