Hydraulic jump stilling basins of sudden expansion are one of the energy dissipater structures which are used where the available tail water depth is less than the required sequent depths. The purpose of present study was to experimental investigation the effect of shear stress caused by a rough bed on S-jump's characteristics in a abruptly expanding stilling basin. Experiments were performed in a horizontal laboratory flume with a rectangular cross section, a Plexiglas wall and floor with an expanding ratio of 0.33, a Froude number range of 5.38 to 10.78. The density of the rough block was 12.33. The results showed that the shear stress of the rough bed in the expanded stilling basin was more than 13.5 times the shear stress of the prismatic stilling basin with a smooth bed. Secondary depth and the length of the s-jump on the rough bed were reduced 20% and 16%, respectively. Also in expanding stilling basins with rough beds the secondary depth reduces 58.5%. Increased shear stress has little impact on jump efficiency, so that the effect of expanding on jump efficiency is greater than the effect of increased shear stress. Expanding stilling basins with a smooth and rough bed are moderate compared with the prismatic basin increased the jump efficiency 23.5 and 28.7%, respectively.
