عنوان مقاله [English]
Depreciation of excess energy in structures such as stilling basin is one of the most important topics considered by hydraulic engineers. Since hydraulic jump has known as one of the important factors in the loss of energy in the lower end of the spillway and the gates, it is important to form and control it in the stilling basin. Therefore, the use of expanding section can be one of the ways to ensure the formation of a jump in the stilling basin. Non-prismatic channels (expanding sections), in comparison to the prismatic channels, cause more loss of current energy, reduce the secondary depth and increase the length of the hydraulic jump. Therefore roughness bed decreases the secondary depth and the jump length, in the present study, non-continuous trapezoidal roughness elements were examined at two relative roughness height and in non-prismatic channels and the S-jump characteristics were analyzed. A total of 99 experiments were carried out in prismatic channels and non-prismatic channels with expanding ratios of 0.67, 0.5 and 0.33, the range of Froude numbers from 4 to 12 and in the horizontal rectangular channel which is 30 cm wide, 45 cm deep and 5 m long. The results of present study show that the shear stress of expanding section with a rough bed in relative height of roughness elements by the ratio was about 11.12 times that on smooth bed in prismatic channels. Also, the expanding section with a rough bed caused a 12 to 17 percent reduction in the conjugated depth and a 7 percent decrease in jump length in comparison to the expanding section with the smooth bed. For the relative height with the ratio , the shear stress of the expanding section with a rough bed was 13.2 times that on smooth bed in prismatic channel. The conjugated depth and length of jump in the expanding section with rough bed decreased by 20-30% and 13% respectively. The non-prismatic channel with the rough bed at an expanding ratio of 0.33 in comparison to prismatic channel with a smooth bed caused 59.5% reduction in the secondary depth and increased jump efficiency about 29%.