Shear stress ratio in unprotected bed in conditions with and without rip rap in bridge opening

one of the major causes of river bridge destruction is flooding below the base surface and flooding. In some cases, the bed is reinforced with rock material to counteract this phenomenon. This method has many advantages and disadvantages. One of the disadvantages of using bedrock that is equivalent to the natural bedding is that the heterogeneous bedding roughness against the bedding material causes disturbance to the flow and creates strong secondary streams. These currents can lead to an increase in shear stress and scour depth in the unprotected bed. In this paper, a conceptual model for the determination of shear stress ratio in unprotected bed with and without rocking is presented. In deriving the conceptual model, numerical simulation tests are performed on a flexible bed. In addition to presenting the performance validation of the Flow3D numerical model, the model was used to simulate flow in two conditions with and without rocket bed. In rigid bed conditions, computational and hardware costs are significantly reduced. The shear stress ratio in the unprotected bed was calculated and the fitted quadratic polynomial equation for different roughness ratios was presented. The coefficient of determination in each case indicates a good fit of the stress ratio in both flexible and rigid bed modes.