Also in this topic: Tsunami flooding and evacuation zones at popular beaches
Tsunami are giant waves that can threaten people and property near coastal areas. They can occur after large disturbances such as earthquakes, volcanic eruptions, and deep sea landslides. Find out more about tsunami and why they are a natural hazard in coastal areas of the Waikato region.
Figure 1 below shows the amplitude, trough and crest of a tsunami wave in relation to the sea floor and still water level. Note how the wave shape changes and the height increases as it approaches the coastline.
Figure 1: Change in tsunami wave shape and height as it approaches land.
Tsunami are a threat to people and property in coastal and low-lying estuarine areas. The waves travel quickly, rapidly flooding and damaging coastal communities, picking up debris as they go. Fast moving waves can quickly destroy homes and communities. Tsunami also create seiching in harbours and confined estuaries.
Tsunami waves can travel inland along river beds as continuous single standing waves due to the energy contained within the wave. This puts smaller inland communities at risk and contaminates rivers with saltwater.
A tsunami can threaten 'lifelines' services such as water, power, telecommunication and transportation networks. Find out more about managing Lifelines to deal with unexpected emergencies or natural hazards.
There are two types of tsunami that affect the New Zealand coastline:
While Distant source tsunami are unlikely to cause significant inundation, the effects of the tsunami ‘surge’ up estuaries, harbours and marinas can still be significant, such as those that occurred in the Santa Cruz Harbour in California following the 2011 Japan tsunami. Although the tsunami ‘waves’ in this event were relatively small in height (compared to open coast wind generated waves), the amount of energy contained within the waves, even after travelling some 14 hours across the Pacific Ocean caused significant damage to boats and marina infrastructure.
Distant source tsunami are often triggered from sites along the Pacific Ocean’s ‘Ring of Fire’ where tectonic plates slide against and under one another (subduction), often causing earthquakes. Along this ‘Ring of Fire’ there are four main sources for large earthquakes that have affected the New Zealand coastline in the past:
Figure 2 below shows the travel-time in hours for a tsunami originating either from Alaska or Chile:
Figure 2: Tsunami source areas and travel-times (hours).
The long travel times of distant source tsunami makes it easier to predict their effects, and evacuate low-lying coastal communities.
Near source tsunami can be caused by:
Near source tsunami have a travel time of up to three hours, but can be an hour or less. This makes it more difficult to evacuate people to safe, high areas before the tsunami reaches the coast.
The effects of a tsunami are likely to be experienced by all coastal communities within the Waikato Region. To reduce the risks to these communities Waikato Regional Council:
Crown Research Institutes are researching historical tsunami and their effects on the coastline.
For policy information on natural hazards, see our Regional Policy Statement.(external link)
Work on assessing and communicating tsunami hazards and risks is being progressively developed by Waikato Regional Council in partnership with Thames-Coromandel District Council and Hauraki District Council.
Scientific work from 2006 to 2011 initially focussed on Whitianga because it is most at risk from the impacts of tsunami. Since 2011, studies and public open days have been completed for Tairua and Pauanui, Whangamata and Whiritoa, Matarangi and Whangapoua. Opito Bay, Otama Beach, Kuaotunu/Rings Beach, Kennedy Bay and Cooks Beach. Tsunami hazard modelling has also been completed for Port Charles, and for the western coast of the Coromandel Peninsula and the Firth of Thames.
Using the latest information, knowledge and tsunami computer models, scientists have been able to determine that the greatest risk of significant tsunami inundation to the Coromandel's eastern coastline will come from earthquakes occurring along the Tonga-Kermadec Trench, and that local faults may present the biggest risk with the Firth of Thames.
A 2015 study to identify the risk of tsunami affecting Port Waikato, Raglan Harbour and Aotea Harbour has found wave heights will be relatively small, but inundation (flooding) may affect low-lying areas.