Manning's value of n is affected by many variables.  Suspended load, sedimentary grain size, presence of bedrock or boulder in the stream channel, variations in channel width and depth, and overall cyanosis of the stream channel can all affect the manning n value

The roughness coefficient provided by Manning depends on various factors such as bed material roughness, channel variation, density and plant type involved, cross section geometry, a channel at which it is made and last but not least the open channel flow area surface changes.

the friction coefficient can affected by thi factor.