Unusual flood events from an Alpine glacier: observations and deductions on generating mechanisms

Abstract

Observations are presented on a particularly unusual sequence of flood events witnessed at Bas Glacier d'Arolia, Switzerland, in July 1987. The sequence was triggered by heavy rain storms, and involved a supraglacial "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 geomorphological data are used to assess the significance of the floods and to deduce 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 outburst is explained as an extreme "spring even t". Hydraulic jacking is implicated, but not proven, during both events. Whilst the flood sequence was triggered by an intense storm, englacially stored waters are believed to have contributed most of the flood waters.

Relationships between flow hydraulics, sediment supply, bedload transport and channel stability in the proglacial virkisa river, iceland

“…This lack of a strong diurnal discharge cycle is not uncommon in environments where daylight persists for 24 hours and where the weather is often overcast. In the absence of such a diurnal discharge cycle peak discharge levels are commonly associated with intense rainfall events (Busskamp and Hasholt 1996) or less frequently with glacial outburst floods (Warburton and Fenn 1994).…”

JÖkulhlaups: A reassessment of floodwater flow through glaciers

“…Although I have considered physical processes during drainage of a subglacial lake, meltwater reservoirs can also form (1) from multiple, water‐filled subglacial cavities [ Kamb et al , 1985; Warburton and Fenn , 1994; Walder and Driedger , 1995]; (2) where glacier ice overlies a geothermal field or hydrothermal system [ Björnsson , 1988; Pierson , 1989]; (3) on the glacier surface, when meltwater accumulation in depressions exceeds ice permeability [ Björnsson , 1976; Boon and Sharp , 2003]; and (4) in ice‐marginal locations where suitable glaciohydraulic and topographic conditions allow meltwater to accumulate against the flank of a glacier [ Björnsson , 1976; Walder and Costa , 1996]. Large ice‐marginal lakes such as Grænalón (∼5 × 10 8 m 3 , Skeiðarárjökull) and Hidden Creek Lake (∼3 × 10 7 m 3 , Kennicott Glacier, Alaska, United States) exist because P i at the base of the ice dam is high enough to force some intraglacial meltwater to flow toward the lake.…”

“…Supraglacial outbursts created during jökulhlaups emanate from a variety of outlet morphologies. Documented outlet types include (1) moulins [ Warburton and Fenn , 1994]; (2) surface crevasses [ Goodsell et al , 2003], ice folia, tension veins, and shear planes [ Goodwin , 1988]; (3) exposed englacial conduits [ Thouret , 1990]; and (4) discrete, flood‐induced fractures [ Liestøl , 1977]. Artesian outpourings 1–2 m higher than the ice surface (i.e., P w ≈ 1.1 P i ) are common for supraglacial outlets that evacuate subglacial floodwater [e.g., Goodwin , 1988].…”

Rapid sediment entrainment and englacial deposition during jökulhlaups

“…A sudden influx of water to the glacier bed can generate basal hydraulic pressures greater than ice overburden pressure. This process is commonly observed during surging (Iken and others, 1983), spring events (Skidmore and Sharp, 1999), intense rainfall (Barrett and Collins, 1997) and jo« kulhlaups (Warburton and Fenn, 1994). If the rate of increase in hydraulic pressure is exponential, it is possible to generate massive short-term deficits between overburden and hydraulic pressure (Bindschadler,1983), creating ideal conditions for fracturing by hydraulic action.…”

“…To achieve this threshold, water must be supplied to intraglacial drainage at a rate faster than it can escape (Ro« thlisberger and Lang, 1987). Given a rapid increase in basal water pressure, the threshold for hydraulic fracturing can easily be achieved (see Warburton and Fenn, 1994). Therefore, the key determinant for hydraulic fracturing is the rate of increase in intraglacial water pressure, and not the overall magnitude of flooding.…”