Pulsations in surf zone currents on a high energy mesotidal beach in New Zealand

Abstract

The exchange of material between the surf zone and continental shelf can be driven by pulsations in rip current velocities. However, there is a poor understanding of the relationship of these pulsations to surf zone morphology and material exchange. Moreover, understanding of rip current dynamics has focused mainly on single-barred beaches in an intermediate state, and there have been few studies on high energy beaches. Therefore, this paper undertakes preliminary research on surf zone current velocity pulsations, on a high energy beach in New Zealand. This initial analysis presents results from two days of measurements using Acoustic Doppler Velocimeters and Lagrangian GPS drifters. Drifters revealed pulsations in current velocities on the order of ∼0.5-2 m s-1 throughout the surf zone, whether inside a rip current circulation cell or not. More infragravity wave energy was associated with constant pulsations in current velocity, and lower infragravity energy with pulsation bursts, lasting 5-10 minutes, interspersed with periods of relatively constant velocity lasting 15-25 minutes. However, higher wave conditions also reduced the exit rate from the surf zone.