I recall vividly the first time I saw Togo’s coast in 2013, and how taken aback I was by the gravity of the erosion. I had been invited by the government of Togo to visit Lomé, the capital of Togo, where I worked on a disaster risk management project. While the trip was not specifically to inspect coastal erosion, the ministry staff took us to the communities most affected. The trip was both profound and disturbing.
Togo has only 56 kilometers of coast that includes the towns of Agodeke, Bagida, and Aneho, all east of Lomé. It is clear the sea is advancing onto the land and taking everything and anything in its way—as if there had been a cyclone or earthquake causing the devastation. While coastal erosion is a natural phenomenon of the sea progressively advancing inland, the rate of advancement is dependent on many factors related to the amount of sediment transported by the ocean currents. Of course, human intervention can accelerate erosion.
I stayed near the border with Ghana, where locals from Lomé often spend their weekends. Here, there is no tangible erosion and the beach is spacious. Surprisingly, this part of the coastal area in West Lomé experiences the opposite phenomenon to erosion, known as costal accretion, which is the process of sediment accumulation in the coastal zone.
We wondered if we could put in place a mechanism of sand transportation from west to east, thus helping the coastal zone east of Lomé begin a process of replenishment. If only the science was that easy. First we needed to understand the sedimentation patterns’ role in the phenomenon of erosion and accretion in the coastal zone of Togo, especially the sediment transportation by the ocean currents and their distribution to coastal areas.
Togo is one among many hotspots of coastal erosion in the Gulf of Guinea. Communities in neighboring countries, such as Benin, Cote d’Ivoire, and Ghana are also confronted with erosion. And, as it turns out, they all share the same river of sand that knows no borders.
National development plans need to be informed by a regional understanding of the “give and take” of coastal erosion, especially as sea levels rise.
The West Africa “Sand River” Study
With funding from the Water Partnership Program, a World Bank trust fund, the West Africa Coastal Areas (WACA) program sought to establish a large-scale sediment budget for the West African Coast. This work, entitled “A Quantitative Assessment of Human Interventions and Climate Change on the West African Sediment Budget,” was entrusted to Deltares, an independent institute for applied research in water, deltas, coastal regions, and river basins.
According to Dr. Alessio Giardino, project leader at Deltares, the barrier island ecosystem of West Africa is maintained by a strong, wave-driven longshore transport of sand which can be compared to a “sand river.” This sand originates from rivers and large coastal sand deposits.
The West African coast mainly consists of a narrow, low-lying coastal strip covering several thousand kilometers. Large cities have developed here on this low-lying back barrier strip, including Abidjan, Accra, Lomé, and Contonou. These capital cities are growing rapidly and in need of sustainable infrastructure. Yet their physical stability is maintained only by the longshore transport of sand that we call a “sand river.”
The highest rates of coastal retreat—in the order of 10 meters a year or more—often occurs in the most urbanized areas. Along with sea level rise, this will lead to the disruption of the coastal barrier if no erosion control measures are taken. Today, however, much of the sand is retained behind river dams and interrupted by port jetties. For these reasons the sandy coastal barrier is eroding in several locations.
The study focuses on quantifying a large-scale sediment budget for Cote D’Ivoire, Ghana, Togo, and Benin. A sediment budget is a quantitative assessment of the volumes of sand moving along the coastline within the “sand river,” which are brought into the coastal systems by rivers and removed from the coastal system by mining and blocked by manmade structures.
The effects of climate change on the large-scale capacity of sediment transport was also analyzed by modelling from the current increase in storm intensity, change in wave direction, and sea level rise.
The Deltares study set up a quantitative and consistent large-scale sediment budget study to assist the region in the consequences of future development decisions. Using a unique modelling framework for Cote D’Ivoire, Ghana, Togo and Benin, the study provides the first quantitative work about how the sand moves along the entire coastline of the “sand river.” The modelling tools can be used to understand the impact of new human interventions, as well as the impact of changing existing infrastructure.
The study demonstrates that sea-level rise may well become the largest source of erosion in West Africa, by the end of the century (in the most extreme scenario). It also found that governments and developers need to take great care regarding the sustainability of major ports. The effects of these ports—such as leeside erosion—are larger than previously thought, and extend as much as 50 kilometers along the coast. Their impact can be felt in neighboring countries.
We hope the reverberations of this study include ongoing knowledge sharing and awareness building among the countries involved, as we continue consultation and validation workshops.
A public on-line viewer is now available for data collection and to facilitate communications with local stakeholders. Coastal observation will boost West Africa’s knowledge of wind and wave strength and how they influence the coastline in the long term. With the support of WACA, countries are gaining access to better data for their decision making along the coast.