Physical and mechanical characteristics of the earthen barns used for food crop storage in the North-West of Benin (West Africa)

Atacora and Donga are two departments regarded as the barns of Benin. In these two regions, the surplus of food crops harvested by farmers is stocked in earthen barns whose primary construction material is kneaded clay soil with an admixture of vegetable straws (fonio, rice…) pounded beforehand. The following research is designed to scrutinize the variation of compression strength and/or tensile strength of the used earthen material with respect to the optimum straw content, and the fluctuation of the fracture related parameters. Laboratory tests have shown that an optimum straw content of 2% provides the highest compression strength with the best mechanical performances in terms of cohesion and internal angle of friction. Tensile strength, however, continues to increase with higher straw percentages with no breakpoint indication. Moreover, the laboratory determination of the composite material’s modulus of elasticity has proven that the used soil does not have a linear elastic behavior

Influence de la deformation plastique sur les caracteristiques mecaniques des materiaux geologiques : Cas des gneiss migmatitiques de la commune de Djidja au centre du Benin (Golfe de guinee, Afrique de l’ouest)

La zone interne de la chaîne panafricaine des Dahoméyides renferme des gneiss migmatitiques et des granites qui sont exploités pour les bâtiments et travaux publics. Ces deux roches de genèse différente, ont fait l’objet d’études pétrographique, structurale et géotechnique dans le but de déterminer l’influence de la déformationplastique sur les caractéristiques mécaniques des roches. Les granites étudiés n’ont pas subi une déformation plastique. Ils ont une structure équante à faiblement orientée, une texture grenue à grains fins et ils sont formés de quartz, feldspaths et micas. Par contre, les gneiss migmatitiques sont déformés de manière plastique ; ce qui s’est traduit par la structure foliée et la texture granolépidoblastique. Ils contiennent du quartz, des feldspaths, des micas et des grenats et renferment des  leucosomes quartzo-feldspathiques. L’étude des propriétés mécaniques de ces deux roches révèle que la résistance à la  sésagrégation est plus faible pour les gneiss migmatitiques 6% que pour les granites 5- 8%. Les essais de Los Angeles indiquent des valeurs plus élevées pour les gneiss migmatitiques 23-27%, 23-25.2% et 23-24% que pour les granites 21-23%, 20-22% et 19.5-20.5% respectivement pour les classes granulaires 4/6, 6/10 et 10/14. Les résultats des tests de Micro-Deval montrent des valeurs plus élevées pour les gneiss migmatitiques 8.4% que pour les granites 6.2%. La résistance en compression uniaxiale est plus élevée 226.5 MPa pour les granites que les gneiss migmatitiques 147.6 MPa. Ces résultats traduisent l’effet de la déformation plastique, marquée par le développement d’une architecture structurale foliée, sur les caractéristiques mécaniques des matériaux géologiques. Mots-clés : mécanique, gneiss migmatitique, granite, déformation plastique.   English Title: Effects of the plastic deformation on the mechanical characteristics of geological  materials: Case of migmatitic gneisses of Djidja town in centre-Benin (Guinee gulf, West Africa) The inner zone of Dahomeyides fold belt contains migmatitic gneisses and granites that are used for the bulding and public works. These two rocks of different genesis, have been studied petrographically, structurally and geotechnically in order to determine the effects of plastic deformation on the mechanical characteristics of rocks. The studied granites have not been deformed plastically. They have equante to weakly oriented structure, fine grained texture and made of quartz, feldspar and micas.  The migmatitic gneisses are deformed plastically that inducing foliated structure and granolepidoblastic texture. They formed by quartz, feldspar, micas and garnet and contain quartz-feldspar leucosomes. The mechanical properties study of these two rocks reveal that the resistance to the desagregation is low for the migmatitic gneisses 6% than in the granite 5-8%. The Los Angeles tests indicate high values for the migmatitic gneisses 23-27%, 23-25.2%, 23- 24% than in the granite 21-23%, 20-22%,  19.5-20.5% respectively for the 4/6, 6/10 and 10/14 granular classes. The results of Micro-Deval tests show high values for the migmatitic gneisses 8.4% than in the granites 6.2%. The resistance to the uniaxial compression is high for the granites 226.5 MPa than in the migmatitic gneisses 147.6 MPa. These results are in agreement with the effect of plastic deformation, marked by the development of foliated structural architecture on the mechanical characteristics of geological materials. Keywords: mechanical, migmatitic gneiss, granite, plastic deformation. &nbsp

Morphodynamic characterisation of the human-impacted Bight of Benin sand barrier coast, West Africa

International audienc

Influence des caracteristiques petro-structurales sur les proprietes mecaniques des quartzites de l’atacora au Benin (Golfe de Guinee, Afrique de l’Ouest)

