Sediment coarsening in the Yellow River subaqueous delta: Regional patterns, causes, and implications

Sediment coarsening in submerged deltas is commonly attributed to seabed erosion because of insufficient sediment input. The Yellow River subaqueous delta (YRSD) has exhibited distinct coarsening patterns following both accretion and erosion events. To investigate these contrasting mechanisms, grain size distributions, elevation changes, and bottom shear stress patterns were analyzed across the delta from 1992 to 2022. The results revealed distinct sedimentary patterns among the abandoned YRSD, active YRSD, southern Laizhou Bay, and adjacent Bohai Sea, with average median grain size (D50) increases of 17, 17, 6, and 0 μm, respectively. Sediment coarsening occurred primarily from 1992 to 2000, when the river mouth position was artificially altered and fluvial sediment grain size increased from 16 to 29 μm. From 1992 to 2015, the active YRSD experienced accretion at a rate of 7.8 mm/yr. Moreover, the abandoned YRSD and southern Laizhou Bay experienced significant erosion. The erosion rates were −5.1 and −1.0 mm/yr, respectively. This led to the identification of two mechanisms of sediment coarsening: erosion-driven coarsening in sediment-deficient areas and accretion-driven coarsening where the input sediment grain size increased. Although marine processes did not intensify during this period, the bottom shear stress distribution changed substantially due to morphological evolution, with correlation coefficients between grain size and shear stress showing increasing trends in littoral zones. This strengthening relationship, coupled with the declining fluvial sediment load, demonstrates the YRSD transition from river-dominated to wave-dominated processes, providing important insight into delta evolution under changing sediment regimes. The insights gained can guide Yellow River Delta management through targeted strategies and provide essential evidence for predicting delta evolution.