Cohesive-soil scour under horizontal jets: influence of soil resistance and jet submergence

Abstract Predicting cohesive-soil scour under hydraulic horizontal jets remains a critical challenge due to the coupled influence of soil strength and flow confinement. This study investigates how soil resistance, modulated by moisture content, and jet submergence jointly control the depth and geometry of scour produced by horizontal turbulent jets. Forty-nine laboratory experiments were performed in a recirculating flume at the National Hydraulics Laboratory (UNI, Peru) using low-plasticity cohesive soil (CL) from Pucallpa, Peru. Soil properties were characterized through particle size analysis, Atterberg limits, pinhole testing, and direct shear tests, which revealed a pronounced reduction in shear strength with increasing moisture content. The experiments systematically varied jet submergence ratio ( h / D  = 0–4), outlet velocity (1.93–3.63 m/s), and soil moisture content, while maintaining constant nozzle diameter ( D  = 25.4 mm) and controlled hydraulic and soil conditions. Results show a strong inverse relationship between maximum scour depth and soil resistance, confirming that bed strength is the dominant control on erosion. Beds with lower shear strength experienced substantially deeper scour across all hydraulic conditions. Jet submergence exerted a secondary influence on maximum scour but significantly affected scour geometry, with submerged jets producing longer and wider cavities while preserving a relatively stable location of maximum depth. Scour progressed rapidly, reaching more than half of its final depth within the first 30 min before approaching equilibrium asymptotically. These results show that accurate cohesive-soil scour prediction requires prioritizing moisture-controlled soil strength over hydraulic submergence. The empirical relationships developed provide practical tools for engineering applications such as outlets and dredging structures.