15 research outputs found

    Influence of the Southern Atlantic Central Water on the distribution of salinity and oxygen in the northeast tropical Atlantic Ocean. Deep Sea Res.

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    Abstract-Two main features of the Southern Atlantic Central Water allow its extention to be followed in the northern hemisphere: a salinity minimum located on CJ, = 26.8 and an oxygen minimum. The salinity minimum has been associated with the coastal upwelling undercurrent off the northwestern coast of Africa but it is not an exclusive feature of this undercurrent south of Cap Blanc. The lowest values of the oxygen minimum are observed in summer in the centre of the Guinea Dome

    On the circulation in the upper layer of the western equatorial Atlantic

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    Flash rip dynamics on a high-energy low-tide-terraced beach (Grand Popo, Benin, West Africa)

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    International audienceRip currents are wave-driven intense seaward-flowing jets of water that are important to both beach morphodynamics and the overall ecosystem. Rip currents are also the leading deadly hazard to recreational beach users worldwide. More specifically, the African region is reported to have the highest rates of drowning in the world, yet both the occurrence and the type of rips developing along the African beaches are unknown. In February 2013, a 12-day field experiment was performed at the high-energy low-tide-terraced sandy beach of Grand Popo beach (Benin, West Africa). Human drifter data and video imagery are combined to address wave-driven circulation and rip current activity. Results show two prevailing rip current types. (1) Low-energy (~0.2-0.4 m/s) swash rips, with short life-spans of about 1 minute, extend about 5-10 m offshore and occur preferably at mid to high tide at fixed locations in the center of beach cusps. (2) Higher-energy (0.2 - 0.8 m/s) surfzone flash rips become active with the onset of intense wave breaking across the low-tide terrace. They tend to migrate downdrift with alonger time-span of about 2-5 minutes. The relatively weak longshore current (0.2 - 0.55 m/s) measured during the experiment suggests that flash rips were driven by vorticity generated by wave breaking rather than shear instabilities of the longshore current. Swash rips and flash rips are common at Grand Popo and often co-exist. We propose a conceptual model of both flash and swash rip activity on this stretch of the West African coast

    Beach cusp dynamics on a reflective beach

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    International audienceBeach cusps are common feature of steep reflective and intermediate beaches. However very few observations reported double coupled cusp systems. Here we present a data set of observations of a beach exhibiting two sets of beach cusps. Data were collected at Grand Popo Beach (Benin, West Africa) in February 2013. Daily topographic survey along a 380m long stretch of shore allowed observing the dynamic of the two set of beach cusps. At the beginning of the field survey, we clearly observe two sets of cusps : the upper beach cusps system is relatively asymetric with a typical wavelength of about 45 m while the lower beach cusps system is relatively symmetric with a typical wave length slightly shorter (about 35 m). After two days, we measured the total destruction of the lower set of beach cusps while the upper set of beach cusps was only partially des tructed. Data suggest that destruction of the lowerbeach cusp system may be related to persistent accretions conditions and/or calm conditions but probably also to the transition from wave- driven circulation dominated by weak alongshore currents with flash and swash rips, to a second period characterized by dominant longshore current further increasing in speed, and rare swash rips. On the other hand the disappearance of the western upper beach cusp might be related to an accretion pattern and to the merging of two individual features. Our observations, consistent with previous works, suggest that beach cusps certainly arise as a result of some combination of erosion and accretion

    Rip currents and circulation on a high-energy low-tide-terraced beach (Grand Popo, Benin, West Africa)

    No full text
    Rip currents are wave-driven intense seaward-flowing jets of water that are important to both beach morphodynamics and the overall ecosystem. Rip currents are also the leading deadly hazard to recreational beach users worldwide. More specifically, the African region is reported to have the highest rates of drowning in the world, yet both the occurrence and the type of rips developing along the African beaches are unknown. In February 2013, a 12-day field experiment was performed at the high-energy low-tide-terraced sandy beach of Grand Popo beach (Benin, West Africa). Human drifter data and video imagery are combined to address wave-driven circulation and rip current activity. Results show two prevailing rip current types. (1) Low-energy (similar to 0.2-0.4 m/s) swash rips, with short life-spans of about 1 minute, extend about 5-10 m offshore and occur preferably at mid to high tide at fixed locations in the center of beach cusps. (2) Higher-energy (0.2-.8 m/s) surfzone flash rips become active with the onset of intense wave breaking across the low-tide terrace. They tend to migrate downdrift with a longer time-span of about 2-5 minutes. The relatively weak longshore current (0.2 - 0.55 m's) measured during the experiment suggests that flash rips were driven by vortices generated by wave breaking rather than shear instabilities of the longshore current. Swash rips and flash rips are common at Grand Popo and often co-exist We propose a conceptual model of both flash and swash rip activity on this stretch of the West African coas

