UNUSUAL FLOOD EVENTS FROM AN ALPINE GLACIER - OBSERVATIONS AND DEDUCTIONS ON GENERATING MECHANISMS

Observations are presented on a particularly unusual sequence of flood events witnessed at Bas Glacier d'Arolla, Switzerland, in July 1987. The sequence was triggered by heavy rain storms, and involved a suprag lacial ''overflow event'' (water cascading from moulins over the snout of the glacier) succeeded, following a series of ''mini-floods'', by a subglacial ''outburst event''. Available hydrological and geomorphol ogical data are used to assess the significance of the floods and to d educe likely explanations for each phase of the flood-event sequence. Bottom-up surcharging of a poorly developed subglacial drainage system is the preferred explanation for the overflow event. The subglacial o utburst is explained as an extreme ''spring event''. Hydraulic jacking is implicated, but not proven, during both events. Whilst the flood s equence was triggered by an intense storm, englacially stored waters a re believed to have contributed most of the flood waters.