Deux variétés de quartzites de l’unité litho-tectonique de l’Atacora dans la chaîne panafricaine des Dahoméyides au Nord-Ouest du Bénin ont fait l’objet d’études pétrostructurales et mécaniques en vue de la détermination de l’influence de la minéralogie et de la déformation des roches sur leurs propriétés géotechniques. Les quartzites étudiés proviennent de Pouya et Bérécingou dans la commune de Natitingou. Les quartzites sont à quartz + orthose. Ils contiennent en plus environ 10 % de muscovite à Pouya tandis qu’ils en en sont presque dépourvus à Bérécingou. De plus les quartzites de Bérécingou et Pouya présentent un bon débitage, un réseau de fracturations et une composition pétrographique permettant l’obtention de plaquettes de 1 à 15 cm d’épaisseur. Les essais géotechniques révèlent que les coefficients de Micro Deval MDE obtenus au niveau des sites de Bérécingou et Pouya sont respectivement de 6,4% et 8,3% suggérant que les quartzites de ces deux sites résistent excellemment à l’usure par frottement. Les coefficients de Los Angeles LA sont de 12,96% et 23,6% respectivement pour les quartzites de Bérécingou et Pouya ; ceci indique une excellente résistance à la fragmentation par chocs et à l’usure par frottements réciproques pour les quartzites de Bérécingou et une résistance satisfaisante à la fragmentation par chocs et à l’usure par frottements réciproques des quartzites de Pouya. Cette variation de propriétés mécaniques entre ces deux types de quartzite est attribuée à l’influence de la tectonique locale et de la minéralogie.Mots clés : Bénin, Atacora, minéralogie, structure, mécanique, quartzites. Influence of petro-structural characteristics on mechanical properties of quartzits from Atacora in Benin (Guinea Gulf, West Africa)ABSTRACTThe petrographic, structural and mechanical characteristic of two types of quartzite from the Atacora structural unit of Dahomeyides pan-African belt were study in order to determine the influence of mineralogy and tectonic on their geotechnical properties. The studied quartzites are sampled at Bérécingou and Pouya in the Natitingou municipality. Both quartzites contain quartz and orthose. Additionally, the quartzites from Pouya contain 10% of muscovite whereas those from Bérécingou are nearly mica free. Moreover the quartzites of Pouya and Bérécingou have a good breakdown, fracturation network and petrographic feature allowing the production wafer quartzites having 1 to 15 cm thickness. The geotechnical test reveal that the Micro Deval coefficients (MDE) obtained at Bérécingou and Pouya are respectively of 6.4% and 8.3% suggesting that the quartzites of these sites excellently resist to frictional wear. The Los Angeles (LA) coefficients are of 12.96% and 23.6% respectively for quartzite from Bérécingou and Pouya. This indicates an excellent resistance to fragmentation by shock and reciprocal frictional wear for the quartzite from Bérécingou and satisfactory resistance to fragmentation by shock and reciprocal frictional wear for the quartzite from Pouya. The change in mechanical properties of these two types of quartzite is attributed to the effect local tectonic and mineralogical composition.Keywords: Benin, Atacora, mineralogy, structure, mechanical, quartzits

Response of the Bight of Benin (Gulf of Guinea, West Africa) coastline to anthropogenic and natural forcing, Part 2: Sources and patterns of sediment supply, sediment cells, and recent shoreline change

International audienceThe Bight of Benin in the Gulf of Guinea, West Africa, forms an embayment between the Volta River delta in the west (Ghana) and the Niger River delta (Nigeria) in the east. The bight coast comprises sandy beaches backed by Holocene beach-ridge barriers. Incident swell waves, beachface gradient and the unidirectional longshore sand transport from west to east are intimately linked, generating a classic example of a strongly wave-dominated drift-aligned coast. The stability of this coast, which hosts several major cities in addition to three large international deepwater ports, has been strongly affected by human activities. We analyzed shoreline mobility and coastal area change over the period 1990-2015. Our results show how the stability of this coast has been strongly affected by the three ports therein, and by natural and human-altered shoreline dynamics related to the Volta River delta and to distributaries at the northwestern flank of the Niger delta. The combination of these factors has impacted alongshore sediment redistribution by segmenting the previously unrestrained longshore transport of sand that prevailed along this open coast. The result is a mixture of natural and artificial sediment cells increasingly dominated by shoreline stretches subject to erosion, endangering parts of the rapidly expanding port cities of Lomé (Togo), Cotonou (Benin) and Lagos (Nigeria), coastal roads and infrastructure, and numerous villages. Post-2000, the entire bight shoreline has undergone a significant decrease in accretion, which is here attributed to an overall diminution of sand supply via the longshore transport system. We attribute this diminution to the progressive depletion of sand-sized bedload supplied to the coast through the main Volta river channel downstream of the Akosombo dam, built between 1961 and 1965. Sand mining to cater for urban construction in Lomé, Cotonou and Lagos has also contributed locally to beach sediment budget depletion. Although alongshore sediment supply from the Volta River has been the dominant source of sand for the stability or progradation of the Bight of Benin coast, potential sand supply from the shoreface, and the future impacts of sea-level rise on this increasingly vulnerable coast are also important. The continued operation of the three ports and of existing river dams, and sea-level rise, will lead to sustained shoreline erosion along the Bight of Benin in the coming decades

