Modeled Atoll Shoreline and Run-up Changes in Response to Sea-level Rise and Changing Wave Directions under Large Wave Conditions: Wake and Midway Atolls, Northwestern Hawaiian Islands

Low-lying atoll islands are dynamic features expected to respond to changes in waves and sea level in a changing climate. Increased run-up and/or island erosion would threaten infrastructure and island communities. However, it is unclear how island erosion and run-up patterns will respond to climate change, making it difficult to prepare for the future. Large winter and summer wave conditions were modeled at two atolls, Wake and Midway in the Northwestern Hawaiian Islands, using Delft3D where sea level was varied at intervals between 0.0 to +2.0 m and incident wave directions from –15.0° to +15.0° from current conditions, to simulate potential climate change effects. The longshore sediment flux was calculated using the CERC empirical formulation and resulting potential erosion rates were calculated by the divergence of the longshore drift between scenarios; similarly, changes in run-up between scenarios were calculated using an empirical approach. The results are variable, but generally the ends of islands exhibit the most change under sea-level rise or rotating wave directions. Islands are projected to accrete on seaward, leeward, and lagoon shorelines due to sea-level rise, whereas island ends predominantly erode. Increases in run-up are greatest along island ends and smallest along seaward shorelines. Shorelines at the ends of islands show the greatest response to incident wave direction changes, whereas seaward and leeward shorelines become predominantly accretionary with an increasing wave angle. Run-up increases as shorelines become more directly exposed to incident waves; however, this response is greatest at the ends of islands. These simulations suggest island ends will experience the most morphological change and run-up increases due to climate change.