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2.1 preliminary investigation
For the beginning of this study it was decided to carry out preliminary investigation based on Pannala city to identify the real impacts due to floods events. By interviewing the residences and the related government authorities such as Irrigation Department of Katugampala Division and pradeshiya sabha information were collected to identify the gravity and necessity of this study based on this Pannala city. Based on their opinion it was understood that need of this project is very essential. There is a high flood risk of this area which is increasing times to time. Flood vulnerability of this area is so high due to many reasons such as high flood intensities, low elevation, sand and clay mining of large extent and illegal wetland fillings.
2.2 selection of the catchment
Study area was selected based on the Pannala city for this study. It was decided to perform flood model analysis to mapping inundation area. To perform flood forecasting model it was needed various flood scenarios of return periods 25 years, 50 years and 100 years. As well as we needed to identify upstream and downstream boundary conditions of specified area and daily average river discharge values for calibrate and validate the food model. When considering the availability of the rainfall and river discharge data it was planned to select Giriulla gauging station as the upstream boundary condition and Badalgama gauging station as the downstream boundary condition.
2.3 water basin delineation (ARC GIS)
Spatial data preparation was done using Arc GIS 10.5 software package. It is one of the best spatial data preparation software. Both hydrology and hydraulic model associated with the topographical information of the catchment. Both software systems consists of processor programs to facilitate graphical editing and mapping which simulate channel and flood plain details. Digital elevation model represent the topography as series of a small tiles. DEM was downloaded by Alaska satellite facility (https://www.asf.alaska.edu/) with 12.5 meter resolution.
As per the study based on “EVENT BASED FLOOD MODELING IN LOWER KELANI BASIN” the system of FLOW-2D system consists of processor programs to enable graphical editing and mapping which simulate stream and flood zone topography. The Grid Developer System (GDS) produces a grid system that signify the landscape as a series of small grid. The GDS was used to produce 250 m x 250 m grids covering the lower basin area. The required topographical data derived from the data which were taken by Department of Survey, Colombo; 1:10,000 map layers were used to develop Digital Elevation model.10