New insight on grain-size distribution changes during the net offshore migration process of an oblique sand bar

Offshore oblique bars typically follow a net offshore migration (NOM) pattern. However, this kinematic view overlooks how migration relates to sand-grain dynamics, as revealed by grain-size distribution (GSD). In the Gulf of Lions (France), slightly oblique bars identified by LIDAR were analyzed through detailed GSD studies. Formed under southward longshore transport, these sediments show mixed grain populations, deconvoluted using the LNDOGS program. High skewness highlights multimodal distributions. Among the four sedimentary types (STs), ST1 and ST2 are most associated with seaward migration, with ST2 evolving from ST1 via minor variants (SVs). As bars merge with the lower shoreface (LSh) and disappear, GSD distinctions fade, shifting toward unimodal, symmetrical patterns under shoaling influence. The apparent paradox—regional southward transport versus northward bar migration—is explained by the interplay of resuspension, sorting, offshore export, and downdrift transport (RSOED). Together, these processes couple sediment pathways and morphology. This innovative GSD-based approach yields a hypothetical qualitative model based on empirical knowledge of processes not measured in the field that links bar form and composition, offering new insight into mechanisms of net offshore bar migration. • Deconvolution analysis of multimodal sand grain size distributions highlights skewness as overarching. • The sands of the oblique bars are mixtures of components that interact during grain size changes. • A combined RSOED (resuspension, sorting, offshore export, downdrift) mechanism explains grain-size changes. • RSOED addresses the apparent antinomy of updrift mobility of oblique bars and downdrift movement of constituent sands.