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Climate change Coastal Hazards and Climate Change: A Guidance Manual for Local Governmentin New Zealand Online version
16 Factsheet 5: Tides
Tides are generated by gravitational forces exerted by both the Sun and Moon on the Earth’s oceans. Ocean tide waves then propagate onto the continental shelf and into estuaries and harbours, being modified by wave shoaling (where the tidal wave slows down and increases in tide range as the water becomes shallower), friction from the seabed and constrictions such as estuary entrances, river mouths and straits. Tides are entirely predictable and can be predicted for any day or period many years in advance. The tide range (the difference between high and low waters) varies around New Zealand, reaching 3.5–4 m on the west coast but only 1–2 m on the east coast. A tide mark commonly used to characterise high tides is mean high water spring (MHWS), which is also used to define the coastal planning boundary. MHWS is traditionally calculated for nautical purposes as the long-term average of the highest high tide that occurs just after every new [N] and full [F] moon (ie, spring tides). Normally, only about 10–20% of all high tides would exceed such a MHWS mark. MHWS is a simple concept and values for it are widely available. Yet, New Zealand tides along the central–eastern coasts don’t easily fit with the commonly-used nautical MHWS definition. For example, at Kaikoura, c. 50% of high tides exceed the nautical MHWS level. |
 Spring tide range (in m) around the coast of New Zealand. |
 Comparison of tide range characteristics between Kaikoura (east coast) and Foxton (west coast). P=perigee, N=new moon and F=full moon. |
The reason is that there is little difference between the fortnightly neap and spring tides along the central–eastern region. Instead, the highest tides occur once a month (every 27.5 days), when the Moon’s elliptical orbit takes it closest to the Earth (ie, when the Moon is in its perigee [P]). Therefore, in estuaries and open coast locations on the east coast from Otago to Bay of Plenty, a better ‘hazard’ definition of the peak monthly tides is to use a ‘pragmatical’ MHWS, such that only 10% or 12% of local high tides exceed it; or use the mean high perigean-spring tide level (a higher tide that occurs in clusters peaking about every 7 months, often referred as a ‘king tide’, when a perigean and spring tide combine).
Tide prediction resources |
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www.niwascience.co.nz/services/free/tides. Open coast tide predictions at any location around New Zealand for any time period since 1830. www.hydro.linz.govt.nz/tides/majports/index.asp. Tide predictions at standard and secondary ports for the following 12 months. |
