SCALE EFFECTS IN RUBBLE-MOUND BREAKWATERS

Abstract

In conducting model tests of wave transmission through permeable rubble-mound breakwaters, it is impossible to satisfy simultaneously the Froude and Reynolds criteria for dynamic similarity. The common practice has been to scale the wave parameters and breakwater dimensions in accordance with the Froude Number, and to use large models. This study represents an attempt to develop theoretical expressions for the coefficients of reflection and transmission as functions of the effective porosity of the breakwater structure, as influenced by the Reynolds-dependent boundary layer growth an the pores. These expressions use linear wave theory and boundary layer theory to estimate the effective decrease in pore diameter due to growth of the displacement boundary layer thickness in the pore. The theoretical expressions were compared with experimental results from a series of three model tests with breakwaters having vertical faces and using gravel with diameters of 1.37 in., 0.762 in., and 0.324 in. respectively. The prototype to model ratios (using the largest model as the prototype) were 1/1.80 and 1/4.23 respectively. The experimental results show clearly the existence of scale effects in both coefficients of reflection and transmission. The theoretical expressions were found to overestimate the scale effect in reflection and to underestimate it in transmission.