Laboratory study of wave transformation and wave-induced setup over reef islands under the effect of tidal current

To study the characteristics of wave motions over reef islands under the effect of tidal current, laboratory experiments were performed in a wave-current flume to investigate the wave transformation and wave-induced setup in the presence of tidal current. A series of tidal flowrates were tested, and three flow regimes (shoreward, seaward and no-flow) were compared. Results show that the seaward flow moves the breaking point to the offshore side while the shoreward flow moves it to the coastal side. The turbulence in the surf zone associated with the seaward flow is stronger than that with the shoreward flow. Incident wave energy around the breaking point transferred from primary waves to their higher harmonics, and this type of transfer is more evident with the seaward flow. The tidal current has insignificant effect on wave reflection, transmission and energy dissipation. The shoreward flow decreased the wave-induced setup on the reef flat while the seaward flow increased it, and there is strong linear relationship between the maximum wave setup and the tidal flowrate. This study may provide theoretical guidance for construction and maintenance projects around those reef islands.