Jean-Pierre Lefebvre

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

ABSTRACT Almar, R., Hounkonnou, N., Anthony, E., Castelle, B., Senechal, N., Laibi, R., Mensah-Senoo, T., Degbe, G., Quenum, M., Dorel, M., Chuchla, R., Lefebvre, J-P, du Penhoat, Y., Laryea, W.S., Zodehougan, G., Sohou, Z., and Appeaning Addo, K., Kestenare, E., 2014. The Grand Popo beach 2013 experiment, Benin, West Africa: from short timescale processes to their integrated impact over long-term coastal evolution. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 651–656, ISSN 0749-0208. 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 term. 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 ~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.

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

ABSTRACT Castelle, B., Almar, R., Dorel, M., Lefebvre, J.P., Sénéchal, N., Anthony, E.J., Laibi, R., Chuchla, R., du Penhoat, Y., 2014. Rip currents and circulation on a high-energy low-tide-terraced beach (Grand Popo, Benin, West Africa). In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 633–638, ISSN 0749-0208. 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 (~ 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 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 coast.

Contribution of swash processes generated by low energy wind waves in the recovery of a beach impacted by extreme events: Nha Trang, Vietnam

ABSTRACT Lefebvre, J.-P., Almar, R., Viet, N.T., Uu, D.V., Thuan, D.H., Binh, L.T., Ibaceta, R., Duc,N.V., 2014. Contribution of swash processes generated by low energy wind waves in the recovery of a beach impacted by extreme events: Nha Trang, Vietnam . In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 663–668, ISSN 0749-0208. Nha Trang beach experiences southerly sediment drift during winter monsoons and northerly sediment drift during summer monsoons. In addition, the area is likely to be impacted by tropical storms or typhoons. Due to the presence of islands at the south east border of the bay, the strongest impact on the shoreline apart from extreme events is due to NE swell from October to April. The mechanism responsible for the sediment drift generated by low energetic locally generated wind waves is insufficiently understood. It involves the functioning of the swash zone for weak conditions. Two field experiments were scheduled before and after the period of cyclonic activity. The aim of the first experiment was to describe the sites bathymetry and the geomorphology of the upper beach and hydrologic functioning of the bay. A new method of measurements in the swash and surf zone based on processing of data extracted from HD video processing was tested successfully for wind wave conditions in a reflective beach. Here, we present some data obtained during the field experiment at different time scales. The data provides a first quantification of the impact on sediment transport from typical low energy conditions which are encountered during spring and summer in Nha Trang.

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

ABSTRACT Senechal, N., Laibi, R.A., Almar, R., Castelle, B., Biausque, M., Lefebvre, E. J. Anthony, J.-P., Dorel, M., Chuchla, R., Hounkonnou, M.H., Du Penhoat, Y., 2014. Observed destruction of a beach cusp system in presence of a double-coupled cusp system: the example of Grand Popo, Benin. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 669–674, ISSN 0749-0208. 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 m 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.