Flow resistance in fluvial open channels,especially in steep gravel-bed channels,still presents challenges to researchers and engineers.This article presents some new data from both the flume experiments and field measurements.Data analysis using the divided hydraulic radius approach shows that the relative roughness plays a significant role in the bed form resistance.A new set of formulas that incorporate the relative roughness are proposed.As compared with several existing formulas,the proposed formulas can be used to better estimate the bed form resistance.