The recent series of severe seismic events affecting New Zealand and the Pacific Rim are stark reminders of the ever-present threat of a catastrophic tsunami; among them the 7.1 Mw Te Araroa earthquake and the 7.8 Mw Kaikōura and Solomon Islands earthquakes.
Tauranga City Council (TCC) is firmly established as Australasia’s leader in tsunami evacuation planning, a pioneer providing its at-risk coastal communities with innovative self-evacuation strategies and solutions - among them a Southern Hemisphere first - vertical evacuation stations (VES).
More than 46,000 Tauranga people live on the coastal plain spanning from Mount Maunganui (Mauao) tō Pāpāmoa (Wairākei). In the warmer months, the population is swelled by thousands of visitors.
A tsunami generated by an earthquake in Japan, Alaska or South America takes more than 12 hours to reach New Zealand, allowing plenty of time for warnings and planned evacuations. A wave generated by a very large earthquake resulting from a local source – such as a rupture of the Kermadec Trench - would potentially inundate parts of Mauao and Wairākei within 60 minutes.
Until recently, tens of thousands of lives would have been at risk from a local source tsunami – particularly at Wairākei, where a considerable part of the back dune landforms have been levelled by development earthworks.
TCC’s dedication to the task and considered judgement in developing a “start to finish” tsunami risk management process – without benefit of established standards or guidelines to refer to – has assured the safety of its coastal population while creating a credible blueprint for others to follow.
Tonkin + Taylor has worked alongside TCC (its client), Emergency Management Bay of Plenty, New Zealand Police and the New Zealand Transport Agency (NZTA) to drastically reduce the risk to lives.
New Zealand’s Civil Defence Emergency Management Act focuses on the sustainable management of hazards, resilient communities and ensuring the safety of people, property and infrastructure. Local authorities are required to pay particular attention to avoiding or mitigating natural hazards, including tsunami.
For the past four years, Paul Baunton, TCC’s Manager: Emergency Management, has been focussed on building safety and resilience - providing Tauranga’s at-risk communities with easily accessible safe zones that can be reached quickly, using sign-posted evacuation routes.
In 2014, work began with GNS Science undertaking research to quantify the ‘maximum credible slip’ event and the size of the resultant tsunami that could be expected to reach the Bay of Plenty coastline.
“My role has been to lead development of public safety for tsunami through understanding the extent of the tsunami risk to the City, what the potential impacts are and then develop a strategy that ensures the community is provided with safe areas, safe assembly locations and routes to reach these safe locations,” Baunton says.
Tonkin + Taylor’s numerical modelling simulated the tsunami flow over the land and into the harbour, while identifying areas above and outside the maximum expected flow extents.
Using a GIS model, T+T was able to determine how long it would take residents to reach safe areas, how many would use the evacuation routes and how many would arrive at the various safe zones. The modelling also gave TCC a solid foundation for decision-making on continued development of land in eastern Pāpāmoa.
A pedestrian-based evacuation network was developed using the existing roading network, walkways, reserves and cycleways to reach safe areas. It was vital that evacuation stations could be reached quickly, on foot, as NZTA modelling revealed vehicular traffic would gridlock the roads within minutes.
Sixteen safe assembly locations were developed at strategic points. Some use distance to keep residents safe, while VES such as Gordon Spratt High Ground, employ the raising of land and are the first of their kind to be constructed outside of Japan and Pacific North-West America. Natural topographical features such as Mount Maunganui (Mauao) and Mount Drury also offer a safe retreat.
Royal Palms’ wooden footbridges and the earth bund stretching alongside Pāpāmoa College’s sports field are seemingly insignificant features, but also vital to preserving lives.
The bridges - built to Importance Level 4 - are designed to withstand the most brutal of natural forces, their foundations driven 18-30m into the ground. Meanwhile, the massive bund offers a safe haven for 3,600 evacuees.
Prominent signs are constant reminders of the potential tsunami risk while clearly detailing the likely extents of the inundation zone. Others point to the nearest assembly area.
Public education and buy-in to the project has been essential; there have been a number of open days and the feedback has been extremely positive.
Ongoing works planned for completion by 2022 include three more VES for Wairākei, along with further evacuation signage and possible additional bridges and evacuation routes.
Tonkin + Taylor applauds Paul Baunton and TCC for their work. The Council is now focusing on regional and distant source tsunami risk emergency management planning and alerting.
Standing strong: Paul Baunton, Tauranga City Council's Manager: Emergency Management, on one of Royal Palms' Importance Level 4 bridges.