Tsunami Towered Nearly Four Stories As It Hit Samoa
Saturday, 5 December, 2009
Wellington - The tsunami which killed 180 people in American Samoa, Tonga and Samoa after a magnitude 8.3 earthquake in the Tonga Trench on September 29 towered nearly four storeys high as it hit, New Zealand scientists say.
Once on the beach, it reached 14m above mean sea level in Samoa and 10m in American Samoa, and penetrated as far as 700m inland, according to the scientists who have spent two weeks assessing the tsunami's impact on communities, individuals, infrastructure, and the environment.
The earthquake, 190km from Apia generated two to three significant waves, with the second wave larger than the first, according to witnesses.
The first wave arrived in Samoa 10 minutes after the quake, and then hit American Samoa 10 minutes later, said the researchers from the National Institute of Water and Atmospheric Research (Niwa) and GNS Science who spent six days in American Samoa. They were in Samoa for another week as part of a United Nations international tsunami survey team.
Results of their field work were of interest in New Zealand and overseas, said GNS Science spokesman John Callan.
"This size of tsunami is also possible for New Zealand, equivalent to about a one-in-500 year event for the most populated parts."
Niwa has already carried out computer modelling to assess the impact in New Zealand of a tsunami from relatively close source, such as the Tonga-Kermadec trench, and found a tsunami from further away -- such as a big South American earthquake -- would swamp Auckland motorways, coastal roads and low-lying bays.
The Niwa report for the Auckland Regional Council predicted the impact of the wave along vulnerable coastal areas.
Niwa's Dr Shona van Zijll de Jong said the Samoan project involved a coordinated team of international scientists who broke new ground in disaster loss assessment.
Preliminary results from their survey of the physical and human effects of the tsunami had direct relevance for New Zealand .
Varying degrees of damage to buildings showed reinforcement was important -- traditional light timber buildings were completely destroyed by floodwater of 1.5m or higher, but the use of minimal reinforced-concrete columns reduced the damage levels significantly.
Building damage depended on water depth, structural strength, shielding, foundations, the quality of building materials and workmanship, and whether the builders had kept to building codes.
Plants, trees, and mangroves on the foreshore reduced the speed and depth of water flows, increasing the chances of human survival and lower levels of building damage.
"The same thing will be true in New Zealand ... solidly constructed buildings which are appropriately located will survive much better than flimsy buildings right on the beach," said Dr Stefan Reese of Niwa.
"It's also clear that practices such as flattening sand dunes or removing beach vegetation would increase the potential for tsunami damage."http://www.voxy.co.nz/national/tsunami-towered-nearly-four-storeys-it-hit-samoa/5/32331