Analysis of the effect of climate change impacts on floods in Kelani river basin, Sri Lanka

Abstract

Sri Lanka is highly vulnerable to climate change impacts, including rising land and sea temperatures, changing precipitation patterns, more extreme weather events, and sea-level rise. Notably, climate change has been observed to increase flood frequency, expand flood areas, and intensify flood damages. Previous research in Sri Lanka has mainly focused on rainfall estimation using weather models and examining climate change scenarios. This study aims to improve flood forecasting by analyzing climate change-induced changes in rainfall depths from Intensity-Duration-Frequency (IDF) curves and considering design sea levels. The objective is to gain insights into future flood characteristics, specifically the projected increases in discharges and water levels. The HEC-HMS Hydrological modelling tool was selected for the hydrological modelling of the entire Kelani Basin, while the HEC-RAS model was used for flood modelling in the Lower Kelani Basin which is downstream from Glencourse. HEC-HMS simulating discharges from rainfall inputs that served as boundary conditions for the HEC-RAS model. The verified models are utilized to simulate the 50-year design rainfall dataset lasting 3 days, incorporating published IDF equations from selected rain gauge locations along with the calibrated models. Rainfall depth multipliers of 1.100, 1.122, and 1.140 were applied to the design rainfall dataset for the RCP4.5, RCP6.0, and RCP8.5 projections, respectively. Simulations also considered sea-level rise values of 0.47 m, 0.48 m, and 0.63 m corresponding to the respective climate change projection scenarios. Calibration and validation of the three HEC-HMS models (Kelani Upper, Kelani Middle, and Kelani Lower) and the HEC-RAS Flood model for Lower Kelani (downstream to Glencourse) Basin were successfully calibrated using 2016 May and validated using 2017 May flood event data. The Nash Efficiency values during calibration were 0.79, 0.95, and 0.85 for the Kelani Upper, Kelani Middle, and Kelani Lower models, respectively. During validation, the Nash Efficiency values were 0.87, 0.85, and 0.25, respectively. The calibration Nash Efficiency values for the HEC-RAS model were 0.57, 0.56, and 0.52, and the validation Nash Efficiency values were 0.80, 0.57, and 0.53 for the respective models considering Hanwella Discharges, Hanwella Water Levels and Nagalagama Street Water levels, respectively. The research concluded that, under climate change projections, the Glencourse Peak Discharge is projected to increase by approximately 13.3% to 16.2%. Similarly, at Hanwella, the peak discharge is expected to increase by approximately 6.4% to 8.8%, while the maximum water level is anticipated to rise by approximately 3.1% to 4.2%. Moreover, the maximum water level at Nagalagama Street is likely to experience an increase of around 16.2% to 21.7% under climate change projections.