Background and Aim: Heavy and consecutive rains at early April of 2018 led to severe floods in large parts of Iran, especially in the Karkheh basin, which was accompanied by huge damages. The average rainfall in the Karkhe dam basin for the event of April 4-7, 2018 was
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Background and Aim: Heavy and consecutive rains at early April of 2018 led to severe floods in large parts of Iran, especially in the Karkheh basin, which was accompanied by huge damages. The average rainfall in the Karkhe dam basin for the event of April 4-7, 2018 was about 87 mm, and for the event of April 11-17, 2018, it was nearly 108 mm. For the flood management by the reservoir, estimation of the peak discharge and flood hydrograph is essential in order to predict the hydrological behavior of the basin. Rainfall-runoff models that are used to simulate flood hydrographs are one of the methods of estimating runoff and a suitable tool for investigating and evaluating hydrological processes, water resources, and flood management.Method: Since the estimation of peak discharge and flood hydrograph has great important to predict the hydrological behavior of the basin and also to take the necessary measures to reduce the flood risk, the present study was conducted by using HEC-HMS model to simulate the rainfall-runoff events during 2007-2018 in the Karkheh Basin .By using this model capabilities and the data from some hydrometric and meteorological stations in the basin, the volume and peak discharge of floods in that period were estimated. Because Seymareh dam impoundment has started since 2013; two separate basin models were developed and for running the model, 11 flood events were obtained then, the basin parameters were calibrated based on six events and the others were used for validation. In the process of developing the basin model, the SCS Curve Number method is used to calculate basin runoff losses and convert rainfall to runoff, the Clark Unit Hydrograph method and the Return flow method to calculate the base flow, the Muskingum method for hydrological routing, and the Weighted average method for spatial data analysis of rainfall. The Outlet Structure method was used for routing the reservoirs of Karkheh and Seymareh dams.Results: Comparing the initial simulation results of the model with the observed values at the outlet of the basin and some hydrometric stations of the basin showed that the hydrograph model overestimates the flow. Therefore, using the residual squaredsum objective function, basin parameters (CN, time concentration, storage coefficient, initial absorption, and recession constant) were calibrated. After calibration of parameters, the results showed that the calculated hydrographs were in good agreement with the Observational hydrographs in the Karkheh and Seymareh dams. Next, to check the accuracy and confirm the results, the model was validated by the five new rainfall events and to evaluate the efficiency of the model used in this stage, the Nash-Sutcliffe indices and the simulated variance coefficient were used.Conclusion: Comparing the calculated results with the flood observational values (peak discharge) using the correlation coefficient (R2) showed that there is a relatively good agreement between simulation and observation in sub-basins 5, 2, 7, and 1 (0.92, 0.73, 0.73 and 0.70, respectively). Also, the model efficiency index values in the validation period for the Nash-Sutcliffe index (0.33-0.99) and simulation variance coefficient (0-0.73) for the outlet of sub-basins 9, 6, 5, 1, and 8 are favorable and the HEC-HMS model approximately can provide an acceptable estimation of the flood hydrograph. So, it can be well-analyzed how the way flood events are formed in the Karkheh basin. Also, the sensitivity analysis of the model parameters showed that the curve number parameter (CN) has a greater effect on the changes in the objective function than other basin parameters.
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