Community Vulnerability to Elevated Sea Level and Coastal Tsunami Events in Otago
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Community vulnerability to elevated sea level and coastal tsunami events in Otago Otago Regional Council Private Bag 1954, 70 Stafford St, Dunedin 9054 Phone 03 474 0827 Fax 03 479 0015 Freephone 0800 474 082 www.orc.govt.nz © Copyright for this publication is held by the Otago Regional Council. This publication may be reproduced in whole or in part provided the source is fully and clearly acknowledged. ISBN: 978 0 478 37630 2 Published July 2012 Prepared by Michael Goldsmith, Manager Natural Hazards, Otago Regional Council Community vulnerability to elevated sea level and coastal tsunami events in Otago i Executive summary The Otago coastline extends 480km from Chaslands in the south to the mouth of the Waitaki River in the north. Approximately 124,000 people (64% of Otago’s population) live within five kilometres of this coastline. A number of the communities situated along the coast have a level of hazard exposure to elevated sea level (or storm surge) and tsunami events. This report assesses the vulnerability (rather than the risk) 1 of these coastal communities to these hazards. The report draws on tsunami and storm surge modelling undertaken by National Institute of Water and Atmosphere (NIWA) for the Otago Regional Council (ORC) in 2007/08, coastal topography data and local knowledge of each community. This information has been used to assess how people and the communities in which they live would be affected during credible, high magnitude tsunami and elevated sea level events. It is intended that this information will: • increase community awareness of elevated sea level and tsunami hazard • inform decision making on the development of warning systems and evacuation plans • assist with the selection of land-use planning and development controls • increase the resilience of infrastructure and utilities (‘lifelines’). Community vulnerability to the following three scenarios has been assessed in some detail: a 1:500-year elevated sea level event, a 1:500-year far-field (South American) tsunami event and a 1:600-year near-field (Puysegur Trench) tsunami. The vulnerability of each community to lower magnitude, higher frequency events has also been assessed. The impact of events that have a higher probability of occurring is, in many cases, only marginally smaller than the three high magnitude scenarios outlined above. Therefore, many coastal communities have a level of exposure to elevated sea level and tsunami events that are likely to occur frequently. Several communities investigated in this report are situated on low-lying land that is afforded a degree of protection from coastal dune systems, flood banks or sea walls. Cross section profiles and topographic data show that a breach or failure of these protective features during an elevated sea level or tsunami event could lead to widespread inundation of these areas. The lower Clutha Delta and South Dunedin areas are particularly vulnerable in this respect. This report does not assess the likelihood of those protective features failing, but highlights the important part they play in mitigating the risk. Other communities, such as Pounawea, Toko Mouth, Taieri Mouth, Long Beach, Karitane and Kakanui, also have a high degree of exposure to elevated sea level and tsunami events because of their low-lying nature and their proximity to the coast or river mouths. The level of vulnerability for each community is dependent, in part, on the amount of warning time before a tsunami arrives. In many cases, the time between a near-field earthquake (such as the modelled event for the Puysegur Trench) occurring, and the resulting tsunami affecting the coast, may be less than two hours. Remote communities 1 See Glossary (at back) for the definition of ‘vulnerability’, ‘risk’ and other terms used in this report. ii Community vulnerability to elevated sea level and coastal tsunami events in Otago with limited communication links may be particularly at risk from these types of event if evacuation is the primary risk mitigation measure. In summary, this report describes the varying degrees of vulnerability to elevated sea level and tsunami hazard along the Otago coastline. Community vulnerability to elevated sea level and coastal tsunami events in Otago iii Contents 1. Introduction ....................................................................................................... 1 1.1 Outline of report ..................................................................................... 1 2. Background ........................................................................................................ 3 3. Methodology ....................................................................................................... 4 3.1 Selection of study sites ........................................................................... 4 3.2 Elevated sea level and tsunami scenarios ................................................ 4 3.3 Combined probability ............................................................................. 5 3.4 The relationship between water depth and velocity ................................. 6 3.5 Sources of additional information ........................................................... 7 4. Elevated sea level ............................................................................................... 9 4.1 Elevated sea level scenarios .................................................................. 10 4.2 Elevated sea level and river flooding .................................................... 11 5. Tsunami hazard ............................................................................................... 13 5.1 What is a tsunami? ............................................................................... 13 5.2 Terms associated with tsunami events .................................................. 14 5.3 Sources of tsunami on the Otago coast ................................................. 15 5.4 Modelled tsunami scenarios ................................................................. 19 5.5 Uncertainty of tsunami modelling ......................................................... 20 6. The impact of future sea level rise ................................................................... 21 7. Papatowai ......................................................................................................... 22 7.1 Community description ........................................................................ 22 7.2 Effect of elevated sea level and tsunami events..................................... 22 7.3 Community vulnerability ...................................................................... 25 7.4 Summary .............................................................................................. 26 8. The Catlins ....................................................................................................... 27 8.1 Community description ........................................................................ 27 8.2 Effect of elevated sea level and tsunami events..................................... 29 8.3 Community vulnerability ...................................................................... 32 8.4 Summary .............................................................................................. 34 9. Kaka Point and Lower Clutha ........................................................................ 35 9.1 Community description ........................................................................ 35 9.2 Effect of elevated sea level and tsunami events..................................... 35 9.3 Community vulnerability ...................................................................... 39 9.4 Summary .............................................................................................. 42 10. Toko Mouth ...................................................................................................... 43 10.1 Community description ........................................................................ 43 10.2 Effect of elevated sea level and tsunami events..................................... 43 10.3 Community vulnerability ...................................................................... 46 10.4 Summary .............................................................................................. 47 11. Taieri Mouth .................................................................................................... 48 11.1 Community description ........................................................................ 48 iv Community vulnerability to elevated sea level and coastal tsunami events in Otago 11.2 Effect of elevated sea level and tsunami events..................................... 48 11.3 Community vulnerability ...................................................................... 51 11.4 Summary .............................................................................................. 52 12. Brighton ........................................................................................................... 53 12.1 Community description ........................................................................ 53 12.2 Effect of elevated sea level and tsunami events..................................... 53 12.3 Community vulnerability ...................................................................... 56 12.4 Summary .............................................................................................. 56 13. St Kilda/St Clair ..............................................................................................