A review on coastal erosion and flooding risks and best management practices in West Africa : what has been done and should be done

The West African coast is vulnerable to natural hazards and human interventions. Although various measures have been taken at different scales, mostly at the local level, there is a need to improve management at the regional level. We examine these actions and possible solutions from different perspectives and provide conclusions and recommendations on the integration of solutions to improve coastal management. From North West Mauritania to across the Gulf of Guinea a system of coastal zoning that can be managed holistically is encouraged. The development of holistic planning is seen as a sustainable approach to management that seeks to link users/processes together rather than focus on a single particular issue and solution. Strengthening, monitoring, promoting the observation network and generalising open data centralisation and exchange for a better understanding of coastal dynamics and pressures is encouraged. There is a need for capacity building, expertise and federative actions. Furthermore, the need to identify and involve not only stakeholders, but also communities and scientists with multilevel inputs. All must agree on coordinated plans to achieve stakeholder objectives, using an approach adapted to the multi-spatial scale (e.g at the scale of sediment cells, integrating from the sources of sediment in river basins to their redistribution along the coast, perturbed by climate changes and anthropic stresses), so that only regional solutions are appropriate and will be effective. These must follow sustainable strategies with a multi-temporal sequenced solution and anticipate changes, or adaptive solutions using solutions in synergy with different time frames as well as managing natural and human systems responsibly. A plan that considers changes in coastal systems and anticipates impacts and adapts plans accordingly will be key

Influence of sea level variability on a micro-tidal beach

Global climate change in response to buildup of human-induced greenhouse gases in the atmosphere has already resulted in several visible consequences such as an increase in ocean heat content and melting of glaciers. The relative effect of sea level and wave variability on shoreline changes along the coastline of James Town, Ghana (Gulf of Guinea), was investigated between 2013 and 2014. Data were gathered at James Town, over an 8-month period (2013-2014) from nearshore video installation, Era-Interim hindcast, and satellite altimetry. Data were evaluated over daily and monthly timescale variations. Though beach changes are not directly driven by sea level variability, its action is key in modulating waves influence. The results show that the shoreline responds in decreasing order to sea level variations (86%), waves (9%), and tidal cycles (5%) on daily bases. Wind-induced setup has limited effect on the shoreline. The observed most important component of sea level anomaly at this tropical "storm free" coast is the influence from inverse barometer. Decadal observations from satellite show that sea level anomaly is continuously rising in the Gulf of Guinea subregion with expected strong consequence for this urbanized low lying sandy coast

Shoreline and beach cusps dynamics at the low tide terraced Grand Popo Beach, Benin (West Africa) : a statistical approach

This paper presents the description of a 2-dimensional shoreline and 3-dimensional shoreline behaviour around a 'storm free' microtidal beach from a 3-year video observation using empirical orthogonal function (EOF) analysis. The EOF method is applied successfully to delineate 2D and 3D developments that show the beach evolution influenced by the action of waves and tides. The EOF model results are in agreement with measurements that reflect the morphological concepts for the 2D and 3D cases: the first temporal EOF mode which accounts for an overwhelming 80% of shoreline variation reflects the alongshore-averaged cross-shore migration (2D) largely influenced by waves while the higher eigenfunctions with 20% shoreline variation reflect beach deformation/irregularities (3D). Shoreline location is correlated to cross-shore energy flow from waves, while 3D is associated to the longshore energy flow. The influence of wave height increases cross-shore uniform migration and beach seasonality. The results show that with EOF method, beach cusps are clearly identified for 3D developments to be evaluated. Beach cusps are well observed at low tides on video images. Tide and cusps increase the importance of short term events and deformation; at the lowest high tide mark, beach cusps were observed on higher spatial functions of EOF. This indicates that a study of these morphological features as a coupled system, promotes their contribution in predicting 2D and 3D shoreline changes

Two and three-dimensional shoreline behaviour at a MESO-MACROTIDAL barred beach

The present work investigates cross-shore shoreline migration as well as its alongshore variability (with deformation) on timescales of days to years using 6 years of time-averaged video images. The variability of the shoreline is estimated through empirical statistical methods with comprehensive reference to three scales of variability. At the meso-to macro-tidal barred Biscarrosse beach, shoreline responds in decreasing order at seasonal (winter/summer cycles, 52%), event (storms, 28%) and inter-annual scales. Whereas seasonal evolution is dominated by wave climate modulation, short-term evolution is influenced by tidal range and surf-zone sandbar characteristics. The influence of tide range and sandbars increases when timescale decreases. This is even more the case for the alongshore deformation of the shoreline which is dominated by short-term evolution. An EOF analysis reveals that the first mode of shoreline change time series is associated with cross-shore migration and explains 58% of the shoreline variability. The rest of the modes are associated to deformation which explain 42% of shoreline variability

