Hoist the flag: A boundary rip current case study on swimmer safety in the Scheveningen harbour area

Abstract

Each year several fatalities due to rip currents are reported in the Netherlands. One of these accidents was during May 11th in 2020 near the Scheveningen harbour, were several surfers lost their lives. During the hydrodynamic and meteorological accident were quite severe, with a significant wave height of 3.0m and wind speeds of 11 m/s, which is likely to have resulted in a boundary rip at the northern mole. On top of that extraordinary amounts of sea foam amassed at the northern harbour mole, disorienting anyone who got trapped in the area. It is likely that due to the wind and wave conditions and the resulting rip current it was near impossible for the surfers to exit the vicinity of the mole, especially with thick layer of foam. The Scheveningen harbour area was modelled in Delft3D and subjected to tide wind and waves. Assessment of the influence of wind, waves and tide on rip current intensity was performed by comparing the model results from a combined run with the rip velocities obtained by superimposing rip velocities per individual forcing condition. Lastly, the model was run with conditions resembling the aforementioned accident as to check whether the model was able hindcast this situation. The main wind and wave directions found to lead to offshore directed rip transport come from 30-70 degrees relative to shore normal. Retentive rips also occur, mainly when the wind-wave forcing comes from -70 to 0 degrees relative to shore normal, although rips occur less frequently in this range.