Rafael Almar

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.

Longshore drift cell development on the human-impacted Bight of Benin sand barrier coast, West Africa

ABSTRACT Laïbi, R. A., Anthony, E. J., Almar, N., Castelle, B., Senechal, Kestenare, E., 2014. Longshore drift cell development on the human-impacted Bight of Benin sand barrier coast, 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. 078–083, ISSN 0749-0208. The Bight of Benin is an open, microtidal, wave-dominated coast forming a 500 km-long mild embayment in the Gulf of Guinea, in West Africa, between the Volta River delta in Ghana, to the west, and the western confines of the Niger River delta in Nigeria to the east. The bight is exposed to energetic swells from the South Atlantic, and is characterised by Holocene sand barriers bounding lagoons. The barrier system has been sourced essentially by sand supplied through the Volta River delta, terminus of a large river catchment of 397,000 km2, although wave energy conditions and sand mineralogy also suggest inputs from the nearshore shelf. The long-term pattern of barrier progradation in the Bight of Benin culminated in a mildly embayed coast wherein incident wave behaviour, beachface gradient and the longshore sand transport system were intimately linked, generating what may be classified as an ‘equilibrium drift-aligned’ coast with a unique and homogeneous longshore drift cell stretching from the Volta River delta to the Niger River delta. This coast has, however, been significantly impacted over the last 50 years by the construction of three deepwater ports in Lomé (Togo), Cotonou (Benin) and Lagos (Nigeria) that have intercepted sand supply, as well as by a major dam on the Volta River, resulting in destabilization of the former single drift cell on this coast. The ensuing multi-cellular structure is characterised by long sectors of rampant coastal erosion that threatens parts of these cities, coastal villages and infrastructure.

Wavelet-Based Optical Flow Estimation of Instant Surface Currents From Shore-Based and UAV Videos

Instant fields of surface current are retrieved from shore-based and unmanned aerial vehicle videos by an optical flow (OF) method named “Typhoon.” This computer vision algorithm estimates dense 2-D 2-component velocity fields from the observable motion of foam patterns in the surf zone. Despite challenging image data resolution and quality, comparison of OF surface current estimates with measurements by an acoustic Doppler velocimeter reveals its ability to capture both wave-to-wave fluctuations and low-frequency variations. The method is also successfully applied to the monitoring of a “flash rip” event. This paper shows clearly the high potential of this method in the nearshore, where the rapid development of webcams and drones offers a large number of applications for swimming and surfing safety, engineering and naval security, and research purpose, by providing quantitative information.

Video monitoring and field measurements of a rapidly evolving coastal system: the river mouth and sand spit of the Mataquito River in Chile

ABSTRACT Cienfuegos R., Villagran M. ,Aguilera J.C., Catalán P., Castelle B., Almar R,, 2014. Video monitoring and field measurements of a rapidly evolving coastal system: the river mouth and sand spit of the Mataquito River in Chile. 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. 639–644, ISSN 0749-0208. The understanding of morphological processes controlling the evolution of sand spit reformation after a tsunami impact is a challenging and interesting topic, especially in highly energetic and micro tidal environments. A field campaign performed during December 2012 at the Mataquito River mouth in Chile, allowed us to simultaneously monitor topo-bathymetry evolution, wave climate, tidal range, swash zone dynamics and upper beach face evolution over a portion of its sand spit. A video system was set up for a continuous and long-term monitoring of the evolution of the river mouth and sand spit. Primarily, in this work we focus on the application of a video-derived shoreline detection method to assess shoreline evolution and beach cusps migration at hourly scales. We test the method performance on short-term episodic migration of beach cusps recorded during the campaign. Beach face variations at a daily scale were observed, which can be attributed to the migration of beach cusps in the alongshore direction, and linked to wave forcing and alongshore sediment transport.

Video-Based Detection of Shorelines at Complex Meso-Macro Tidal Beaches

Abstract Almar, R.; Ranasinghe, R.; Sénéchal, N.; Bonneton, P.; Roelvink, D.; Bryan, K.R.; Marieu, V., and Parisot, J-P., 2012. Video-based detection of shorelines at complex meso–macro tidal beaches. Remote video imagery is widely used to acquire measurements of intertidal topography by means of shoreline detection, but, up to now, problems of accuracy were still encountered in the challenging case of energetic waves in nonuniform, meso–macro tidal environments. Unique, simultaneous, video-based and global positioning system (GPS)–based measurements of shoreline were undertaken at Truc Vert (France), a beach with such characteristics. An innovative video method, referred to herein as the Minimum Shoreline Variability (MSV) method, was developed to cope with highly variable spatiotemporal shoreline properties. The comparison of video-based and GPS-derived shoreline data sets showed that using images averaged over short periods (30 s), rather than the traditionally used 10-min averaged images, significantly improved the accuracy of shoreline determination. A local video-derived, swash-based shoreline correction was also developed to correct for the MSV error, which was found to be linearly correlated to local swash length. By combining shorter time-averaged images and video derived local swash correction factors, the horizontal root mean square error associated with MSV shorelines was reduced to 1.2 m, which is equivalent to errors reported at more uniform, microtidal, and less-energetic beaches.

