Along-channel Variation of Sediment Flux Mechanisms in a Shallow, Macrotidal Estuary with an Estuarine Dam: Geum Estuary, Korea

During the Holocene, deglaciation and marine transgression drowned river valleys and formed estuaries. During the Anthropocene, humans have altered the forcing and geometry of estuaries. Human impacts, such as the construction of estuarine dams, can have consequences to the sediment fluxes and depositional rates in estuaries, but these consequences are not well understood. To better understand the sediment fluxes and depositional rates in an Anthropocene estuary, observations were collected in the shallow, macrotidal Geum Estuary, Korea, where an estuarine dam constructed during 1983-1988 and which began operation in 1994 is known to have resulted in enhanced deposition. Observations were collected over a neap-spring cycle by six Acoustic Doppler Current Profilers (ADCPs) along the 22 km main channel and by one ADCP along the 10 km distributary channel which bifurcates 8 km seaward of the estuarine dam. ADCP backscatter was calibrated with suspended sediment concentration to estimate the suspended sediment flux. The fluxes were decomposed into correlation flux (on the tidal timescale) and mean-flow flux (on a longer timescale) terms. Results showed that the sediment fluxes at all stations were directed landward resulting in deposition. The landward correlation flux due to tidal asymmetry during spring tides was the primary flux mechanism along the main channel. The seaward mean-flow flux during episodic dam discharge was a secondary mechanism which was only important in the inner 8 km section. The sediment fluxes were modified by periodic stratification which occurred in the central estuary, as well as by waves, which were important only during wave events and near the shallow, exposed channel bifurcation. Overall, the evidence suggests that shallow, macrotidal estuaries with relatively small discharge may experience enhanced landward sediment fluxes and depositional rates in response to the construction of an estuarine dam because of the increase in the tidal asymmetry and settling-scour lag mechanisms and the decrease in flushing. From a morphological point of view, the evidence suggests that estuarine dams in the Anthropocene may result in abrupt channel disequilibrium, and in response to the decrease in tidal prism the channel cross-sectional areas and estuarine volume as a whole are reduced.