Long-term stability of Wellington tide gauge, 1911-2001, incorporating May 2001 levelling survey to Wellington fundamental benchmark K80

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Beavan, R.J. 2001 Long-term stability of Wellington tide gauge, 1911-2001, incorporating May 2001 levelling survey to Wellington fundamental benchmark K80 . Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 2001/09 18 p.

Abstract: As part of a project to correct long-term trends in sea level for tectonic and other vertical motions near tide gauges, we have remeasured a levelling survey between the Wellington tide gauge and several nearby benchmarks set in or close to bedrock. The levelling survey was previously measured in 1971 and 1980. Since the tide gauge was established at its present sites on Queen's Wharf in 1970, we find that the location of the gauge has been subsiding at an average of 0.2 - 0.25 millimetres per year relative to nearby benchmark K80, which is founded in bedrock. This rate is some 10% of present estimates of global sea level rise, so is a small but significant correction to the long-term sea level signal estimated from Wellington tide gauge data. Analyses of earlier levelling data indicate relative stability from 1911-1945 of the original tide gauge location, but indicate subsidence rates up to several tenths of a millimetre per year between 1945-1971 at the 1944-1970 tide gauge location. These are subsidence rates of the tide gauge location relative to the nearby bedrock benchmarks, and do not take account of possible regional tectonic subsidence or uplift. Benchmark K80 was established as the Wellington fundamental in 1970, prior to the destruction of the previous fundamental benchmark (known as ''2B''). Because of the mid-city location and the possibility of developemnt in the area, four separate stainless steel marks were established within a radius of 75 metres to jointly hold the level of K80. We find that only two of these marks have survived to 2001 without disturbance, and we recommend that additional marks are established to assure long-term survival of K80. (auth)