Recent shoreline changes in the Volta River delta, West Africa: the roles of natural processes and human impacts

The Volta River delta developed as an asymmetric lobe in a tectonic offset on the coast of Ghana. The delta comprises a large curvilinear spit that widens in its central portion due to the adjunction of successive sandy beach ridges. The appearance of a distinct spit, in lieu of a continuous barrier from the present mouth of the Volta River to the Bight of Benin coast, may be an outgrowth of a natural change in the location of the mouth of the Volta. The spit marks a segmentation of the unique sand drift cell that hitherto prevailed on this bight coast. Spit growth has been accompanied by a wave of erosion over the last century of the immediate downdrift sector of the bight coast, endangering the town of Keta. Erosion since the 1960s may have been aggravated by the construction of the Akosombo hydropower dam. The tip of the spit has recently welded to the shoreline, thus assuring resumption of sand supply from the Volta towards the rest of this formerly sand-starved sector of the bight coast. Blocking of sediment by the Akosombo Dam is, in due course, likely to become the overarching factor in delta shoreline stability.

WAVE CELERITY FROM VIDEO IMAGING: A NEW METHOD

Video systems are emerging tools to monitor high frequency nearshore morphodynamics. Bathymetry can be inverted from surface wave celerity. We present here a new and robust method to estimate this celerity. The method is a time and space cross-correlation based on time-stack images. The celerity estimation is local, within a distance smaller than typical encountered wavelength and uses a time-domain correlation that integrates all wave effects within the duration of the time-stack. A validation is given from available in-situ data of the Pre-ECORS experiment. Global RMS error is lower than 0.2 m/s for the available period of comparison. Thus, the method seems to be well adapted to complex morphologies. Sources of error on the celerity estimation in the nearshore area are also investigated.

What dominates sea level at the coast: a case study for the Gulf of Guinea

Sea level variations and extreme events are a major threat for coastal zones. This threat is expected to worsen with time because low-lying coastal areas are expected to become more vulnerable to flooding and land loss as sea level rises in response to climate change. Sea level variations in the coastal ocean result from a combination of different processes that act at different spatial and temporal scales. In this study, the relative importance of processes causing coastal sea level variability at different time-scales is evaluated. Contributions from the altimetry-derived sea-level (including the sea level rise due to the ocean warming and land ice loss in response to climate change), dynamical atmospheric forcing induced sea level (surges), wave-induced run-up and set-up, and astronomical tides are estimated from observational datasets and reanalyses. As these processes impact the coast differently, evaluating their importance is essential for assessment of the local coastline vulnerability. A case study is developed in the Gulf of Guinea over the 1993–2012 period. The leading contributors to sea level variability off Cotonou differ depending on the time-scales considered. The trend is largely dominated by processes included in altimetric data and to a lesser extent by swell-waves run-up. The latter dominates interannual variations. Swell-waves run-up and tides dominate subannual variability. Extreme events are due to the conjunction of high tides and large swell run-up, exhibiting a clear seasonal cycle with more events in boreal summer and a trend mostly related to the trend in altimetric-derived sea-level.

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.

Under-estimated wave contribution to coastal sea-level rise

Coastal communities are threatened by sea-level changes operating at various spatial scales; global to regional variations are associated with glacier and ice sheet loss and ocean thermal expansion, while smaller coastal-scale variations are also related to atmospheric surges, tides and waves. Here, using 23 years (1993–2015) of global coastal sea-level observations, we examine the contribution of these latter processes to long-term sea-level rise, which, to date, have been relatively less explored. It is found that wave contributions can strongly dampen or enhance the effects of thermal expansion and land ice loss on coastal water-level changes at interannual-to-multidecadal timescales. Along the US West Coast, for example, negative wave-induced trends dominate, leading to negative net water-level trends. Accurate estimates of past, present and future coastal sea-level rise therefore need to consider low-frequency contributions of wave set-up and swash.Large-scale sea-level rise is primarily dominated by thermal expansion and ice melt. However, wave processes are found to significantly influence local sea-level trends at the coast, amplifying or reducing steric and eustatic contributions.