Flood Hazard Mapping in Lower Reach of Kelani River

After the Tsunami, disaster management strategies have undergone rapid changes. Flood hazard mapping has, particularly, been realized as one of foremost tasks to be accomplished in support of disaster management and sustainable development. As a pioneering effort, the lower reach of Kelani River which covers the islandis capital, Colombo, and two densely populated districts of the country. Colombo and Gampaha districts which are frequently and considerably affected by flood events, were chosen for flood hazard mapping together with the part of the slightly affected adjoining Kegalle district. The study covers all most all the potential flood area in Kelani river. The objective of the study were, 1) Prepare flood depth and flood extend maps for different return periods, 2) Identify flood management and flood-disaster mitigation strategies.

This exercise comprises four basic steps,1) Generation of Terrain model in the study area to work in Arc View, 2) Determination of flood discharges for different return periods, 3) Flow profile generation using Hydrodynamic model; HEC RAS and 4) Generation of flood area and depth maps by exporting the hydrodynamic model results to ArcView. The one dimensional steady flow module of HEC-RAS Software was utilized for water surface profile, flood area, and flood depth calculations. The requisite geo-spatial modeling of the flood plain was carried out conveniently by implementing HEC-GeoRAS module in the ArcView environment using a triangular irregular network (TIN) model. Peak flows associated with selected return periods were obtained from a frequency analysis and the calibration run was executed simulating the 50 year return period flood event.

Comparison of field records corresponding to the above event of 1989 indicates that the modeling of flow and terrain conditions is very successful. Accordingly, a set of flood hazard maps associated with 10 year, 20 year, and 50 year return period events. Results 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. Model results were compared and verified against the records available in the Irrigation Department in addition to the field verification.