Introduction

Erosion is a phenomenon that occurs due to the passage of fluid flow, especially water, at the contact boundaries with other objects in hydraulic structures. The basis for this phenomenon is the creation of a vacuum at the contact boundaries of two environments due to changes in fluid velocity. Fluid at the contact boundaries with other environments is affected by the roughness and shape of the environment with which it is in contact. In hydraulic structures, this phenomenon can damage the stability and durability of the hydraulic structure because the water fluid can wash the soil at the bottom and around the foundations of hydraulic structures, causing erosion of the walls and banks of rivers and carrying the eroded materials with it in the direction of flow. Local erosion is a phenomenon that occurs due to the interaction of water flow and soil in rivers, streams and downstream of hydraulic structures. Gates and weirs are widely used in open channels to control, regulate flow and stabilize the bottom. Due to the flow caused by the jet passing over or under the structures, there is a possibility of creating scour holes downstream of the structures, which may endanger the stability of the structure; therefore, determining the characteristics of scour holes has been of interest to flow hydraulic researchers. In order to minimize the problems in spillways and gates and also to increase their benefits, a combined spillway-gate structure can be used so that the water flow can pass both over the spillway and under the gate at the same time. This combined device can eliminate the problems caused by erosion and sedimentation.

Methodology

To achieve the objectives of this research, experiments will be conducted in a laboratory flume. Experiments will be conducted with clear water and different hydraulic conditions. In order to investigate this phenomenon more precisely, experiments will be conducted for different flow modes over the spillway and under the gate with different discharges and different backwater depths. After the experiments are completed, the created bed profile will be measured. After the experiments and data collection, the data related to the scour measurement during the experiment will be entered into the Excel program and the bed profile data will be entered into the Surfer software. Then, based on the dimensional analysis, the relevant dimensionless parameters will be calculated and the results will be analyzed.To conduct the experiments in this research, an aluminum spillway model with a thickness of 1 mm, a width of 50 cm, a crest length and a height of 40 and 20 cm, respectively, was used in the laboratory of the Water Engineering Group of Tahkim Gostar Company, and a combined gate and spillway were installed at a distance of 140 cm from the beginning of the channel. The experiments were conducted in a flume with a length of 10 m, a width and a height of 0.5 m. The height of the channel from the laboratory floor is 1.3 m and the slope of the channel floor is 0.001. The material of the channel floor and skeleton is iron and the walls are glass. To measure the flow rate, a sharp-edged triangular weir with an apex angle of 90 degrees made of metal was used at the end of the downstream basin. Also, this flume was equipped with a valve at the downstream with the ability to control the depth of the backwater and the jump. The length of the rigid bottom was 60 cm, the depth of the sediments was 17 cm and its length was 210 cm. In this research, silica sand materials were considered with three uniform grain sizes with an average particle diameter of 1.6, 3.3 and 6.2 mm. The depth of the backwater was adjusted by a sliding valve installed at the end of the laboratory flume. At the end of the test period (6 hours), the equilibrium time was 120 minutes (90% scour depth). Simultaneously with closing the end valve, the flow was stopped and after complete drainage of the sediment bed, the bed profile was measured with a depth gauge with an accuracy of 1 mm and the above process was repeated for the next test.

Results and Discussion

The changes in the Froude number are affected by the speed and depth of the flow in the measurements. However, it is necessary to draw a diagram of these changes due to the effect of dimensionless parameters on the value of this characteristic for different measured discharges. These parameters include the average particle size, scouring cross-section width, scouring cross-section length, scouring cross-section depth for different values of sediment diameters, which need to be evaluated. It is noteworthy that the overall trend of changes in the Froude number for changes in discharge under the same conditions is increasing, but it is necessary to determine the changes in the parameters in dimensionless conditions with respect to the flow depth. The range of changes in the Froude number for changes in ds/H with respect to changes in discharge at different depths of the tributaries indicates that with decreasing discharge, the increasing trend of ds/H changes is maintained first with a steep slope and then with a gentle slope. The changes in the Froude number with respect to the Ws/H trend at different tested discharges also showed that the Froude number initially had an upward trend with respect to this dimensionless parameter, but from the point related to the change in the spectrum, a downward trend with a steep slope begins for all three discharges. The important point is that in the final values for all three discharges, this parameter ends at approximately the same location. This is due to the high impact of the channel width on the flow rate changes. In the evaluation of the trend of changes in the length of the scour position relative to the flow depth at different values of the Froude number, it was determined that despite the gradual decrease in the Froude number for the relative increase in this dimensionless parameter, the range of flow rate changes had a minor effect on this.

Conclusion

The changes in the above measured values were re-measured and examined by changing the size of the sediments related to the bed and also applying the same discharges. It is noteworthy that with the increase in the size of the sediment particles, despite the minor nature of some changes, the effect of the armor coating on the bed increased at larger sizes, which allows for better control over the scouring performance. Another effective parameter in the process of flow changes for different values of descent, depth of the outlet and measured discharges is the value related to the d50 of the sediment particles in question. In general, d50 represents the average size of the particles present in the bed, the determination of which leads to a better view of the scouring mechanism. The studies in this study indicated that as the d50 of the bed sediments increases and the relative size of the particles becomes larger, the scouring performance in creating the scouring hole also occurs with a delay and over a longer period of time, and the conditions can be improved. These changes have undergone a decreasing trend over time with increasing flow rate, indicating that an increase in Froude number due to an increase in flow rate for a constant value of channel characteristics leads to an increase in the amount of leaching.