Experimental tests were carried out to investigate the effective scouring parameters downstream from stepped spillways with different flow rates and step sizes. The results indicated that the flow regime plays an important role in scour-hole dimensions such that the minimum scouring depth happens in the nappe flow regime. Moreover, step size and tailwater depth are essential parameters for maximum scouring depth. Increasing tailwater depth from 6.31 cm to 8.54 cm and then to 11.82 cm decreases the scouring depth by 18.56% and 11.42%, respectively. These alterations also decrease the scouring length by 31.43% and 16.55%, respectively. By increasing the flow rate, the particle Froude number will increase, and the increased momentum of the flow promotes scouring. In addition, the results show that scouring at the sidewalls is higher than in the middle of the cross-section. Finally, an empirical formula with root mean square error = 0.107 and R2 = 0.974 is proposed to predict the maximum scouring depth downstream from the stepped spillways. Comparisons were made between the proposed formula and experimental results. This comparison demonstrated that the formula can predict souring depth to within 3.86% and 9.31% relative and maximum errors, respectively.
