TABLE OF CONTENTS Long waves (or "infra-gravity" waves, or IG-waves for short) are waves with significantly longer periods than the peak frequency of the incident wave spectrum. Field measurements show that such waves occur very frequently on many beaches. The measurements show that close to the shore - well inside the surfzone - the major part of the wave energy is often concentrated in spectral components with wave periods much longer than the dominating periods (generally in the range 20 300s) of the incoming wave motion. Therefore it is natural to suspect that some mechanism is present in the very nearshore region that transforms short wave energy into longer wave energy.
Different mechanisms have been considered for their generation.
One mechanism is the height variation of incoming waves which Munk (1949) suggested create shoreward mass transport under high wave groups that, at the break point where wave groups are destroyed, transfers into long waves. The long waves are reflected from the shore and become free waves called surf beat. Tucker (1950) found negative correlation between incident wave amplitude and low frequency motion. This observation was later by Longuet-Higgins and Stewart (1962) shown to agree with the forced set-down waves under wave groups that is a consequence of the radiation stress variations. This mechanism is analysed in a section below.
Symonds et al. (1982) pointed out that varying wave heights causes the break point of the waves to vary with height. This means varying the point where the...
