Geomorphic Hazards associated with Glacial Change, Aoraki/Mount Cook region Southern Alps, New Zealand
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Glacial floods and mass movements of ice, rock or debris are a significant hazard in many populated mountainous regions, often with devastating impacts upon human settlements and infrastructure. In response to atmospheric warming, glacial retreat and permafrost thaw are expected to alter high mountain geomorphic processes, and related instabilities. In the Aoraki/Mount Cook region of New Zealand's Southern Alps, a first investigation of geomorphic hazards associated with glacial change is undertaken and is based primarily on the use of remote sensing and Geographic Information Systems (GIS) for mapping, modelling, and analysing related processes and terrain.
Following a comprehensive review of available techniques, remote sensing methods involving the use Advanced Spaceborne Thermal Emission and Radiometer (ASTER) imagery were applied to map glacial ice, lakes and debris accumulations in the Aoraki/Mount Cook region. Glacial lakes were mapped from two separate classification techniques using visible near infrared wavelengths, capturing highly turbid and clearer water bodies. Large volume (10⁶– 10⁸ m³) proglacial lakes have developed rapidly over recent decades, with an overall 20 % increase in lake area recorded between 2002 and 2006, increasing the potential for large mass movement impacts and flooding from displaced water. Where significant long-term glacial recession has occurred, steep moraines have been exposed, and large talus slopes occupy formerly glaciated slopes at higher elevations. At the regional-scale, these potential source areas for debris instabilities were distinguished from surrounding bedrock slopes based on image texture variance. For debris and ice covered slopes, potentially unstable situations were classified using critical slope thresholds established from international studies.
GIS-based flow routing was used to explore possible intersections between zones of human use and mass movement or flood events, assuming worst-case, probable maximum runout distances. Where glacial lakes are dammed by steep moraine or outwash gravel, primarily in cirque basins east of the Main Divide, modelled debris flows initiated by potential flood events did not reach any infrastructure. Other potential peri- and para-glacial debris flows from steep moraines or talus slopes can reach main roads and buildings. The direct hazard from ice avalanches is restricted to backcountry huts and walking tracks, but impacts into large glacial lakes are possible, and could produce a far reaching hazard, with modelled clear water flood-waves capable of reaching village infrastructure and main roads both east and west of the Main Divide. A numerical modelling approach for simulating large bedrock failures has been introduced, and offers potential with which to examine possible lake impacts and related scenarios.
Over 500 bedrock slope failures were analysed within a GIS inventory, revealing distinct patterns in geological and topographic distribution. Rock avalanches have occurred most frequently from greywacke slopes about and east of the Main Divide, particularly from slopes steeper than 50°, and appear the only large-magnitude failure mechanism above 2500 m. In the schist terrain west of the Main Divide, and at lower elevations, other failure types predominate. The prehistoric distribution of all failure types suggests a preference for slopes facing west to northwest, and is likely to be strongly influenced by earthquake generated failures. Over the past 100 years, seismicity has not been a factor, and the most failures have been as rock avalanches from slopes facing east to southeast, particularly evident from the glaciated, and potentially permafrost affected hangingwall of the Main Divide Fault Zone. An initial estimate of permafrost distribution based on topo-climatic relationships and calibrated locally using mean annual air temperature suggested permafrost may extend down to elevations of 3000 m on sunny slopes, and as low as 2200 m on shaded slopes near the Main Divide. A network of 15 near-surface rock temperature sensors was installed on steep rock walls, revealing marginal permafrost conditions (approaching 0 °C) extending over a much larger elevation range, occurring even where air temperature is likely to remain positive, owing to extreme topographic shading. From 19 rock failures observed over the past 100 years, 13 detachment zones were located on slopes characterized by marginal permafrost conditions, including a sequence of 4 failures that occurred during summer 2007/08, in which modelled bedrock temperatures near the base of the detachments were in the range of 1.4 to +2.5 °C.
Ongoing monitoring of glacial and permafrost conditions in the Aoraki/Mount Cook region is encouraged, with more than 45 km2 of extremely steep slopes (>50°) currently ice covered or above modelled permafrost elevation limits. Approaches towards modelling and analysing glacial hazards in this region are considered to be most applicable within other remote mountain regions, where seismicity and steep topography combine with possible destabilizing influences of glacial recession and permafrost degradation.