Abstarct: In this study, a numerical model has been developed to investigate the transport of sediment loads of bed and suspended load in steep slopes, including overflows. For this purpose, a wide trapezium overflow has been used. In this study, the SSIIM software, which uses 3D flow and sediment equations, is used.SSIIM software is designed baxsed on finite volume approach with three-dimensional organized network and Navier-Stokes and transfer and release equations for sediment transportion are solved. In order to verify the validity of the results and the numerical strength of the model, the results were compared with the laboratory data. The results showed that for an overflow, the increase of the flow due to the increase of the flow velocity due to the depth reduction on the slope of the overflow; results in transfer and transport of sediments on the overflow.Due to the gradient effect on sediment movement in the upstream of the overflow, all sediments are transferred suspendedly over the overflow. No significant sediment deposits can be identified for the upstream overflow area, and deposition of sediment in the downstream is expected to occur in the velocity reduction region. The results show that the SSIIM model can predict the sediment transport pattern well around the overflow.Then, the sediment flow on the overflow were investigated under 8 discharges with the values of 0.015, 0.02, 0.025, 0.031, 0.036, 0.043, 0.049, 0.057 (m3/s) and different diameters of sediments was investigated.For each discharge, the particle diameter increases to some extent that other particles cannot pass through the surface, and the bed transformation is formed only before the overflow. As a result, the results of this numerical experiment showed that the diameter of the sedimentary particles that were not able to cross overflow were respectively 0.00017, 0.00033, 0.00065, 0.0016, 0.0026, 0.0052, 0.0072 and 0.011 meters.Finally, the results showed that in the constant flow conditions, increasing the diameter of sediments, the output force to the input force increases in sediment movement. Sedimentary particles fall and other particles are not able to cross the overflow.Then, the relationship of the results are obtained to estimate the maximum diameter of the sedimentary particles passing through the overflow.