Modelling study of human-induced historical changes in sediment provenance and transport in a macro-tidal estuary.

Human-induced changes in the sedimentation process and hydrodynamics were investigated using rare-earth element (REE) signatures in two sediment cores (K1 and K12) and 51 surface sediment samples sediment records, in a medium-scale estuary, the Yalu River Estuary (YRE), China. According to the vertical profiles of concentration and fractionation factors of the REE, the local sedimentary environment experienced dramatic change and subsequently becoming homogeneous around 1975. The binary diagram of REE chemistry in the YRE and discrimination function (DF) between the two cores also demonstrate that the properties of deposits in K1 and K12 became similar to each other after the 1970s. Such deposition change appears to be related to human activity in this area, including blocking water channels and land reclamation around Chouduan Island. A three-dimensional (3-D) coastal model was then established using the Finite Volume Coastal Ocean Model (FVCOM) to explore the changes in hydrodynamics and sediment transport caused by these human activities. Model results show that the tidal choking effects was enhanced in the East Branch after the land reclamation, resulting in a decreased tidal range and stronger tidal currents after 1975 in that area. Both vertical and horizontal mixing processes were strengthened due to the enhanced tidal choking effects. As a consequence, sediments in the East Branch area became more well-mixed and homogeneous after 1975 due to the enhanced mixing process. Landward sediment flux caused by tidal pumping decreased significantly after the land reclamation. Then the FVCOM model was incorporated into a newly developed 3-D Particle Tracking Model (TrackMPD) for the first time to investigate the changes in sediment provenance. Trajectories of released particles showed that sediments in this area was likely from multiple sources before 1970s but turned into a uniform mixture after that. Materials from the Western Korean Bay cannot be transported into the estuary after land reclamation. Conclusions from this study indicate that the anthropogenic influence can dramatically change the sedimentary environment in a medium scale estuary over a short period. Furthermore, this study proposes the utility of using an unstructured coastal model to resolve the marine geology problem.