https://cirp.usace.army.mil/products/files/doc/Hallermeier1981-ProfileZonation.pdf

A profile zonation for seasonal sand beaches from wave climate

Field measurements of vertical pore-pressure gradients within the bed are used to quantify instantaneous (8 Hz) rates of swash infiltration-exfiltration across the beach face. Cyclic infiltration-exfiltration is associated with individual swash events, with observed vertical flow rates O(10−3) m/s. Rates of net swash-groundwater exchange (i.e., through-bed flow integrated over several swash cycles) are two orders of magnitude smaller. At the timescale of individual swashes, vertical pore-pressure gradients within the beach face are much greater than horizontal pore-pressure gradients. This permits application of the numerical solution of Darcy's law for one-dimensional vertical flow to model fluctuating pore pressures (and hence vertical through-bed flow). Vertical flow through a porous bed modifies sediment mobility in (at least) two ways: (1) Seepage forces change the effective weight of surficial sediments, and (2) boundary layer “thickening” or “thinning” result in altered bed shear stresses. By considering these two (opposing) effects separately, a new Shields parameter is derived that incorporates terms for through-bed flow. Simulation of time-varying seepage force and bed stress effects over an uprush-backwash cycle suggests that the effect of altered bed stresses dominates over the change in effective weight and that infiltration-exfiltration effects are most important during uprush. Simulated transport rates are increased by up to 40% of the peak transport rate during uprush and reduced by 10% during backrush. In summary, swash infiltration-exfiltration across a saturated beach face enhances the net upslope transport of sediment.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98JC02606

Swash infiltration-exfiltration and sediment transport

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.

Wave-induced advective transport below a rippled water-sediment interface

Boundary layers arising from flows which oscillate parallel to a permeable bed, and are subject to oscillating percolation of the same frequency as the bed parallel flow, referred to here as ‘ventilated oscillatory boundary layers’, are the subject of this laboratory study. These boundary layers are intended to approximate naturally occurring wave boundary layers over permeable beds. Measurements of boundary-layer velocities, bed stress and turbulent flow properties are presented. It is observed that suction (flow into the bed) enhances the near-bed velocities and bed stress while injection (flow out of the bed) leads to a reduction in these quantities. As the ventilated oscillatory boundary layer experiences both these phenomenon in one full cycle, the result is a net stress and a net boundary-layer velocity in an otherwise symmetric flow. While production of turbulence attributable to injection is enhanced, the finite time required for this to occur leads to a greater vertically averaged turbulence in the suction half-cycle. Turbulence generated in the suction half-cycle is maintained in a compact layer much closer to the bed. These effects appear to hold for ranging from 105 to 106 and for oscillations other than sinusoidal.

Ventilated oscillatory boundary layers

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S002211209400193X

Karl E. B. Lofquist

Papers

An effect of permeability on sand transport by waves