MECHANISMS OF DAMAGE DURING TYPHOON HAIYAN: STORM SURGE, WAVES, AND 'TSUNAMI-LIKE' SURF BEAT

摘要

Typhoons cause damage by a variety of mechanisms: wind, large waves, and storm surge. Storm surge itself has three components: pressure-driven setup (the inverse barometer effect), wind-driven setup (wind pushing water onshore), and breaking-wave-induced setup (breaking waves pushing water onshore). Numerical modeling shows that in the fringing-reef-protected town of Hemani during Typhoon Haiyan, pressure- and wind-driven setup were small, while breaking-wave-induced setup varied in time as the envelope of large and small incident wave groups (sets), resulting in an infragravity oscillation. The surf beat contained energy in a frequency range close to the natural resonant mode of the reef, which amplified the energy of the surf beat (Nakaza and Hino, 1991; Nwogu and Demirbilek, 2010). During propagation over the reef, the amplified surf beat steepened nonlinearly, and impacted the town as a tsunami-like bore, resulting in extensive damage and casualties. Since coastal hazard planning presently relies on phase-averaged wave modeling, infragravity surges such as the one that struck Hernani are not currently being accounted for, highlighting the necessity for a change of policy and the adoption of phase-resolving wave models in regions protected by fringing coral reefs.