Bed Roughness

Bed roughness develops due to stream surface relief at the base of a flowing fluid and it exerts frictional effect on the flow in the stream. Bed roughness is considered to be smooth or rough depending on whether sediment particles project through the viscous sublayer at the base of the flow. Roughness generally increases with increasing particle size.

There are two components of bed roughness: (a) grain roughness which relates to the effects of the individual grains making up the channel bed and (b) form roughness which refers to features of bed forms such as ripples and dunes, created when certain alluvial substrates are moulded by the flow. Grain roughness refers to the shear forces created by sediment particles at the flow boundary. Grain roughness is mainly a function of relative roughness which is defined as the ratio of depth of flow and bed material size. Grain roughness can be the dominant component of the bed roughness where stream beds consist of gravel or cobbles.

The shape...