Sedimentary Impacts of Anthropogenic Alterations on the Yeongsan Estuary, South Korea

초록

Over the last half-century, coastal zones within the Republic of Korea (S. Korea) have experienced a wide range of engineered coastal modifications including construction of seawalls, extensive land reclamation, and installation of estuarine dams. The Yeongsan Estuary has experienced all of these modifications and provides an ideal case study on how sedimentation changes within a macrotidal estuary in response to these alterations. Combined, these alterations have considerably modulated the timing and intensity of river discharge, prevented natural tidal exchange, modified the shoreline profile, and altered the transport of sediment and organic matter within the coastal zone. These impacts have been investigated using 30 gravity cores analyzed for 210Pb radioisotope geochronology, laser diffraction particle size analyses, δ13C and δ15N isotope ratio mass spectrometry, and X-radiography. Average sediment accumulation rates range from 0.9 ± 0.6 cm yr-1 to 10.0 ± 2.9 cm yr-1, with the highest rates proximal to the downstream side of the dam, and some areas determined to be either actively eroding or recently dredged. These results are supported by comparison of multiple bathymetric surveys, and CHIRP seismic data suggest an order of magnitude increase from average Holocene sediment accumulation rates. Side scan sonagrams collected adjacent to the dam reveal distinctive scouring, transitioning to areas accumulating fine-grained sediments. Shifts in the organic matter source inputs are apparent in pre/post-dam sediments and reflect the occluding of tidal influence above the dam, resulting in increasingly terrestrial dominated signatures. Additionally, a time series of cores collected during periods of limited and high discharge analyzed for 7Be, indicates sediment deposition occurs episodically corresponding to high discharge dam releases. Our observations record a shift in depositional environments as a response to an extensive array of anthropogenic alterations.