Critical elevation levels for flooding due to sea-level rise in Hawai‘i

Coastal strand and wetland habitats in the Hawaiian Islands are often intensively managed to restore and maintain biodiversity. Due to the low gradient of most coastal plain environments, the rate and aerial extent of sea-level rise (SLR) impact will rapidly accelerate once the height of the sea surface exceeds a critical elevation. Here, we develop this concept by calculating a SLR critical elevation and joint uncertainty that distinguishes between slow and rapid phases of flooding. We apply the methodology to three coastal wetlands on the Hawaiian Islands of Maui and O‘ahu to exemplify the applicability of this methodology for wetlands in the Pacific island region. Using high-resolution LiDAR digital elevation models, flooded areas are mapped and ranked from high (80 %) to low (2.5 %) risk based upon the percent probability of flooding under the B1, A2, and A1Fl emissions scenarios. As the rate of flooding transitioned from the slow to rapid phase, the area (expressed as a percentage of the total) at a high risk of flooding under the A1Fl scenario increased from 21.0 to 53.3 % (south Maui), 0.3 to 18.2 % (north Maui), and 1.7 to 15.9 % (north O‘ahu). At the same time, low risk areas increased from 34.1 to 80.2, 17.7 to 46.9, and 15.4 to 46.3 %. The critical elevation of SLR may have already passed (2003) on south Maui, and decision makers on North Maui and O‘ahu may have approximately 37 years (2050) to develop, and implement adaptation strategies that meet the challenges of SLR in advance of the largest impacts.