Wave-induced internal pumping as a sediment sorting mechanism and its implications for deltaic stratigraphic evolution: Insights from the Yellow River Delta

Abstract

Sediment grain size and stratigraphic evolution in river deltas are critical for understanding shoreline stability and associated engineering risks. The role of wave-induced sediment responses in regulating deltaic grain size distributions and stratigraphic organization remains poorly constrained. This study combines analyses of undisturbed sediment samples from depositional lobes of different ages with in situ wave-loading experiments in the Yellow River Delta to investigate surface grain size patterns and their sorting mechanisms. The results reveal a pronounced “older-coarser, younger-finer” configuration and a transition from cohesive to non-cohesive textures, reflecting the combined influence of external hydrodynamics and human activities. More importantly, periodic wave loading induces a significant internal seabed response, here termed internal pumping, which drives the upward migration of fine particles and produces a short-lived surface inverse fining that cannot be explained by conventional hydrodynamic sorting alone. Based on these observations, a coupled sediment-sorting framework for river deltas is proposed: hydrodynamic sorting, flow-induced shallow infiltration, and deeper internal pumping act together to regulate grain size distributions and deltaic stratigraphic evolution under conditions of reduced fluvial sediment supply and intensified nearshore energy. This mechanistic framework elucidates previously overlooked internal processes in coastal morphodynamics and provides a sedimentological basis for understanding how wave-induced sediment response governs grain size sorting and stratigraphic evolution in deltaic systems.