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:
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).