Monochromatic Bi-directional Waves and Their Effect on the Nearshore Circulation

The formation of bimodal spectra sea states is not uncommon and their effects on the coast has not yet been sufficiently elucidated. As a consequence, groups of interference waves (constructive and destructive) are generated, leading to complex circulation patterns in the nearshore. Rip currents are one example, which may be formed either normally or obliquely in relation to the coast, flowing offshore. These currents are responsible for numerous drowning or near drowning events with beachgoers. Therefore, the present work aims to perform a small-scale model of the formation of rip currents, interacting with fixed bed and with movable bed, through the generation of a monochromatic-bidirectional wave pattern. The experiments were carried out in a wave basin of 25 x 15 m that has an oblique wall from which the original wave was reflected. This generated a bidirectional wave pattern as it interferes with the original incident wave. The main observations during the study were a static wave pattern of constructive and destructive wave interference with distinct orbital velocities and several effects on the sand beach. These lead to the formation of different features: (a) cusps near the beach line, (b) crossbars, (c) crescentic holes, (d) breaker banks, (e) bedforms with alongshore oriented crests (f) bedforms with cross-shore oriented crests and (g) bedforms with 3D crests, in addition to smaller bottom ripples. In addition, a mean water level alongshore variation was observed, validating the theoretical model.