Flood is a natural disaster which is able to damage human beings, animals and other properties than other natural disasters such as landslides, droughts, wind storms etc. So, the analysis of risk of flood is very important to get many decisions as to mitigate above problems. Suitable modelling approach can use as a solution of this risks(Heimhuber, Hannemann, & Rieger, 2015). Changes in climatic variables, especially temperature and humidity, are probably the main drivers of the change in precipitation and extreme hydrological risk. Climate change can affect the intensity, frequency, duration and scope of risks and the vulnerability of communities to disasters.(Ntajal, Lamptey, Mahamadou, & Nyarko, 2017).There is the flood which can occur in river and surrounding of the river basins when the flowrate exceeds the capacity of the river, particularly at bends or meanders on the path of the river. Occurrence of flood in Sri Lanka is mainly due to excessive of rainfall during monsoon period(Central & Consultancy, 2014) Therefore, this study is based on the available data of the study area.
Recently, usage of HEC-HMS and HEC-RAS software has been increased for flood analysis. Those can use to manage spatially distributed data and the distributed basin model (Hashemyan, Khaleghi, & Kamyar, 2015). The use of these hydrological and hydrological parameters of the river and river basin can analyse the risk of flooding in selected river basin in Sri Lanka.
The HEC model is used to simulate the surface runoff of the catchment due to precipitation by interconnecting hydrologic and hydraulic components. It is primarily appropriate to flood simulations. In HEC-HMS, the river basin model includes different functions of the precipitation-runoff process of the basin.
An element of this application is depicted a surface runoff. Each of the elements is assigned a variable in HEC-HMS which defines the attribute of the element and mathematical relationship of the actual processes and also the result of the modelling process is the computation of stream flow hydrographs at the catchment outlet(Oleyiblo & Li, 2010).