Assessment of the Vulnerability of the Southwestern Coast of Benin to the Risk of Coastal Erosion and Flooding

The coastal zone of Benin is inherited from the last marine oscillations of the Quaternary. A rich and very fragile environment, it presented until the 1960s, a shoreline in dynamic equilibrium over the entire 125 km of coastal line. Since the 1960s, with the construction of important development infrastructures (ports, dams, groins), the Beninese coast is now subject to risks of coastal erosion and seasonal flooding due to the overflow of lagoon water bodies. The present study, based on socio-economic surveys in the communes of Ouidah, Comè and Grand Popo, exposes the extent of coastal risks and socio-economic and environmental damage in the southwestern coastal zone of Benin. The results show that in terms of land, 2.9 ha and 5.7 ha of land have been permanently lost to coastal erosion in the communes of Ouidah and Grand Popo respectively. Similarly, 212 ha of crops of all types were affected by the flooding, including 35 ha destroyed, i.e. 6.67 ha, 11.3 ha in Comè, 4.67 ha Ouidah and 14 ha Grand Popo. Also, 6435 buildings were affected, and 4235 huts were damaged. In addition, working tools, food stocks and other items are counted among the losses recorded by coastal hazards with their corollaries of diseases. The cost of losses and damages in the 08 districts amount to 418,000,000f cfa of which 266,000,000f cfa of damage and 152,000,000f cfa of loss.

Hydro-Sedimentary Dynamics of the Grand-Popo Lagoon Located Upstream of the Mobile Embouchure “Bouche du Roy” in South-Western Benin

The problems of flooding, bank erosion and even the breaching of the barrier beach by the Grand-Popo lagoon are a concern in the south-western Beninese river-lagoon complex. The present study aims to improve the knowledge of the factors controlling the hydro-sedimentary functioning of this lagoon of Grand-Popo. It was based on bathymetric and sedimentological studies and the analysis of physical-chemical parameters of the water and allowed to know the morphology of the lagoon bottom and the distribution of sedimentary facies according to the intensity of the water current. The salinity of the lagoon waters shows a west-east gradient passing thus from 0‰ to 0.78‰ in the main direction of flow. Over the whole lagoon system, the average liquid flows vary from -38.499 to 159.13 m3/s. The bathymetry indicates depths varying from 0.4 to 6.5 m, revealing type V and type U lagoon bottoms which reflect acute bottom concavities marked by hollowing under the effect of strong currents or by sandy terraces resulting from the continuous input of sediments carried by the Mono River. These sediments are heterogeneously distributed in the lagoon bottom and vary from free sand to mud. Given the complexity of the factors that control the hydro-sedimentary functioning of the Grand-Popo lagoon, it is recommended that periodic hydrological and bathymetric monitoring be carried out to locate exceptional variations in water levels to prevent not only the risks of overflowing lagoon waters and the flooding that follows but also spectacular erosion of the lagoon banks.