5. Acknowledgements
This work is partly funded by the “Agence de l’Eau ArtoisPicardie”, the “Region Nord Pas-de-Calais”, the “Conseil Général du Nord”, and the Town Hall of Villeneuve d’Ascq. This study is part of the first-author (St C. Dehou) Ph.D. thesis, and results from the cooperation between the University of Lille1 (France) and the Univer- sity of Bangui (Central African Republic). This collaboration and the Grant-in Aid to Mr. St C. Dehou for his scientific research are financially supported by the Embassy of France to Bangui.