STUDENTS BECOME HUMAN SENSORS TO EXAMINE THE HAZARDS OF RIP CURRENTS
NEWS
Rip currents are dangerous currents in the surf zone that pull swimmers offshore into deeper water. This problem is well known to the lifeguards in the U.K., U.S., Australia and in Egmond aan Zee in the Netherlands. However, beachgoers are seldom aware of this hazard. In a joint project of TU Delft, Deltares and Shore Monitoring and co-funded by Flood Control 2015 and Building with Nature, the rip currents at Egmond aan Zee are studied in more detail.
Gundula Winter is doing her MSc research on this topic. She is graduating at the department of Coastal Engineering at the Faculty of Civil Engineering, TU Delft under the supervision of Prof. Marcel Stive, Matthieu de Schipper (TU Delft) and Ap van Dongeren (Deltares). From August 22 to 26, Gundula and a team of fellow students and colleagues set up camp at Egmond aan Zee to investigate rip currents in the field during the measurement campaign SEAREX.
Students are getting ready to become a rip current sensor.
Photo: www.muien.nl (2011 Willem Verbeek)
What is a rip current?
When waves approach the shore they break on the so-called breaker bar. This process can be easily observed at the beach. However, what is not visible to the naked eye is that breaking waves push the mean water level up against the shoreline. This so-called set up is in the order of ten centimetres and fluctuates with the incoming wave groups (sets of high and low waves). Also, the bar is not perfectly uniform, but is interrupted by occasional rip channels. As a result the water level set up differs along the shore and in time. The differences are small (orders of centimetres over a hundred meter length), but this is enough to drive an alongshore flow, much like a river. These currents converge at the rip channels into an offshore directed flow, the so-called rip current.
What makes rip currents dangerous?
Rip currents are hard to identify by inexperienced people because the current speed can be deceptively small. Moreover, the water in a rip current seems even calmer than in its surroundings. This leads to the underestimation of the dangers by beachgoers. However, a large number of rescues by the life guards in Egmond aan Zee are associated with rip currents. For the assessment of swimmer safety it is thus crucial to understand when these currents can occur as scientists and life guards have posed different hypotheses, and how strong they can be.
How to measure rip currents?
The rip currents were measured with drifter floats that were tracked by GPS data loggers. The drifters float with the surface currents and thus, provide a picture of the prevailing flow pattern in the surf zone. The drifters were released in the surf zone and left floating until they either crossed defined boundaries or stranded in shallow water. In addition to the drifter floats, human drifters (who were told not to swim but to lay still and float) were equipped with the same GPS units to compare the behaviour of swimmers and drifters. A high resolution survey of the study site was performed from a jet-ski to map the underlying bathymetry.
Are rip currents present at the Dutch coast?
The bathymetry survey revealed the existence of two rip channels to the North and South of the surf lifesaver station at Egmond aan Zee. It was in these two rip channels where the experiments were performed. During the whole week the team experienced low wave conditions. Nevertheless, both drifter floats and human drifters were observed to float offshore and thus proved the existence of rip currents even during low energy conditions. A variety of rip current conditions were measured with the strongest being approximately 0.6 m/s. On average these currents extended about 100 m offshore from the bar crest.
The information and data gained from this project will help us understand the nature of rip currents and enable us to make a model-based predictive system for rip current occurrence. A first version has been developed and can be viewed on “muienradar.nl”.
For more information please contact Gundula Winter or Ap van Dongeren