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residual currents in tidal estuaries and coastal embayments have been recognized as fundamental factors which affect the long-term transport processes. it has been pointed out by previous studies that it is more relevant to use a lagrangian mean velocity than an eulerian mean velocity to determine the movements of water masses. under weakly nonlinear approximation, the parameter k, which is the ratio of the net displacement of a labeled water mass in one tidal cycle to the tidal excursion, is assumed to be small. solutions for tides, tidal current, and residual current have been considered for two-dimensional, barotropic estuaries and coastal seas. particular attention has been paid to the distinction between the lagrangian and eulerian residual currents. when k is small, the first-order lagrangian residual is shown to be the sum of the eulerian residual current and the stokes drift. the lagrangian residual drift velocity or the second-order lagrangian residual current has been shown to be dependent on the phase of tidal current. the lagrangian drift velocity is induced by nonlinear interactions between tides, tidal currents, and the first-order residual currents, and it takes the form of an ellipse on al hodograph plane. several examples are given to further demonstate the unique properties of the lagrangian residual current.