Abstract: The overbank flow in a compound channel is studied.And the relationship between the friction factor and one dimensional in-bank flow friction factor is established.The dimensionless eddy viscosity is found increasing with the growing friction factor on flood plains.The effect of secondary flow is expressed in the same form as the Reynolds shear stress so that they can be combined into the apparent shear stress and determined through the dimensionless apparent eddy viscosity,which can be calculated by the relationship of the dimensionless apparent eddy viscosity and the friction factor similar to that of the di mensionless eddy viscosity and the friction factor.The symmetric overbank flows with smooth boundary are computed and the results agree with the experimental data.