Tsunami in the Seychelles: Assessing mitigation mechanisms

Abstract

A spatial statistical model determining the possible tsunami impact mitigation by coastal vegetation developed after the December 2004 earthquake event for Aceh, Indonesia, was adapted and tested under the conditions of the Seychelles in the western Indian Ocean to find out whether comparable protective effects of vegetation existed. The waves generated at the three main populated islands, Mahé, Praslin and La Digue averaged less than 2 m at the shoreline allowing assessment of effects of vegetation in mitigating impact of a low intensity tsunami event. Semi structured interviews with local witnesses of the event, field measurements of local features, as well as secondary data sources provided information on wave height at the shoreline, landscape characteristics as well as tsunami impacts inland, i.e. structural damage and maximum flood distance. Observation points at maximum flood distance and where damage occurred were geo-located and entered into a GIS. Vegetation density and resistance coefficients were determined on site and their spatial distribution through visual interpretation of Google Earth® pre tsunami imagery. Data was analyzed using a generalized linear mixed model, acknowledging the spatial distribution of data. As for Aceh, the results indicated that distance from the settlement to the shoreline was the most important factor to avoid adverse tsunami effects. In contrast to Aceh, a direct effect of coastal vegetation was not observed (p > 0.05). A significant protective effect, however, was apparent through sand dunes, stabilized by vegetation, reducing damage to buildings by up to 30% where initial water height at shoreline was 3 m. Maximum flood distance was not affected by sand dunes but was reduced (p = 0.013) by existing seawalls. A coastal planning that encompasses the promotion and protection of multi-use coastal vegetation, i.e. generating income through tourism, providing food products and protecting natural physical barriers is encouraged in order to enhance local resilience against natural wave events such as those of the 2004 tsunami.