Spring-neap and spatial variability of horizontal density gradient in Sumjin River Estuary, Korea

The vertical profiles of velocity are mainly controlled by the estuarine circulation, which depends critically on the spring-neap variations of horizontal density gradients (dρ/dx). Therefore the spatial and temporal variation of horizontal density gradient is very important to understand the physical dynamics of an estuary.

The longitudinal salinity and temperature data were obtained using a CTD profiler at high and low waters from 2004 to 2007 and at middle ebb and flood tides in 2013, 2015 during spring and neap tides covering four distinct seasons. The observation took one and half hour to complete the whole transects. This long-term salinity and temperature data were used to find out the variations of horizontal density gradient in Sumjin river estuary. Depth mean salinity was used to calculate and understand the temporal and spatial variation of the horizontal density gradient along the estuary. It is found in middle estuary that the horizontal density gradient is generally larger during spring tide than that during neap tide. In contrast, the horizontal density gradient was found to be greater during neap tide than during spring tide in estuary mouth and head. This middle region is distinguished from estuary mouth and head according to horizontal density gradient and flushing rate. This study indicates that the horizontal density gradient varies with location in estuary and the difference of horizontal density gradient between spring and neap tide is maximum in middle estuary.