Defining the position of MHWS is important as it is used to delineate the landward jurisdictional boundary of the Coastal Marine Area (CMA) under the Resource Management Act 1991 and the Foreshore and Seabed Act 2004. However, defining MHWS is not a straightforward task, particularly if an accurate definition is required. There are a variety of quantitative and qualitative definitions of what constitutes a MHWS level in use:
- MHWS: The traditional nautical approach is based on a quantitative ‘tidal harmonic’ definition of MHWS typically1 as the average of pairs of successive high waters in a 24-hour period in each semi-lunation (approximately every 14 days) at New and Full Moon (or in mathematical terms the sum of M2 (lunar) and S2 (solar) tide constituents. However, for central areas of the eastern coast of New Zealand, such a definition results in high tides that exceed such a MHWS level much more frequently than would be pragmatic for defining the boundary of the CMA.
- MHWPS: This upper-level MHWS is related to the higher perigean-spring tides that occur in clusters for a few months, peaking approximately every 7 months when a Full or New Moon coincide closely with the Moon’s perigee (king tides). Around New Zealand, such a tide height is exceeded by between 3% and 12% of high tides.
- MHWS-10 and MHWS-12: These definitions are based on an appropriate percentile of the high tides that would exceed a MHWS level. So, 10% of high tides exceed MHWS-10 and 12% of high tides exceed MHWS-12.
- Practical application of natural indicators: A range of natural indicators can be used to provide an qualitative assessment of MHWS, including toe of the dune, toe of the cliff, edge of vegetation, highest line of driftwood, tide marks on fence posts and, for estuaries, the seaward edge of glasswort (Salicornia australis) or other salt marsh plants.2
Max high water – the maximum high tide level over 100 years.
Text description of figure: The percentage of time that high tide levels exceed different levels above mean level of the sea over a 100 year prediction of tides at Kaikoura. On the right-hand side of the plot are different high tide level definitions corresponding to their respective levels above mean level of the sea at Kaikoura:
Mean High Water Apogean Neap Tide level which is exceeded by 97% of all high tides.
Mean High Water Neap level which is exceeded by 56% of all high tides.
Mean High Water Spring level which is exceeded by 43% of all high tides.
Mean High water Spring twelve percent which is exceeded by 12% of all high tides
Mean High Water Perigean Spring level which is exceeded 11% of all high tides
An exceedence curve of high tides for a 100-year period at Kaikoura showing the different levels relative to mean level of the sea (MLOS) for different definitions of MHWS – MHWS (traditional approach); MHWS-12 – level exceeded by 12% of high tides; MHWPS – mean high water perigean-spring tide. Also shown are neap high tide markers (MHWN, MHWAN).
Both Land Information New Zealand and the Environment Court have emphasised that there is no single definitive method that can be used to establish a natural boundary such as MHWS; the method used will have to depend on the particular issue under consideration and natural characteristics of the location.
1 Bell RG. 2007. Use of exceedence curves for defining MHWS and future sea-level rise. In: Coast and Ports 2007: Proceedings of the 17th Australasian Conference on Coastal and Ocean Engineering. Melbourne, 17–20 July 2007.
2 Baker RF, Watkins M. 1991. Guidance notes for the determination of mean high water mark for land title surveys. Report published by the Professional Development Committee of the NZ Institute of Surveyors. 12 p. + Appendices. www.surveyors.org.nz/Documents/ MeanHighWaterMark-LandTitleSurveys(1).PDF (23 April 2008).