Flow characteristics at the rip current neck under low energy conditions

Abstract

This paper presents the flow signature of the rip current neck as determined by fourteen near-bottom velocity records, with a comparison made to 63 recordings in the surf zone. The field experiments, including wave monitoring and morphological mapping, were conducted under relatively calm, dissipative wave conditions ( T sig = 3–6 s, H sig = 0.3-1.1 m) at Herzliya beach, Israel, on the Mediterranean shore. The current data were analysed for shore-normal and resultant components. The processing of the time series in the form of instantaneous and smoothed velocities, provided spectral energy, orbital and current velocity, specific current discharge and directional distribution data. The unique characteristic of the neck environment was found to be its continuous unidirectional seaward flow. Due to the channeling effect, this steady offshore drift constituted the fastest flow in the surf zone with a typically narrow (35°–94°) directional range. Through spectral analysis of the unidirectional return flow the morphodynamic process signature at the rip neck was defined by the relative contribution from the incident waves (78%), the subharmonic edge waves (14%) and the infragravity oscillations (8%). The pulsatory, jet-like rip model with concentration of energy at low frequencies is not corroborated by this study. The studied environment of the rip and bar pattern represented an inherited beach morphology, at an initial accretional, although not yet reflective, stage.