Equivalent Roughness of Alluvial Bed

The eguivalent roughness of a movable bed is considered. Relationships are given which can be used to determine the equivalent bed roughness from the bed material size and the bed-form dimensions. In the case of a plane bed the equivalent roughness is related to the D90 of the bed material. Based on flume and field data, the equivalent roughness may vary from 1-10 D90. Similar values were reported by other research-workers. In the case of bed forms the equivalent roughness is related to the average height and length of the bed forms. Both flume and field data were used to determine a functional relationship. The proposed relationship yields values which are considerably smaller than other existing relationships.