Mechanisms for sediment convergence in altered microtidal Nakdong Estuary, South Korea

초록

Previous sedimentological work has shown that an order of magnitude higher sedimentation (2-6 cm/yr) than sea-level rise resulted in, due to the emplacement of estuarine dams and land reclamation, at altered macrotidal Nakdong estuary of Korea [Williams et al., 2013]. To investigate the timing and rate of sedimentation, two autonomous sonar altimeters were deployed nearly a year. In the west Nakdong where no freshwater discharge occurs, the sedimentation was gradual up to 4 cm over a year which corresponds to long-term sedimentation. On the other hand, the episodic sedimentation up to 6 cm, which also corresponds to the long-term sedimentation rate, occurred in two months during the summer in the main Eulsuk channel where high energy discharge was allowed through the estuarine dam. To examine mechanisms of rapid sedimentation in the main Eulsuk channel, current velocity profiles were measured by Acoustic Doppler Current Profilers (ADCP) at three sites during increased discharge and wave activity events, and through spring and neap tidal cycles. Suspended sediment concentrations were obtained by converting ADCP backscatter profiles. These observations were decomposed into tidal pumping and advection terms to determine the primary factors controlling suspended sediment flux throughout the estuary. Inherently, results reveal that the highest sediment fluxes occur by mean advection during large discharge events. However, interestingly the cumulative sediment flux at the mouth of the estuary is of lower magnitude than adjacent to the dam, resulting in deposition of large volumes of sediment within the estuary. Suspension of sediments by waves and tidal currents also cause a flux into the estuary in the absence of strong discharge, which suggests the main mechanism for the gradual deposition, in the absence of freshwater discharge, in the west Nakdong.