— EDGE WAVE AMPLITUDE 

 -— INCIDENT WAVE AMPLITUDE 



stilt-water level 



Figure 1 . Schematic representation of incident wave saturation and infragravity wave 

 resonance (after Holman, 1983) 



morphology, such as crescentic and welded bars, has been attributed to the 

 interaction of two or more long (edge) waves (Bowen and Inman 1971, Holman 

 and Bowen 1982). 



Another field in which infragravity motions are of interest is the shipping 

 industry, where harbor oscillations with periods on the order of several minutes 

 are common. If the infragravity wave forcing is at periods close to the natural 

 mode of the harbor, then resonant amplification can occur. When excessive, 

 these motions can be detrimental to mooring of ships, and transfer of cargos, and 

 they can decrease the lifetime of mooring facilities. These oscillations also 

 adversely influence tide measurements, particularly for tidal stations located near 

 antinodes of the oscillation. Since tide gauges are typically designed to filter out 

 waves at incident wave periods (<25 s), infragravity waves and harbor 

 oscillations with longer periods are not filtered and contaminate the tide records. 

 Furthermore, accurate water depth measurements (relative to a datum) cannot be 

 made with fathometers, which is a serious problem for surveying and dredging 

 operations. Typically a survey vessel uses the mean water level of a nearby tide 

 gauge as a reference datum. If either the vessel or tide gauge is located near an 

 antinode, then significant errors in survey accuracy will be recorded. 



Chapter 1 Introduction 



