University of Twente Proceedings


Rip Current Observations at Egmond aan Zee


Winter, G. and Dongeren, A.R. van and Schipper, M.A. de and Thiel de Vries, J.S.M. van (2012) Rip Current Observations at Egmond aan Zee. In: NCK-days 2012 : Crossing borders in coastal research., 13 March 2012 - 16 March 2012, Enschede, the Netherlands .

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Event: NCK-days 2012 : Crossing borders in coastal research., 13 March 2012 - 16 March 2012, Enschede, the Netherlands
Abstract:Rip currents are narrow, seaward directed flows in the surf zone that can pose a serious threat to swimmers. This issue has received attention particularly on swell dominated coasts (such as the US, Australia, France and UK) where numerous field experiments have been undertaken. However, the threat of rip currents is less recognised on wind-sea dominated coasts such as the North Sea, even though a consistent number of swimmers drift offshore (in rip currents) and require rescue by surf lifeguards each year (for example at Egmond aan Zee, The Netherlands). In August 2011, a five day field experiment SEAREX (Swimmer Safety at Egmond – A Rip current Experiment) was conducted. Lagrangian velocity measurements were taken with drifter instruments and human drifters that were tracked via GPS. Three flow patterns were observed in the experiment: (1) a locally governed circulation cell, (2) a pattern in which the drifter initially floats offshore and then is advected by a strong tidal alongshore-directed current and (3) a meandering longshore current between the shoreline and the bar. A variety of rip current velocities were measured with the strongest being approximately 0.6 m/s. The field data was hindcasted with the numerical model XBeach. Based on this model the sensitivity of rip currents towards wave height, water level and rip channel depth was investigated. Both field and model data show that offshore velocities in a rip increase with increasing wave height and decreasing water level, but that the rip channel depth imposes an upper limit on the rip current velocity.
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