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.
A tsunami is made up of a series of travelling ocean waves of extremely long wavelength. They are triggered by large disturbances such as earthquakes, undersea volcanic eruptions or deep sea landslides.
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 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. A fast moving wave over 10 metres high 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. This puts smaller inland communities at risk and contaminates rivers with saltwater.
A tsunami can threaten 'lifeline' 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. The following video https://www.youtube.com/watch?v=ge7SvB-KFnA(external link) shows the effects of the 2011 Japanese tsunami on Santa Cruze Harbour, California. Although the tsunami ‘waves’ are 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 can cause significant damage to boats and marina infrastructure.
Distance 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 distance source tsunami makes it easier to predict their effects, and evacuate low-lying coastal communities.
Near source tsunami can be set off by:
Near source tsunami have a travel time of one or two hours. This makes it more difficult to evacuate people to safe, high areas before the tsunami reaches the coast.
Several 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)
The Eastern Coromandel Tsunami Strategy is being progressively developed by Waikato Regional Council and Thames-Coromandel District Council.
Work on the strategy initially focussed on Whitianga because it’s the location most at risk from the impacts of tsunami. Since then studies and public open days have been completed for Tairua, Pauanui, Whangamata, Whiritoa, Matarangi and Whangapoua. Other area currently under assessment include Opito Bay, Otama, Rings Beach and Kuaotunu. 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. In such cases, the travel time between the source and impact is between one and three hours.
Over time, the councils will be working with each major settlement on the Coromandel’s east coast communities.
A 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.
Despite the low likelihood of a tsunami event in these areas, tsunami may produce strong surges and currents, particularly at the entrance to the harbours, making it dangerous to be on or in the water.
The study, funded by Waikato District Council, Waikato Regional Council and WEL Networks, has been carried out by Jose Borrero of Raglan-based international marine and freshwater experts, eCoast.