Current State of Flooding and Water Quality of Nokoue Lake in Benin (Ouest Africa)

Abstract

A thorough literature review was conducted in order to discover the previous studies that have been done in relation to the problems of flooding and water pollution of Nokoue lake (Benin, West Africa), to identify the limitations of these previous studies, to draw lessons in order to propose how to overcome them. This bibliographic synthesis has shown that previous studies related to the flooding issue have focused on the dynamics of understanding the genesis of flooding. These studies range from the oldest conducted in 1979 to the most recent conducted in 2016. The major results of these studies in relation to floods are hydrodynamic techniques allowing to evaluate in a punctual way the hydrological regime of the Nokoue lake. In other words, they are solutions allowing to reconstitute the floods generated by the river waters. It appeared that these results of flood reconstitution are difficult to generalize because of the complexity of the hydrographic network of Nokoue lake. The hydrodynamics of Nokoue lake is influenced differently by its main water tributaries (the Sô River, the Ouémé River, the Porto Novo Lagoon and the Atlantic Ocean). Therefore, a decision-making issue on planning and flood management must be motivated by a decision support tool. It is moreover recommended in these studies the resumption of the process of hydraulic techniques periodically during the period of high water in order to take advantage of the hydrological behavior of Nokoue lake in terms of overflow which are at the origin of the floods. The need for innovative decision-making solutions on flood planning and management in order to reduce disasters is still a priority for decision-makers. These innovative solutions are undoubtedly the flood forecasting. On the water quality of Nokoue lake, the oldest study conducted dates back to 1987 and the most recent one in 2018. In these studies, water quality is evaluated in two seasons: the low water season (December to mid-May) and the high-water season. It was found that the salinity of the lake increased significantly with an average of 16 ppt during the low water period while during the high-water period, freshwater inflows from the tributaries invaded the lake and prevented water from the Atlantic Ocean from entering. The concentrations of inorganic nutrients in the lake increase from low water to high water. However, the quantification to date of nutrient loads from external sources as well as internal sources to the lake have not been quantified. This implies that further research on the water quality of the lake could be conducted. Prevention actions should consider a predictive monitoring of the water quality index of Nokoue lake.