Ultra-rough oceanic surfaces, such as oyster reefs, are characterized by densely-packed, sharp-edged roughness elements that induce high frictional resistance on the ambient flows. To effectively employ, for example, oyster reefs as a nature-based solution in coastal protection, a detailed understanding of the frictional wave energy dissipation processes is necessary. This work reports on an experimental study in which six surrogates of very to ultra-rough oceanic bed surfaces were subjected to regular waves. The influences of different sharpness' of roughness elements (bluntly-shaped, sharp-edged, and a combination thereof) and relative spacing between elements compared to the near-bed horizontal excursion amplitude, lambda/ab, on the wave attenuation have been investigated. Turbulence is 2-27 times larger for sharp-edged surfaces and 1 to 18 times larger for mix surfaces than those of bluntly-shaped surfaces. Maximum bed shear stresses, hydraulic roughness lengths, and wave friction factors are likewise significantly larger for sharp-edged compared to bluntly-shaped surfaces. These observations indicate that the sharp edges are crucial for frictional energy dissipation. Comparing the maximum bed shear stresses determined from wave height reductions to those determined from velocity measurements indicates that in addition to turbulent kinetic energy (TKE), periodic form-induced stresses significantly contribute to the overall bed shear stresses. This study provides new insight into the frictional dissipation processes of oscillating flows encountering ultra-rough surfaces. Oyster reefs and other ultra-rough bed surfaces near a shore significantly reduce wave heights of passing waves. Integrated into a nature-based coastal protection system, they can reduce the requirement for artificial structures (e.g., seawalls and breakwaters). However, the processes causing the wave height reductions have not been comprehensively investigated. Oyster reefs have ultra-rough surfaces, with edges so sharp they can cut rubber boots. As a model of those surfaces, we investigated the influence of different shapes of elements (sharp, blunt, and a combination thereof) on wave height reductions to address this feature of ultra-rough surfaces. We found that the sharp-edged elements cause significantly stronger turbulence in the surrounding flow, which leads to more substantial wave height reductions. We also found that the spacing between the elements in relation to the wave length influences the wave height reduction. Furthermore, we compared two methods of estimating the shear stress near the bed and found similar trends but different magnitudes of the results for the sharp-edged surfaces. The results improve the understanding of underlying processes of wave height reductions caused by ultra-rough bed surfaces. It is suggested to consider the bed roughness more prominently when designing oyster reefs as a coastal protection measure. Physical modeling is used to investigate the influence of the sharpness of ultra-rough surfaces on wave energy dissipation Sharp-edged roughness elements induce stronger turbulence production rates and wave height reductions compared to bluntly-shaped elements Both turbulent and wake kinetic energy are necessary for an accurate estimation of bed shear stress for ultra-rough surfaces
