The HEC-HMS model uses a graphical interface to build the semi-distributed basin model for flood simulation. For each sub-basin of the selected river basin, the hydrological model is forced by the use of a single hyetograph. Using the kriging method, the spatial distributions of precipitation are produced from hourly metrology values for the upper part of the Kelani river. Then, for each sub-basin, the series of precipitation per hour are calculated. The Soil Conservation Service (SCS-CN) curve number method is used for calculation of runoff volumes in the rainfall-runoff model.The HEC-RAS model, when presented with the appropriate hydraulic and geometric data, calculates the profiles of the water surface.

The original reference for the method of determining roughness coefficient in sections is Cowan’s method because it includes several factors that control the roughness coefficient. Next, the HEC-Geo RAS extension is used to prepare and enter geometric information about the scope of these data: flow path, left and right bank, and cross sections that, in the form of new data layers, are entered in the HEC-RAS. Then, during the import of HEC-HMS output hydrographs and the introducing roughness, convergence and divergence coefficients, the HEC-RAS model is executed and the results of the hydraulic analysis and extracting of flood zones and flood depths are performed. Finally, floodplains are determined for return periods of 10, 20 and 50 years.