This paper studies flow resistance characteristics of corrugated circular drainage pipes through experimental measurements and numerical calculations under turbulent uniform open-channel flow conditions. The Manning roughness coefficient n is determined for corrugated pipes where semicircular ribs are placed on the inner surface of the pipe periodically with different height-to-pitch ratios. Numerical simulations are conducted for circular pipes having diameters of 0.6, 0.8, 1.2, 1.6, and 2.2 m and a slope of 7% with discharges varying between 0.1 and 2.5 m(3)/s. Results of the numerical calculations are validated in terms of Manning's n by full-scale experiments. In this way, experiments are conducted for smooth and corrugated pipes of 0.8 m diameter, where two distinct inner corrugation profiles and various flow rates are employed. It is found that the Manning roughness coefficient can be increased significantly, which is a favorable outcome with regard to reducing the abrasive effect of water by lowering its velocity to more plausible values for steep slopes in an underground pipe network. Considering the numerical results, a new correlation between Manning's n and the height-to-pitch ratio of corrugation is proposed. (C) 2017 American Society of Civil Engineers.