MASS WASTING PROCESSES, GEOMORPHIC RESPONSES, AND TEMPORAL EVOLUTION OF ICY DEBRIS FANS ALONG THE LA PEROUSE AND DOUGLAS GLACIERS, SOUTHERN ALPS, NEW ZEALAND

Ice-dominated mass wasting processes from deteriorating alpine ice-caps are constructing icy debris fans at outlets of incised bedrock catchments along valley glaciers margins. Previous short-term field observations provide limited understanding of the nature and rate of depositional processes. Recent eight-month time-lapse imagery (Jan. - Sept. 2013) permits the investigation of frequency, areas, and volumetric contributions of mass wasting deposits to 7 fans in the Southern Alps of New Zealand.

Time-lapse imagery and field observations document remarkably high depositional rates, primarily by ice avalanches (96%); debris flows, slush flows, and rockfalls account for 4% of recorded events. Time-lapse imagery only records major events that emerge onto fans, but direct field observations also document smaller events that occur in catchments. During 9 months, 2 fans at La Perouse Glacier received 303 major depositional events (85-218 events per fan, 0.4-0.9 events/day per fan) covering 2675% of the fan area (653-2022% per fan). During 8 months, 5 fans at Douglas Glacier received 653 events (15-300 events per fan, 0.2 to 1.6/day per fan) covering 6151% (313-2102% per fan). Field observations on 2 fans at La Perouse experienced 7 events over 2 days (3-4 events/day per fan). At Douglas, 5 fans received 210 events over 2 days (9-40 events/day per fan), >90% of these were in the catchments. Frequency of ice avalanches correlates with deposit areas, but areas vary significantly. It is not uncommon for 5-7 depositional events to cover >50% of a fan area; on especially active days, deposits may cover 80->100%.

Ice avalanches occurred throughout the observation period, but the pace decreases during winter. Most depositional events occur after significant rainfall. However, in warmer months icecap melting may be the primary cause of ice avalanches in the absence of rainfall. Debris and slush flows are only mobilized after major rainfall. At both sites, differences in timing, frequency, and depositional process types between fans are attributable to variations in catchment morphology and linkages to the ice/snow supply in the névé regions.

For the first time, the evolution processes of icy debris fans is documented over several seasons, providing information on the dynamics of contributed material to valley glaciers annually.