Geological stability assessment, Port of Lyttelton tide gauge, Canterbury, New Zealand

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Barrell, D.J.A. 2001 Geological stability assessment, Port of Lyttelton tide gauge, Canterbury, New Zealand . Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 2001/20 37 p.

Abstract: Situated on Banks Peninsula, Port of Lyttelton lies in Lyttelton Harbour, a former stream valley drowned by the post-glacial rise in sea level. A layer of post-glacial marine silt (harbour silt) overlies Pleistocene-age deposits of valley-fill alluvium and windblown loess that mantle the Miocene-age volcanic bedrock. Reclamation fill placed as part of port development covers the original shoreline and expends out over the harbour silt. The tectonic regime of the Lyttelton area is a matter of some debate. Unambiguous evidence from eastern Christchurch indicates long-term subsidence. Some recent studies have suggested tectonic stability over at least the past 120,000 years but the evidence for stability is not conclusive. The matter remains unresolved. Possibilities range from tectonic stability (nil uplift or subsidence) to subsidence of the order of 0.2 to 0.3 mm/year. Even if Lyttelton is affected by long-term subsidence, there is no information on whether subsidence is steady, or occurs in episodes. Precise regional geodetic surveying over a 30-year period may bring some closure to the issue. Since the late 1950’s the tide gauge and tide pole have been on the No.1 Wharf, which is piled down to a probably firm foundation on valley-fill alluvium deposits. However, the current reference benchmark is on fill overlying harbour silt, and this should be a cause for concern. There is a benchmark on in-situ volcanic rock some 200 m northeast of the tide gauge that would be a suitable alternative reference point. Prior to the late 1950’s the tide gauge was on the A Wharf, which lies beside No.1 Wharf. No information has been found on the A wharf construction, or survey reference system. By analogy with the No.1 Wharf, it is likely that A Wharf was seated on valley-fill alluvium, and thus had a reasonably secure foundation. An investigation focused on survey records may uncover information on the earlier history of the tide gauge. (auth)