Experimental Studies on Burial of Finite-Length Cylinders under Oscillatory Flow

Self-burial of finite-length cylinders under pure oscillatory flow was studied with a large oscillating water-sediment tunnel (LOWST). Cylindrical models were placed on a $60\mathrm{cm}$ deep sandy bed and measurements of cylinder burial were made with the aid of video cameras. Thirty experiments were completed for a wide range of flow conditions to investigate burial due to scour of cylinders having length-to-diameter ratios between two and four. Experimental results showed a strong correlation between the Shields parameter and the equilibrium burial depth. On the other hand, the correlation between the equilibrium burial depth and the Keulegan-Carpenter number, commonly used to determine equilibrium scour depth of pipelines, was found to be less strong for the current case of short cylinders. Length-to-diameter ratio, density, and initial burial of the models were not found to influence the equilibrium burial depth. A time interval, necessary for burial initiation, was defined. The time scale for the burial of a cylindrical object was also identified and characterized by a semiempirical equation. Based on the observations, a new equation to estimate the burial of finite-length cylinders is advanced and a procedure to estimate the burial process is proposed.