Quaternary geology and landslide dam hazard assessment of the Shotover Gorge, Otago.

Formation and failures of landslide dams cause significant immediate and longer-term geomorphic impacts in mountain catchments. In the Shotover River of Otago, New Zealand, these impacts would affect not only the gorge and lower reaches of the Shotover River, but could increase flood risk to Lake Wakatipu and Queenstown (population 15,000) near the Shotover Delta. This thesis addresses unknown or poorly understood factors of landslide dam hazard in the Shotover River, developing and modelling geologically-founded scenarios for future landslide dam events, dambreak flooding and the long-term sediment aggradation. Geological and geomorphic investigations include mapping of Quaternary geological deposits and geomorphic features, surveying of terrace and fan surfaces, and optically stimulated luminescence (OSL) dating of sedimentary units. Quaternary deposits and landforms of the Shotover Gorge are dominated by events of the last glacial maximum and the postglacial period. Glaciolacustrine sediments in the upper Shotover Gorge show an early Last Glacial Maximum (29.4 ± 3.4 ka; late MIS3/early MIS2) Shotover Glacier advance, followed by formation of an extensive proglacial lake during glacial retreat. Alluvial terraces in the Shotover Gorge and Moonlight Creek are interpreted as remnants of an aggradation surface formed during the LGM, in response to advance of the Wakatipu Glacier into the Wakatipu Basin and the lower Shotover Gorge. Formation of the modern Shotover Gorge has occurred in the postglacial period, incising in response to tectonic uplift. There was no evidence identified for large (1M m3) landslide dam formation in the Shotover Gorge, although smaller events have been noted historically. From the discovery of alluvial gold in the 1860s, extensive hydraulic sluicing of alluvial terrace gravels took place in the Shotover Valley, with this sediment influx causing a sediment pulse with widespread aggradation in the Shotover River. This sediment pulse had a volume of at least 20-40M m3 and caused significant geomorphic changes, including aggradation of several metres in the lower Shotover River. The degradation phase of this sediment pulse is ongoing, with channel incision and a narrowing of the active riverbed. Extensive evidence of landslide activity on the steep slopes in the Shotover Gorge indicates the potential for future landslide occurrence, and the narrow gorge can be easily blocked by landslide debris to form a dam. Any future landslide dam formed in the Shotover Gorge is expected to be short-lived and to fail catastrophically, largely due to the erosive power of river flows from the large, high-rainfall catchment area. Modelling of peak dambreak flows and routing of dambreak flood waves demonstrates that dambreak floods resulting from sudden failure of a Shotover Gorge landslide dam could be very large (>15,000 m3/s). These are potentially greater than an order of magnitude larger than the largest known meteorological flood events (1,500 m3/s), and capable of generating flood waves that would impact on the residential area at the Shotover Delta. Estimations of post-dambreak aggradation indicate geomorphic impacts at the Shotover Delta are limited by sediment buffering at the lower Shotover beach sections. However, any large magnitude sediment inputs are expected to increase the flood risk for the Shotover Delta residential area. The largest existing landslide scarps in the Shotover Gorge are estimated as being an order of magnitude greater in volume than those used in developing dambreak flood and aggradation assessments. While these larger events are less frequent, a landslide of this size can occur at any time and would have major geomorphic impacts, posing a substantial threat to downstream locations. The landslide dam hazard component of this thesis provides an example of an assessment of the future threats from a landslide-induced hazard events cascade, including the longer-term and further afield geomorphic impacts.