Experimental Study on Sandy Seabed Scouring Around Submerged Spur-Dike Under the Action of Wave and Current

Abstract

The scouring of piles, piers, pipelines and spur dike under the action of current and wave is a very important problem in coastal engineering. In this paper, we present physical modeling results in a 50 m long and 15 m wide basin with a submerged spur dike and sandy seabed. The median diameter of sediment on the seabed is 0.275 mm sand and the height of the submerged spur dike is 0.208 m the side slope of dike is 1:2. The physical model tests under the combined action of wave and current are carried out, along with tests under the action of only waves. First, the current velocity and direction of upper, middle and lower layers near spur dike were measured. Then, under different test conditions (water depth, current velocity), the shape and development of the scour hole near the head of the spur dike were measured. ADV was used to measure the current velocity and direction in the scour hole, and the flow velocity in the scour hole was analyzed. The results show that after the construction of the spur dike, the flow velocity at the tip of the spur dike increases, and separated shear layer forms downstream from the head of spur dike. a series of vortices are generated in the separated shear layer. When the velocity of flow is greater than the critical incipient velocity of sediment, the scour hole firstly formed at the position where vortices appear. The scour depth and scope of the scour hole near the spur dike head increase with time and gradually reach maximum. When the test water level is 0.245m, the average velocity of upstream flow is 0.32 m/s and the total scour time of wave and current is 20 hours, the maximum depth of the scour hole near the spur dike head reaches 0.21 m. The results also show that the current structure of the vortices in the scour hole is affected by the wave, the maximum depth of the scour hole under the action of current only is deeper than that under the combined action of wave and current in the same scour time. Finally, the concrete block soft mattress is used to protect the seabed near the spur dike, and the scours around the protected area with different protection strategies are compared, the results show that the scour depth of the seabed around the protected area outside the head of spur dike obviously decreases when the total width of the apron protection reaches 5.0m.