Tsunami runup in narrow bays: the case of Samoa 2009 tsunami

Abnormal tsunami amplification and runup in narrow bays is studied with respect to the Samoa tsunami of 29 September 2009. The data of the tide gauge in Pago Pago harbour are used to calculate wave runup in the city of Pago Pago (Tutuila, American Samoa) for two approximations of the bottom topography: a plane beach and a narrow bay. Theoretical estimates of tsunami runup are compared with field survey data for the 2009 Samoa tsunami. It is shown that both formulations result in equally good estimates of runup, having approximately the same difference with the field measurements. However, the narrow bay model presents more wave amplification and, consequently, runup, which is the main observation of the field survey. The differences in estimated shoreline velocity, travel time and wave breaking regime, calculated in the framework of these two approximations, are also discussed. It is concluded that wave runup in narrow bays should be calculated by the corresponding formulas, which should be taken into account by tsunami early warning systems.