Sedimentary Reorganization at the Day River Mouth (Southern Red River Delta, Vietnam) Following Coastal Infrastructure Development: Evidence from EMMA and Grain-Size Trend Analysis

Grain-size distributions of surface sediments provide indicators of sediment provenance and transport, and their comparison through time can reveal changes in the sedimentary organization at river mouths. This study analyzes 113 surface-sediment samples collected in April 2016 and April 2025 from the Day River mouth, southern Red River Delta, Vietnam, under the same seasonal regime at the end of the northeast monsoon. The datasets were collected before and after the construction of a mainland–Con Noi sandbar sea-crossing road and bridge and the subsequent appearance of a T-shaped spur dike system. Grain-size distributions were jointly analyzed using End-member Modelling Analysis (EMMA) and Grain-Size Trend Analysis (GSTA). Three end-members were consistently resolved in both datasets, explaining 86% and 91% of the total variance: EM1, medium silt (10.1–11.6 μm); EM2, very fine sand (88–101 μm); and EM3, fine sand (133–175 μm). EM1 is concentrated along the river channel, river mouth and northern nearshore zone, whereas EM3 is mainly distributed in the southern to southeastern offshore sector and EM2 occupies transitional zones. GSTA identifies two co-existing trends, FB- associated with finer-grained sediment trends and CB+ associated with the relative redistribution and accumulation of coarser material. Between 2016 and 2025, EM1 displayed a higher variance contribution in the 2025 dataset (43%) than in 2016 (35%), whereas EM3 displayed the opposite tendency (35% versus 45%); EM2 remained broadly similar (20–22%). However, statistical comparisons performed on paired stations and within the common sampling domain did not detect significant differences in the overall proportions of the end-members. In contrast, mapped distributions of the end-members revealed descriptive differences in their spatial organization between surveys. The FB- XOR CB+ representation further highlights a more spatially differentiated pattern in 2025, particularly around Con Noi and the southeastern offshore sector. These results suggest that the principal decadal evolution was expressed as a spatial reorganization of persistent sediment components and associated grain-size trend patterns, rather than as a statistically demonstrable change in their overall proportions or the emergence of a new sedimentary regime. The observed patterns are consistent with the combined influence of the fluvial sediment supply, tidal and wave processes, and southward residual longshore sediment transport. Coastal infrastructure constructed between the two surveys may have contributed to local sedimentary reorganization at the Day River mouth.