Introduction: The aim of this investigation is to investigate the influence of various hydraulic parameters on the dimensions of scour holes caused by symmetrical crossing jets on cohesive sediment beds. Scouring, particularly in environments with cohesive beds, is a major factor threatening the stability of hydraulic structures such as spillways and dams. The kinetic energy of jets discharged from these structures, if not sufficiently dissipated, results in the creation of scour holes and ultimately results in bed degradation and structural failure. Conventional energy dissipation methods, such as stilling basins, are often impractical owing to economic constraints or environmental conditions. This research focuses on cohesive sediment beds and examines parameters such as jet crossing angle, distance between the crossing point of two symmetrical jets and water level, and bed moisture content to analyze the dimensions of scour holes under various conditions.
Methodology: To conduct this study, a laboratory flume with dimensions of 16 meters in length, 1 meter in width, and 0.8 meters in height was employed. The sediment bed utilized in this research was constructed using clay soil sourced from the Semnan region. The soil had a relative density of 2.71, a liquid limit of 35, a plastic limit of 20, and an average particle size of 0.024 mm. The soil was prepared at three different moisture levels: 13%, 16%, and 19%. To achieve the desired moisture levels, the soil was first mixed with water and homogenized using a 1500-watt electric mixer. The moistened soil was then stored in plastic bags to maintain its moisture content until testing. The sediment bed was formed layer by layer, with each layer having a thickness of 5 cm. Each layer was compacted using a heavy roller weighing approximately 60 kg. The jets were supplied through 2-inch pipes that ultimately split into two jets with a diameter of 22 mm. A metal frame was employed to precisely adjust the angle and position of the jets, enabling variations in impact angles of 45°, 75°, and 105°, and setting the distance between the crossing point of two symmetrical jets and water level at 5, 7.5, and 10 cm. The vertical angle of the jets was maintained at 85° across all experiments, with a tailwater depth of 9 cm. Each test was conducted for three hours to allow the scour hole to reach equilibrium. After each experiment, the used soil was discarded, and a new bed was prepared for subsequent testing.
Results and Discussion: The experimental results demonstrated that increasing the jet distance between the crossing point of two symmetrical jets and water level significantly decreased the depth, length, and width of the scour holes. This phenomenon is attributed to the decrease in pressure and force exerted by the jets on the bed, as well as the increased air contact area of the jets prior to impact. These factors jointly decreased the kinetic energy, leading to smaller scour hole dimensions. Furthermore, the jet impact angle played a critical role in determining the dimensions of the scour hole. An increase in the crossing angle from 45° to 105° resulted in greater energy dissipation and attenuation of jet force, thereby reducing the dimensions of the scour hole. The smallest depth and degree of the scour hole were observed at 105° impact angle, indicating maximum energy dissipation. Additionally, an increase in bed moisture content contributed significantly to reducing the scour hole dimensions. With higher soil moisture, the bed compressive strength and specific weight increased, which restricted jet penetration and reduced the depth, length, and width of the scour hole. The shape of the scour holes was elliptical, and with increasing the impact angles, the holes elongated along the flow direction. This elongation reflects the longitudinal distribution of jet forces and diminished vertical penetration.
Conclusion: This study showed that parameters like jet crossing angle, distance between the crossing point of the two symmetrical jets and water level, and soil moisture significantly influence scour hole dimensions. Increased distance between the crossing point of the two symmetrical jets and water level and crossing angle reduced hole size owing to lower kinetic energy and higher energy dissipation. Higher soil moisture also enhanced bed resistance, further reducing scour dimensions. These findings aid in optimizing hydraulic structure design and mitigating scour and erosion risks.