Tsunami Inundation Modelling for Bay of Plenty - Executive Summary March 2013

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1 Tsunami Inundation Modelling for Bay of Plenty - Executive Summary March Inundation Modelling for the Bay of Plenty Executive Summary

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3 Tsunami Inundation Modelling for Bay of Plenty - Executive Summary Background The following is a brief and high level summary of four tsunami research papers commissioned by The Bay of Plenty Civil Defence and Emergency Management Group: Review of tsunamigenic sources within the Bay of Plenty Region, GNS Science Consultancy Report 2011/224 (Prasetya and Wang 2011) Tsunami inundation modelling for Tauranga and Mount Maunganui, GNS Science Consultancy Report 2011/193 (Prasetya and Wang 2011) Tsunami inundation modelling for Whakatāne, Ōhope, and Opotiki, GNS Science Consultancy Report 2011/194 (Prasetya and Wang 2011) Modelling of the tsunami risk to Pāpāmoa, Wairakei and Te Tumu and implications for the SmartGrowth Strategy, GNS Science Consultancy Report 2011/294 (Prasetya and Wang 2011) This document is intended to provide an overview of research that was undertaken and the resulting information that was produced. The work was undertaken by the Institute of Geological and Nuclear Sciences Limited (GNS Science) as the national experts in this field. The information presented in this overview is summarised and abridged, the results of the research should be interpreted in the context of the constraints and limitations of the modelling undertaken as detailed in the reports in full. Executive Summary As part of its on-going programme researching natural hazards to improve understanding and increase community resilience, the Bay of Plenty Civil Defence Emergency Management Group in 2011 commissioned GNS Science to further investigate the impact of tsunami on the Bay of Plenty. GNS produced four reports as part of this work: (1) An overview of the sources of tsunami that could affect the Bay of Plenty; And three reports detailing the results of inundation modelling for (2) Tauranga and Mt Maunganui; (3) Pāpāmoa, Wairakei and Te Tumu; and (4) Whakatāne, Ōhope and Opotiki. These reports differ from earlier tsunami inundation studies conducted by NIWA (2006) because they were applied in a more advanced tsunami model and used higher resolution ocean floor terrain and topographic data. They also include lager tsunami scenarios, which are similar in magnitude to the 2011 Tohoku Japan Tsunami. Overall, 13 earthquake scenarios were modelled. Modelling identified that local sources within the Bay of Plenty region (Astrolabe, Volkner Fault and White Island) and offshore Peru sources pose a minimal tsunami threat to the region (no dune overwash). The sand dunes provide an effective barrier to a distant source tsunami (greater than 3 hours warning) where they are vegetated and well established. The greatest threat to the Bay of Plenty is from a rupture along the Kermadec Trench, which was supported by historical evidence in soil profiles, identifying past occurrences of large magnitude tsunami. Waves from these events are expected to reach the coast within 50 to 70 minutes. The research has confirmed previous mapping work that identified low lying areas around ports, harbours, estuaries and rivers are susceptible to tsunami inundation. These areas include Tauranga and Ohiwa Harbours, Maketu Estuary, low lying areas along the Kaituna River, Waiotahi River and Opotiki Township. Areas with limited dune protection and/or vegetation, such as sections of Mount Maunganui to Papamoa, and Ohope are also at increased risk. 1 Inundation Modelling for the Bay of Plenty Executive Summary

