Water and sediment outbursts from advanced Franz Josef Glacier, New Zealand

The Franz Josef Glacier, Westland, New Zealand, has a history of catastrophic sediment-laden outburst floods associated with extreme rainfall events when the glacier toe is advanced over its own sediments. Consideration of these events and inspection of recent sediment deposits suggest that there are three distinct modes of outburst. The first is associated with fans fed by overflow along the glacier margin. As the glacier has advanced across its own forefield gravels, it is inferred that the primary drainage conduit has developed a reach of negative slope. In high flows massive boulders call block the conduit, trapping lesser clasts. The resulting backup of water causes overflows through marginal moulins, producing the fail type of deposit. The second type of outburst deposits massive imbricated boulders at a greater or lesser distance from the glacier portal. In this case, pressure buildup drives the blockage out of the portal where the boulders deposit. Smaller materials are generally carried away. The third type consists of very shallow flows, and produces massive gravel deposits of uncertain provenance. Ill this condition, the excess pressure in the conduit results ill slight uplift of the glacier and widespread discharge of water and sediment below the glacier snouts gravels and smaller sediments are laid clown in a massive deposit across the forefield. The massive, boulder-veneered deposit from the December 1995 outburst is interpreted ill the light of the above mechanisms as a hyperconcentrated flow deposit from hydraulic jacking, overlain by boulders emplaced by a subsequent conduit outburst. A possible association of outbursts with the present advanced position of the glacier is Suggested. Copyright (C) 2003 John Wiley Sons, Ltd.