Understanding the Role of Sharp Edges in the Propagation of Surface Gravity Waves

Abstract

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, λ/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.