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14 Factsheet 3: Coastal inundation (tsunami)

The word tsunami is used internationally, and is a Japanese word meaning ‘harbour wave’ or waves. Tsunamis are generated by a variety of geological disturbances, particularly:

large seafloor earthquakes in which significant uplift or subsidence of the seafloor or coast occurs
submarine landslides (which may be triggered by an earthquake)
volcanic eruptions (eg, under-water explosions or caldera (crater) collapse, pyroclastic flows and atmospheric pressure waves
large coastal-cliff or lakeside landslides
very occasionally, meteorite (bolide) impact.

Tsunami inundation at the Port
of Gisborne in 1960.

The last major remote source tsunami to hit our shores was the Chile tsunami of 1960 that reached 5.5 m high in Lyttleton Harbour, thankfully around low tide. It caused damage at many locations along the east coast.

Tsunamis can be classified either by the distance from their source to the area impacted or, more relevant for emergency management purposes, the travel time to the impacted area and the length scale of impact. For New Zealand, three categories are typically defined:

local source/local impact event – within 60 minutes travel time and affecting several tens of km of coast
regional source/regional impact event – within three hours’ travel time and likely to affect a region or several regions
distant (remote) source/national impact event – longer than three hours’ travel time and likely to affect many regions.

Remains of a four-bedroom house
north of Gisborne destroyed due to
inundation caused by a local source
tsunami in March 1947.

The remains of a four-room house north of Gisborne that was destroyed by a 10-m local source tsunami in March 1947. Three people rode out the tsunami in the house, while two others ran across the road and up the hill with water at their heels. Pouawa, where the bridge was destroyed, is in the distance. Source: Weekly News, 2 April 1947.

Tsunami wave characteristics at any location can vary substantially, depending on several factors, including: the generating mechanism; the location, size and orientation of the initial source (disruption); source-to-locality distance; and local seabed and coastal margin topography. The timing and height of high tide are also critical factors in determining the extent and magnitude of inundation.

Tsunami waves differ from the waves we see breaking on the beach or in the deep ocean, particularly in their length between wave crests. In a tsunami wave-train, the distance between successive wave crests (the wavelength) can vary from several kilometres to over 400 km, compared to around 50–100 metres for waves at the beach. The time between successive tsunami wave crests can vary from several minutes to an hour, rather than several seconds. As tsunami waves reach shallow coastal waters, they slow down and steepen rapidly, sometimes reaching heights of 10 m or more. Shallow bays and harbours tend to focus the waves and cause them to be amplified (or resonate) and slosh back and forth. Tsunami waves that overtop or breach natural coastal beach ridges and barriers can surge considerable distances inland in low-lying areas (100s of metres to a kilometre or more, depending upon the wave height at the shoreline, the wave period and the geographical characteristics of the coastal margin).

Key tsunami definitions
tsunami period (minutes) – the time between successive wave peaks. This can fluctuate during a single event and vary between different locations within the same region. Periods are usually in the range of a few minutes (eg, ‘local source/local impact’ tsunami) to an hour or more for a ‘distant source/national impact’ tsunami tsunami height (m) – taken as the vertical crest-to-trough height of waves, but it is far from constant, and it increases substantially as the wave approaches the shoreline. It is generally used only in conjunction with measurements from a sea-level gauge to express the maximum tsunami height near shore tsunami amplitude (m) – the height difference between a wave crest and the instantaneous sea level at the time of arrival. It is used in tsunami warnings tsunami run-up (m) – a more useful measure; the vertical inundation elevation the seawater reaches above the instantaneous sea level at the time of the tsunami (including the tide). This measure still has the drawback that it depends markedly on the type of wave (rapidly rising and falling, a bore, a breaking wave, the wave period) and on the local slopes of the beach and foreshore areas, so it is site-specific inland penetration (m) – the maximum horizontal distance inland from the shoreline or mean-high-water mark inundated by the tsunami (inundation line). It depends on the tsunami run-up and local topography, barriers and slopes within the coastal margin.

Key tsunami definitions

tsunami period (minutes) – the time between successive wave peaks. This can fluctuate during a single event and vary between different locations within the same region. Periods are usually in the range of a few minutes (eg, ‘local source/local impact’ tsunami) to an hour or more for a ‘distant source/national impact’ tsunami
tsunami height (m) – taken as the vertical crest-to-trough height of waves, but it is far from constant, and it increases substantially as the wave approaches the shoreline. It is generally used only in conjunction with measurements from a sea-level gauge to express the maximum tsunami height near shore
tsunami amplitude (m) – the height difference between a wave crest and the instantaneous sea level at the time of arrival. It is used in tsunami warnings
tsunami run-up (m) – a more useful measure; the vertical inundation elevation the seawater reaches above the instantaneous sea level at the time of the tsunami (including the tide). This measure still has the drawback that it depends markedly on the type of wave (rapidly rising and falling, a bore, a breaking wave, the wave period) and on the local slopes of the beach and foreshore areas, so it is site-specific
inland penetration (m) – the maximum horizontal distance inland from the shoreline or mean-high-water mark inundated by the tsunami (inundation line). It depends on the tsunami run-up and local topography, barriers and slopes within the coastal margin.

The arrival of a tsunami wave-train (ie, it typically isn’t just one wave) is often manifested by an initial draw down of the level of the sea (much faster than the tide). However, the first sign may instead be an initial rise in sea level. The waves that propagate towards the coast seldom break before reaching the nearshore area, and appear to have the whole ocean behind them. Inundation of the coastal margin continues until maximum run-up height is reached before the water temporarily recedes. Other tsunamis occur as an advancing breaking wave front or bore, which is the type of wave most people associate with a tsunami. For the same wave height at the shore, a longer-period tsunami wave-train, such as generated by a remote source (eg, from South America), will cause greater inundation volumes than a shorter-period wave (eg, a local source).



Modelling tsunami waves approaching the Coromandel and Hauraki Gulf coasts: Modelling tsunami inundation requires complex computer models that can simulate: the generation of the tsunami; the way the waves propagate over the ocean; the waves’ interaction with the continental shelf, nearshore seabed and coastline; and ultimately their flow over and retreat from the coastal margin. Such modelling requires detailed nearshore bathymetry information and topography, such as LiDAR data.

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