Risk of extreme rainfall in tne Bandama basin (West Africa): Contribution of multifractal analysis

The floods that have affected many African countries south of the Sahara have not exempted Ivory Coast, especially since the 1970s. The impact of these events has led to major climatic disruptions, particularly the occurrence of extreme rainfall. In the Bandama catchment area, in September 2018 alone, the heavy rains that fell led to a flooding of the Bandama River. The towns of Zenoula and Bouaflé were flooded, plantations destroyed. According to the Agence de Presse Ivoirienne (AIP) on 22 September 2018, a total of 2017 people including 1130 children are affected in Bouaflé. In this context, it is important to better understand how the irregularity and distribution of extreme rainfall occurs in order to adopt preventive measures. However, the lack of studies on climate trends does not allow us to anticipate environmental problems. Indeed, the simplest way to understand this variability, although the necessary concepts and notions must be available, is to consider that there is an elementary and scale-invariant process that reproduces this variability from scale to scale. The possibility of characterising and modelling the variability of precipitation from scale to scale rather than at a given scale has led to a number of studies on multifractal precipitation (Lovejoy et al., 1987; Hubert et al., 2002). The objective of this study is to highlight the occurrence of extreme rainfall in the Bandama catchment using multifractal analysis. For this work, daily data from 2005 to 2010 from 10 stations (Yamoussoukro, Bouaflé, Dimbokro, Ferkessedougou, Niakara, Séguela, Bouaké, Tiassalé, Grand-Lahou, Zuenoula) were considered. Three intensity thresholds (with respective values of 20; 35; 50 mm/d) were studied. For these thresholds, the respective fractal dimension is 0.321; 0.249; 0.232.  The probability of obtaining a rainy day with thresholds set at 50 mm, 35 mm, 20 mm is respectively 0.069; 0.074; 0.093 over one month. The probability of obtaining rainy days for the three thresholds is spatialized over the catchment.

Keywords: Occurrence of extreme rainfall, multifractal, Bandama basin, Ivory Coast