COMPOUNDING MECHANISMS OF SEA LEVEL RISE INDUCED FLOODING PRODUCES DOUBLING OF CRITICAL INFRASTRUCTURE FAILURE IN HONOLULU BY THE 2030S

Through a Mayoral Executive Directive, the City and County of Honolulu has recognized the immediacy of action needed to mitigate sea-level rise (SLR) impacts. Such action requires identification of site-specific SLR vulnerabilities to inform adaptation design and implementation. We respond by providing a method that can be used to inform these efforts.

We apply the method to Honolulu’s Primary Urban Core (PUC) based on the high density of vulnerable assets and to illustrate continued flood vulnerability despite the presence of shoreline armoring. The method includes production of near-term flood scenarios made up of high resolution raster data sets featuring water depth generated by three mechanisms: 1) direct marine flooding, 2) storm drain backflow, and 3) groundwater inundation. Damage associated with the compounded flood mechanisms has not been previously studied, despite producing present-day impacts during elevated tide events. Simulation of dynamic coastal processes (i.e., coastal erosion, wave run-up, changes in land cover) are not considered here since the majority of PUC shorelines have been hardened and are likely to be reinforced as needed. Annual sea level exceedance frequencies calculated using the methods of Thompson et al. (2019) provide time ranges in which exceedance of simulated flooding will occur sporadically and become progressively chronic.

A preliminary damage assessment is conducted to illustrate the utility of the method towards revealing locations of critical infrastructure failure. We find that critical components of infrastructure (i.e., roadway, drainage, cesspools) already experience sporadic failure during elevated tide events. Such failure will be amplified over the coming decades, approximately doubling in extent by the 2030s considering the NOAA Intermediate scenario. Our results underscore the need for timely implementation of adaptation methods to avoid municipal degradation, and further, that the design of such methods will require site specific strategies that account for compounding mechanisms of flooding.