Beach cusp dynamics on a reflective beach

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

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

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 km(2), 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 Lome (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

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

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

Response of the Bight of Benin (Gulf of Guinea, West Africa) coastline to anthropogenic and natural forcing, Part1: Wave climate variability and impacts on the longshore sediment transport

The short, medium and long-term evolution of the sandy coastline of the Bight of Benin in the Gulf of Guinea, West Africa, has become a major regional focal point due to the rapid socio-economic development that is occurring in the region, including rapid urbanization and a sharp increase in harbor-based trade. Harbors have a significant impact on the present evolution of this coast, notably by affecting longshore sediment transport. However, little is known of the environmental drivers, notably the wave climate, that governs longshore sediment transport and the ensuing pattern of shoreline evolution of this coastal zone. This article aims to address this important knowledge gap by providing a general overview of coastal evolution in the Bight of Benin and the physical processes that control this evolution.Here, the 1979-2012 ERA-Interim hindcast is used to understand the temporal dynamics of longshore sediment transport. Oblique waves (annual average Hs=1.36m, Tp=9.6s, S-SW incidence) drive an eastward drift of approximately 500,000m3/yr. The waves driving this large longshore transport can be separated into two components with distinct origins and behavior: wind waves generated locally in the Gulf of Guinea and swell waves generated in the southern hemisphere sub- (30-35°S), and extra-tropics (45-60°S). The analysis undertaken here shows that the contribution to the gross annual longshore transport from swell wave-driven longshore currents is an order of magnitude larger than the local wind wave-driven longshore currents. Swell waves are dominantly generated by westerlies in the 40-60°S zone and to a lesser extent by trade winds at 30-35°S. The longshore sediment drift decay (-5% over 1979-2012) is found to be linked with a decrease in the intensity of westerly winds associated with their southward shift, in addition to a strengthening of the trade winds, which reduces the eastward sediment transport potential. The equatorial fluctuation of the Inter-Tropical Convergence Zone (ITCZ) is found to explain most of the variability in transport induced by wind waves, while the Southern Annular Mode (SAM), an extra-tropical mode, has a predominant influence on transport induced by swell waves. The ITCZ and SAM have, respectively, a negative and positive trend over the period 1979-2012 that explains the decrease in both wind- and swell-wave-induced transport. For future scenarii, General Circulation Models (GCMs) predict a stabilization of the SAM, and, thus, a non-substantial or weak change in longshore sediment transport can be expected on the coast of the Bight of Benin

Response of the Bight of Benin (Gulf of Guinea, West Africa) coastline to anthropogenic and natural forcing : Part 1 : Wave climate variability and impacts on the longshore sediment transport

The short, medium and long-term evolution of the sandy coastline of the Bight of Benin in the Gulf of Guinea, West Africa, has become a major regional focal point due to the rapid socio-economic development that is occurring in the region, including rapid urbanization and a sharp increase in harbor-based trade. Harbors have a significant impact on the present evolution of this coast, notably by affecting longshore sediment transport. However, little is known of the environmental drivers, notably the wave climate, that governs longshore sediment transport and the ensuing pattern of shoreline evolution of this coastal zone. This article aims to address this important knowledge gap by providing a general overview of coastal evolution in the Bight of Benin and the physical processes that control this evolution. Here, the 1979-2012 ERA-Interim hindcast is used to understand the temporal dynamics of longshore sediment transport. Oblique waves (annual average H-s,=1.36 m, T-p=9.6 s, S-SW incidence) drive an eastward drift of approximately 500,000 m(3)/yr. The waves driving this large longshore transport can be separated into two components with distinct origins and behavior: wind waves generated locally in the Gulf of Guinea and swell waves generated in the southern hemisphere sub- (30-35 degrees S), and extra-tropics (45-60 degrees S). The analysis undertaken here shows that the contribution to the gross annual longshore transport from swell wave-driven longshore currents is an order of magnitude larger than the local wind wave-driven longshore currents. Swell waves are dominantly generated by westerlies in the 40-60 degrees S zone and to a lesser extent by trade winds at 30-35 degrees S. The longshore sediment drift decay (-5% over 1979-2012) is found to be linked with a decrease in the intensity of westerly winds associated with their southward shift, in addition to a strengthening of the trade winds, which reduces the eastward sediment transport potential. The equatorial fluctuation of the Inter-Tropical Convergence Zone (ITCZ) is found to explain most of the variability in transport induced by wind waves, while the Southern Annular Mode (SAM), an extra-tropical mode, has a predominant influence on transport induced by swell waves. The ITCZ and SAM have, respectively, a negative and positive trend over the period 1979-2012 that explains the decrease in both wind- and swell-wave-induced transport. For future scenarii, General Circulation Models (GCMs) predict a stabilization of the SAM, and, thus, a non-substantial or weak change in longshore sediment transport can be expected on the coast of the Bight of Benin