Rip currents: 2. Laboratory and field observations

TLDR: In this paper, 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 and it was found that the interaction between these edge waves and the incident waves gave rise to steady flow patterns.

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.