A review of break‐out floods from volcanogenic lakes in New Zealand

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Abstract
New Zealand hosts numerous lakes in its active volcanic areas. These water bodies are developed in calderas, volcano‐tectonic collapse structures, explosion craters, and valleys dammed by lava, pyroclastic, or laharic flows. They range in scale and elevation from small shallow ponds a few metres above sea level in Auckland, through volcano‐tectonic collapse structures such as the Rotorua and Taupo calderas that hold many cubic kilometres of water at a height of a few hundred metres, to the summit Crater Lake of Mt Ruapehu, which contains c. 9 × 10 6 m 3 of hot acidic water at an altitude of 2530 m. The combination of active volcanism and New Zealand's temperate climate means that new lakes can form or old ones overfill rapidly following activity at any volcanic centre. Lakes in volcanic environments are often relatively shortlived features compared with those in tectonic settings, being prone to rapid formation and/or modification/destruction by both primary volcanic processes and the secondary effects of post‐eruptive landscape re‐adjustment. In addition to primary eruption‐related volcano‐hydrologic hazards, significant delayed hazards can also result from partial to total failure of impounding barriers of volcanic material. For example, New Zealand's worst volcanic disaster, the 1953 Tangiwai lahar, which resulted in 151 deaths, was caused by partial failure of the rim of Mt Ruapehu's Crater Lake 8 yr after the 1945 eruption had ended. Most recently, post‐1996 refilling of the lake behind a barrier of unconsolidated tephra laid down during the 1995/96 eruption sequence culminated in breaching and failure of the dam on 18 March 2007, resulting in the release of 1.3 × 10 6 m 3 of water and generation of the largest historic lahar at the volcano. Historical reports and geomorphic/sedimentologic studies demonstrate that many of New Zealand's volcanogenic lakes have been the source of large‐magnitude floods, some of which have caused major re‐organisation of regional drainage networks. Post‐eruption failures of the topographic rim of the Taupo caldera have produced catastrophic releases of up to 60 km 3 of water, whereas intracaldera lakes hosted within the Okataina and Rotorua Volcanic Centres have also been the source of repeated floods. At least four events have been identified from Lake Tarawera (Okataina Volcanic Centre) in response to volcanogenic modification of the lake basin and catchment, most recently following the AD 1886 Tarawera eruption. Paleohydraulic reconstructions of dam‐breach hydrographs and downstream flood discharge rank some New Zealand caldera lake break‐outs as amongst the largest known Holocene floods on Earth.

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