The importance of waves in large-scale coastal compound flooding: A case study of Hurricane Florence (2018)

Wave-driven flooding is often neglected or included in an approximate way in large-scale flood hazard assessments and early warning systems, despite its significant contribution to coastal flood hazards. This study introduces a method to incorporate incident and infragravity wave processes into a fast compound flood model by extending the SFINCS software with the SnapWave stationary wave energy solver. This extension efficiently translates offshore incident and infragravity wave conditions to the nearshore, allowing for the estimation of incident-wave-induced setup and the resolution of wave runup and overtopping. A quadtree approach is employed to optimize the grid resolution for wave processes in the coastal zone. The approach is validated for Hurricane Florence (2018) along the North and South Carolina coastline of the United States, where observed offshore wave heights reached 10 m. The results illustrate that the impact of the hurricane extended hundreds of kilometers beyond the landfall area due to waves, highlighting its importance as coastal flood driver. In 19% of the coastline analyzed, wave contributions surpassed all other flood drivers combined, with waves contributing to an additional flooded area of 226 km2 and a flood volume of 62 million m3. The study also indicates that simpler parameterized methods for including wave-induced setup can lead to significant discrepancies in modeled water depths. The computational efficiency of the extended SFINCS model allows for the simulation of 1,000 km of coastline with limited computational resources. Hereby the critical role of wave effects in coastal compound flood hazard assessments could be demonstrated.