Roughness Factor of Two Type Wave-Dissipating Blocks Estimated by Overtopping Experiments

Abstract

This study estimated the roughness factor of two type wave-dissipating blocks by wave overtopping experiments to calculate the amount of overtopping discharge based on EurOtop. The relationship between the roughness factor γ_f and surf similarity parameter ξ_m-1,0 was also examined. Experiments were conducted using a two-dimensional wave flume, and test blocks were GRASP and SEA-LOCK. The experimental conditions were set referring to Bruce et al. (2009). The input wave was random waves having JONSWAP spectrum of γ=3.3. The boundary of ξ_m-1,0 was set to 4.0, and γ_f was estimated by the least squares method. The relationship between the overtopping discharge q and the crest height R_C was considered. Comparison of γ_f in cases with different layer thickness showed that the case of γ_f with large layer thickness was smaller than the γ_f with small layer thickness’ case. The results of Rock and Cube in the experiments by Bruce et al. indicated that the γ_f value of two-layers was smaller than that of single-layer, the present study was same tendency as existing results. By comparing γ_f in cases with different structure crest width G_C, it was found the case of G_C=150 mm was γ_f =0.36 of which value was about 8% smaller than the case of G_C =104 mm. Consequently, it is considered that the γ_f value of wide structure crest became smaller than that of narrow crest. Under two-layer thickness condition, the roughness factors γ_f of SEA-LOCK were 15% smaller than the γ_f of GRASP. The installed volume porosity of GRASP is 63.5% and SEA-LOCK is 53%, GRASP is 10.5% larger than SEA-LOCK. In addition to the difference in block shape, it is thought that the difference in the installed volume porosity of the block also affected the overtopping discharge.