The Rainfall-related phenomena affect the hydroelectric dams functioning and productivity. In Côte d'Ivoire the trans boundary Bia River basin, hydroelectric amenagements of Ayamé 1 and 2, is no exception. Flooding has caused extensive damage in recent years to these two (2) amenagements. However, the rains nature that caused these events is statistically unknown. This study objective is to identify the rainfall events nature in the Bia basin. The statistical modelling approach has been adopted. The daily rainfall from 1941 to 2000 and rainfall higher threshold than 100 mm from 2001 to 2015 available at the Aboisso, Adiaké, Bianouan, Ayamé and Agnibilékro stations has been used.The results show that the minimum threshold for abnormal rainfall in the Bia basin is 103 mm. The rains nature identification between 2001 and 2015 reveals, eight (08) rainfall events with an occurrence probability greater than 6 years are observed. These events are consisted of four (4) abnormal rains, three (03) very abnormal rains and one (1) exceptional rainfall observed during the years 2004, 2007, 2009 and 2015.