ABSTRACT

Dusek, G. and Seim, H., 2013. Rip current intensity estimates from lifeguard observations.

Lifeguard observations of rip intensity are utilized as an alternative to rip current rescue data to determine the influence of the wave field on rip current activity. Daily rip current intensity observations were made by Kill Devil Hills Ocean Rescue on the Outer Banks of North Carolina over two summers. The daily observations were made at up to 19 alongshore locations over 7.5 km of beach and compared to local wave field observations. The lifeguard intensity observations compare favorably to rip current rescue data when rescues occur; however, rip intensity observations offer significant advantages over rip rescue data: (1) Rip current intensity observations are a continuous data record and provide data even when bathers are not present, and (2) lifeguard observations provide an estimate of rip current intensity, while rescue data only indicate rip current occurrence. Results of this study generally compare favorably to previous research and to expected rip current dynamics. Rip intensity is compared to observed wave spectral statistics (significant wave height, mean direction, peak period, and directional spread) at 12-m depth and to wave height and direction refracted and shoaled to just outside the surf zone to provide a more dynamically relevant comparison. Rip intensity increases with significant wave height (at a threshold value of 0.7 m), wave directions closer to shore normal, and narrower directional spread. The results suggest that wave height determines a baseline intensity level, which is then modulated depending on the direction and spread. No relationship was found between rip intensity and peak period. Rip intensity is analyzed with wave spectral components (wind sea, swell, and multiple swells) and generally yields similar results to the analysis of the bulk spectral measurements.

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