Dealing with hurricane surge flooding in a changing environment: part II. Risk-based coastal adaptation design considering hazard-adaptation interactions

Climate change exacerbates tropical cyclones (TCs) induced flood risk that demands adaptations for vulnerable communities. Current studies often overlook hazard-adaptation interaction when modeling coastal inundation and apply “bathtub” flood modeling method which may overestimate flood risk. Here, we propose the Inundation Modeling-based Overtopping Risk Analysis (IMORA), where we model coastal inundations considering interactions between storm tide hazard and adaptation measures (in this study, a floodwall system) with a physics-based but computationally feasible method. We apply IMORA to assess adaptation strategies for lower Manhattan, NYC. We used large numbers of synthetic TCs to drive a storm tide model (ADCIRC) and an inundation model (SFINCS) to obtain inundations and damages with different floodwall levels. We calculated expected annual damages (EADs) and expected total damages (ETDs) with different floodwall levels. Considering the effect of sea level rise and storm intensification, we estimated ETDs from 2025 to 2100 with no adaptation to be $1.11 billion and $1.75 billion under the moderate emissions scenario (SSP2-4.5) and the high emissions scenario (SSP5-8.5), respectively. We determined optimal floodwall elevations to be 18.25 feet and 18.75 feet with minimum net expected costs (ETD plus investment) of $36.7 million and $38.1 million under SSP2-4.5 and SSP5-8.5, respectively. The obtained optimal design levels are comparable to the design level proposed in USACE’s NYNJ-HATS coastal storm risk management for this region, although USACE’s analysis method is challenged. We find that assuming “bathtub” overestimates ETDs by up to 143% at the floodwall elevation of 18.75 ft compared to applying the IMORA framework.