One of the approaches to reduce flood depth in dams and rivers is the application labyrinth weirs instead of simple linear weirs. Labyrinth weirs have a longer length than linear weirs; therefore, the flood will pass through it with a lower depth. In this study, two types of triangular and semicircular labyrinth weirs were experimentally investigated. Experiments were performed in a flume with a width of 0.8 m and a flow discharge of up to 70 L/s. Eight weir models with a thickness of 4 mm were made, of which four models are triangular weirs and four models are semicircular weirs. Weirs were used in two types, one-cycle and two-cycle. The height of the weirs is 15 cm and the ratio of the weir length to the flume width (L/W) is equal to 1.57. The results showed that the increase in the total water head ratio (HT/P) decreases the discharge coefficient in all models, which is due to the increase in the Nappe interference and the local submergence. Also, increasing the number of cycles in labyrinth weirs has reduced its efficiency. The reason for this decrease is the impact of the issuing jets at the junction of the cycles and the flow disturbance over the weir. The comparison between triangular and circular weirs showed the better efficiency of triangular weirs. In addition, the results showed that the placement of the top of the cycle towards the upstream will cause a better performance in the weir. It seems that the low angle of the weir wall in the downstream with the channel wall causes the Nappe flow to collide with the channel wall and consequent local submergence will ultimately reduce the discharge coefficient.
