Infragravity Wave Motions and Runup over Shallow Fringing Reefs

Abstract

This paper presents the results of a combined laboratory and numerical investigation into the role of infragravity motions in the wave runup process over fringing coral reefs. Laboratory experiments were performed with a reef profile typical of fringing reef systems along the southeast coast of Guam. Spectral analysis of the measured time histories of surface elevation over the reef face and flats show significant changes to the wave energy spectrum shoreward of the break point. Most of the wave energy in the incident wave frequency band is dissipated within a few wavelengths of the reef face with the wave motions over the reef flat and shoreline dominated by oscillations at infragravity periods [O(100s) prototype]. The infragravity wave energy is minimum at the reef crest and increases as waves propagate shoreward over the reef flat and also with increasing water level on the reef. The dominant infragravity mode is the first reef oscillation mode with a wavelength approximately equal to four times the width of the reef flat. This component is resonantly amplified at the shoreline relative to the incident infragravity energy at the reef crest. A numerical model based on the Boussinesq equations is applied to the laboratory data and is able to describe complex changes to the wave spectrum over the reef flat due to nonlinear wave-wave interactions and wave breaking as well as runup at the shoreline.