Stability relations of rip-rap revisited

Practically, all stability relations in use for the design of slopes with rubble or rip-rap protections are of an empirical nature. Based on experimental data, they have been obtained by curve fitting. One of their limitations therefore is that they can only be used within the range of the experiments executed. Two widely used stability relations are those by Hudson and Van der Meer. Iribarren started already 60 years ago with a model based on a, although simple, physical concept. Hudson tried to establish proportionality constants for this concept with tests in a wave flume with regular waves, but did not completely succeed. He finally came with the well-known equation that can be used for preliminary design of breakwaters. For gentle and very steep slopes, the influence of the slope angle is not represented correctly, due to the empirical nature of the equation. The same is true for revetments where the Hudson equation underestimates the wave attack. Moreover, the Hudson equation does not take into account the wave period and some other parameters that influence the stability. So, although the results of Van der Meer's relation are quite acceptable for practical use, it is rather unsatisfactory that, despite spectacular breakthroughs in fluid dynamics and hydraulic engineering, we still do not have a stability relation based on insight into the physical processes. The search for a stability relation with a more sound physical basis is therefore still justified, e.g. for geometries differing from simple slopes, and also from an educational point of view. Two completely different pilot projects at Delft University of Technology indicate that efforts to establish such a relation are promising.