Geological stability assessment of the Port Otago tide gauges, Otago Harbour, New Zealand

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Barrell, D.J.A. 2002 Geological stability assessment of the Port Otago tide gauges, Otago Harbour, New Zealand . Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 2001/21 19 p.

Abstract: Port Otago lies in Otago Harbour, and includes deep-water port facilities at Port Chalmers in the middle reaches of the harbour, as well as port facilities at Dunedin, at the southwest end of the harbour. Tide gauging is carried out at both Port Chalmers and at Dunedin. Otago Harbour is a former valley system drowned by the post-glacial rise in sea level. Post-glacial marine sediments overlie Pleistocene-age deposits including valley-fill alluvial gravel, windblown silty loess and silty or gravelly slope colluvium. These deposits mantle Miocene-age volcanic bedrock. At both Port Chalmers and Dunedin, the original shoreline is masked by extensive reclamation fill that has been placed out over the harbour sediment. Long-term rates of vertical tectonic movement are considered to lie in the range of 0.1 +/- 0.2 mm/year. In the Otago Harbour area, some geological evidence favours either stability or very slow subsidence, and any vertical tectonic movement that has occurred will have had an equal effect on both Port Chalmers and the Dunedin port. For practical purposes it can be assumed that there has been approximate tectonic stability over the period of tide gauge records. The Port Chalmers tide gauge system was installed in the late 1970s, and it appears that tides were not recorded at Port Chalmers prior to that time. The tide gauge, reference marks and the GOS survey site appear to have stable foundations. Dunedin tide gauge records span back to 1899, although there is no information on tide gauge location prior to 1922. Between at least 1922 and December 1982, the tide gauge was positioned on the Birch St. Wharf, after which it was suspended below the Otago Harbour Board head office, until its repositioning to its present location on the Victoria Wharf T & U Section in June 1993. Drillhole evidence indicates that all the Dunedin port structures, upon which the tide gauge has been fixed since at least 1922, have stable foundations within a firm substrate of valley-fill deposits. The main reference benchmark on the G Shed at Dunedin also appears to have secure foundations. (auth)