Rip currents hazard (pilot areas) Threats posed by rip currents, as defined using the XBeach mode

Rip currents are currents occurring in the coastal zone that flow seawards at a right angle to the shoreline (Fig. 1). They are caused by localized, instantaneous build-ups of water between the shore and the sandbar zone. They are strong enough to “rip” though these bars, hence their name. Localized build-ups of water are usually caused by storm waves breaking over the sandbars. Such build-ups can also be generated by long-term longshore waves. The localization of rip currents depends on the shape of the shoreline, near which so-called circulation cells can form.

Fig.1 Diagram showing how rip currents form (http://www.seagrant.umn.edu/coastal_communities/rip) Fig.2 How to escape from a rip current (http://gosouthonline.co.za/nsri-giving-advice-on-rip-currents/)

Methodology

Determining the possible occurrence of rip currents is based in the SatBałtyk System on the assumption that they form when waves break up continuously over a sandbar. The frequency of breaking waves in the energy spectrum of waves approaching the shore is defined using the XBeach prognostic model. This is then compared with the breaking waves visible on photographs taken by the camera installed at Dziwnówek for the Dziwnów Spit region or on photographs/films taken by the cameras recording wave-breaking during small storm events in 15-minute series from the Jamno and Bukowo Spits and the Hel Peninsula. Finally, the threshold value of this parameter is calculated: if this is exceeded, rip currents may occur.

Validation

Comparison of the threshold frequencies of breaking waves defined by the model with the breaking waves visible on photographs taken by the camera installed at Dziwnówek for the Dziwnów Spit region or on photographs/films taken by the cameras recording wave-breaking during small storm events in 15-minute series from the Jamno and Bukowo Spits and the Hel Peninsula. More information: Validation of rip currents.

Interesting phenomena Rip currents are particularly dangerous to swimmers. Every year in the USA more than 100 people drown as a result of rip currents, and more than 80% of rescue actions take place because of these currents. Rip currents can flow as fast as 1-2 m/s. A swimmer who gets caught up in a rip current does not normally have the strength to swim against it. The ensuing panic, rapid tiring and poor swimming skills usually lead to a tragic finale. Therefore, if someone finds him/herself in a rip current, the first thing is NOT to panic and NOT to struggle against the current but let him/herself be carried until the current’s energy has dissipated. Only then should one try to swim back to the shore; alternatively, one can try to swim parallel to the shore (at right-angles to the rip current) in order to escape from the rip current (Fig. 2).

On ocean coasts rip currents are particularly dangerous; on the Polish coast the dangers posed by these currents are somewhat less. Rip currents on the Baltic coast have not yet been thoroughly studied. Even so, lifeguards from the Polish coastal rescue service (WOPR) have said that the danger from rip currents is particularly acute when strong onshore winds accompany wave breaking. During interviews, lifeguards usually refer to rip currents as backward or return currents. More information can be found at (http://www.tvn24.pl/wiadomosci-z-kraju,3/niebezpieczne-prady-zwrotne-na-baltyku-tonacy-gina-po-cichu,341667.html).

Link to the parameter in SatBaltic System:

Rip currents hazard (pilot areas)