Video oberservation of waves and shoreline change on the microtidal James Town Beach in Ghana

The morphology of sandy beaches is highly dynamic. They are influenced by the geology of the coastal area and external hydrodynamic forcing. On long timescales (years to decades), it is more efficient and convenient to monitor beach evolution through remote sensing technics rather than through direct field measurements. Erosion is a major problem along the coastline of Ghana with over 25 erosion hotspots, including James town. Here, tides, ECMWF EraInterim re-analysis wave data and images covering the beach area have been obtained for the 2013-2014 period. This paper presents preliminary results of the first efforts in processing video-derived observations of waves and shoreline change in Ghana. The pilot application shows a strong potential of the video system in providing fair quality wave data for beach management purposes where video wave characteristics are in good agreement with EraInterim global reanalysis (daily RMSE = 0.8 m and 0.7 m for H-b and T-p, respectively). Shorelines extracted from video suggest large monthly variability driven by wave seasonality while shoreline change shows a subsequent erosion/accretion cycle

Shoreline resilience to individual storms and storm clusters on a meso-macrotidal barred beach

This study investigates the impact of individual storms and storm clusters on shoreline recovery for the meso-to macrotidal, barred Biscarrosse beach in SW France, using 6 years of daily video observations. While the study area experienced 60 storms during the 6-year study period, only 36 storms were analysed due to gaps in the video data. Based on the 36 individual storms and 13 storm clusters analysed, our results show that clustering impact is cumulatively weak and shoreline retreat is governed by the first storms in clusters, while the impact of subsequent events is less pronounced. The average post-storm beach recovery period at this site is 9 days, consistent with observations at other beaches. Apart from the dominant effect of present storm conditions, shoreline dynamics are also significantly affected by previous storm influence, while recovery is strongly modulated by tidal range and the bar location. Our results reveal that not only is the storm energy important but also the frequency of recurrence (storms result in greater retreat when time intervals between them are longer), which suggests an interaction between short storm events and longer-term evolution

Assessment of the evolution of Cameroon coastline : an overview from 1986 to 2015

The coast of Cameroon is located at the bottom of the Gulf of Guinea with varied nearshore environments and oceanic forcing influenced by the presence of several islands. It is also the area of important river flows. Here, the global evolution of the Cameroonian coastline and hydrodynamic between 1986 and 2015 is investigated using satellite images and ECMWF Eralnterim re-analysis wave data. Seven areas of important cross-shore changes have been identified with only one case of human-induced variation that corresponds to the construction of a new habour at Kribi. This paper presents the results of using SYMPHONIE model with DOUALA26 configuration to assess the changes in Cameroon estuaries. The other areas are mainly located at the mouths of the rivers and at the entrance of Cameroon and Rio Del Rey esruaries. The results of the circulation model show that the convergence of Wouri, Dibamba river flows and littoral drift corresponds to the accumulation of sediments observed at "Souleyba", while the eroded area at "Cap Cameroun" corresponds to the estuary output current. This work provides an understanding of erosive or accretive coastal processes using barotropic currents modeling in estuaries

African coastal camera network efforts at monitoring ocean, climate, and human impacts

(IF 4.99; Q1)International audienceRegular and long-term monitoring of coastal areas is a prerequisite to avoiding or mitigating the impacts of climate and human-driven hazards. In Africa, where populations and infrastructures are particularly exposed to risk, there is an urgent need to establish coastal monitoring, as observations are generally scarce. Measurement campaigns and very high-resolution satellite imagery are costly, while freely available satellite observations have temporal and spatial resolutions that are not suited to capture the event scale. To address the gap, a network of low-cost, multi-variable, shore-based video camera systems has been installed along the African coasts. Here, we present this network and its principle of sharing data, methods, and results obtained, building toward the implementation of a common integrated coastal management policy between countries. Further, we list new contributions to the understanding of still poorly documented African beaches' evolution, waves, and sea level impacts. This network is a solid platform for the development of inter-disciplinary observations for resources and ecology (such as fisheries, and sargassum landing), erosion and flooding, early warning systems during extreme events, and science-based coastal infrastructure management for sustainable future coasts

COASTVAR 2015-2018 : caractérisation de la variabilité littorale en Afrique de l'Ouest par une étude multi-échelle et multi-méthode [résumé]

International Conference AWA (ICAWA), Dakar, SEN, 09-/12/2014 - 11/12/201

COASTVAR 2015-2018 : caractérisation de la variabilité littorale en Afrique de l'Ouest par une étude multi-échelle et multi-méthode [résumé]

International Conference AWA (ICAWA), Dakar, SEN, 09-/12/2014 - 11/12/201