Two-and Three-Dimensional Laboratory Studies of Wave Breaking, Dissipation, Setup, and Runup on Reefs

Physical model studies were performed at a 1-to-50 model to prototype scale in a three-dimensional (3-D) basin and a two-dimensional (2-D) flume to measure wave transformation, setup, and runup on a generalized reef to improve understanding of the processes.Incident wave and water level conditions encompassed a range of combinations representative of the geographical locations where reefs are present.Results from the 3-D experiment indicated higher frequency wave energy dissipated across the reef, whereas low frequency wave energy remained nearly constant.Three-dimensional patterns were observed on the reef, which was attributed to non-symmetrical bathymetry, wave-wave interactions, wave refraction, and reflection.Additional tests with an angled channel cut into the reef showed net offshore flow in the channel and near the reef centerline, where a net alongshore velocity also was measured.Wave heights were similar to the non-channel tests, but longer-period waves with higher incident heights generally showed greater heights in the shoaling region.A cross-section of the 3-D reef was duplicated and installed in a wave tank.The 2-D experiments were performed for similar wave and water level conditions as in the three-dimensional tests.Reef slopes of 1:2.5 and 1:5 were examined.Transformation results were similar to the 3-D tests except that a strong return flow was observed which increased breaker height.Reef roughness was varied for both the 2-D and 3-D tests, but had little effect on the waves, indicating that a rougher surface would be necessary to reduce wave height.