Flood Hazard Mapping in the Lower Reach of the Kelani River, Sri Lanka

The objectives of this project were: 1) To prepare the flood hazard maps for 10yr, 20yr and 50yr return periods, 2) To identify flood encroachments on different landuse types.

      The lower reach of the Kelani River that covers the suburbs of the island’s capital Colombo is a densely populated area and very frequently affected by floods. The one dimensional steady flow module of HEC-RAS was used for determining the water surface profile, flood area, and flood depth. The requisite geo-spatial modeling of the flood plain was accomplished by using HEC-GeoRAS module in the ArcView environment. The landcover map of the study area was updated using the ALOS data for estimating hydraulic parameters such as Manning’s Roughness Coefficient. Peak flows associated with various return periods were obtained from a frequency analysis. Model results and field records corresponding to the flood event of 1989 was compared and a good correlation was found. Flood hazard maps for 10, 20 and 50 years return periods were achieved and results show that the corresponding flood extends were of 60, 77, and 94 km2, respectively. Geospatial tools have also facilitated to identify the flood encroachments on different landuse types and the number of buildings they would affect.