Introduction Flooding is a natural disaster whose damage to human beings is higher than other natural disasters, such as drought and famine, and is a major concern in many regions of the world. Therefore, we must select appropriate and effective methods for flood management and control by understanding the hydraulic flow in the flood plains. The change in land use can cause changes in hydrological conditions, such as the frequency of floods, the base flow and the average annual discharge. As it is not possible to fully protect against the risk of flooding, it is necessary to live in floods and apply new policies on land use management and residential development on the river bank to reduce the impact of their destruction. The knowledge of floods and the study of their behavior require sufficient data on the hydrological condition of the basin and the discharge of the river, and it is not possible to reach this objective in seasonal rivers and regions where there is no constant flow, so the implementation of hydrological models is useful for rainfall simulation. runoff process and hydraulic models for the analysis of runoff flow in the river and its distribution.

Traditional rain gauges provide a fine enough resolution for accurate runoff calculations and flood warnings. Therefore, flood modelling requires predictions of distributed models related to planning and insurance. Recently, interest in flood analysis has been increased using powerful HEC-HMS and HEC-RAS tools to manage spatially distributed data and the distributed basin model. The use of these hydrological and hydrological parameters of the watershed can analyze the risk of flooding in my selected watershed in Sri Lanka. Problem Statement In Sri Lanka flood is the one of major disaster which harms human beings, wild lives, public and private properties, environment etc. Due to many reasons there would be occurred flood in river basins. So, in this case, analyze of risk of flood in river basin is very important to mitigate the lot of problems.

 Significance of the research Floods are natural occurring processes that are difficult to prevent but can be managed to reduce its social and economic impacts. Flooding is a threat to life and leads to damage of property. Therefore, it is very important that flooding risks be considered during any planning process. There are areas that are more susceptible to flooding than others. Good planning and management can help in curbing the risks of flooding. In simple terms, flood risk analysis is an assessment of the various risks in relational to residential, industrial and commercial land uses.

It is a requirement as part of any planning application for larger sites for development especially in areas that are in flood-prone areas. The analysis can also be used by insurers to act as a way of reducing premiums in insurance. Flood risk analysis considers the impacts of flooding and how the proposed development project may affect the area. In addition, the analysis includes a recommendation on how the flooding risks can be improved after the development.These strategies help in that they reduce the vulnerability of the area to flooding. The flood risk analysis requires information on the development of the area; existing and the proposed development information. This is important in determining whether the proposal will be allowed to proceed or not. The analysis ensures that future risks caused by floods are avoided or can be managed and dealt with in good time and in an affordable way.

The analysis is also, like an assessment that provides not only recommendations but also includes the management of the risks posed by floods. Clearly, there are many significances of flood risk analysis. Whether the application is for a major commercial purpose, individual development purposes, industrial or even small extension of our residential land, analysis is inevitable. Scope of the study The one type of floods occurs in rivers when the flow rate exceeds the capacity of the river channel, particularly at bends or meanders in the waterway. Floods often cause damage to homes and businesses if they are in the natural flood plains of rivers.

While river basin flood damage can be eliminated by moving away from rivers and other bodies of water, people have traditionally lived and worked by rivers because the land is usually flat and fertile and because rivers provide easy travel and access to commerce and industry. In my study area is limited around analysis of river basin flooding.  Aims and Objectives                      The aim of the research is to identify formal flood risk areas and other areas of significant local flood risk of selected river basin.  Also there are objectives in my research.·         Observation of previous and present flood heights and inundated areas·         Mapping inundated areas and flood heights for future flood scenarios·         Long-term land use planning and regulation·         Engineering design and construction of structures to control or withstand flooding·         Intermediate-term monitoring, forecasting, and emergency-response planning·         Short-term monitoring, warning, and response operations.      Methodology The hydrological model inter-comparison study proposed in the present work is carried out by using two different physically-based rainfall-runoff models to generate simulated discharges.  These are: (i)                 HEC-HMS(ii)              HEC-RAS HEC-HMS model: HEC-HMS model for flood simulation utilizes a graphical interface to build the semi-distributed watershed model. For each sub-basin in the mentioned watershed, the hydrological model is forced by using a single hyetograph.

Firstly, using kriging method, rainfall spatial distributions are generated from hourly values get from metrology department for upper section of the river. Then, for each sub-basin the hourly rainfall series are calculated. The Soil Conservation Service Curve Number method (SCS-CN) is used to calculation of runoff volumes in the rainfall-runoff model. Development of HEC-RAS flood simulation Model (Geometric simulation of river): The HEC-RAS model when presented with the appropriate hydraulic and geometric data calculates water-surface profiles.

The original reference for the method to determining the roughness coefficient in reaches is Cowan method because it includes several factors control the roughness coefficient. Then HEC-Geo RAS extension is used to preparing and inputting geometric information about the reach that these data are include flow path, left and right bank and cross sections that in the form of new data layers are entered HEC-RAS model. Then while importing of the output hydrographs resulted from HEC-HMS and introducing the roughness, channel convergence and divergence coefficients, HEC-RAS model is run and the results of the hydraulic analysis and extracting of flood zones and flood depth is done in ARCVIEW software and the flood plains is determined for return periods of 10, 20 and 50 years.  This can be summarised as follows.

                       Time schedule & work plan Estimated cost Task Cost  (Rs.) Metrological data and survey data 10000 Printing 1000 Traveling 1000                        Estimated total cost = Rs.12000   References Hashemyan, F., Khaleghi, M. R., & Kamyar, M. (2015). Combination of HEC-HMS and HEC-RAS models in GIS in order to Simulate Flood ( Case study?: Khoshke Rudan river in Fars province , Iran ), 4(8), 122–127.

Heimhuber, V., Hannemann, J., & Rieger, W.

(2015). Flood Risk Management in Remote and Impoverished Areas—A Case Study of Onaville, Haiti, 3832–3860.

3390/w7073832Horritt, M. S., & Bates, P. D. (2002). Evaluation of 1D and 2D numerical models for predicting river flood inundation, 268, 87–99.

Oleyiblo, J. O., & Li, Z.

(2010). Application of HEC-HMS for flood forecasting in Misai and Wan ‘ an catchments in China. Water Science and Engineering, 3(1), 14–22.

issn.1674-2370.2010.01.002Plate, E. J.

(2002). Flood risk and flood management, 267, 2–11. 


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