Cross-shore currents in the surf zone: Rips or undertow?

Abstract

Although the dynamics and kinematics of various types of mean cross-shore current flows in the surf zone (undertow and rip currents) are fairly well understood, the causes for transitions occurring between these two types of mean circulation patterns remain obscure. On longshore barred beaches, such transitions involve the formation, degeneration, or both of rip channels. In this paper, field evidence is presented to suggest that transitions between undertow and rip current (cell) circulations could depend on the magnitude of the wave-induced onshore mass transport across a longshore bar, rip channel spacing, and trough cross-sectional area. The results are based on data obtained from four field experiments on the Danish and Dutch North Sea coasts, which encompassed a range of incident wave energy conditions. Two of the data sets demonstrate transitions between cell and undertow circulations. Calculated onshore-directed mass transports in the two circulation types were plotted against At/yr, where At is trough cross-sectional area and yr is longshore distance from the measurement position to a rip channel. The two types of circulation are separated in parameter space by a straight line with a slope of 1 (m/s). The observations support a model proposed previously, in which optimum rip spacings exist that depend on the balance between onshore discharge and longshore pressure gradients caused by irregular bar bathymetry. This simple morphodynamic model indicates that both hydrodynamic conditions and existing bathymetry are critical in determining the type of mean current circulation.