Effect of Freshwater Discharge From Estuary Dam on Residual Circulation in Bifurcated Channel

Abstract

The residual structure of the inner part of the Yeongsan River estuary has been reported to be a multi‐layer vertical structure, unlike the two‐layer vertical structure of a typical estuary, due to irregular freshwater discharge. The inner estuary has a main channel connected to the open sea, and a bifurcated channel system (sub‐channel) in its central part, which can affect the residual current. In this study, field observations of three‐dimensional velocity and salinity were conducted to understand the discharge effects on the residual current at bifurcated channels. The residual flows were estimated using least‐squares harmonic analysis. The relative contributions of advection acceleration and friction terms to nonlinear effects were evaluated using along‐channel momentum balance. In the absence of freshwater discharge, the flow patterns were dominated by tidal dynamics in the sub‐channel rather than the main channel, whereas most flow patterns during freshwater discharge were dominated by near‐surface strong velocity in the main channel. A multi‐layer vertical residual structure was observed during the non‐discharge period in the main channel, which was caused by the insertion of upstream residual flow from the sub‐channel. Nonlinear effects occurred at the bifurcation point of the two channels. During the non‐discharge period, the advection term was most dominant, whereas the vertical friction term was dominant during the discharge period. Moreover, the magnitude of the horizontal friction term was larger than that of the advection term. Therefore, freshwater discharge can lead to an increase in the friction term, which in turn can markedly affect the residual circulation.