Wave Breaking on an Opposing Current

estimate wave breaking on an opposing current, such as at coastal inlet entrances. PROBLEM: Waves propagating into a tidal inlet will shoal and break because of changes in water depth, focusing by shoals, and interactions with an opposing (ebb) current. On an ebb current, waves steepen because their height increases and length decreases. The wave steepening can intensify wave breaking, causing a navigation hazard and inducing a wave-driven current and sediment transport. Most wave-breaking criteria are based on a maximum ratio of wave height to water depth. But, in regions where waves break because of steepening on an ebb current, the height-to-depth criterion may significantly underestimate wave breaking and overestimate wave height. CETN IV-9 (Smith 1997) describes the process of wave-current interaction in one dimension and gives the equations for calculating wave-height transformation on an ebb (opposing) or flood (following) current. Extending the discussion in CETN IV-9, the present CETN provides guidance on estimating wave breaking that may occur as waves transform on an opposing current. Before the method for calculating breaking is described, the governing equation, wave-action conservation, is reviewed (see also CETN IV-9), and wave shoaling on a current is discussed. CONSERVATION OF WAVE ACTION: Wave height in the presence of a current is governed by the conservation of wave action (Jonsson 1990, and others). The one-dimensional conservation of wave action equation is given by: ()+æ