Shore Protection against Sea Level Rise and Tropical Cyclones in Small Island States

Abstract

The abrupt phase shift of the El Nino Southern Oscillation (ENSO) resulted in several cyclones during 2004–2005 in the Southeast Pacific and east of the International Date Line. During a four-week period in February 2005, cyclones Meena, Nancy, Olaf, and Percy struck the Cook Islands. Because these cyclones were stronger than Cyclone Sally in 1987, which is reputed to have caused the most damage to Rarotonga in the past, the government was well prepared for the storm and its effects. Evacuation shelters were set-up throughout the island and no injuries or casualties were reported. In contrast, the storm surge and high waves of Sally devastated houses, shops, and the airport on the seafront in north Rarotonga. Since Sally, the government of the Cook Islands has worked together with the Japan International Cooperation Agency (JICA) to specifically protect the shoreline from storm surge; the outcome of their work provides scientific information regarding vulnerability to future hypothetical cyclone events. The purpose of this study is to reduce such vulnerability by assessing the cyclone of the 100-year return period and to evaluate the allowable seawall height. Mathematical models are proposed to estimate the following components: (1) wave attenuation and set-up on a coral reef; (2) wave run-up on a foreshore slope; and (3) irregular wave-overtopping of a seawall. These mathematical models are verified from the data of Cyclone Sally.