The longshore flow component in low-energy rip channels: The Mediterranean, Israel

Abstract

We examined in the field the near-bed longshore (v) flow component in rip channels at the Herzliyya beach, Israel, on the Southeastern Mediterranean. Field experiments, conducted under relatively calm wave conditions (T_sig = 3–6 s, H_sig = 0.3–1.1 m) included current and wave monitoring and morphological mapping. Time series of instantaneous and smoothed shore-parallel current velocities and of the orbital components, were analysed, including statistical and spectral processing.

The peak instantaneous velocities of waves and currents combined were in excess of the threshold motion of the local sand, taken as 0.3 m/s. Some velocities even exceeded the threshold for sheet flow transport ∼0.9 m/s. The feeder and the neck demonstrated the highest longshore flow components, whereas the rip head, typically showed the lowest longshore velocities. The unsteadiness of the net longshore current was very high. Feeders typically showed a mean unidirectional longshore current. However, directional fluctuations gradually increased towards the rip head.

The data suggest a clear spectral structure; most energy (46–67%) was in the gravity range with 13–35% subharmonic and minor (13–16%) in the infragravity band. Systematic cross-shore spatial trends of the energy spectra components were linked to bar morphology and to exposure to incident waves.