Tides

Tide forecasts

Predictions of high and low tides anywhere in New Zealand’s marine area are based on computer model simulations of the tide behaviour in the SW Pacific. These predictions are only intended for information, and are NOT to be used for navigation or engineering design as heights are not surveyed in to the local Chart Datum. For enquiries, contact Rob Bell (r.bell@niwa.co.nz).

For tide predictions at NZ ports, see the Land Information NZ (LINZ) web site.

Sea-level network

The sea-level recorder at Anawhata being serviced by helicopter.

NIWA coordinates a network of open coast sea-level recorders around the New Zealand coast. The NIWA sea-level recorder network is supported by some regional and district councils, University of Canterbury and by the National Tidal Facility (Adelaide). The primary motivation is too collect high-quality and accurate measurements of sea level and tides for scientific studies of storm surge, tides and global warming at open coast sites that are unaffected by port activities or rivers. “Sea level on the move?” is an article on the network and why the need for high quality data. The data is also used for processing hydrographic surveys of the sea bed topography.

Some of the recorders have also been tsunami enabled to store sea-level measurements quickly at 1-minute intervals to better describe the various waves that make up a tsunami event.

Tidal model of New Zealand’s EEZ

Close-up photos of some moss species found deep within lakes, including one with minute, scale-like leaves. (Photos: M. de Winton)

Contrasting tides at Kaikoura (east) versus Foxton (west). P= Moon’s perigee, and N,F = New and Full Moon (Click to enlarge)

Overall, tides account for about 95% of the variation in sea level we see on the NZ coast. The nature of the tides varies considerably from place to place around NZ. The most noticeable difference is between the east and west coasts (see plot). On our west coasts and in Tasman/Golden Bay, we have large tidal ranges (up to 4 m) during spring tides (following New of Full Moon), and much smaller neap tides, when the Moon is in the 1st and last quarter phases. Contrast that with the mid-eastern coasts and Chatham Islands, where the biggest tides occur only once a month. These are called perigean tides, so-called because they occur when the Moon is closest to the Earth (i.e., its perigee) in its elliptical or non-circular orbit every 27.5 days.

The rising and falling tide we observe at the beach or in an estuary actually comprises many different tides (or constituents) of various strengths and cycle times, each resulting from a particular feature of the gravitational attraction by the Moon and Sun on Earth’s waters. NIWA has built a computer tidal model to simulate 16 of the most important tides across the entire Exclusive Economic Zone (EEZ) around New Zealand. The model has been calibrated with measurements from the sea-level network and is the basis of the tide forecasting service.

Three of the important tidal constituents from the model are illustrated by a static co-tidal image, summarising the tide amplitude (or half-range) and the phase (or timing of high water), and a movie of the tide wave height as it regularly cycles around New Zealand.

Lunar Semi-diurnal Tide (M2), occurring every 12.4 hours

Highest M2 tides occur in the western Cook Strait area from Taranaki to Nelson. This tide wave propagates anti-clockwise around New Zealand.

Click here to see a static image of an M2 tide

Click here to see the M2 tide in action (135KB animation)

Solar Semidiurnal Tide (S2), occurring every 12.0 hours

This tide propagates around New Zealand in a similar fashion to M2, but meets opposing waves on the east coast. The tide and opposing waves cancel each other out to produce low solar tides on the east coast.

Click here to see a static image of an S2 tide

Click here to see the S2 tide in action (90KB animation)

Diurnal Tide (K1), occurring once a day every 23.9 hours

Cause of the daily variation between successive high tide heights.

Click here to see a static image of a K1 tide

Click here to see the K1 tide in action (139KB animation)

The NZ tide model is described in more detail in Computer Models Define Tide Variability published in The Industrial Physicist and further tide model animations featured as Image of the Month in the Tecplot Contours online magazine.

For more background information on tides, visit these websites:

Red Alert days for coastal flooding

Foreshore Rd in Colac Bay, awash after a storm surge event on 16 April 1999 (Photo: D. Bradley)

Most New Zealand cities are situated on the coast and many of these are beside rivers. Therefore, they are at risk from flooding, either from the sea itself, or from rivers discharging into the sea. Yet, a major contributor to coastal flooding around New Zealand are high tides, which are deterministic and able to be forecast well in advance. Most rivers have protection against inundation from the tide itself, but IF a storm surge or river flood occurs at the same time as the largest tides, serious flooding can occur. The amplitude and height of the tide is governed by the positions of the Moon and Sun relative to Earth and these positions are well-known in advance. Hence, estimates are easily made of dates during the year when the potential for coastal flooding is greatest–they occur a few days after Full or New Moon when the Moon is also closest to the Earth (in its perigee). These tides are called perigean-spring tides.

Thus, a set of RED-ALERT dates can be generated when even a minor storm surge or river flood could cause coastal flooding in low-lying areas because the tides are extreme for those dates (see photo). Conversely, another set of CAREFREE dates can be generated when there is little danger of flooding at the coast unless a large storm surge or river flood occurs. Of course, a really major flood or storm surge event may cause coastal flooding at any time, no matter what the tide. No one is surprised when such events cause flooding. However, many people were surprised on 17 April and 15 June 1999 when relatively minor storm surge events coincided exactly with perigean spring tides to cause widespread coastal flooding from Dargaville to Christchurch. Yet these dates were predicted in a set of RED-ALERT dates for that year.

Predicted Red-alert and Carefree dates apply not only for New Zealand, but almost everywhere around the world. Of course, the details differ from place to place and depend upon the pattern of tidal propagation. In some places, the dates of maximum tide height will be a day or two different from those forecasted and the high tide height on some of those days may be nothing out of the ordinary.

High and low tides for the entire year are summarised in a single plot for several sites from the sea-level network, with reference to Full and New Moon and the Moon’s perigee.