Vulnerability of Colombo Suburbs for Kelani River Floods

The lower basin of Kelani River, encompassing the highly populated Colombo metropolitan area, frequently suffers severe inundation due to high river discharges and localized heavy rainfall. This research utilized the HEC-RAS 2D model to analyze water levels in the Kelani River, assess flood extents, and make reliable future predictions despite limited data availability The model was calibrated and validated by comparing observed and simulated water levels during the May 2008 and June 2016 flood events, demonstrating satisfactory performance with a Root Mean Square Error of 0.134 m, coefficient of correlation of 0.951, Nash-Sutcliffe efficiency of 0.914, and a goodness-of-fit index of 70.1% for inundation extents. Additionally, an ArcGIS model was employed to generate a flood susceptibility map for the lower basin of the Kelani River. The study included identifying flood-prone areas in Colombo suburbs, assessing the flood risk through simulating hypothetical flood bund breaches, estimating the flood extent, evaluating the efficiency of existing flood bunds, and assessing the vulnerability of areas protected by flood bunds on the left bank of Kelani River during 50 - and 100 -year Kelani floods and a 50-year rainfall in Colombo metropolitan area. This analysis provides valuable insights into the potential advantages of investing in infrastructure improvements to mitigate flood risk in the Colombo Suburbs.