This study presents the observation result of vertical velocity profiles found in a small flood of the Mukawa River in Hokkaido in August 1996. The result shows that the velocity profiles in the rising stage of the flood deviate from the logarithmic law and have a steep velocity gradient; the profiles in the peak stage of the flood follow the log-law; the profiles in the falling stage again increase the velocity gradient.
The characteristic property was analyzed together with experimental unsteady flow data, by using 2-D momentum equation with the parabolic type eddy viscosity coefficient. The theoretical velocity profile derived from the equation, which has a form of Legendre differential equation, well agreed with the observation data. From the fact, it was concluded that the deviation of the velocity profile of the flood from the log-law depends on the effect of flow acceleration of the flood.