My problem in sediment transport model

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My problem in sediment transport model

Mehdi Rooyvaran
 Hello there!
Modeling sediment transport with HEC-RAS I confronted with an observation which I don’t know the reason. I use HEC-RAs 5.3.3. The model has been developed to simulate decreasing trend of sediment deposition in reservoir of a large dam. The measure to decrease sedimentation in reservoir is to set up a series of consequent short weirs along the upstream river. For this means, 10 years of daily flowrate alongside with floods have been considered. Since short weirs need to be dredge after a few years, modeling has been performed year by year. So that the resulting geometry at end of the year is saved and has been used as the initial geometry of next year.
Now the question is: if instead of year by year running, one runs the model in the context of an integrated time course of 10 years, results would be considerably different. This fact is also observed in short weirs those have very negligible sedimentation behind them. WHY?
It is worth to note that for modeling, fallowing items are considered:
1- Armor layer has been modeled.
2- Yang method is selected for solution of sediment transport.
3- Sediment load that reaches to the reservoir of large dams is desired value which renders significant difference in case of using different mentioned methods.