Shear Stress Ratio on the Unprotected Bed in Conditions with and without Riprap at the Bridge Span

Scour at the shallow abutment foundation during floods is a common cause of bridge failure. In some cases, the bed is reinforced with riprap. There are several advantages and disadvantages of using this method. One of the disadvantages of using flush riprap aprons with the bed is that heterogeneous roughness (riprap versus native bed materials) alters the flow and manifesting as strong secondary currents. This flow can increase the shear stress and the scour depth in the unprotected bed. To study the subject, a conceptual model for determining the shear stress ratio in the unprotected bed in both with and without riprap has been investigated. In developing the conceptual model, numerical simulation tests in a flexible bed have been used. Then, the validation of Flow3D numerical model is presented and the mentioned model is used to simulate the flow in two cases with and without riprap in the rigid bed. In a rigid bed compare to flexible bed, computational and hardware costs are significantly reduced. The shear stress ratio in the unprotected bed is calculated and the quadratic polynomial equation is fitted for different roughness ratios. The coefficient of determination in each case indicates the appropriate correlation of the stress ratio in the two modes of flexible and rigid bed.