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A new relationship between the roughness height and the main hydrodynamic and sediment parameters for plane beds under oscillatory conditions is presented. In order to derive such a relationship, a large data base encompassing plane bed experiments was compiled from previous investigations and analyzed. Different methods to estimate the roughness height were investigated. Comparisons between the data and different existing predictive formulas for the bed roughness obtained from the literature were also performed. A relationship involving the grain size and Shields parameter only, which is commonly proposed, appeared to be insufficient to characterize the roughness. The roughness height was also found to be a function of the sediment density and the settling velocity. A critical Shields parameter was identified up to which the effective roughness ratio is proportional to the Shields parameter only. The new empirical equation developed in this study provides the highest predictive skill for all conditions investigated.