The actual fluid flow in the waterways is in contrast to the ideal fluid flow with the creation of shear stresses and on the one hand the amount of these stresses depends on the physical properties of the fluid and the flow behavior and on the other hand depends on the physical characteristics of the waterway, So, these characteristics affect the environmental resistance against the fluid passing through the environment. The exact measurement of the frictional force of the source of the shear stresses is not feasible, and this has led to many investigators to approximate estimate the resistance level. As a result, investigations were applied to optimize the coefficients in transactions governing the flow of open and close channels. Although the experiments showed that the use of uniform flow resistance coefficients for non-uniform flows are usable, But they all confirmed the necessity of modulating the coefficients for non-uniform flows and here, assuming that the being non-uniform in the passing flow on smooth surfaces or interstitial surfaces is hydraulically the formation and growth of the boundary layer, It was attempted to investigate the probable deviation of the manning resistance coefficient in non-uniform flows relative to their amount in uniform flows and The result of the research showed that the resistance coefficients in non-uniform flows were less than uniform flows and the necessity of correction of the resistance coefficients in culverts and dewatering channels.
Keywords: Boundary Layer, Resistance Coefficient, Manning Coefficient, Non-Uniform Flow.