Determination of Flood Risk Thresholds and Analysis of Rainfall Frequency in the Diani Watershed, Forest Guinea

The Republic of Guinea, like most African countries, is affected by extreme weather phenomena (floods, recurrent droughts, heavy rain showers, tornadoes and violent winds). Its forested areas in particular are sensitive to these precipitation-induced flood risks. The aim of this study is to carry out a frequency analysis of extreme rainfall events associated with flooding in order to estimate their return periods in the Diani river watershed. The methodological approach is based on the collection of annual daily rainfall data from synoptic stations (Macenta, N’Zérékoré) and flow data (Diani bridge station) over the period 1995-2024. Precipitation and flow indices, based on McKee’s classification, were used to characterize flood thresholds. Similarly, the return periods of annual daily rainfall and flood flows were determined. The analysis highlights that the daily rainfall threshold for flood risk is an average of 74.2 mm and 88.1 mm respectively for the synoptic stations (N’Zérékoré and Macenta), corresponding to an average flow of 275.3 m3/s for limited risk. Annual daily rainfall ranging from 65.9 to 82.3 mm for the 2-year return period for the N’Zérékoré synoptic station, and from 72.9 to 94.7 mm and 107.4 to 129.1 mm for the 2 and 5-year return periods for the Macenta station, can be described as normal events, corresponding respectively to interval flows of 147.8 to 187.2 m3/s and 210.1 to 249.5 m3/s.