Nature and stability of atoll island shorelines: Gilbert Island chain, Kiribati, equatorial Pacific

Abstract

Islands rimming Pacific atolls typically form narrow, low-lying lands that are commonly perceived to be particularly vulnerable to global changes such as sea-level rise. As these, low islands form the only habitable land for many island nations, understanding the character of shorelines, and the rates and controls that operate to bring about changes, is an issue of central importance. The purpose of this study is to unravel the characteristics of coastal change on atoll islands of the Gilbert Island chain of the equatorial Pacific nation of Kiribati, especially as they relate to autogenic shoreline processes and El Niño/Southern Oscillation variability. Integration of field observations, differential global positioning system data, historical aerial photographs and ultra-high resolution remote sensing images demonstrates the nature, spatial patterns and rates of change from 17 islands on Maiana and Aranuka atolls. The results illustrate that, between 2005 and 2009, ca 50% of the shorelines on these islands displayed a discernable shift in position; some shorelines were accretionary (at net rates up to ca 8myear-1) and others were erosional (up to ca 18myear-1). Long-term net rates of change on Maiana between 1969 and 2009 were lower than short-term net rates measured between 2005 and 2009. Both short-term and long-term observations illustrate some of the greatest change occur near terminations of the largest, north-south oriented islands, associated with longshore movement of coarse sand and gravel. Direct hits by tropical depressions and marked seasonality, factors interpreted as being essential in island growth and shoreline dynamics elsewhere, do not directly impact these equatorial atolls and can be eliminated as fundamental controls on shoreline dynamics. Similarly, observations over four years suggested that shoreline variability probably is not influenced directly by marked sea-level change, although a recent increase in the rates of shoreline change could reflect instability related to the cumulative effect of a long-term increase in the rate of sea-level rise. Within this framework of global change, local anthropogenic effects, autogenic shoreline processes and El Niño/Southern Oscillation-influenced wind and wave variability control many aspects of these dynamic shorelines. These results provide quantitative insights into the character and variability of rates of shoreline change, information essential for evaluating and mitigating the vulnerability of island nations such as Kiribati. © 2011 The Author. Journal compilation © 2011 International Association of Sedimentologists.