Introduction:
In the context of energy dissipation in stepped spillways, various geometric and hydraulic parameters play a role. These include the flow rate, step height, number of steps, inception point of free aeration, and width of the spillway. Researchers have explored different approaches to enhance energy dissipation in stepped spillways. These approaches involve modifying the spillway's geometry and structure, as well as introducing obstacles and roughness on the bottom and edges of the steps. Regarding changes in geometry, researchers have investigated altering the spillway angle, the angle of the bottom of the steps, and the transverse slope of the steps. However, previous research has not specifically investigated the location and shape of obstacles on the steps to assess their impact on wear and flow characteristics. Therefore, further research in this area is necessary.

Methodology:
In this research, a flume with dimensions of 10 meters in length, 1.2 meters in width, and 1 meter in height was used. The first 3 meters of the flume had a height of 1.2 meters. The maximum flow rate of the flume was 150 liters per second, and the flow rate parameter (dc/h) ranged from 0.37 to 1.06 to cover all three flow regimes in the stepped spillway. Two depth gauges with a measurement accuracy of ±1 mm were used to measure the depth of the spillway downstream and the depth of water upstream at the entrance of the spillway. The spillway had a total height of 87 cm, 8 steps, with a step height of 10.9 cm and a step length of 20.9 cm. The obstacles used in this research included a continuous obstacle with a square cross-section (CO), a right-angled triangle with a chord in the upstream (TU) and downstream (TD), an isosceles triangle (IT), and a combination of a square and a triangular barrier in the upstream direction (MTU) and downstream (MTD). The relative heights (h_o/h) ranged from 0.19 to 0.56, and the relative edge distance (L_o/L) ranged from 0.19 to 0.48. The obstacles were placed on all steps, and in some experiments, obstacles were placed only on step 5.

Results and discussion
According to the results of the flow boundaries, it can be seen that the placement of continuous obstacles with different shapes and heights in different places, due to the variability of the mentioned parameters, causes the displacement of the boundaries of the beginning of the flow. The change of flow boundaries under the influence of obstacles (continuous and discontinuous) at the edge of a stepped spillway was also reported by Kökpinar (2004) and Asghari Pari & Kordnaeij (2020 & 2021). According to the results of the present research, it can be stated that in the examined range, the placement of a continuous barrier with different cross-section shapes with different height and location at the bottom of the step also causes the change of the beginning of the flow boundaries compared to the control state, and in general, the tendency of the flow to expand And the durability is more in the transitional range.
The results of energy dissipation for arrangements that have energy dissipation equal to or greater than the control state are given in Figure 1. In all the arrangements of the current research, in the nappe flow regime, the placement of obstacles with different shapes, heights and locations on the bottom of the steps causes an increase in energy dissipation compared to the control condition.
In the transitional and skimming flow regimes of all three states, there was no change, increase and decrease in energy dissipation compared to the control state. In the skimming and transitional flow regime, the increase in energy dissipation was not observed only in the combined MTU and MTD arrangements, and in other arrangements, it was different according to the height and location of the obstacle.

Conclusion
- The placement of a continuous obstacle with different cross-sectional shapes with different heights and locations on the bootom of the step spillway changes the beginning of the flow boundaries compared to the flat step (F.S) and generally increases the tendency of the flow to expand and remain longer in the transition range.
- The use of square obstacles (CO) has moved the inception point of free aeration (IP) to the downstream side compared to the flat step (F.S), but the triangular obstacles (TU, TD, IT) have moved the IP unchanged or to the upstream side compared to the F.S.. In the combined triangle and square obstacles (MTU, MTD), the IP has not changed compared to the F.S,. Also, all the arrangements that have been able to move the IP upstream compared to the F.S. have created more energy dissipation in the skimmimg flow regime than the F.S.
- For the nappe flow regime, the energy dissipation results show the effectiveness of placing the obstacle with different shapes and heights in different places on the steps, but in the transition and skimming flow regimes, according to the arrangement used, energy dissipation has three modes of no change, increase and Or had a decrease in energy dissipation.
- The results of BIV analysis show that according to the shape of the obstacle, the height of the obstacle and the location of the obstacle, the dimensions of the formed areas are different compared to the F.S. At a slope of 1:2, if a continuous barrier is placed on the edge of the step, only a rotating zone. is formed on the step, but with the distance of the barrier from the edge of the step in the range of lo/l 0.38 and 0.48, in addition to the rotating zone, the mixing zone is also formed. Is.