4 Modeling for extreme events from the Kermadec Trench, similar to the Japan Tohoku tsunami in size, identified that all of coastal Bay of Plenty will experience tsunami flooding and these scenarios are considered high hazards. However while it is now considered that these sized events are possible, the probability of them occurring is less certain. Because tsunami research is relatively new, the knowledge about the severity and frequency of local and distant source tsunami is incomplete. Generally, the greater the magnitude of an event, the lower the probability of it occurring. GNS is currently carrying out work to further understand the probability of different magnitude events at a national level. This work is due to be released in 2013/later this year. It is important to note that this information is a refinement of previous tsunami mapping work and while our detail of knowledge has improved, it does not significantly change our earlier understanding of the tsunami hazard to the region. The current research reinforces our previous understanding we are living on a coastline where tsunami could occur at any time. It is important to be prepared and know what the warning signs and alerting mechanisms are for a tsunami. Being prepared and able to determine these without the aid of authorities can give you vital time to safely evacuate. The extent of flooding from tsunami varies within the region, but is generally within the tsunamievacuation zone boundaries previously mapped ( Further mapping of the new modeling results is required to refine the evacuation maps. Some of the key points to come out of this research are: 1. Bay of Plenty is highly susceptible to a tsunami generated from the Kermadec Trench. 2. Local sources within the Bay of Plenty region (e.g. Astrolabe, Volkner Fault and White Island) pose a minimal tsunami threat to the region. 3. A major earthquake and landslip in the magnitude of the Tohoku, Japan earthquake is possible and would result in extensive inundation across the region. However, the greater the magnitude of an event, the lower the probability of it occurring. 4. There would be around 50 to 70 minutes from an earthquake on the Kermadec Trench before the first wave arrives, and waves could be as big as 6 to 10m. 5. Low lying areas between Mount Maunganui and Mt Drury are susceptible to inundation which could also occur along Marine Parade, Ōmanu Beach and low lying Sulphur Point and Otumoetai areas in Tauranga. 6. Mt Drury and Tauranga Airport do not suffer from tsunami inundation. 7. Sand dunes provide an effective barrier against tsunami. 8. The offshore islands and seabed topography play a significant role in amplifying or reducing the tsunami impacts on certain local areas. White Island and its surrounding area direct most of the tsunami energy towards Thornton, Matatā and Pikowai, and Ōpōtiki while deflecting the energy away from the Whakatāne area. 9. The narrow river entrance at Whakatāne provides some protection from tsunami inundation. 10. Low and degraded dunes at Pāpāmoa increase the risk of tsunami inundation in this area. 11. There are implications for the SmartGrowth future development areas of Te Tumu and Wairakei and further research has been undertaken in these areas. Technology and research techniques into tsunami will continue to improve with time and while there is potential for huge level of research to be undertaken in this area, it is only one of 22 hazards for the region. Tsunami inundation information calculated from the GNS reports serves as a good resource to understand the location and extent of impact on the Bay of Plenty region. The information highlights potential areas for further investigation. The following list key areas for possible future tsunami research in the region: 1) Model these scenarios for the coast north of Mount Maunganui to Waihī Beach. 2) Link this research with the national probabilistic information that is due for release in ) Compare GNS model results to tsunami evacuation zone maps using GIS to identify any areas where adjustments may be necessary (potentially Matatā/Thornton, West End Ōhope, and Ōpōtiki Township). Inundation Modelling for the Bay of Plenty Executive Summary 2

5 4) Further investigate the resilience of dune systems to tsunami. While this modelling identifies dune systems as an effective barrier, there is scope for improving the way dunes are modelled in these types of scenarios. 5) Further investigation is required to understand co-subsidence and local faulting impacts (vertical movement of the land) especially in the eastern Bay. 6) Improved understanding is required of the effects of tsunami in and around harbours, estuaries and ports, on currents, waves and velocities. 7) Further evacuation modelling and mapping for developed and undeveloped areas. This information has been shared with Local Authorities and our neighbouring regions. It has been and will be used to assist in policy, planning and decision making by Local and Regional Councils. Since these reports were commissioned, SmartGrowth has commissioned a further two reports to identify how land use planning and other mitigation measures can be used to reduce the risk of tsunami in areas of future development. For more information on what to do and where to go in the event of a tsunami you can visit the following websites: Bay of Plenty Civil Defence and Emergency Management Western Bay District Council Tauranga City Council Whakatāne District Council Ōpōtiki District Council Ministry of Civil Defence and Emergency Management GNS Science Inundation Modelling for the Bay of Plenty Executive Summary