Abstarct: The flushing of the reservoir is a process which scours the previously deposited sediments in the reservoir and transport them through low-level sluice gates. In order to reduce the wasting of the storage water of reservoirs during desiltation, this phenomenon should be studied by physical or mathematical model. In this study, a one dimensional numerical model was developed to study and simulate the flushing process. In order to achieve the objective of this study, The Saint Venant equations and the governing flushing equation (Beta method) were solved with two methods including one stage and coupling methods. More over, the results of developed numerical models were compared with experimental results of Jafari,(2003) and also with HEC-RAS model. The results show that by increasing the outflow discharge, the sediment transport rate increased. Also, the compassion results of the mathematical model with two methods of one stage and coupling methods and also HEC-RAS model with experimental data show that these models with coefficient of determination 971% , 89% and 98 % respectively have good accuracy to predict flushing of reservoirs.