The Stability of Tide Gauges in the South Pacific Determined from Multiepoch Geodetic Levelling, 1992 to 2010

Abstract

Tide gauge data forms the basis for determining global or local sea level rise with respect to a global geocentric reference frame. Data from repeated precise levelling connections between the tide gauges and a series of coastal and inland benchmarks, including a Continuous GPS (CGPS) benchmarks, is used to determine the stability of tide gauges at 12 locations in the South Pacific. The method for determining this stability is based on using a constant velocity model which minimises the net movement amongst a set of datum benchmarks surveyed since the installation of the tide gauges. Tide gauges were found to be sinking, relative to the CGPS benchmark, in Pohnpei (FSM), Samoa, Vanuatu, Tonga, Nauru, Tuvalu, Fiji and Cook Islands (Is); listed in order of their sinking rate. A maximum of-1.01  0.63 mm/yr is computed at Pohnpei (FSM) and the minimum at Cook Is was statistically insignificant. The tide gauge levels were rising, relative to the CGPS benchmark in Solomon Is, Manus Is (PNG), Kiribati and Marshall Is, with a maximum of 3.12  0.49 mm/yr at Solomon Is and a minimum at Marshall Is, which was statistically insignificant. However, these estimates are unreliable for the Solomon Is and Marshall Is, which have recently established CGPS benchmarks and have been surveyed less than 3 times. In Tonga and Cook Is, the tide gauges were found to be disturbed or affected by survey errors whereas the Vanuatu results were affected by earthquakes. It was also found that the constant velocity model did not fit the observations at the tide gauges in Tonga, Cook Is, Fiji, Marshall Is and Vanuatu, which had large variations in their velocities. This is an indicator of the high frequency (short period) motion of the tide gauge structure, which cannot be measured by the levelling method since they have a higher frequency than the time interval between levelling surveys.

Determination of the new vertical datum of Slovenia

“…Z mareografi opazujemo spreminjanje morske gladine, ki je vsota plimovanja in vertikalnih pomikov mareografa, saj so mareografi običajno postavljeni na pomolih, ki niso stabilni (Wöppelmann et al, 2006;Tervo, Poutanen in Koivula, 2007;Braitenberg et al, 2011;Sterle et al, 2017). Spremljanje morske gladine je tako obremenjeno tudi s posedanjem mareografa, ki je zajeto v izmerjeni morski gladini na mareografu, zato je nujno, da ga izločimo iz izračunane srednje morske gladine (preglednica 5) (Bevis, Scherer in Merrifield, 2002;Wöppelmann et al, 2006;Hannah, 2010;Santamaría-Gómez, Bouin in Wöppelmann, 2012;Deo, Govind in El-Mowafy, 2013). Za korekten izračun višinskega datuma mora biti natančnost določitve vertikalnega pomika večja od natančnosti določitve srednje morske gladine (Deo, Govind in El-Mowafy, 2013).…”

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“…Kljub temu se moramo zavedati, da so bili številni pomoli, na katere so postavljeni ali prestavljeni mareografi, med dolgoletnimi opazovanji nivoja morja obnovljeni, dograjeni ali kako drugače preoblikovani. Za zanesljivo spremljanje spreminjanja nivoja morja potrebujemo dobro dokumentirane podatke o vseh posegih na mareografih in njihovih vertikalnih premikih (Wöppelmann, Zerbini in Marcos, 2006;Hannah, 2010;Deo, Govind in El-Mowafy, 2013).…”

“…Nivelmanska mreža naj bi bila ponovno izmerjena po preteku 1,5 leta oziroma vsako leto v daljšem obdobju od 10 do 20 let (Bevis, Scherer in Merrifield, 2002;Deo, Govind in El-Mowafy, 2013;Gill, Weston in Smith, 2015).…”

“…Težava pa nastane, ko izbrani domnevno stabilni reper ni tudi dejansko stabilen. Takrat je pomembno, da najprej določimo stabilne reperje v nivelmanski mreži (Deo, Govind in El-Mowafy, 2013). Med izvedbo projekta Posodobitev prostorske podatkovne infrastrukture za zmanjšanje tveganj in posledic poplav smo v okviru podprojekta Razvoj geodetskega referenčnega sistema 2014 opravili analizo stabilnosti reperjev na podlagi razlik višin reperjev, ki jih pridobimo s prosto izravnavo nivelmanske mreže in razlik merjenih višinskih razlik med posameznimi reperji.…”

“…Znano je, da se pomoli posedajo, in ti lokalni premiki običajno niso povezani z nestabilnostjo litosfere. Takrat je smiselno stalno delujoče postaje GNSS postaviti v bližini mareografov, na območja, kjer so vertikalni premiki povezani s premiki litosfere (Bevis, Scherer in Merrifield, 2002;Wöppelmann, Zerbini in Marcos et al, 2006;Buble , Bennett in Hreisdóttir, 2010;Deo, Govind in El-Mowafy, 2013;Dawidowicz, 2014). Pomembno je tudi, da je referenčna točka GNSS povezana z nivelmansko mrežo mareografa z ustrezno natančno geodetsko izmero (Bevis, Scherer in Merrifield, 2002).…”