Comparison of Two-Dimensional and Three-Dimensional Beach Profiles

Scale effects are always a major concern in mobile-bed hydraulic-model studies. These scale effects can be minimized by using large models (i.e., almost prototype size), but large models require large and costly modeling facilities. Several large “super flumes” have been built for two-dimensional (2D) testing, but right now a comparable large “super basin” does not exist. Thus, beach processes can be tested in large 2D models, but not yet in three-dimensional (3D) models. The research in super flumes assumes that 2D-wave-flume experiments are representative of the 3D prototypes. Two aspects of that assumption are tested in this paper. Six sets of parallel tests were performed in both a 3D wave basin and a 2D wave flume. It was found that the time scales for morphological development are adequately modeled in the 2D flume, and that there are some differences in the resulting beach profile shapes. The 3D profiles correspond closely with the prototype observations; the 2D profiles are affected by exaggerated turbulence generated when the incoming wave and the downrush of the previous waves meet head-on. The use of the term equilibrium for fluid flow, sediment flow, and profile formation is also discussed in this paper.