Rip Current Characteristics at the Dutch Coast

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 was conducted at Egmond aan Zee. Lagrangian velocities in the surf zone were measured with drifter instruments and human drifters that were tracked via GPS. An extensive dataset of measurements was collected from which parameters that govern the strength of rip currents and affect their mean flow properties were identified. 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 long-shore current and (3) a meandering longshore current. A variety of rip current velocities were measured with the strongest being approximately 0.6 m/s. A statistically significant correlation between the ratio of offshore wave height over water depth on the bar and rip current speeds was established from the data. A 2-dimensional hydrostatic XBeach model was validated against laboratory rip current experiments and field data from Egmond aan Zee. A sensitivity analysis was performed to test a range of parameters that were categorised into two groups of different time scales: hourly and daily altering hydrodynamic parameters, and daily to weekly (sometimes longer) varying geometric parameters. Various hydrodynamic scenarios of wave height, wave period, wave angle and tidal water level were tested to evaluate the influence on rip current initiation and mean flow properties. Additionally, the importance of wave and tidal driven longshore currents was investigated. The key geometric parameters tested were channel width and depth. A reduction of the hydrodynamic parameters along with simplification of the model bathymetry allowed for identification of the governing rip current parameters. The sensitivity analysis demonstrated that rip currents strengthen with increasing wave height, decreasing water depth over the updrift bar and increasing channel depth. The influence of the wave period was of secondary importance and the wave angle did not affect the offshore rip current velocity for site specific channel dimensions. The wave angle was observed to only have an impact for relatively narrow channels (relative to the forcing). This study revealed similarities and differences of rip currents at the Dutch coast and rip currents at previous field sites. The driving parameters of rip currents were identical; however, the flow patterns differed. While in previous field experiments drifters were predominantly retained within the surf zone, most drifters at Egmond aan Zee were ejected from the surf zone and did not return shoreward. Offshore of the channel the drifter behaviour was governed by the tidal current that advected the drifters alongshore. In case of weak tidal currents (slack water) the rip currents extended far offshore.