Wave height, setup and currents around a detached breakwater submitted to regular or random wave forcing

Abstract

Wave height, set-up and currents were measured in the laboratory around a detached breakwater erected on a 1 in 50 plane beach and subjected to regular unidirectional waves, random unidirectional waves and directional random waves. A comparison is made between regular and random wave cases which had equal incident wave energy. While few differences are noticed between unidirectional and directional random waves, wave height, setup and current variations are smoother for random waves than for regular waves. For regular wave conditions, the location and the extent of the eddy currents behind the breakwater are strongly constrained by the breaking line location; a steep gradient of the current is observed across it. The circulating flow observed in the lee of the breakwater surrounds a wide eddy centre with almost quiescent fluid. This is interpreted as a result of the significant reduction of eddy diffusivity outside the surf zone. For comparison with numerical modelling results, an extensive investigation of one regular wave case was conducted including determination of the vertical structure of the currents. It is shown that currents inshore from the breakwater display limited variations over depth.