Proposal of a Wave-Steepness-Dependent Effective Correction Factor for the EurOtop Overtopping Formula for Tetrapod-Armored Rubble-Mound Structures

This study investigated the applicability of the EurOtop mean wave overtopping formula using the results of 270 hydraulic model tests conducted on rubble-mound structures with two-layer regular tetrapod armor and crown structures of vertical and return-wall types. Since the roughness reduction factor for tetrapods in EurOtop was derived under specific armor placement and structural conditions, it was used in this study as a reference value for comparison. Constant effective correction factors were first determined separately for vertical-and return-wall-type crown structures to account for the mean prediction bias, and the relationship between the residual prediction errors and wave steepness was subsequently analyzed. Effective correction factors were also back-calculated for individual experimental conditions, and the predictive performance of exponential, inverse, and linear correction functions expressed in terms of wave steepness was compared. The results showed that the nonlinear correction functions improved the prediction accuracy of mean wave overtopping discharge compared with the constant correction factor. The proposed factor should be interpreted not as an intrinsic roughness factor of the armor layer, but as an empirical effective overtopping correction factor incorporating the effects of armor placement, crown-wall configuration, and wave conditions. Its applicability should therefore be limited to the range of experimental conditions examined in this study.