An Analysis of Stream Flow and Flood Frequency: A Case Study from Downstream of Kelani River Basin, Sri Lanka

River floods in Sri Lanka are mainly associated with extreme rainfall events. The Kelani and Kalu rivers are recorded the highest flood frequencies and the accompanying flood damages among the river basins in wet zone (UNDP, 2011). Therefore, the specific objective of the study is to estimate the temporal probability of occurrence of flood events in downstream of Kelani river basin. Secondary data were used for the study. Daily discharges data were obtained from Hanwella gauging station for the period of 1990 to 2019 from the Department of Irrigation, Sri Lanka. Trend analysis, normal distribution and flood frequency analysis have been used. The results of the study revealed that there was a bi-modal pattern of discharges that occurred in June and October. The results also indicated that ten return periods were covered the total period of 30 years, and there was a 97 per cent probability of flood occurrence almost annually and 64 per cent probability of occurring once every two years. This study, therefore, was recommended to design flood control structures for mitigating flood risk; and to determine the economic value of flood control projects and the effect of encroachments on flood plain. Keywords: floods, frequency, downstream, return period, probability