Scour and flow around pile supported wharf under waves

The design of large marine structures are essential because of their large application fields, such as bridge piers, offshore gravity platforms and pile supported backfilling zones. In order to design these types of marine structures, the flow mechanism around the structures must be well understood and analyzed. Since the flow around the structure will cause an increment of the local sediment transport, eventually it will cause scour formation around the structure. The structures built in a marine environment are exposed to waves, currents, or combination of them. However, in the marine environment waves are generally more dominant than currents. The current induced scour and wave induced scour have different mechanisms; therefore they must be investigated separately. Pile supported reclamation zones can be built as a pile row. Piles are attached to each other or with a gap, which fills with sheet between the pile surfaces. The surcharge and wave loads acting on the structure must be analyzed properly, especially when the erodible bottom condition is valid, since scour can be a great threat to the structural stability. In this study, in order to understand the flow mechanism and scour process around pile-supported wharfs, a series of experiments were conducted in İstanbul Technical University Hydraulics Laboratory. Experiments were held in a regular wave flume, which is 22.5 m in length, 1 m in width and 0.5 m in height. Two twin cylinders, which has 22.5 cm diameter, were used as experimental model. They were flush mounted to the flume wall and flume base. First of all, rigid bed experiments were conducted. Secondly, actual erodible bed experiments were made for different gap ratios. In all the experiments, wave conditions were the same and water depth was 34.5 cm. Pressure measurements, velocity measurements and bottom scanning were made for all the tests in order to get a better understanding of the flow mechanism around the cylinder(s). Pressure measurements were made with five transducers which are located 5 cm above the bed and placed on the surface of the cylinder with 45° intervals. Velocity measurements were made at the undisturbed bed with an Acoustic Doppler Velocimeter (Vectrino). The bottom was scanned before and after the experiments in order to determine the scour and deposition depths around the structure. In all the tests, the maximum scour depth was seen at the incoming wave side, between the flume wall and cylinder. In this zone, vortex formation was observed and it was the cause of scour. However, the scour around the cylinder except this region was independent from the vortex generation. The scour around the cylinder was because of the wave induced steady streaming mechanism. The results of the experiments indicated that, where x is the distance between the cylinders and D is the pile diameter, the most critical case of scour depth was observed in the cases of x/D=1.5 for steady streaming zone. For this test, the maximum scour depth to the cylinder diameter ratio, was S/D=0.09~0.13. The results also showed that there is a relation between the pore pressure and deposition.