Multi-layer structure in the Youngsan Estuary, Korea

Repeated hydrographic observations were undertaken to examine the spatial structure and temporal variation of temperature and salinity in the Youngsan Estuary during summer. A multi-layer structure with four water masses was observed. Fresh water was frequently observed at the surface in summer, when a large volume of river water was discharged from the sea dike. The coldest water, which occupied the entire water column due to vertical mixing in spring, was preserved in the middle layer during summer. The bottom layer, which was warmer than the middle layer, became warmer in summer due to an intrusion of warm saline water, along the bottom from the open sea. The surface water was a mixture of discharged fresh water and cold water from the middle layer. Long-term monitoring data showed that the spreading of discharged fresh warm water over the pre-existing surface water and the intrusion of warm saline water along the bottom from the open sea produced a multi-layer structure. Large Richardson number, due to strong stratification and a weak tidal current, suggested that the multi-layer structure remained throughout the summer. The multi-layer structure disappeared in fall due to an inflow of cold water from the river and surface cooling.