HR: 0800h
AN: U11B-0831    [Abstracts]
TI: Effects of the 26 December 2004 Indian Ocean Tsunami in the Republic of Seychelles
AU: * Jackson, L E
EM: lijackso@nrcan.gc.ca
AF: Geological Survey of Canada (GSC Pacific), 101-605 Robson Street, Vancouver, BC V6B 5J3 Canada
AU: Barrie, J V
EM: vbarrie@nrcan.gc.ca
AF: Geological Survey of Canada (GSC-Pacific), 9860 West Saanich Road, Sidney, BC V8L 4B2 Canada
AU: Forbes, D L
EM: dforbes@nrcan.gc.ca
AF: Geological Survey of Canada (GSC-Atlantic), Bedford Institute of Oceanography, Box 1006, Dartmouth, NS B2Y 4A2 Canada
AU: Shaw, J
EM: johnshaw@nrcan.gc.ca
AF: Geological Survey of Canada (GSC-Atlantic), Bedford Institute of Oceanography, Box 1006, Dartmouth, NS B2Y 4A2 Canada
AU: Manson, G K
EM: gmanson@nrcan.gc.ca
AF: Geological Survey of Canada (GSC-Atlantic), Bedford Institute of Oceanography, Box 1006, Dartmouth, NS B2Y 4A2 Canada
AU: Schmidt, M
EM: mschmidt@nrcan.gc.ca
AF: Geological Survey of Canada (GSC-Pacific), 9860 West Saanich Road, Sidney, BC V8L 4B2 Canada
AB: The Dec. 26, 2004 Indian Ocean tsunami impacted Mahé and Praslin islands as a sequence of waves at intervals of tens of minutes to hours. The first tsunami wave struck at low tide, but others occurred through several tidal cycles, so that some subsequent waves arrived at high tide. The first indication of the tsunami on the Mahé tide gauge (sampling interval 4 minutes) was a rise in water level to lower than higher high water at large tides between 08:08 and 08:12 UTC(between 12:08 and 12:12 local time). This was followed by a maximum withdrawal of water in all areas. This level was not recorded by the tide gauge at Mahé, because the stilling well went dry, but evidence from observers indicates that it dropped as much as 4 m below mean sea level. The subsequent highest water levels, highest run-ups, and maximum distances inland that tsunami flooding reached were in coastal lowlands generally facing east toward the source of the tsunami. The highest flood levels on Mahé ranged from ~1.6 m to >4.4 m above mean sea level. On Praslin, they ranged from ~1.8 m to 3.6 m. The shallow (<200 m) shelf platform surrounding the granitic islands played an important role in determining the tsunami wave direction and amplitude at the shoreline. The shoaling waves were refracted, causing them to approach the islands from various directions, and amplified so as to cause higher run-up in specific coastal embayments. Consequently, tsunami inundation and damage were not confined to east-facing shores. Run-up and damage were locally as severe along shores of Mahé and Praslin facing away from the source of the tsunami. Some observers on the west sides of both islands reported water approaching from two directions (northwest and southeast). Furthermore, the timing of maximum inundation varied around the archipelago as tsunami waves arrived at different times in the tidal cycle: the maximum inundation at Anse-à-la-Mouche (on the west side of Mahé) occurred about 4 hours after the initial tsunami wave reached the archipelago, whereas the highest water level in the city of Victoria (on the northeast side of Mahé) occurred about 16 hours after the first arrival (but with much lower wave energy). Damage to public works was greatest in the Victoria area. Lateral spread failures developed in artificial fills forming the fishing port. Liquefaction was induced in these fills by cyclic inundation, saturation and rapid draw-down. Washouts occurred on two sections of highway causeway crossing reclaimed land south of Victoria due to the rapid drainage of tsunami floodwaters. Similar erosion caused structural failure of hotel buildings on Praslin. Elsewhere, the greatest damage was coincident with preexisting modification of the coast by development including: removal of natural beach berms, construction of hotel structures adjacent to the high-water mark or seaward over the beach, and placement of roads immediately adjacent to beaches. The damaging effects of the tsunami were confined to the granitic islands of Seychelles archipelago. The lack of impact on the atolls is due to the deep water surrounding them: this resulted in minimal shoaling and amplification of the long wavelength and low-amplitude tsunami waves.
UR: http://www.ioc.unesco.org/iosurveys/seychelles/sey1.htm
DE: 4564 Tsunamis and storm surges
DE: 9340 Indian Ocean
SC: Union [U]
MN: Fall Meeting 2005