Application of a synthetic cyclone method for assessment of tropical cyclone storm tides in Samoa

Tropical cyclone-induced storm surges cause damaging impacts in coastal regions. The present study uses a stochastic cyclone modelling approach to evaluate the likelihoods of storm tides, the combination of storm surges and astronomical tides, for Samoa. Cyclones that occurred in the vicinity of Samoa from 1969 to 2009 are used to build a stochastic tropical cyclone data set, and an analytic cyclone model and hydrodynamic model are used to model storm tides under average, La Nia and El Nio cyclone and sea level conditions for present climate conditions as well as cyclone and sea level conditions relevant for 2055, and storm tide return periods are estimated. We find that extreme storm tides exhibit relatively modest variation around the coastline of Samoa owing to the uniform width of the shelf surrounding the coastlines of two main islands of Savai'i and Upolu. The frequency of cyclones and hence storm tides during El Nio conditions is similar to the frequency for all seasons, but is considerably lower in La Nia conditions. For the future, tropical cyclones are assumed to undergo decreased frequency and increased intensity. This is found to lower the storm tide height for return periods < 100 years and increase it for return periods greater than about 200 years. Sea level rise is shown to have a larger influence on storm tides than future changes to tropical cyclones. Considering the aggregated probabilities of storm tides occurring at the national scale, we find that the likelihood of a storm tide occurring that locally exceeds a 1-in-100-year level (i.e. an event with a 1 % annual exceedance probability) has a 6 % probability of occurring somewhere along the entire coastline of Samoa. Such information may be useful for those involved in coastal management and disaster response for which there may be a need to consider the overall likelihood that a nation may have to respond to such a disaster.