Experimental study on local scour around large-diameter submerged cylinders under combined waves and current action

The characteristics of local scour around large-diameter submerged cylinders are quite different from the non-submerged ones under combined wave and current conditions. Local scour model tests are carried out in a wave-current flume. A submerged cylinder model was installed in a flat sand bed. The flow velocity around the model was measured,and the development of the scour depth was recorded under various waves,current velocities,current directions and submergence rates. A laser scanner was used to generate the geometry of the equilibrium scour hole. The influence of the submergence rate S_r and the relative cylindrical height h_c/D on the equilibrium scour depth is analyzed. The results show that under the same incident wave current conditions,as the relative cylindrical height h_c/D decreases,the scour development rate,the scour hole radius and the scour hole depth decrease. When the relative cylindrical height h_c/D is larger than 1,the scour depth increases rapidly as the cylinder height increases until a critical value of h_c/D is reached. The quasi-equilibrium scour hole is in the shape of a "flipped spoon". The scour hole is symmetrically distributed along the left and right sides of the cylinder. The front of the cylinder is a semi-annular scour hole. A "saddle-shaped" dune is formed due to accumulation of sand scoured away from the cylinder.