Oblique Wave Transmission Over Low-Crested Structures

Wave transmission over low-crested structures has often been subject for research, as the wave field behind these structures determines what will happen in this area. Detached low-crested structures are often parallel to the coastline and in most cases wave attack will be perpendicular to this coastline and therefore, perpendicular to the structure. This situation can be simulated by small scale physical modeling in a wave flume. Results have been given by Van der Meer and Daemen (1994) and d'Angremond, van der Meer and de Jong (1996). Recent research, including all data of the above given references and new extensive data sets, has enlarged the insight on the topic, see Briganti et al. (2003). The results from 2D tests are: 1) prediction formulae for the wave transmission coefficient K_t and 2) description of change of spectral shape due to wave transmission. In quite some situations low-crested structures are not parallel to the coast. T-shaped groynes are an example, but also breakwaters for a harbour where only under very extreme storm surge the structure can be considered as low-crested. In these situations wave attack is very often not perpendicular, to. the alignment of the structure and in many situations even quite oblique wave attack and transmission occurs. But what will be the difference with perpendicular attack? More in detail: 1) Are the prediction formulae for K_t still valid? 2) Is the spectral change (more energy to high frequencies) similar to perpendicular wave attack? 3) Is there any influence of short-crestedness of waves? and 4) Are wave directions similar in front of the structure and after transmission? Only a three dimensional investigation in a short-crested wave basin can give answer to these questions. Within the EU-project DELOS these tests have been performed and are the subject of this paper.