Convex coastline induced rip currents at the Sand Engine

This study investigates the hydrodynamics at a convex coastline. The Sand Engine, a man made peninsula in the Netherlands, is used as a case scenario. It is hypothesized that a rip current can develop under certain wave conditions due to the change in coastline orientation. The main goal of this research is to obtain a better understanding of the relevant processes for this rip current to develop near convex coastlines and to investigate if these currents pose a hazard for swimmer safety. On March 8, 2012, a field experiment was conducted at the Sand Engine to measure flow velocities in the surfzone. In this experiment GPS tracked buoys were released in the surfzone. Two drifter deployments are presented in this study, one of these showed a circular flow pattern, the second a large offshore directed current, with a maximum velocity of 0.8 [ms?1]. The data set, from the fieldwork, was then used to optimize and test the abilities of a numerical model, Delft3D. A hindcast, of the conditions during the field experiment, in Delft3D gave results that were in good agreement with the field data. Furthermore the model results showed that the offshore current from the second deployment was partly due to the coastline convexity. A simplified alongshore uniform model is constructed in Delft3D to test a variety of scenario’s. Most scenarios focused on coastline convexity (between 35.5 and 0 [deg]) and wave directions as it was found likely that these are the most relevant processes. Other scenarios tested the influence of a tide, a rip channel, wave height and bottom profile. For this model it was found that the most relevant processes for a rip current at a convex coastline are; (1) Wave direction, when waves approach the shore along the mean shore normal the offshore velocity is highest. (2) Coastline convexity, a more convex coastline inducing a higher offshore current velocity and has a larger range of wave direction that induce a rip current. (3) Wave height, higher waves give more offshore flow. (4) The tide, tidal current extends the range of wave directions that induce a rip current. Moreover it was found that a rip channel induces higher rip current velocities. The implications for swimmer safety were examined by predicting the rip current strength under typical wave conditions during summer beach days. It was concluded that a convex change in coastline orientation larger then 15 [deg] results in an enhanced risk for swimmer safety. Up to a 5 times higher rip current frequency is anticipated on summer beach days due to coastline convexity.Hydraulic EngineeringCivil Engineering and Geoscience