Swash infiltration‐exfiltration and sediment transport

Abstract

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.

Swash‐aquifer interaction in the vicinity of the water table exit point on a sandy beach

“…The present data indicates that the rate of decrease is directly related to the distance between the shoreline and the exit point; the greater the distance the faster the rate of decline. Future modeling of swash-aquifer interactions in the vicinity and landward of the exit point should therefore include the horizontal shore-normal dimension and not be limited to simplified 1D vertical applications [e.g., Turner and Masselink, 1998;Baldock et al, 2001].…”

“…Figure 4 (bottom) shows the corresponding pore pressure response at two different elevations in terms of a pressure head in meters relative to the sand surface (h = 0). The head at the two elevations is seen to be quite similar and considering that a significant body of work on swash-induced pore pressure gradients already exists in the literature [e.g., Turner and Nielsen, 1997;Horn et al, 1998;Turner and Masselink, 1998;Baldock et al, 2001;Butt et al, 2001], the following discussions will focus on the general trends seen in the pore pressure response rather than on hydraulic gradients.…”

“…[9] Simple 1D vertical diffusion type groundwater flow models have been shown to adequately predict swashinduced pore pressure fluctuations seaward of the exit point within the seepage face [Turner and Masselink, 1998;Baldock et al, 2001]. However, as Baldock et al [2001] pointed out, their model was unable to predict the pore pressure fluctuations observed at a point frequently transgressed by the exit point.…”

On the role of infiltration and exfiltration in swash zone boundary layer dynamics

“…In addition, the local mass and momentum transfer at the sediment-water interface of the beach varies on swash-by-swash and tidal cycle time scales [Li et al, 1997;Turner and Nielsen, 1997]. These variations lead to infiltration/exfiltration effects and groundwater fluctuations that may be important for sediment mobility [Turner and Masselink, 1998;Butt et al, 2001;Nielsen et al, 2001]. Thus, determining the effects of bed sediment characteristics (i.e., permeability and porosity) on boundary layer evolution is an important step toward improved understanding and predictive capability of swash-zone sediment transport.…”

“…Earlier works [Nielsen, 1992;Conley and Inman, 1994;Turner and Nielsen, 1997;Turner and Masselink, 1998;Hughes and Turner, 1999;Butt et al, 2001] related the behavior of the streamlines of the flow with the infiltration/exfiltration of water into/from ventilated natural beds. From these and other studies [e.g., Packwood and Peregrine, 1979;Li et al, 2000;Nielsen et al, 2001;Karambas, 2003] it is established that one of the main effects of the infiltrating/exfiltrating flows relates to the mass and momentum transfer that result from these motions, affecting the magnitudes of the bed shear stress and the BL thickness.…”