Controls on flash rip current hazard on low-tide terraced tropical beaches in West Africa

2018

Article référencé dans le Web of Science WOS:000457252000013

Scott T., Castelle B., Almar Rafaël, Senechal N., Floc'h F., Detandt G.

Journal of Coastal Research, 2018, (81 No spécial), p. 92-99 ISSN 0749-0208

Flash rip currents are transient non-fixed offshore-directed jets of water often associated with migrating surf-zone eddies. They can occur on featureless (alongshore-uniform) beaches and are essentially hydrodynamically controlled. Due to their unpredictable nature, they represent one of the most significant hazards to bathers across surf zones worldwide, especially in Tropical West Africa, where high levels of flash rip activity and a lack of a lifeguard service lead to high drowning rates. Recent work has shed light on two contrasting driving mechanisms for non-fixed rips: (1) cascading vortices generated by wave breaking, and (2) shear instabilities of the longshore current. This research provides the first quantitative scientific understanding of wave-driven currents in Tropical West Africa. Specifically, the study focussed on the dynamics of flash rips occurring along alongshore-uniform beach morphology, through the collection of unique Lagrangian field measurements of flash rips. During two periods, February 2013 and March 2014, 13 days of field data collection occurred at Grand Popo beach on the southerly-facing Bight of Benin coast (Gulf of Guinea, West Africa). The experiments measured flash rip currents, nearshore hydrodynamics and morphology at a moderately energetic (long-term average significant wave height, Hs = 1.36 m, and peak wave period Tp = 9.4 s), sandy alongshore-uniform low-tide terraced beach exposed to South Atlantic groundswells and local wind-waves. A suite of GPS drifters, dye-tracking and video confirmed the presence and characteristics of low high energy flash rips (up to 0.7 m/s) in the surf zone, while in-situ acoustic wave and current measurements from the surf zone and inner shelf provided metocean forcing conditions (0.8