Risk assessment for population, agriculture, and infrastructure in the Rotorua District from a rhyolitic eruption at the Okataina Volcanic Centre

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Kaye, G.D. 2007 Risk assessment for population, agriculture, and infrastructure in the Rotorua District from a rhyolitic eruption at the Okataina Volcanic Centre. Lower Hutt, N.Z.: GNS Science. GNS Science report 2007/39 62 p.

Abstract: A rhyolitic volcanic eruption from the Okataina Volcanic Centre (OVC), similar to either of the two most recent significant events (0.7 ka Kaharoa or 5 ka Whakatane), would have devastating consequences for the entire central North Island of New Zealand. Numerous qualitative risk assessments for all aspects of New Zealand society under threat from volcanic eruptions have been undertaken in the last decade, but have fallen short of taking a quantitative approach via combining scenarios, hazard models, and vulnerability assessment. This paper presents a quantitative approach applied to three major inventory sectors in the Rotorua District: population, agriculture, and infrastructure. Risk is estimated from tephra and pyroclastic density current (PDC) hazards during a scenario rhyolite eruption at the Okataina Volcanic Centre similar in scale to the 0.7 ka (1314 AD) Kaharoa event. The ASHFALL model of Hurst (1994) along with the EXPLORIS pyroclastic density current model of Toyos et al. (2007) are used to determine the intensity of tephra and PDC hazards (respectively) in the scenario eruption. Fragility functions or hazard intensity / damage curves developed for the RiskScape natural hazard risk assessment tool (Kaye, 2007b; King and Bell, 2005) are used to determine expected damage to inventory in the three inventory classes. Two opposing wind directions are used to model a best- and worst-case tephra fall from the perspective of Rotorua City. Impact in each of the three inventory classes varies greatly with wind direction, with more significant effects occurring in easterly winds that carry the bulk of the tephra directly from the vents to the densely populated urban areas. Tephra poses the greater hazard than PDCs to all three classes of inventory because of the wider distribution of the hazard throughout the District. Determination of financial exposure is left to a future risk assessment, although an example methodology is given via determination of financial risk to forestry. (auth)