Decoupling natural and man-made impacts on the morphologic and sedimentologic changes in the tidal flats, Saemangeum area, west coast of Korea: Implications for benthic ecosystem stability.

Abstract The morphodynamics of tidal flats responds mainly to the nonlinear interaction between tides and seasonal wave activity. Man-made activities such as reclamation further complicate the morphodynamics by disturbing physical processes acting on the tidal flats. Decoupling the anthropogenic influence from the natural forcing on the tidal-flat morphodynamics is crucial in assessing the adverse effects of man-made activities. Still, it remains a challenge due to inherent difficulties in characterizing spatiotemporal variability of the tidal-flat morphology. A three-year-long field survey using unmanned aviation vehicle (UAV)-assisted photogrammetry, sedimentological and benthic fauna data was conducted on the Shinsi tidal flats near the Saemangeum dike, west coast of Korea, to evaluate the relative significance between natural and anthropogenic influence on the morphologic changes of the tidal flats and benthic community structures. The Shinsi tidal flats exhibited non-seasonal sedimentation patterns and experienced overall erosion despite their sheltered location from the offshore waves. The Saemangeum dike contributed to the sustained erosion by reflecting offshore waves toward the tidal flats during winter to spring. Heavy rainfalls also promoted erosion of the tidal flats in summer. The small-scale embankments complicated the spatial sedimentation trend by protecting tidal flats from offshore waves or promoting erosion with shoaling waves. Notable mud deposition occurred during winter, incompatible with the general hydrodynamic condition, resulting from intensive dredging activity inside and outside the Saemanguem dike. The proliferation of the opportunistic species followed the mud deposition for several months until the enhanced wave activity removed the muddy sediments from the flats. Overall erosion dominance and the temporary occurrence of opportunistic species imply that the Shinsi tidal flats are subject to non-seasonal changes in sedimentology and morphology due to artificial structures and man-made activity, leading to the instability of the benthic communities in the tidal flats.