Rip currents: 2. Laboratory and field observations

TLDR

Standing edge waves of the same frequency as the incident waves are generated on the beach. The study experimentally investigated nearshore circulation on a plane beach under a uniform, normally incident wave train. The relative importance of edge‑wave modes is estimated by the parameter ω²xb/(g tan β), linking wave frequency, beach slope, and surf‑zone width. Edge‑wave and incident‑wave interaction generates steady circulation cells and rip currents at alternate antinodes spaced by the edge‑wave longshore wavelength, with one mode often dominating, and Gulf of California observations confirm this mechanism.

Abstract

The nearshore circulation of water on a plane beach exposed to a uniform wave train, normally incident on the beach, was investigated experimentally in the laboratory. The incident waves generated standing edge waves on the beach of the same frequency as the incoming waves. The interaction between these edge waves and the incident waves gave rise to steady flow patterns (nearshore circulation cells) consisting of an onshore flow toward the breakers, a longshore current in the surf zone, and an offshore flow in relatively strong, narrow rip currents. The rip currents were found to occur at alternate antinodes of the edge waves, and the spacing of the rip current was therefore equal to the longshore wavelength of the edge waves. Although the incoming wave may interact with all the possible edge wave modes of the same frequency, it was found that the interaction with one particular mode is often dominant. A useful estimate of the relative importance of the modes is given by the parameter w2xb/(g tan β), where ω is the radian frequency of the edge waves, tan β is the beach slope, and xb is the width of the surf zone. Field observations made in the Gulf of California strongly suggest that this mechanism is important on real beaches.