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Nowadays there are many articles and the researches have been written based on the flood forecasting. Because it is an essential and timely topic considering the natural disasters occurring in Sri Lanka and its effects. . Many advancements have taken place in flood modelling and flood mapping over the past few years. Researchers preferred many of methods and means as well as the soft wares to achieve above objectives.
Flood hazards mapping is the essential task coming under flood mitigation. These flood inundation maps have multiple uses.8
• Land use planning and Land management both by authorities and users
• Watershed management
• Water management planning
• Hazard assessment on local level
• Emergency planning and management
• Planning of technical measures
• Overall awareness building
• Taking flood insurance and under willing of flood risk
Flood hazard maps are targeted to the following groups.
• National regional or local land-use planners.
• Flood managers (usually the Irrigation Department)
• DMC, District and Divisional Secretaries
• Public at large
When considering about flood vulnerable areas in Sri Lanka only few areas in the island can be called entirely free from flood. The case of flood vulnerability is a subjective measure in practice. People tend to regards an area free from flood which is undergoes flooding at very rare intervals without regarding the intervals. The vulnerable areas for flooding have some common characteristics. Usually they were reported as high rainfall intensities and low elevation and close to a stream. Presence of any or all of these factors does not make and area necessarily flood plain. As well as absence of them also does not make an area free from flood. Figure 5.26 illustrate the flood events reported in different districts of Sri Lanka from 1990 to 2011. Figure 5.27 illustrate the same information graphically. These figures were derived based on data available with the Disaster Management center and considered all type of flood hazards.8

Figure 5.27 Districts based flood hazards in Sri Lanka from 1990-2011

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Figure 5.28 floods events by districts 1990-2011

2.2 Maha Oya flood plain
When considering Maha Oya, there are no much articles and researches based on the subject of flood forecasting model. So it’s very essential to going through such research based on the Maha Oya river basin. But there is a considerable amount of researches that have been performed in Sri Lanka based in the Kelani River. As well as there are so many water quality researches performed based on the Maha Oya. These researchers and articles are very helpful to gain related data for our study flood modeling in Maha Oya.
1.3 HEC-RAS
1.4 HEC-HMS
HEC-HMS (Hydrologic Engineering Centre e Hydrologic Modelling System) model was developed by the U. S. Army Corps of Engineers (Feldman, 2000) that would be used for several hydrological simulations. The HEC-HMS model is applied to investigate urban flooding, flood frequency, flood warning system coming up with, reservoir, spillway capability stream restoration. (US Army Corps of Engineers, 2008). The proliferation of private computers and therefore the development of the HEC-1model of the U.S. Army Corps of Engineers in 1998 to a graphical user interface (graphical user interface) based mostly easy HECHMS model is on the market within the property right, have come back as another useful tool to the sector hydrologists. Sadly, the HEC-HMS model, or any of the various watershed models for that matter, has found several takers because of the uncertainty concerned within the estimation of parameters of the models. But parameters estimation on a regional scale a minimum of could also be potential to change over to watershed models just like the HEC-HMS and make the most of the high speed computer programs than programme exercises (Kalita, 2008)
The HEC-HMS contains four main elements. 1) associate analytical model to calculate land flow runoff similarly as channel routing,2) a sophisticated graphical program illustrating hydrologic system elements with interactive options, 3) a system for storing and managing information, specifically giant, time variable information sets, and 4) a method for displaying and reportage model outputs(Bajwa and Tim, 2002). This model isn’t mark and valid for the Sri Lankan watersheds and want reliable information inputs to check the suitableness of the model for the study location and purpose. Activity of rainfall-runoff models with regard to native observational information is employed to boost model foregone conclusion. When model results match ascertained values from stream-flow measure, users have larger confidence within the responsibility of the model (Muthukrishnan et al., 2006)
1.5 Modelling parameters
As per the research “Application of the HEC-HMS model for runoff simulation in a tropical catchment” employed three different approaches to calibrate and validate the HEC-HMS model to Attanagalu catchment and generate long term flow data for the Attanagalu oya and its tributaries. When talking about human influences on river flows and designing ecologically sustainable water management approaches hydrological simulation and employing computer models has advanced rapidly and computerized model have become essential tools. The HEC-HMS is a reliable model developed by US Army Corps of Engineers that could be used to many hydrological simulations .The model is not calibrated and validated based on the Sri Lankan water sheds and need relevant data s the inputs to check weather sustainability of the model for the specified location and requirement. For this study the researcher used Twenty year daily rainfall data from five rain gauging stations scattered within the Attanagalu Oya catchment and monthly evaporation data for the same years for the agro meteorological station Henarathgoda together with daily flow data at Dunamale from 2005 to 2010 as the data. As well as GIS layers were prepared by using Arc GIS 9.2 that were needed as input data for flow simulation and used HEC-HMS 3.4 to calibrate of the Dunamale sub catchment using daily flow data from 2005 to 2007. The model was calibrated adjusting three different methods. The model calibration was performed for each methods to identify the most accurate hydrological parameters for the catchment. Service Curve Number loss method, the deficit constant loss method (the Snyder unit hydrograph method and the Clark unit hydrograph method) in order to determine the most suitable simulation method to the study catchment. In this study the calibrated models were validated with 2008 to 2010 rainfall and flow data sets. The simulated flows from each methods were tested the relative error, residual method and statistically employing the coefficient of performance. As the final outcome of this study researcher proved that the. The Snyder unit hydrograph method simulates flows more reliably than the Clark unit hydrograph method. As the loss method, the SCS Curve Number method does not perform well.9
According to the research “Event Based Flood Modelling in lower Kelani basin” a two dimensional food simulation model FLO-2D is applied to model the floods in lower Maha oya basin. The model was applied to simulate five flood events occurred in the basin during the recent past, viz, in 2012, 2013, 2014, 2015, 2016, 2017. Model calibration was carried out for the flood event occurred in 2014, by comparing the discharge hydrograph at Badalgama gauging station. The other four events were used for validation. Nash-Sutcliffe efficiency and Normalized Objective Function was used to compare the simulated and observed discharge at Badalgama gauging station. Above mentioned parameters indicated that there is a good agreement between the observed and simulated flows at Badalgama gauging station. Inundation extent shown in flood inundation maps published by Disaster Management Centre and flow heights related to the specified locations in study area were used to compare the simulated inundation extents with the observed for the flood events in 2014 and 2015 respectively. Simulation results are matching with the observed extents for the flood events. The calibrated model as the final outcome can be used to forecast flood inundation extents at Badalgama catchment area in lower Maha Oya basin.10

By referring the study performed by J.S.M.Fowze et al. (2008) the requisite geo-spatial modeling of the flood plain was possible by applying HEC-GeoRAS services in ArcView environment and implementing a Triangular Irregular Model (TIN) model. Then the data was remotely sensed by ALOS to update the land use map for the basin selected attributing the study area with required hydraulic parameters. Peak flows corresponding to certain return periods were acquired from a frequency analysis and the calibration run was carried out by simulating the 50 year return period flood event.

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