THE PENETRATION OF SHORT-CRESTED WAVES THROUGH A GAP

Abstract

The propagation of long-crested and short-crested waves through a gap such as between two breakwaters is affected by diffraction. If the water depth is not uniform, refraction may also affect this penetration. It is shown with a navigation channel between two breakwaters (academic case and a realistic harbour case) that the effects of diffraction are small compared to those of refraction in regions where waves can penetrate with refraction only (e.g. where wave rays can penetrate). This region is relative large for short-crested waves. Outside these regions diffraction is dominant. With another example of waves penetrating through a tidal gap against a strong ebb current, it is shown that the short-crestedness of the waves destroys to a very large extent the waved guide effect of such currents. Both set of experiments show that the inclusion of short-crestedness in the wave computations causes some smoothing of the wave field, and that diffraction is a minor effect in regions with considerable wave motion. This is of some scientific and economic relevance as refraction can readily be combined in wave models with other physical phenomena such as wave generation and dissipation. Moreover, such models require less computer capacity than diffraction models which do not readily absorb wave generation and dissipation.