SIMULATING 21ST CENTURY INUNDATION OF WAIKIKI AND HONOLULU, HAWAII

Global mean sea level rise (SLR) threatens coastal communities and strand ecosystems. According to the National Research Council (NRC 2012) sea level may rise (relative to 2000) 8-23 cm by 2030, 18-48 cm by 2050, and 50-140 cm by end of the century. We use a digital elevation model (DEM) of FEMA 2007 LiDAR to simulate the impacts of SLR on portions of the Waikiki and Honolulu communities. We differentiate marine vs. groundwater inundation at MHHW on the DEM and characterize the vertical uncertainty (linear error at 95% CI) of the LiDAR data at three increments of SLR. Based on projections by NRC 2012, Vermeer and Rahmstorf 2009, and Rignot et al. 2011, we estimate SLR impacts at 1 ft (mid-century), 2 ft (mid late-century), and 3 ft (late century). The resulting spatial information is applied to parcel assessments and transportation infrastructure to evaluate impact and adaptation options. We find fundamental differences between parcels threatened by marine vs. groundwater inundation. Marine inundation implies across-land flow that may be directly mitigated by barriers and other diversionary approaches. Groundwater inundation implies the development of severe drainage problems, potentially mitigated by pumping. Groundwater inundation is more widespread and may arrive earlier than marine inundation because the coastal plain water table sits somewhat above mean sea level. Additionally, simulation of wave overtopping indicates that the fixed elevation of the coastal plain is breached by the annual maximum deep-water wave (of a 26 year directional buoy record) when SLR reaches approx. 60 cm above present. The combined vulnerability to marine and groundwater inundation and wave overtopping indicates that significant portions of the Honolulu and Waikiki business districts are vulnerable to SLR in the second half of the century.