The influence of swash infiltration–exfiltration on beach face sediment transport: onshore or offshore?

Abstract Measurements were obtained from the swash-zone of a high-energy macrotidal dissipative beach of pore-pressure at four levels below the bed, and cross-shore velocity at a single height above the bed. Time-series from relatively high (Hs≈2.0 m) energy conditions were chosen for analysis from the mid-swash-zone at high tide. Calculation of upwards-directed pore-pressure gradients shows that, in this case, fluidisation of the upper layer of sediment, leading to enhanced backwash transport, is unlikely. An apparent conflict exists in the literature regarding the net effect of infiltration–exfiltration on the sediment transport, through the combined effects of stabilisation–destabilisation and boundary layer modification. This is examined for the data collected using a modified Shields parameter. Inferred instantaneous transport rates integrated over a single swash cycle show a decrease in uprush transport of 10.5% and an increase in backwash transport of 4.5%. Sensitivity tests suggest that there is a critical grain size at which the influence of infiltration–exfiltration changes from offshore to onshore. The exact value of this grain size is highly sensitive to the method used to estimate the friction factor.