Measurement of Infragravity Edge Waves on the East Coast of Taiwan Using a Single Instrument

Traditionally, an edge wave is measured along a long and straight coast with pressure-gauge arrays to acquire the frequency-wavenumber for dispersion relationships. Ideal measurement locations are limited, and the cost of purchasing and operating some 20 instruments emerges as a threshold requirement. In the present study, one instrument measuring both the bottom pressure and the flow velocity was used to retrieve edge-wave information on the east coast of Taiwan outside the Hua-Lien Harbor. Using the progressive linear-wave assumption for irrotational and inviscid shallow water provided the pressure-flow relationships for both the infragravity edge and leaky waves. The relationship for edge waves was helpful in identifying the edge-wave mode, whereas the relationship for the leaky waves eliminated the possibility of incorrectly interpreting a leaky wave as an edge wave. Applying this approach to field measurements on the east coast of Taiwan shows that a Stokes edge wave is a significant component in the band of period 70-120 seconds before the landfall of a typhoon coming from the southeast but still hundreds of kilometers from Hua-Lien Harbor. This result suggests that the approach being presented can be applied in the real ocean, and a unique method for taking into account edge waves may need to be considered for the resonance problem that occurs in Hua-Lien Harbor during passing storms.