Control of the high velosity and kinetic energy of the flow at downstream of the hydraulic structures in order to prevent the erosion of the channel bed is one of the most important concerns of hydraulic engineers. For this reason, energy dissipation structures are used.The sudden symmetrical expanding channel is an energy-dissipation structure that requires minimal tailwater depth for the formation of hydraulic jumps. Rough beds are important for reducing stilling basin dimensions and the effect of the roughness elements impact the hydraulic jump. The effect of bat-shaped elements in a sudden expansion channel have been investigated here. The experiments were performed in a rectangular channel with symmetric expansion ratios of 0.67, 0.5, and 0.33, and a Froude number that ranged from 4.6 to 11.3. The results show that the shear stress in a rough bed of a sudden expansion channel was more than 12 times greater than the shear stress in a smooth prismatic channel. Also, the secondary depth and S-jump length in a rough bed compared to the smooth bed decreased by 22% and 9-13%, respectively. Finally, several equations were developed to predict the hydraulic jump on the rough bed. The correlations have R2 values of more than 0.988 and NRMSE values of less than 2.5%. These highly accurate equations are easy and simple to apply for the design of enlarged stilling basins.
