Oscillatory Ripple Geometry

Abstract

Ripple geometry provides a bed roughness which governs the bed shear stress and the profile of eddy viscosity or diffusion coefficient required for the description of the sediment concentration profile or rate of sediment transport. An analysis is made of ripple geometries using known tabulated laboratory and field data as well as water tunnel data obtained by the writer. Three different types of ripples can be identified: sand, light sediment, and oscillating bed ripples. They reflect different effects of the fluid shear stress and fluid acceleration on bed sediments. It is also shown that, when the bed shear stress or the amplitude of the water particle motion relative to the sediment size is small, the resulting vortices are more regular with low turbulence intensity, and the generated ripples are two dimensional; they are called growing or vortex ripples, which are strongly influenced by the amplitude of the water particle motion. At the other extreme, when the bed shear stress or the amplitude of the water particle motion relative to the sediment size is large, the resulting vortices are very irregular with high turbulence intensity, and the generated ripples are three dimensional; they are called decaying or turbulent ripples, which correlate well with the relative magnitude of the fluid shear stress and fluid acceleration.