Combined infragravity wave and sea‐swell runup over fringing reefs by super typhoon Haiyan

TLDR

Super typhoon Haiyan, one of the strongest recorded, struck the Philippines on 8 November 2013, delivering extreme storm waves that battered the Eastern Samar coast. A Boussinesq‑type wave model, validated with laboratory data, was used to simulate wave transformation and runup over shallow fringing reefs, employing offshore boundary conditions from a wave hindcast. Field surveys recorded 6–14 m overwash heights and widespread inundation despite reef protection, while the model revealed that extreme runup results from a superposition of infragravity waves and sea swells whose balance is governed by reef width and beach slope, causing site‑specific flood characteristics and extreme runup on steep beaches fronted by narrow reefs, with infragravity dominance over wide reefs producing bores.

Abstract

Abstract Super typhoon Haiyan struck the Philippines on 8 November 2013, marking one of the strongest typhoons at landfall in recorded history. Extreme storm waves attacked the Pacific coast of Eastern Samar where the violent typhoon first made landfall. Our field survey confirmed that storm overwash heights of 6–14 m above mean sea level were distributed along the southeastern coast and extensive inundation occurred in some coastal villages in spite of natural protection by wide fringing reefs. A wave model based on Boussinesq‐type equations is constructed to simulate wave transformation over shallow fringing reefs and validated against existing laboratory data. Wave propagation and runup on the Eastern Samar coast are then reproduced using offshore boundary conditions based on a wave hindcast. The model results suggest that extreme waves on the shore are characterized as a superposition of the infragravity wave and sea‐swell components. The balance of the two components is strongly affected by the reef width and beach slope through wave breaking, frictional dissipation, reef‐flat resonances, and resonant runup amplification. Therefore, flood characteristics significantly differ from site to site due to a large variation of the two topographic parameters on the hilly coast. Strong coupling of infragravity waves and sea swells produces extreme runup on steep beaches fronted by narrow reefs, whereas the infragravity waves become dominant over wide reefs and they evolve into bores on steep beaches.