Flash Rip Statistics from Video Images

Abstract

Floc'h, F.; Mabiala, G.R.; Almar, R.; Castelle, B.; Hall, N.; Du Penhoat, Y., Scott, T., and Delacourt, C., 2018. Flash rip statistics from video images. 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. 100–106. Coconut Creek (Florida), ISSN 0749-0208.The coast of the Gulf of Guinea is exposed to high-energy waves generating strong nearshore currents both driving erosion and posing a threat to recreational beach goers and fishermen. Along this coast, the coexistence of swash and flash rip currents at Grand-Popo beach (Benin) was evidenced in a previous study using a limited number of Lagrangian drifter measurements. In this paper, a method based on color processing of video images from a permanent station has been developed to detect the presence of rip currents via a change of turbidity. Rip current characteristics and their link with tide and wave forcing over a 7-day period are derived. During this experiment, a total of 434 events has been counted during daylight. The majority of rips occurred at low tide and migrates downdrift. Flash rip activity is maximized for shore-normal wave incidence and mild wave conditions showing that flash rips are not driven by shear instabilities of the longshore current at this site. However, the lack of strong variability in wave forcing throughout the experiment prevents a more thorough wave-rip relationship investigation, which motivates the application of the method for a longer period with a wide wave-tide spectrum.