Post-tsunami survey of Niuatoputapu Island, Tonga, following the 30th September, 2009, South Pacific tsunami

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Wilson, K.J.; Power, W.L.; Nishimura, Y.; 'Atelea Kautoke, R.; Vaiomo'unga, R.; Mori, H.; Pongi, 'A.; Fifita, M.; Vaoahi, M.; Teukava, S. 2009 Post-tsunami survey of Niuatoputapu Island, Tonga, following the 30th September, 2009, South Pacific tsunami. Lower Hutt, N.Z.: GNS Science. GNS Science report 2009/71 25 p.

Abstract: The Tongan island of Niuatoputapu was severely affected by the 30th September, 2009, South Pacific tsunami. Located c. 190 km from the earthquake epicentre, there were 9 fatalities on Niuatoputapu, over half of the houses were destroyed and the healthcare and communications facilities were badly damaged. Post-tsunami surveys were undertaken by Tongan, New Zealand and Japanese teams, this report is a compilation of results from those surveys. Three basic tsunami wave parameters were measured by the survey teams: inundation distance, runup elevation and flow depths. The inundation distance of the tsunami was measured at 105 locations on Niuatoputapu with a greater density of measurements around the villages. The maximum recorded inundation distance was 910 m on the southeastern coastline of the island. Inundation distances in the villages were typically 200 – 500 m. Interpretation of post-tsunami satellite images suggests there had been inundation up to 1100 m inland at the southeast of Niuatoputapu and that 46% of the island had been inundated by the tsunami. The tsunami runup elevation was measured at 29 locations with a maximum runup of 4.7 m above mean sea level at the village of Falehau in the northwest of Niuatoputapu. Runup elevations were lower on the eastern coastline due to the gentle topography. The flow height of the tsunami reached a maximum of 16.9 m above mean sea level at Toma, on the southeast coast of Niuatoputapu. Flow heights were typically between 8-15 m along the northern peninsula and eastern coastline of Niuatoputapu. Flow heights decreased by about half, to between 4 – 7 m above mean sea level along the western coastline of Niuatoputapu where the villages of Hihifo, Vaipoa and Falehau are located. Topographic profiles show that tsunami flow depths at Hihifo were 2.5 – 3.5 m above ground level near the shoreline, remained at c. 2 m above ground level up to 300 m inland at Hihifo and then decreased rapidly towards the inundation limit. Flow direction indicators suggest the strongest wave of the tsunami came from a northeast to east direction; the tsunami flow wrapped around the northern and southern points of the island and inundated the west coast from variable directions. Tsunami impact to the villages of Niuatoputapu was severe but some of the greatest damage was evident in the unpopulated, forested areas of the eastern and northern coastline of Niuatoputapu. In these areas swathes of mature forest were completely destroyed, debris piles of trees and vegetation were built up on land and in the lagoon, the shoreline was significantly scoured and the land surface was stripped of soil. (auth)