Swash infiltration-exfiltration and sediment transport

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.