A Revision of the Scaling Method for Core Material in Rubble-mound Breakwaters

When constructing a physical model of permeable coastal structures at reduced scale (1:λ), the problem of an incorrect representation of the laminar, viscous flow resistance needs to be addressed. For the case of a rubble-mound breakwater, a common method is to scale the finer materials in the core and possibly the underlayer(s) with a distorted scale factor λ^c < λ. The key in finding λ^c is the characterization of the porous flow field in the breakwater core. Since the flow field varies both in space and time, a practical method has been proposed (Burcharth et al., 1999), making use of a simplified pore pressure calculation model to predict wave-induced pore pressures and derive porous flow velocities from these pressures. The pore pressure calculation method in question has been critically re-examined, revealing a reduced accuracy when applied in broadly-varying wave conditions. To overcome these shortcomings, an improved pressure calculation model has been derived (Vanneste and Troch, 2012), based on a more extensive set of large-scale pore pressure measurements and a more detailed theoretical formulation. In the following, the revised core scaling procedure based on the new pressure calculation model is demonstrated on the case of the Zeebrugge breakwater (Belgium). A comparison with the existing procedure is given, as well as a critical discussion on the choice of hydraulic resistance parameters of the core material. The latter seem to have the largest impact on the outcome of the scaling procedure, since the similarity in the flow field holds for pressure gradients, and not the pressure level itself.