Introduction:
Scour in cohesive sediment beds under hydraulic jets is a critical challenge in hydraulic engineering due to the complex interplay of jet energy and sediment properties. Cohesive beds, dominated by fine-grained clay, exhibit unique erosion resistance driven by inter-particle bonding, making scour prediction and control vital for the stability of hydraulic structures like dams and plunge pools. The novelty of this study lies in its comparative analysis of scour induced by single and symmetric crossing jets on a purely cohesive clay bed, a relatively underexplored area. Unlike previous research focusing on non-cohesive or mixed beds, this study investigates the specific effects of jet configuration, impingement angles, bed moisture, and nozzle distance on scour dimensions under controlled conditions. By examining a cohesive bed with 90% fines and 10% sand, the study addresses a critical gap in understanding scour dynamics in fine-grained sediments. The experimental findings aim to provide innovative insights into optimizing plunge pool design and developing effective scour control strategies, enhancing the resilience of hydraulic infrastructure in cohesive sediment environments.
 Methods:
Experiments were conducted in a 16 m long, 1 m wide, and 0.8 m high rectangular flume with transparent plexiglas walls for direct observation of scour processes. The flume included an adjustable tailgate to maintain a 12 cm tailwater depth, and a pumping system to provide a 2 L/s steady flow rate, measured by an electromagnetic flowmeter. Circular nozzles (2.2 cm diameter for crossing jets; 3.11 cm equivalent diameter for single jets) were mounted on adjustable supports to maintain a fixed vertical angle of 85° for both single and crossing jets, with crossing angles set at 45°, 75°, and 105° for crossing jets only. The nozzle distances from the water surface were adjusted to 8, 10.5, and 13 cm. The cohesive bed, with 90% clay (illite, montmorillonite, kaolinite) and 10% fine sand, had a median particle size of 0.0126 mm, plastic limit of 20%, and liquid limit of 35%. Bed moisture contents (13%, 16%, 19%) were prepared by mixing soil with water using an electric mixer, followed by compaction in six 5 cm layers to a 30 cm thickness. A hand roller ensured uniform compaction, and a vane shear test verified bed shear strength. Scour dimensions (depth, length, width) were measured using a laser meter with 1 mm accuracy.The experiments were conducted for 180 minutes to ensure that scour had reached equilibrium. Dimensional analysis focused on scour characteristics as functions of crossing angle, moisture content, and nozzle distance, with constant flow rate, tailwater depth, and vertical jet angle.
 Results:
The results revealed distinct scour patterns between single and crossing jets. Single jets, due to concentrated kinetic energy, produced deeper scour holes, with depths up to 30% greater than those from crossing jets. At a dimensionless fall height of 4.18 and 13% moisture content, single jets produced maximum scour depth.  In contrast, crossing jets at 105° and 19% moisture content exhibited minimal scour, while no scour observed at higher moisture levels (16% and 19%) and a fall height of 4.18 due to increased bed resistance. Crossing jets reduced scour depth by 45–66% at 75° and up to 100% at 105° compared with single jets, attributed to energy dissipation from jet collision, which forms a radial flow with reduced shear stress. Moisture content significantly influenced scour, with a 12–55% reduction in depth and 33–40% in length as moisture increased from 13% to 19%. This reduction is attributed to the reflecting enhanced cohesion from montmorillonite clay in the bed material. Scour length followed similar trends, with single jets producing longer holes (up to 14% increase with fall height) and crossing jets reducing length by 19–100% at 105°. Width was also lower for crossing jets, with reductions of 24–100% at higher angles and moisture levels. These findings align with previous studies, confirming that increased fall height reduces relative scour depth due to energy dissipation. The study highlights the effectiveness of crossing jets in minimizing scour on cohesive beds, particularly at higher impingement angles and moisture contents, providing a basis for optimizing plunge pool designs.
 Conclusion:
This study highlights the contrasting scour behaviors of single and crossing jets on cohesive sediment beds, emphasizing the role of jet configuration, bed moisture, and nozzle distance in scour dynamics. Regarding the results, single jets, with focused energy, produced deeper, longer, and wider scour holes, with maximum dimensions at 13% moisture and higher fall heights. Crossing jets, particularly at 105°, significantly reduced scour (up to 100% in depth, length, and width) due to energy dissipation, with minimal scour at 19% moisture. Increased moisture content enhanced bed cohesion, reducing scour dimensions across both jet types. These findings offer valuable insights for hydraulic engineers, enabling the design of plunge pools and spillways that mitigate scour risks in cohesive bed environments. The results advocate for the use of crossing jets in scenarios requiring scour control, particularly in high-moisture conditions. Future research should explore numerical modeling to validate these experimental outcomes and investigate additional parameters, such as varying flow rates and bed compositions, to further refine scour prediction models for cohesive sediments.