WAVE-INDUCED ADVECTIVE TRANSPORT BELOW A RIPPLED WATER-SEDIMENT INTERFACE

The exchange between the water column and the sediment bed and the transport inside the permeable sediment layer are important processes in the cycles of chemical elements. In this paper we examine quantitatively the effects of modulations in the profile of the water-sediment interface on this exchange and on the advective transport inside the sediment bed. The flow field inside a sediment layer bounded between spatially periodic ripples on top and an impermeable bottom surface is modeled using Darcy's law. The forcing is due to progressive gravity waves in the water above. The results of two different models for the pressure variation imposed on the upper boundary are compared. The two pressure profiles are derived from potential flow theory and from a numerical solution to the Navier-Stokes equations for the oscillatory flow over a rippled bed. From an analytic solution to the two-dimensional model, the trajectories of pore water particles immediately below the ripple profile are found to be quite different from the simple elliptical pattern found below a flat bed. The shapes of these trajectories can be quite complicated and vary considerably both along the length of the ripple and over the depth of the sediment layer close to its surface. The total exchange across the water-sediment interface, averaged over one wave period, is significantly higher across a rippled interface than across a flat bed. This difference increases with increasing ripple slope and the strength of the wave motion, and it decreases with increasing thickness of the sediment layer relative to the length of the gravity wave. Since rippled bed forms are commonly found in coastal waters, the increase in the total exchange across a rippled water-sediment boundary can enhance the exchange of solutes due to "wave pumping." Immediately below the water-sediment interface, circulation cells with net advective transport over a wave period are found. Such net advection patterns can lead to spatial (in the horizontal direction) inhomogeneities of the vertical concentration (or temperature) profiles if the overlying water column and/or the sediment bed act(s) as source or sink. This gives a plausible physical mechanism to explain the spatial variations in vertical concentration profiles found in field measurements.