ASSESSING MORPHOLOGIC CONTROLS ON ATOLL ISLAND SHORELINE STABILITY DUE TO SEA-LEVEL RISE

Atoll islands’ shoreline stability depends in part on how island and reef morphology affect incident wave energy. In addition, sea-level rise (SLR) may change incident wave energy and, thus, shoreline stability. It is unclear how atoll morphology influences shoreline erosion and/or accretion patterns, and how these relationships may respond to SLR. Schematic atolls with varying morphologies were used to evaluate the impact of individual parameters on patterns of erosion and/or accretion using a physics-based numerical model and empirical formula. The magnitude of erosion or accretion was found to increase with SLR; shorelines that were initially erosive generally became more erosive, and shorelines that were initially accretive generally become more accretive with SLR. The morphologic parameters that significantly influenced shoreline stability were reef flat width, reef flat depth, island width, and atoll diameter. Narrower reef flats, deeper reef flats, narrower islands, and smaller atoll diameters were associated with increased magnitude of erosion and/or accretion. Windward islands are projected to extend toward their longitudinal ends and migrate toward the lagoon due to SLR, whereas leeward islands erode along lagoon shorelines and extend toward their longitudinal ends. Oblique islands, oriented parallel to the incident deep-water wave direction, are forecasted to migrate leeward along the reef flat and toward the lagoon. These results suggest that small islands on small atolls are most at risk for decreased coastline stability with SLR. These findings make it possible to evaluate the relative risk of coastal change hazards to atolls islands due to SLR and help prioritize mitigation efforts.