Flows along rivers, through pipes and irrigation channels encounter resistance that is proportional to the roughness of bounding walls. Roughness can vary considerably from smooth steel, to concrete or sand, pebbles, and even large boulders. In traditional hydraulics the influence of roughness has been cataloged in the form of a roughness coefficient based on data obtained from a wide range of field and laboratory observations.
Roughness coefficients are typically defined in one of two ways: (1) Ch¾zy’s resistance coefficient, or (2) Manning’s n. To understand these terms we must first state the conditions under which they are defined. For this discussion, and much more information about flow resistance caused by roughness, the reader is referred to Open Channel Hydraulics by Ven Te Chow and published by McGraw-Hill, reissued 1988.
Flow losses are defined in terms of a uniform flow state defined as steady flow with a fixed discharge and flow depth. In practice this usually means the flow in a channel of uniform cross section and having a constant slope such that the gravitational acceleration down the slope is balanced by frictional resistance at the boundaries of the channel.