The bar at the Monobe River mouth composed of gravel and cobbles narrows the width at the outlet to the sea. The July 2018 flood greatly enlarged the opening width of the river mouth and formed a spit-like depositional landforms at the coastal area. The sediment transport of gravel particles and cobbles are essentially non-equilibrium phenomena due to the influence of the unevenness of bed surface configurations and the local bed gradients. This paper proposes an analysis method that can

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... rate three-dimensional flood currents and non-equilibrium motion of gravel particles and cobbles by improving the bed variation analysis method for stony-bed rivers and combining it with the Q3D-FEBS. Moreover, it is shown that the proposed analysis method can explain the opening and deformation mechanism of the river mouth bar than the conventional analysis method based on the equilibrium bed load formula.