THE INFLUENCE OF CLIMATE OSCILLATIONS ON ALLUVIAL FAN MORPHOLOGY IN THE SOUTH ISLAND OF NEW ZEALAND

Alluvial fans pose a significant threat to nearby human settlements and infrastructure due to their potential to produce floods, debris flows and channel avulsion events. Climate has been shown to be one of the primary drivers of alluvial fan morphology around the world. In New Zealand, climate oscillations, such as the Interdecadal Pacific Oscillation (IPO), have been shown to exert a control over annual rainfall and river discharge in the South Island. There is limited research exploring how these changes in hydrology associated with the IPO affect alluvial fan morphology in the South Island. Here, I utilise historic aerial photography and satellite imagery to map a range of morphometric variables for a sample of 28 alluvial fans in the South Island. The morphometrics are then compared with the IPO record and relevant catchment hydrological data. Using these data, I explore the relationship between alluvial fan morphology and the hydrologic cycle as it pertains to the IPO. A comparison of morphometric trends between alluvial fans west and east of the main divide is also undertaken. I show that during a positive IPO phase (higher than average rainfall), alluvial fans located west of the main divide show an increased chance of channel avulsion and maintain a greater number of active channels than during a negative IPO phase (lower than average rainfall). I also demonstrate that alluvial fan morphology in the South Island of New Zealand can change on short timescales associated with natural decadal-scale climate oscillations. The findings of this study have implications for alluvial fan hazard management in the South Island, particularly on the west coast.