Dissipation of incident forced long waves in the surf zone—Implications for the concept of “bound” wave release at short wave breaking

Abstract

A review of laboratory data sets on surf beat is presented, with a focus on the dissipation of long wave energy in the surf zone. It is frequently assumed that incident forced long waves, or “bound” long waves, are released from short wave groups when the short waves break, subsequently propagating to the shore as a free wave. Free long waves may additionally be generated by the moving short wave breakpoint. Convincing evidence of the release of forced long waves as a result of short wave breaking is lacking, while there appears to be strong evidence to the contrary from a range of recent laboratory experiments. The data from the laboratory experiments are also consistent with field observations of strong nearshore dissipation of long waves. These data are also consistent with Longuet-Higgins and Stewart (1962), who suggest that the forced long wave may reduce in amplitude following short wave breaking, not that it might be released as a free wave. In contrast, forced long waves can be progressively “released” from the groups when the short waves are in shallow water, since these conditions correspond to those where the forced long wave satisfies the free wave dispersion relationship. This frequently occurs prior to short wave breaking for mild wave conditions, but here it is shown that these conditions are not usually satisfied at the short wave breakpoint for storm conditions. Energy transfers between free and forced waves are also discussed with regard the data. A surf beat similarity parameter that incorporates both relative beach slope and short wave steepness is suggested, which distinguishes between different long wave forcing regimes inside the surf zone.