Modeling the impacts of a changing climate on cross-shore sediment transport: Kwajalein Atoll, Marshall Islands

Abstract

Atoll reef islands primarily consist of unconsolidated sediment, and their ocean-facing shorelines are maintained by sediment produced on their fringing reefs. Changes in the waves that propagate across the fringing reef can alter net cross-shore sediment transport rates and thus affect the sediment budget and morphology of atoll reef islands. Here we investigate the influence of sea-level rise and projected wave climate change over the coming century on potential cross-shore sediment transport across the fringing reef on Kwajalein Island, Republic of the Marshall Islands, using a phase-resolving hydrodynamic model. For the current reef flat geometry, sea level, and wave climate, on- and off-shore components of the potential bedload transport are nearly balanced. Mean annual wave energy incident on the island is projected to decrease by 12% by 2100 CE, and we find this decrease has negligible influence on potential cross-shore sediment transport rates on the reef flat and beach, as waves on the reef flat are strongly depth-limited. However, 0.5-2.0 m of sea-level rise leads to greater high-frequency wave heights, skewness, and associated shear stress on the reef flat, with a decrease of these parameters on the fore reef. These changes in hydrodynamics increase potential sediment transport from the outer reef flat to the beach, but decrease potential sediment inputs onshore from the fore reef. The effect of projected wave climate change and sea-level rise on reef flat and reef island morphology will depend on the availability and production of sediment on the fore reef and reef flat itself.