West Africa

BOX 5.2

Sediment Budgets and Numerical Modeling in West Africa

West African coasts have undergone high rates of coastal retreat because of changes in the wave-driven alongshore transport of sand. These changes are largely caused by human developments such as river dams or harbor jetties and have led to severe erosion on the order of 10 meters per year at some locations. In a large-scale project, the World Bank’s West Africa Coastal Areas Management Program (WACA) partnered with the Dutch research company Deltaresa to set up a coastal sediment budget for Benin, Côte d’Ivoire, Ghana, and Togo. This involved estimating annual alongshore sediment transport capacity using numerical modeling to quantitatively assess the effects of different human interventions.

Illustrating the implications of interventions on the evolution of the coast and transboundary effects, together with the effects of climate change, the model provided an important awareness-raising tool for decision-makers. The project developed a digital coastal viewer to facilitate communications to the different stakeholders who were both affected by changes in the coastal landscape and responsible for these changes.

The numerical approach in the modeling exercise developed three submodels in an integrated framework:

• A large-scale wave model was employed to simulate wave propagation from offshore to nearshore, taking into account wind generation, dissipation, and nonlinear wave-wave interactions. It covered the complete West African coast and included 15 spatially more explicit models to ensure sufficient spatial resolution.

• A hydrological model that takes into account land use and soil maps as well as meteorological data was used to compute the annual runoff that influences the sediment yield from West Africa’s major rivers.

• A shoreline-evolution model was employed to simulate coastal evolution. It calculates the magnitude of alongshore sediment transport for specific locations along the coast, considering wave-induced drift, tidal flow, or both. It used information on coastal orientation and local wave angles to carry out long-term simulations at large spatial scales.

The analysis allows simulation of the impacts of different changes in the coastal landscape under different climatic or meteorological conditions. Scenarios that can be assessed include major anthropogenic developments such as ports or dams, rising sea levels, and changes in wave height and direction due to climate change. Hence, it provides researchers and policy makers with an efficient tool to model and investigate the effects of changes in coastal development and measures deemed to reduce sediment blockage or coastal erosion. Including the effects of climate change allows for the evaluation of shoreline changes in the long term. Given its large scale, the simulations lend themselves to the analysis of possible transboundary effects of coastal interventions at specific hot spots.

Source: Giardino et al. 2018. a. For more information, see the Deltares website: https://www.deltares.nl/en/.