Article révisé par les pairs
Résumé : A well‐developed subglacial drainage system consisting of large cavities developed in the lee of bedrock steps connected together by a network of Nye channels is exposed on an area of recently deglaciated limestone bedrock in front of Glacier de Tsanfleuron, Switzerland. This system covers some 51 per cent of the bedrock surface area, and is believed to have transported the bulk of supraglacially‐derived meltwaters through the glacier. Using the cavity hydraulics model of Kamb (1987), it is shown that the geometry of the system rendered it stable against collapse by meltback of channel roofs into a tunnel‐dominated system. For likely combinations of glacier geometry and meltwater discharge, the steady state water pressure in this system would have been only a small fraction of that required for flotation, and for discharges of less than about 0·5–5 m3 s−1 water would have flowed at atmospheric pressure. The system appears to have adjusted to varying discharges by a combination of varying water pressure and changing the total cross‐sectional area of flow by altering the number of active channels connecting cavities. Glacier sliding velocity would have been independent of meltwater discharge for discharges at which water flowed at atmospheric pressure, but would have risen with increasing discharge for higher flows. Velocities on the order of 0·1 m d−1 are predicted for a realistic range of discharges and effective pressures, and these are believed to be plausible. Episodes of enhanced sliding in glaciers with similar drainage systems could be triggered by a rise in meltwater discharge across the threshold between flows at atmospheric pressure and flow under pressure from the glacier. Copyright © 1989 John Wiley & Sons, Ltd