Characterization of episodic rip current pulsations in the inner shelf during RCEX 2007
O'Neill, Andrea C.
Thornton, Edward B.
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As the Navy thrusts operations into global "brown water" environments, a more complete understanding of the phenomena ships and swimmers will encounter in nearshore regions is necessary. Rip currents remain infamous and important characteristics of the nearshore environment. These events not only impinge upon swimmers' safety, but may play a key role transferring water, containing nutrients, biologics, and even shore-/ship-borne pollution, between the surf zone and open ocean environments. Vertical and temporal behavior of rip currents outside of the surf zone is poorly understood due to a paucity of comprehensive observations. Observations of two upward-looking Acoustic Doppler Current Profilers (ADCP) deployed in 3 m and 5 m water depth within a rip current (nominally 1.0 and 1.2 surf zone widths from the shoreline respectively) were obtained during April-May 2008 as part of the Rip Current EXperiment (RCEX) at Sand City, Monterey Bay, CA. The ADCPs sampled continuously at 1 Hz. Energetic seaward-directed episodic pulses associated with the rip current obtained velocities up to 0.5 m/s with a frequency of occurrence varying from 1-15 times a day depending upon coincident wave and tidal conditions. Vertical variations of the episodic rip current pulsations ranged depthuniform to surface-dominated. Cross-rotary analysis and complex correlation, performed in the vertical to describe rotational behavior and temporal lags, show rip currents in the inner shelf exhibit more rotation, up to 20 degrees in both CW and CCW directions, than in the surf zone. High coherence is limited to nearsurface levels in the inner shelf, versus more depth-uniform values in the surf zone. Mean vertical profiles show these phenomena exhibit significant shear and structure.
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