Geological investigations conducted over a vast seabed offshore the giant Saemangeum Dike on the west coast of Korea indicate that the surface area of fine to very fine sands has rapidly expanded southward in recent years (2002–2008). To unravel the processes responsible for this phenomenon, two field campaigns of hydrodynamic measurements were carried out at the same station with an instrumented tripod during May 2007 and January 2008. The measurements show the definite occurrence of wind-generated residual currents that were more distinctive and consistent during winter compared with the relatively weak, inconsistent analogues of the late-spring season. The simple algebraic calculations for bedload transport based on a cubic relationship between the bedload transport rate and steady currents suggest that the southward movements of surface sands result from interplay of wintertime residual currents and macrotidal currents. In addition, numerical model experiments illuminate that the dike construction has increased the N-S component of tidal currents over the study area. The sediment transport evaluations further suggest that this artificial increase in tidal currents in the N-S direction could considerably enhance southward sand transport.