Q. Effects of sewer sediment on hydraulic performance?

The presence of sediment in sewers has contrary effects on the hydraulic performance of sewers. In case of sewage flow carrying sediment without deposition, presence of sediment in the flow causes a small increase in energy loss. 

In case sewer invert already comprises a bed of sediment deposit, it decreases the cross sectional area of sewers and subsequently for a given discharge the velocity increases. Head losses associated with this velocity increase. Furthermore the increase in bed resistance persuaded by the rough nature of sediment deposit decreases the pipe flow capacity of sewers. 

For sewers that are partially full, presence of sediment bed enhances higher frictional resistance and results in increasing the flow depths and subsequent decrease of velocity. Reduction of velocity will result in further deposition of sediment because of the decrease of sediment carrying capacity if increase of capacity of sewers produced by the presence of sediment bed doesn't exceed the reduction in flow caused by bed roughness.