Flood hazard mapping in lower reach of Kelani River, Sri Lanka

Abstract

Although floods are the most frequent natural disasters in Sri Lanka they had received little attention until the tsunami of December 2004. After the Tsunami, however, disaster management strategies underwent rapid changes and under this framework flood hazard mapping has, particularly, been realized as one of the foremost tasks to be accomplished in support of disaster management activities. As a pilot study, the lower reach of Kelani River which covers the suburbs of the island's capital, Colombo, was chosen for flood hazard mapping as it is frequently affected. The one dimensional steady flow component of HEC-RAS Software was utilized for the study. The requisite geo-spatial modeling of the flood plain was carried out by implementing HEC-GeoRAS utility in ArcView environment and using a TIN model. The land-use map of the study area was updated using the data remotely sensed by ALOS for attributing the study area with necessary hydraulic parameters. Peak flows associated with considerate return periods were obtained from a frequency analysis and the calibration run was executed simulating the 50 year return period flood event. Model results and field records corresponding to the above event of 1989 was compared and it was found that results are very satisfactory. Accordingly, the first ever set of flood hazard maps associated with 10 year, 20 year, and 50 year return period events to be adopted by the government of Sri Lanka were prepared successfully. Statistics show that the study area would be subjected to flood encroachments of 60, 77, and 94 square kilometers for events with return periods of 10 years, 20 years, and 50 years respectively. Modern day geospatial tools have also facilitated to identify with ease, flood encroachments on different land use types and the number of buildings they would affect.