In order to clarify the sediment transport mechanism due to a wave, fluid motion over equilibrium and nonequilibrium sand-ripples has been investigated by using both video image processing analysis and numerical analysis. Generally, the velocity fields obtained by the image processing analysis do not satisfy the law of mass conservation, so they are corrected by using the mass-consistent model proposed by C. A. Sherman. The numerical analysis has been made by using the continuity and Reynolds equations. Both Dean's stream function method and one-dimensional analysis for the turbulent boundary layer are used to determine the boundary conditions.
The change in sea bottom topography is really induced by the interaction between fluid and sediment. Sediment transport causes changes of the sea bottom topography, which affect the flow fields, waves and sand motion itself. Such a chain relation is the main property of bottom topography changes in coasts. One of the principal factors in sediment transport is the shear stress by the flow. The shear stress scours the sediments, which will be suspended by the vortex and turbulence, and eventually be carried by wave motion and drift current. This transport mechanism goes on until the wave looses a considerable amount of its mechanical energy due to viscosity, releasing the sand, and thus forming deposits along its way. In addition, sand-ripples form on a shallow sea bed and cause complicated turbulent boundary layer flow. The turbulent boundary layer flow along rippled-bed under waves is characterized by eddy current. When the steepness of sand ripples becomes large, streamlines tend to separate from the crest and establish a vortex-generating regime. The vortices are formed in the leeward side of ripple crests and lift up a large amount of sediment.
