Surface current variability in the Keum River Estuary (South Korea) during summer 2002 as observed by high-frequency radar and coastal monitoring buoy

High-frequency (HF) radar observations of surface currents were conducted for 3 months during summer 2002 in the Keum River estuary. A comparison between HF radar-derived currents and directly measured ones form a buoy showed that the regression slope is close to 1 and the correlation coefficient greater than 0.86, with an RMS difference less than 13 cm/s which is less than 17% of the tidal current. This fairly good agreement allows us to use HF radar observation in investigating the surface flow and circulation in this tidal-current-dominant coastal-plume area. To examine the spatial variation in tidal current characteristics, as well as currents associated with non-tidal forcing, the HF radar-derived currents were separated into tidal and sub-tidal frequency currents. The overall pattern of M2-current ellipse distribution in the study area showed a counterclockwise rotation, with the offshore maximum current direction to the northeast. Eccentricity, the direction of maximum current, and the phase of net motion of the ellipse changed near the estuary mouth and near the gap of the Saemangeum reclamation tide dyke due to the complex coastal geometry and the out-flowing jet during the ebb period.The Eulerian mean current field in July was similar to the circulation patterns suggested in the previous studies using the CTD data and numerical models, implying that the region of northwestward flow may have developed by the rivers discharges. The short-term variation in the surface sub-tidal flow field was governed primarily by that of wind forcing and was responsible for the changes in the hydrographic conditions in the surface layer off the Keum River estuary mouth. The spatially averaged current, which is calculated after extracting the Eulerian mean currents from the sub-tidal flow, flowed almost to the right of wind during 22 days for an observer looking down-wind-stream, with the mean angle of 33°, indicating predominant Ekman drift. The sub-tidal current observed at the buoy showed that there is an almost unidirectional, persistent current irrespective of the variable wind forcing, producing strong Eulerian mean jet from the northern dyke gap. This current could be formed due to firstly the residual tidal currents generated by the open edges of the dyke and secondly the offshore discharge of the diluted water through the gap. However, when the daily river discharge increased greatly, the sub-tidal current speed at the buoy increased as much as 21 cm/s under the very strong southerly wind condition. This event is interpreted by relaxation process of the trapped coastal water by wind and the large river discharge.