The growth and erosion of alternate bars under unsteady flow conditions is an important topic in river morphodynamic, as alternate bars exhibit regular morphologies and similar sizes within the channel. This study employs a mathematical model to simulate the growth and erosion process of alternate bars under repeated hydrographs. The results reveal that the characteristics and evolution of alternate bars in unsteady flows demonstrates a strong time-dependence. With an appropriate cycle value, the variation of sandbars over time will reach a subtle consistency, but such phenomenon can’t be reached if the cycle is too long or too short. We believe such phenomenon is caused since the response of alternate bars is not strong enough to destroy the original wavelength within a limited cycle, suggesting that the hydrograph cycle that achieves this equilibrium state must be moderately long such that the timescale of the hydrograph is similar but slightly smaller than the bar growth.

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