Tsunami

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Existing tsunami evacuation maps - how has the mapping been done?

The existing tsunami mapping for the whole of the District was done for the purpose of displaying evacuation zones. This mapping is based on a simple method that assumes a constant decrease in tsunami wave height as it moves inland. For example, based on a 1m decrease in wave height for every 200m inland, a 5m high tsunami would reach 1km inland.

New tsunami maps - how has the mapping been done?

In 2013 the Institute of Geological and Nuclear Sciences Limited (GNS Science) undertook tsunami modelling and reported on the potential tsunami wave amplitude heights for all coastal towns around New Zealand (i.e. National Tsunami Hazard Model). As part of the tsunami modelling GNS Science took into account the knowledge gained from the unexpected large earthquake and tsunami event in Japan in 2011. In particular it considered the effect of the Kermadec Trench, located north-east of New Zealand. The Kermadec Trench is a subduction zone, similar to the environment where the great Japan event took place.

The new tsunami mapping completed by Regional Council for Maketu, Pukehina and Little Waihi builds on the GNS (2013) work and mapping the inundation extent inland. The maps show areas susceptible to tsunami inundation from an earthquake source along the Kermadec Trench. The new tsunami mapping is based on a more accurate modelling method. The hydrodynamic model accounts for changes in both offshore bathymetry (water depth) and land elevation. The new tsunami modelling results provide tsunami height, speed and arrival time in addition to just the area inundated. The new tsunami mapping shows a similar inundation area to the existing evacuation zones maps.

The scenarios modelled were a 0.2% Annual Exceedance Probability (AEP), 0.1% AEP, 0.04% AEP and maximum credible event (worst case). AEP is explained in the FAQ's below. This mapping also takes into account climate change over a 100 year timeframe.

The purpose of this new mapping is to provide tsunami hazard mapping for risk assessments to support land use planning under the Regional Policy Statement (RPS). We will then work on implementing mitigation actions to reduce tsunami risk where required. These actions may include enhancing evacuation routes, increased signage, improved education and preparedness, and District Plan changes.

We will be working with the Regional Council and Emergency Management Bay of Plenty on assessing the need for any updates to the existing tsunami evacuation maps.

Technical Reports

Here is the technical report explaining the evacuation maps:

Here is the technical report supporting the new tsunami maps:

What further work is being done?

Council and the Regional Council are modelling tsunami at Waihi Beach as a part of the same project that modelled tsunami for Maketu, Pukehina and Little Waihi. Council and the Regional Council are also modelling tsunami for the Tauranga Harbour. These reviews will take into account climate change. They will show us what a 0.2% AEP, 0.1% AEP, 0.04% AEP and maximum credible event will look like if they happen in 100 years' time. When the mapping is completed, risk assessments and changes to the District Plan will also be required.

Frequently asked questions about tsunami

What is an Annual Exceedance Probability (AEP)?

This is a statistical term referring to the likelihood of a natural hazard event occurring in any year. As a generic example, the event might be 300mm of rainfall in 24 hours. If such an event has a 2% chance of occurring in any year it would be described as a 2% AEP rainfall event. This means it would be expected to occur on average once in every 50 years. Although this is sometimes referred to as a 1 in 50 year flood, it could actually happen at any time. For example, it might not occur again for 80 years or it could even occur twice in one year.

AEP terms associated with tsunami include:

• 0.2% AEP event = expected to occur on average once in every 500 years
• 0.1% AEP event = expected to occur on average once in every 1,000 years
• 0.04% AEP event = expected to occur on average once in every 2,500 years

How long would it take a tsunami to reach us and how should we respond?

A tsunami triggered from a major Kermadec Trench earthquake would arrive in less than one hour. The first and best tsunami warning is an earthquake. Do not wait for official warnings. Natural tsunami warning signs are:

• Any earthquake that lasts a long time (more than a minute)
• A big earthquake that is very strong (knocks you off your feet or is very difficult to stand up)
• Strange ocean behaviour. Like loud or strange noises, sudden change in sea level, or ocean drawing away from the shore.

If any of these things happen, don't wait for an official warning. Grab your emergency pack and walk as quickly as you are able to a safer location or to high ground. Remember LONG, STRONG, GONE. Refer to the Get Ready Get Thru website for more information.

Has this information been available for long?

Tsunami inundation maps are not new. They have been displayed on the Council and the Civil Defence Emergency Management (CDEM) Bay of Plenty Council websites since 2010. The maps that were published in November 2010 were the most recent and reliable information that we had at that time about tsunami inundation. The new maps build on our previous knowledge.

Where would a maximum credible event come from?

The maximum credible tsunami would be generated by an earthquake somewhere along the Kermadec Trench. To produce a tsunami of this scale, the earthquake would need to be greater than magnitude MW 9. This earthquake is predicted to last a long time and be felt very strongly at the Western Bay of Plenty.

What about tsunami from other areas?

Most other tsunami scenarios that have been modelled don't come anywhere near the Kermadec scenario. While most tsunami have potential to affect the marine and beach areas, only a rare tsunami could potentially overtop some of the dunes along our coastline. A tsunami from White Island is unlikely to overtop the dunes because it won't be large enough.

Could a tsunami exceed the maximum credible event?

The tsunami maps are based on our current best knowledge. The knowledge includes in-depth studies undertaken in 2004, 2013 and 2017 by NIWA, GNS Science and eCoast Marine Consulting and Research respectively. These studies are peer reviewed by technical experts to ensure they are accurate and utilise the most up to date information and methods of research available. Nature has a way of surprising us and our knowledge is always being added to. The maps are updated whenever new information comes to hand.

What if I need assistance?

With less than an hour after a major earthquake before a tsunami arrives, emergency services will not be able to get you out in time. The reality is that you're on your own. The community's best chance to survive a tsunami is to work together as a community. Make an evacuation plan with your neighbours, especially if you know they will need help getting to a safe area. If you live in a retirement village or gated community, make sure you know what their emergency plan is.

Why do I need an emergency pack?

Tsunami can arrive in several waves over a long period of time, and the first wave is not always the biggest. That is why you need your emergency pack. Pack anything you think you will need, like medicine and a water bottle. You must be prepared to wait for many hours before the water subsides.

Where can I find tsunami evacuation maps for other areas?

Maps for the wider Bay of Plenty region are listed at bopcivildefence.govt.nz.

What are the requirements if I am constructing a habitable building or shed/garage?

There are no requirements at this point.

Got a different question?

If you have any further questions, please contact Mark Ivamy, Natural Hazards Advisor at the Regional Council on 0800 884 881 ext 8372 or at mark.ivamy@boprc.govt.nz.