The value of Manning's n is affected by many variables. Factors like suspended load, sediment grain size, presence of bedrock or boulders in the stream channel, variations in channel width and depth, and overall sinuosity of the stream channel can all affect Manning's n value.

a certain flow, then the manning roughness Coefficient comes into action.

The roughness coefficient given by manning depends on various factors like the roughness of the bed Material, variations in channel, Density and type of vegetation involved, Cross section geometry, One channel is meandered to the degree to which it is done and last but most important is Open channel flow area surface changes.

These are all the factors that can affect the friction coefficient.