The Morphological Response of Atoll Islands to Sea-Level Rise. Part 2: Application of the Modified Shoreface Translation Model (STM).

The morphological response of atoll islands to sea-level rise is of paramount importance to low-lying atoll nations, which have limited land area. These islands are situated on horizontal non-erodable reef platforms backed by deep lagoons. To date, there have been few attempts to model atoll island response to changes in sea level. This paper uses the modified shoreline translation model (STM) for non-erodable and horizontal substrate surfaces to examine the likely mode of island response to changing sea level using morphological surveys from two islands on Tarawa atoll, Kiribati. Model simulations using a range of sea-level rise scenarios show that atoll islands can undergo a range of morphological responses from the standard Bruun response to Barrier rollover. The mode and relative magnitude of response was shown to be sensitive to island morphology. Low elevation and narrow islands were shown to be most sensitive to changes in sea level. These islands undergo barrier rollover and migrate across the reef flat toward the lagoon and become increasingly unstable as sea-level rise increases. That is, since islands are low in elevation and have lagoons behind them, beach sediments will be displaced to the top of islands or through inlets into lagoons. Higher elevation islands are shown to be more stable and undergo a combination of Bruun and barrier recession responses as sea level increases. Simulations indicate that washover processes allow the island surface to keep pace with sea level rise. Under total washover barrier rollover also implies no change in island width although islands become very unstable rapidly migrating across the reef flat. Under restricted washover conditions (wide islands) simulations indicate upgrowth of the island margin and island narrowing as the barrier recedes. All simulations indicate recession of the island shoreline away from the reef edge. Relative differences in magnitude of change indicate that while low narrow islands experience significant recession, higher islands are relatively stable and are able to cope with sea level rise. Significantly results dispel a number of misconceptions regarding possible atoll island response to sea level showing that: atoll islands will respond in a variety of ways depending on the physical characteristics of islands; conglomerate platform does not necessarily provide stability for islands; and that total loss of atoll islands is unlikely as islands will be able to morphologically adjust to changes in sea level ensuring their future persistence. [ABSTRACT FROM AUTHOR]