Seychelles Tsunami Impacts in the Western Indian Ocean

Temporal and spatial characteristics of the December 2004 tsunami in the Republic of Seychelles,Western Indian Ocean are described, with particular reference to the detailed water level record from the Pointe La Rue tide-gauge, Mahé, and tsunami run-up characteristics on Mahé and Praslin. Assessments of tsunami impacts on coastal and shallow marine environments in the granitic islands of the Northern Seychelles, and on the coral islands of selected locations in the Southern Seychelles, are reported. The lack of noticeable impacts within the southern islands compared to those further north appears to be related to both reduced tsunami wave heights to the south and to differences in regional bathymetry, the tsunami being accentuated by the shelf seas of the Seychelles Bank in the north and not amplified around the southern islands surrounded by deep water.

INTRODUCTION

At some 5000 km from Sumatra, the 115 islands of the Republic of the Seychelles were not in the front line of tsunami impacts. Only two tsunami-related fatalities were reported. Nevertheless, the tsunami did have a considerable infrastructural and economic impact, notably on the northern granitic islands. There was prolonged flooding of the capital, Victoria, as a result of the blocking of the storm drainage system by sediments mobilized by the tsunami, fissuring and failure of dock walls at Port Victoria from repeated inundation and drawdown cycles on unconsolidated fills (Plates 1, 2), washouts of key transport routes by the drainage of tsunami waters from coastal lagoons (Plates 3, 4), disruptions to water supply and sewerage networks (with in the case of the latter attendant pollution problems) and extensive structural damage to houses, hotels, restaurants and other beach-front infrastructure. Total estimates of damage have been assessed at US$30 million (UNEP, 2005) due to both structural damage and loss of earnings following the event. The tsunami was said to have damaged 94 fishing boats, a third of the entire fishing fleet, around Mahé and fish catches for January 2005 dropped by 30% compared to previous catches for this month (Payet, 2005, pers. comm.). Here we document the temporal and spatial characteristics of the tsunami in the Seychelles and review its impact on geomorphology and shallow marine ecosystems. We draw heavily on the Canadian United Nations Educational, Scientific and Cultural Organization (UNESCO) mission to the Seychelles (Jackson et al., 2005) and on the International Union for the Conservation for Nature and Natural Resources (IUCN) report (Obura and Abdulla, 2005), supplemented by our own observations in Mahé (Stoddart and Hagan, 1 and 4/2005) and the remote southern islands of the Amirantes, Alphonse/St. François and Providence Bank (Hagan, 1/2005), Aldabra and Assumption (Stoddart 4/2005)…