All-inclusive coastal defence scenarios for Saint Louis, Senegal
French consultancy Egis Eau and Dutch research institute Deltares are preparing future scenarios on coastal development for the city of Saint Louis, Senegal.
The input for the scenarios is based on two years of monitoring and modelling the coastal processes and the Senegal river.
Caught between ocean and river
The coastal city Saint Louis is caught between two threats. It is located on a sand spit at the mouth of the the Sénégal River, where high river levels cause flooding during the wet season. At the same time, the city borders on the Atlantic Ocean, which means that storms and waves also cause flooding and coastal erosion on the narrow, vulnerable coast where many people live.
‘The problems are big, but we have collected a lot of data to arrive at the best solutions,” says Pieter Koen Tonnon, senior advisor at Deltares. The problems increased after 2013, when the city made a passage from the river to the sea in order to drain the excess river water more quickly.
‘That crossing has started to grow uncontrollably, becomes wider and moves along the coast’, Tonnon continues. ‘That brings along additional problems. Villages have been wiped out, saltwater intrusion is a problem for crop irrigation and fishermen can no longer catch freshwater fish due to saltwater intrusion into the river. They move to the sea and that leads to dangerous situations for the fishermen.’ Many fishermen have already drowned due to the unpredictable sea.
Deltares started setting up an environmental monitoring and modelling system two years ago. By conducting three large monitoring campaigns, a lot of knowledge has been gained about the waves, currents and discharge of the river.
The results have been used to set up models of the system, to predict the effects of waves and storms. Currently Deltares is assessing all options to make the area safer and how to counter the salt intrusion, for example.
A second project focuses on reducing erosion on the coast near Saint-Louis. 'Every year they lose four to five meters of coast, that’s a lot. While a lot of people live there’, Tonnon adds.
Tonnon expects that the first measures, such as strengthening the coast with vegetation and emergency nourishments, will take place within a year. What is also special about this project is that Deltares transfers knowledge and expertise to local parties, whereby the measuring equipment and models have also been transferred. “We have organised training courses together with IHE - Delft to educate the local parties. They can now monitor themselves and get to work with the models we have developed. Because the more measurements and analysis they do themselves, the better they understand the situation there.”
The work is done in close collaboration with other parties, such as the World Bank, who finances the projects. The World Bank is very interested in West Africa and has set up many programs to combat coastal erosion and other problems in that area.