The effect of accelerating sea levels on the evolution of natural and developed barriers

Abstract:

Although barriers and barrier islands are dynamic and geologically ephemeral, many are developed with established communities depending on their survival. As sea level rises, natural barriers can aggrade and migrate landward through overwash deposition; however, when sea level rises too rapidly, barriers may drown by either not growing tall or wide enough for overwash to maintain a subaerial landform. Often numerical models of barrier island evolution test scenarios of barrier stability or drowning for a given constant rate of sea level rise. However, over the coming century and beyond, anthropogenically driven climate change is expected to make sea level rise at an increasing rate, which means that barriers will experience changing, and worsening driving conditions. Here, we assess barrier response to accelerating sea level under different scenarios (RCP2.5, RCP4.5, and RCP8.5) using the LTA, a simple, morphodynamic cross-shore barrier evolution model. We find that barriers drown more often and earlier when forced by accelerating sea level rise and that the style of drowning often changes from width drowning to height drowning. Coupling the barrier evolution model with a myopic economic model for beach nourishment decisions, we then explore the dynamics of coastal community response to increasing rates of sea level rise, including the resulting change in management decisions and economic impact. Overall, our model experiments suggest rapid onset of threshold changes in barrier evolution for both natural and managed conditions when faced with continued increases in the rate of sea level rise. Understanding these threshold effects is crucial for the characterization of risks that coastal communities on barrier islands are facing in the coming years, contributing to their climate adaptation planning.