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One of the most challenging hydraulic topics is the study mountainous river flow in natural condition where the relative submergence (the ratio of water depth to the bed roughness length characteristics) is low and Reynolds number is too high. Most of the rivers that cross the mountainous areas have coarse aggregates of sand aggregates. In natural flow conditions, usually depth of water is not high and as a result existence of roughness affects the flow field and velocity profile specially roughness sublayer. Investigation of flow field in this types of rivers and understanding the important parameters of turbulent flow helps to better exploit these types of rivers. In shallow currents, the relative submergence is often less than 20. However, there is an uncertainty regarding this range. Present study, which will be continued by Mohajeri et al. (2015), has investigated the numerical simulation of this type of flow. For this purpose, the gravel bed was modeled physically by importing a digital model into the Flow-3D software. One of the innovations of this research was the importing of a rough bed in a physical manner while in similar researches, the roughness equivalent coefficient was imported as an effective roughness parameter and the bed was not physically modeled. In present study, the study of parts of the flow in numerical simulation order that in laboratory study Due to laboratory limitations was not investigated, was considered. Actually, the main purpose in present study is investigation of turbulent flow characteristics in a low relative submergence in near sandy stone. One of the other points of this research is the investigation of low relative submergence, while most studies had done on high relative submergence. It should be noted, for simulating the k-ԑ RNG that is two equations model and having Medium computing cost, has been used in Flow-3D Software. After validation it was found that Flow-3D was able to simulate these types of flows. After validation and at the end of modeling, it was found that existence of sand stones in these conditions, it affects severely on flow field, turbulent intensity and shear stress. Quantitative investigation of region under sandy sandstone shows that roughness sublayer region not only changes in location, but also vary according to the parameter being studied. Such an observation makes the region's precise and comprehensive definition very difficult and complicated. also existence of  changes  in longitudinal velocity and medium turbulent kinetic energy (T.K.E) is  the  cause  of  formation of  longitudinal strings of variations in velocity and kinetic energy across the channel .According to  the type of turbulent  modeling in this study which was k-ԑ RNG, existence this strings Attributed  to  Swirling together and Arranged sequences near sandy sandstone because these model  Based on isotropic turbulence assumption and  it  hasn’t  ability to investigating and modeling the secondary currents. Finally with investigation of shear velocity contours in bed channel, it was found that maximum shear velocity occurred near maximum elevation sandstone. Also, maximum drag force occurred in this region. Finally, due to small flow fields in near of walls and colliding Flow currents to these walls, the mean of shear velocity and friction increases.

Keywords: Gravel bed, Open-channel, Turbulent flow, Numerical simulation, Shallow flow

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