Nastaran Sahranavard - M.Sc Student of Hydraulic Structures, Bojnourd University , Faculty Civil Engineering, Bojnourd , Iran
Gholamhosein Akbari - Associate Professor of Civil Engineering Department of Bojnourd University, Iran
Elham Mina - Ph.D of Hydraulic Structures, Tarbiat Modares University, Faculty of Environmental Civil Engineering,Tehran, Iran

Dams are an important part of this nation’s infrastructure. When populations at risk are located close to a dam, it is important to accurately predict the breach outflow hydrograph and its timing relative to events in the failure process that could trigger the start of evacuation efforts. In general, a numerical analysis of unsteady flow in a reservoir has to be able to deal either with zones of supercritical and subcritical flow separated by a discontinuity or with flows that are entirely subcritical. For a stationary observer the negative surge appears to be a gentle lowering of the free surface. The Hec-ras model based on the linearized implicit finite -difference model which based on the De St. Venant equations. The various implicit methods of finite differences that have been developed, the Preissmann-type schemes. When the surge occurs, the De St. Venant differential equations are not valid in the neighborhood of the surge. So In most cases the partial differential equations may be solved to obtain The Explicit MacCormack (MOC) model by Method of Characteristics in the neighborhood of the surge. Thus, the models can describe closely the wave formations in the reservoir. They can establish the time and place of surge generation, and they can track the surge whether it is propagating under subcritical, supercritical, or mixed flow conditions. it is very important to understand wave propagation in a reservoir caused by a dam-break. when the failure time is very low, a steep negative wave may be formed. In this case was used software Fortran 90 and write code to solve The Explicit McCormack model. According to This paper provides the Parametric Models methods and numerical model such as MacCormack Explicit scheme and Preissmann Implicit scheme using Hec-Ras to simulate dam break furthermore it used to predict breach outflow hydrographs .Unsteady flow modeling is often used in dam breach analysis due to the need to obtain more detailed or accurate results. Breach characteristics can be estimated in several ways which is describe the physical characteristics of a dam breach, use of those parameters within the unsteady flow routing model Hec-Ras , and the computation breach out flow hydrograph. The breach parameters will directly and substantially affect the estimate of the flows. Due to this illustrate the failure time is very low, as a result the negative surge occurs, However De St. Venant differential equations are not valid in the neighborhood of the surge. In order to evaluate the effective of negative wave on the outflow hydrograph. according to this, the outflow computed by the simplified model with The explicit method of characteristics (MOC). The results of the Hec-Ras simulation, present that it was relatively closely to estimate peak breach discharges with The Explicit McCormack model

dam-break, embankment dam, numerical model, dam overtopping, peak discharge