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.

Extreme flow events have had a significant impact on populations and their activities in recent decades. To reduce the impacts associated with their advent a analysis between extreme hydrological phenomena and climate variability is necessary. However, qualitative data accessing difficulties and explanatory variable definition of extremes hydrological phenomena limit extreme flows rate studies. This study proposes the analysis the variability of the floods and the low flows at the Bianouan hydrometric station. Thus, five (5) floods characteristics variables (QCX5, QXJA, F90p, F95p and F99p) and three (3) for, the low flows (VCN10, QJNA and F10p) were extracted daily flows from the Bianouan hydrometric station from 1 January1962 to 31 December 2005. The variability of these extremes has been studied by the trend analysis (linear regression) and of the stationarity (Pettitt and Hubert tests). The results show that for flood variables the negative trend is very significant and ruptures are observed in 1980. For low flow variables the negative trend is significant and the ruptures are observed in 1981. From this study, we can deduce that the ruptures observed in the rains and flows average in the end 1960s and early 1970s have affected later the extreme flows around the 1980s.