Rip currents under obliquely incident wind waves and tidal longshore currents |
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| Institution: | 1. Deltares, Dept. ZKS and HYE, P.O. Box 177, 2600 MH Delft, The Netherlands;2. Delft University of Technology, P.O. Box 5048, 2600 GA Delft, The Netherlands;1. Faculty of Civil Engineering and Geosciences, Section of Hydraulic Engineering, Delft University of Technology, Delft, The Netherlands;2. UNESCO-IHE Institute for Water Education, Delft, The Netherlands;3. Harbour, Coastal and Offshore Engineering, Deltares, Delft, The Netherlands;1. School of Biological, Earth and Environmental Sciences, UNSW Australia, Sydney, NSW 2052, Australia;2. Water Research Laboratory, School of Civil and Environmental Engineering, UNSW Australia, Manly Vale, NSW 2093, Australia;3. School of Environmental and Life Sciences, University of Newcastle, Callaghan, NSW 2308, Australia;4. Marine Climate Risks Group and Department of Environment and Geography, Macquarie University, NSW 2109, Australia;1. Ocean and Earth Science, National Oceanography Centre, University of Southampton, UK;2. Department of Environmental Sciences, Macquarie University, Australia;3. School of Marine Science and Engineering, Plymouth University, UK;4. UNESCO-IHE, Deltares, Delft, The Netherlands;5. University of Queensland, Australia;6. Water Engineering and Management, Faculty of Engineering Technology, University of Twente, PO Box 217, 7500 AE Enschede, The Netherlands;1. School of Biological, Earth and Environmental Sciences, UNSW Australia, Sydney, NSW, Australia;2. CNRS, UMR EPOC, University of Bordeaux, Pessac, France;3. School of Marine Science and Engineering, Plymouth University, Drake Circus, Plymouth PL4 8AA, UK |
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| Abstract: | A field experiment on the nature of rip currents was conducted on the Dutch coast, which differs from previous rip current study sites because it is a wind-sea dominated environment with mostly obliquely incident waves and tidally-driven longshore currents. During the experiment three distinct flow patterns, obtained with GPS tracked drifter instruments, were observed: (1) a locally governed circulation cell, (2) an offshore current that is deflected shore parallel outside the surf zone and (3) a meandering longshore current. The transition from rip currents (flow patterns 1 and 2) to meandering longshore currents (flow pattern 3) occurred gradually within the tidal cycle with longshore currents prevalent at mid to high tide. Rip currents at this site appeared at depressions in the surf zone bar and typically occurred when the water level fell below NAP (equivalent to MSL), even in the presence of obliquely incident waves and tidally driven longshore currents. Hindcast simulations of the drifter experiments were performed with the numerical model XBeach and showed good agreement with field observations. The model was subsequently used to investigate the influence of tidal water level fluctuations, longshore currents and obliquely incident waves on rip currents.Rip currents were initiated when the water level dropped below a specific threshold with the magnitude of the rip current associated with the water level. The strength of the tidal current and its orientation with respect to the incident waves governed the offshore extent and orientation of the rip current. In contrast to other studies that suggest that rip currents solely occur under shore normal (or slightly oblique waves), in this study both observations and numerical model simulations indicate that rip currents can exist under large angles of wave incidence, when the rip channel is sufficiently wide and the wave height is small. |
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