The surface relief developed at the base of a flowing fluid, comprising bedforms (form roughness), and particles projecting from the sediment carpet (grain roughness). ‘Bed roughness’ is the quantifiable factor which expresses the frictional effect that the bed exerts on the flow. Surfaces whose bed-roughness elements do not project through the viscous sub-layer at the base of the flow are said to be smooth. Surfaces whose bed-roughness elements project through the sub-layer are said to be rough.