Effects of an estuarine dam on sediment flux mechanisms in a shallow, macrotidal estuary

Abstract

While estuarine dams have been noted for often resulting in siltation, the flux mechanism and the effect of the dam on the fluxes is not well understood. To gain process-based insight into the spatiotemporal variation and factors that control sediment flux, high resolution in-situ flux measurements were collected at stations 1.5 km and 6.5 km seaward of the Geum estuarine dam, Korea. Fluxes were calculated based on flow collected by Acoustic Doppler Current Profilers (ADCPs) and suspended sediment concentration derived from ADCP acoustic backscatter which was calibrated with water samples. The fluxes were decomposed into components occurring on timescales longer than the tidal cycle (mean-flow flux) and those occurring on the tidal timescale (correlation flux), and ship-borne surveys were conducted to investigate the role of the dam-induced horizontal salinity gradient to stratification and sediment dynamics. Results suggest that the landward correlation fluxes increased because the dam construction amplified the tide and because the correlation flux was proportional to the tidal amplitude, through tidal asymmetry. The dam discharge was found to control the seaward mean-flow fluxes, which were characteristically unsteady and proportional to the discharged volume. Discharge also was found to result in periodic stratification and flocculation asymmetry. The bed level change between the two stations was a balance between the effects of tidal asymmetry and freshwater discharge. For shallow, macrotidal estuaries with small discharge, estuary siltation is expected primarily due to correlation fluxes, and the effect is enhanced when the dam installation amplifies the tides.