Hydraulic experiments were conducted to clarify a changing process of bar shape formed during flood at usual time. Double-row bar-like shape appeared and reaching a meandering style channel was observed. It became clear that the component which has wavenumber 1 and 0 in the transverse and longitudinal directions respectively with formation of a meandering style channel increases as a result of the wave number analysis of bed forms. Furthermore, although it was examination near the limit of theory, weak nonlinear analysis considering the interference between (1, 1) and (2, 2) components was carried out, and the same result as an experiment result was obtained.