A new approach to determining the long-term trend in relative sea levels
Hannah, John & Rob Bell
New Zealand is a tectonically active country that straddles the Australian and Pacific plates, having over 15,000 km of coastline stretching in latitude from 344 to 477S. Assessing longterm changes in relative sea-level at a regional level is important to future development decisions. In the past, and as recommended by Douglas (1991,1992), relative sea level changes have typically only been determined at sites with at least 50 60 years of almost continuous sea level data. In New Zealand, this has been done at the four main ports of Auckland, Wellington, Lyttelton, and Dunedin. This paper describes a new approach to improve the spatial coverage of reliable estimates of sea level rise at a regional level. Here, data from other regional tide gauges, typically with broken discontinuous records, and previously used only to define local height datums were used. The process involved a comparison of an old historical Mean Sea Level (MSL) datum with a newly defined MSL datum. A simple linear trend was fitted between the two datum points and then the trend assessed for possible bias due to inter-annual and inter-decadal sea level variability. This process, not previously used for trend determination, has enabled new sea level trend estimates to be derived at a further six tide gauge sites around New Zealand. The average relative sea level rise from these gauges is 1.7 0.1 mm.y-1, a result that is entirely consistent with the analyses of the four long-term, primary tide gauge records. Most importantly, the process offers a relatively simple solution for improving the spatial determination of relative sea level trends in data sparse areas of the world.
Event: FIG Working Week 2012 : Territory, environment, and cultural heritage
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