This study focuses on the prediction of technical efficiency of narrow-crested submerged permeable rubble-mound breakwaters, in terms of wave attenuation. A number of existing formulae for estimating wave transmission coefficient for submerged breakwaters can be found in the literature, whereas in this work further improvement for that estimation has been achieved mainly through physical modelling. A series of 2D experiments under scale were conducted for regular and random waves providing data on wave transmission coefficient and respective wave breaking characteristics. A Boussinesq-type wave model capable of simulating wave propagation for regular waves over porous submerged breakwaters was also used in order to provide additional wave transmission information. Data analysis showed that wave breaking mechanism significantly affects wave energy dissipation and, therefore, wave breaking occurrence and type can be directly linked to wave transmission coefficient for a given structure’s geometry and sea state. The result of this work is the proposal of a set of simple semi-empirical equations for predicting wave transmission coefficient over small profile porous submerged breakwaters in relevance to the parameterization of the expected dominant wave breaking mechanism.

Submerged mound breakwaters are viable eco-friendly structures for shore protection, serving as an alternative to their emerged counterparts. Physical modelling is still extensively used to study coastal processes and provide calibration and verification data. The aim of this study is to present a methodology adapted exclusively to submerged structures to tackle the problem of scale effects when determining the size of the core material. Such effects have been adequately studied in emerging mounds, thus forming the rationale for the present work. Hydrodynamic pore pressures inside the underwater body were evaluated using experimental results; these were then input into the extended Forchheimer equation, including the nonlinear effects of velocity upon the drag force induced by the pressure gradient. The experimental results used were satisfactorily compared with other laboratory measurements and with results from two numerical models. The proposed calculation methodology combined with the corresponding hydraulic resistance can yield a correction factor K for model downscaling of the breakwater material. Numerical applications are given along with design nomographs providing the K value. The presented results on the required scale correction to be applied to determine the sizing of the core material in physical models of submerged mound breakwaters deviate significantly from those related to emerging breakwaters, thus justifying the undertaking of the present work.