Controls on Flash Rip Current Hazard on Low-Tide Terraced Tropical Beaches in West Africa

Abstract

Scott, T.; Castelle, B.; Almar, R.; Senechal, N.; Floc'h, F., and Detandt, G., 2018. Controls on flash rip current hazard on low-tide terraced tropical beaches in West Africa. In: Almar, R.; Almeida, L.P.; Trung Viet, N., and Sall, M. (eds.), Tropical Coastal and Estuarine Dynamics. Journal of Coastal Research, Special Issue No. 81, pp. 92–99. Coconut Creek (Florida), 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<Hs<1.6 m; 8<Tp<14 s; 1°<Dp<21° (from shore normal)) and longshore current velocities (0.2–0.8 m/s in surf zone). Throughout a range of wave conditions (height/period/angle/directional-spreading) contrasting flash rip behavioural responses were observed (surf zone exits/offshore extent/offshore-directed velocities). This study (1) supports findings of the first rip experiment conducted at this site (Feb 2013) suggesting that flash rip activity is likely to be driven by wave breaking vorticity generation (flash rip activity high under shore-normal wave forcing) and (2) provides further quantitative insights into the role of specific wave/tide forcing characteristics on flash rip activity, specifically the importance of reduced directional spreading and tidal level in controlling increases in offshore extent of flash rip currents. Finally, (3) the observed ubiquity of significant hazardous transient flash rip activity and identification of key behavioural controls can provide a foundation for any future development of rip hazard prediction tools and effective lifeguarding in West Africa.