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Abstract: The effects of a giant offshore dyke, Saemangeum, have been of special interest on both scientific and environmental aspects. Without any rival worldwide with regard to the ample scale and estuarine setting, the Saemangeum dyke and its vicinity (Saemangeum Area) should be an unprecedented experimental site for monitoring the consequence of the estuarine developments with dykes. In order to unravel geologic changes caused by the Saemangeum dyke, we collected a number of surface sediment samples and bathymetric profiles from a full coverage of the Saemangeum Area, and compared topography and sediment phase on the inter-annual (2002–2006, post-construction) and decadal (1982–2002, between pre- and post-construction) timescales. The dyke construction induced vigorous erosion along the dyke, especially across a few open gaps between dyke segments. These open gaps led to forming deep erosional troughs and the associated channels to the river mouth. At the same time, mild accumulation of sediments occurred on the broad seafloor offshore the erosional zone of dyke sector IV and on shoals between the erosional troughs off dyke sectors I–II. Inside the dyke, tidal flats have become expanded and aggraded with continuously discharged riverine sediments. Sands have been newly introduced into the offshore area of sector IV owing to tidal currents with an orientation forced to be parallel to sector IV (NE–SW), reinforced by high waves during winter. By contrast, the expanded sandy area off sectors I–II resulted from redistribution of pre-existing sands without any new input from offshore. As a whole, the concentrated erosion and sandy-seafloor expansion offshore represent the man-made influences triggered by the Saemangeum dyke. [Copyright &y& Elsevier]

Abstract: The Saemangeum dyke is an enormous offshore structure 33 km long that dams the combined estuary of two rivers on the mid west coast of Korea. Among the numerous environmental issues associated with the dyke construction, it has been of special interest whether the tidal flats in the most reentrant area, Daehangri, just outside the beginning part of the dyke will actively grow or not. This lies in the center of the arguments about positive/negative functions of the dyke. Field measurements, concentrated on hydrodynamics within the bottom boundary layer, and sediment-transport evaluations were conducted in the coastal areas around the Daehangri tidal flats to examine sediment transport and thus source–sink relationships of sediments therein. Deployments of autonomous benthic instruments, TISDOS, during 1–3 weeks indicate that the nearshore sands and muds are transported toward the dyke, whereas sands on tidal flats and beaches are transported from the dyke along the shoreline. Most of the nearshore sediments eventually deposit to form a shallow shoal near the Garyuk opening located in the opposite direction to the Daehangri tidal flats. The rates of sediment transport increase greatly by high waves as compared to those caused by even spring peak tidal currents alone. As a result, the scheme of sediment transport over the study area clearly rules out the Daehangri tidal flats from the newly forming, active sinks of the nearshore sediments triggered artificially by the construction of the Saemangeum dyke. [Copyright &y& Elsevier]