Assessing the Impacts of Coastal Erosion, Passive Inundation, and Dynamic Wave Inundation under Higher Sea Level in Hawaii

The Interagency Climate Adaptation Committee was created by the Hawaii Legislature and Act 83 to investigate community vulnerability to sea level rise (SLR) in Hawaii. To support the committee, we model: (1) coastal erosion; (2) wave inundation; and (3) passive flooding based on the IPCC RCP 8.5 model of SLR over the 21st Century.

Erosion is estimated using a hybrid equilibrium profile model (Anderson et al., 2015) that combines historical rates of shoreline change with a Bruun-type model of beach profile adjustment to SLR. Results are mapped to GIS layers showing the 80th-percentile probability of potential shoreline change at years 2030, 2050, 2075, and 2100. Seasonal wave inundation is modeled using XBeach (Deltares) in non-hydrostatic mode. A seasonal high wave event (Ho=2.3 m, Tp=16 s, Dir=200° for the Ewa test site) is simulated at each heightened sea level (corresponding to the years previously mentioned); which accounts for changes in wave dynamics due to the change in water level over the reef platform. We use a bare earth topo/bathy digital elevation model derived from USACE 2013 LIDAR data surveys and multi-beam and side-scan sonar data from the Hawaii Mapping Research Group at the University of Hawaii. Waves are modeled along one-dimensional profiles spaced 20 m apart. From this, we develop a gridded product of water depth and velocity for use in a vulnerability analysis. Passive flooding due to SLR, the so-called "bath tub" method, is used as a proxy for groundwater inundation of low-lying coastal plains (where the majority of development in Hawaii takes place).

Modeling results are used with other available data in the FEMA Hazus software to estimate exposure and loss of upland assets. Here, we present the three modeling products and a summary of the larger hazard assessment for the Ewa area on the Hawaiian Island of Oahu.