Résumé
Flood is one of the most important natural disasters that cause huge loss of life and properties everyyear around the world. Moreover, the International Federation of the Red Cross and Red CrescentSocieties pointed out that floods were by far the greatest cause of homelessness. In West Africa,many countries are damaged from flooding almost every season. Thus, this study aimed to set aseasonal flood forecast model and carried out an evaluation of the level of risk associated with eachseasonal forecast. HEC-RAS (Hydrologic Engineering Centers River Analysis System) was used todevelop a hydro-dynamical model of Niger river on a 160km reach (80km upstream to80kmdownstream of Niamey), then a simple risk measure was used to calculate the probability ofovertopping the flood protection dykes in Niamey. Results show that the hydro-dynamical modelreproduced well the rating curve over the period 2009-2014. A subsequent copula analysisdemonstrated a dependency between flow on the Niger river and flow on the Sirba River, the maintributary contributing to the seasonal flood at Niamey. The Gumbel copula was found to be the bestamong the tested 5 copulas to represent the dependency between peak flow on the main channel ofthe Niger River and concomitant flow on the Sirba river. It is found that for the six dykes theprobabilities of being overtopped by the flood range from very high (100%) to relatively low (16.67%) over the 34 years of simulation.