Alongshore fluid motions in the swash zone of a sandy and gravel beach

Abstract

Field experiments were conducted on a low-gradient, high-energy sandy beach (Truc Vert, France) and a steep, low-energy gravel beach (Slapton, UK) to examine alongshore-directed currents within the swash zone. At Truc Vert, data were collected over 33 tidal cycles with offshore significant wave heights of 1–4 m and periods of 5–12 s. At Slapton data were collected during 12 tides with wave heights of 0.3–1 m and periods of 4–9 s. The swash motion was predominantly at infragravity frequencies at Truc Vert and incident frequencies at Slapton. During selected tides, strong alongshore currents of the same order of magnitude as the cross-shore flows were a consistent feature of both swash zones; however, while the cross-shore flows were saw-toothed with a distinct flow reversal, the alongshore flows were unidirectional and continuous. On the sand beach, alongshore momentum was advected into the swash from the surf zone by standing long-wave motion resulting in the strongest flows in the lower swash. In contrast on the gravel beach, highly oblique uprush directly forced the longshore-directed motions and resulted in strong flow velocities in the upper swash. On the sandy beach, the cross- and longshore boundary layers were consistently different with strong vertical shear during the uprush for the cross-shore and during the backwash for the alongshore flows. A logarithmic profile better fitted the longshore flows during the backwash, compared to during the uprush for the cross-shore flows, and enabled the drag coefficient to be computed. The alongshore drag coefficient was C d, v = 10 − 3 and of the same order as for the cross-shore, but was generally larger during the backwash compared to the uprush. Sediment transport efficiency during the backwash is potentially enhanced by longshore-directed swash flows. Significant bed shear stress is maintained throughout that cross-shore flow reversal enabling down-slope transport earlier in the swash cycle.