    Observed destruction of a beach cusp system in presence of a double-coupled cusp system : the example of Grand Popo, Benin

    No full text
    Beach cusps are common features of steep reflective and intermediate beaches. However, very few observations have reported double coupled cusp systems. Here, we present a dataset of observations of a beach exhibiting two sets of beach cusps. Data were collected at Grand Popo Beach (Benin, West Africa) in February 2013. Daily topographic surveys along a 380 in long stretch of shore allowed observation of the dynamics of the two sets of beach cusps. At the beginning of the field survey, we clearly observe two sets of cusps; the upper beach cusps system is relatively asymmetric with a typical wavelength of about 45 m, while the lower beach cusps system is relatively symmetric with a typical wave length slightly shorter (about 35 m). After two days, we measured the total destruction of the lower set of beach cusps while the upper set of beach cusps was only partially destroyed. The data suggest that destruction of the lower beach cusp system may be related to persistent accretionary conditions and/or calm conditions but probably also to the transition from wave-driven circulation (dominated by weak alongshore currents with flash and swash rips), to a second period characterized by dominant longshore currents further increasing in speed (with rare swash rips). Conversely, the disappearance of the western upper beach cusp may be related to an accretionary pattern and to the coalescence of two individual features. Our observations, consistent with previous works, suggest that beach cusps certainly arise as a result of some combination of erosion and accretion

    The Grand Popo experiment, Benin

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    International audienceThe first large nearshore field experiment in the G ulf of Guinea was conducted at Grand Popo Beach, Be nin, in February 2013, on an open wave-dominated micro- to meso-tidal coast, located mid-way between Cotonou and Lome harbours. The overall project aims at understanding at multi-scale (from event to interannual) the causes of the dramatic erosion observed throughout the Bight of Benin, and caused by the interaction of a large littoral drift with human engineering works. Grand Popo 2013 experiment was designed to measure the processes over the short term and to test the ability of an installed video system to monitor the evolution of this stretch of coast over the longer term. The beach, characterized by a low-tide terrace and a high tide reflective part, experiences a long swell (Hs=1.6m, Tp=16 s, oblique incidence ~15-20°). Topographic surveys showed a double beach cusp system interaction and repeated surf-zone drifter runs revealed high flash and swash rip activity driven by wave dissipation over the terrace and energetic swash dynamics at the upper reflective beach. Swash was measured over a cusp system at two locations using video poles. Wave reanalyses (ERAInterim) were used to determine the wave climate and its variability, and to quantify sediment transport. This robust methodology is thought to be replicated elsewhere in different coastal environments in West Africa, in particular with the objective to monitor various sites within the framework of the new West African Coastal Observatory

    The Grand Popo beach 2013 experiment, Benin, West Africa: from short timescale processes to their integrated impact over long-term coastal evolution

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    The first large nearshore field experiment in the Gulf of Guinea was conducted at Grand Popo Beach, Benin, in February 2013, on an open wave-dominated micro- to meso-tidal coast, located mid-way between Cotonou and Lome harbours. The overall project aims at understanding at multi-scale (from event to interannual) the causes of the dramatic erosion observed throughout the Bight of Benin, and caused by the interaction of a large littoral drift with human engineering works. Grand Popo 2013 experiment was designed to measure the processes over the short term and to test the ability of an installed video system to monitor the evolution of this stretch of coast over the longer tenn. The beach, characterized by a low-tide terrace and a high tide reflective part, experiences a long swell (Hs=1.6 m, Tp=16 s, oblique incidence similar to 15-20 degrees). Topographic surveys showed a double beach cusp system interaction and repeated surf-zone drifter runs revealed high flash and swash rip activity driven by wave dissipation over the terrace and energetic swash dynamics at the upper reflective beach. Swash was measured over a cusp system at two locations using video poles. Wave reanalyses (ERAInterim) were used to determine the wave climate and its variability, and to quantify sediment transport. This robust methodology is thought to be replicated elsewhere in different coastal environments in West Africa, in particular with the objective to monitor various sites within the framework of the new West African Coastal Observatory
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