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The presence of sediment in sewers has adverse effects on the hydraulic performance of sewers. For the case of sewage flow carrying sediment without deposition, the presence of sediment in the flow causes a small increase in energy loss.
In case the sewer invert already contains a bed of sediment deposit, it reduces the cross sectional area of sewers and consequently for a given discharge the velocity increases. As such, the head losses associated with this velocity increase. Moreover, the increase in bed resistance induced by the rough nature of sediment deposit reduces the pipe flow capacity of sewers.
For sewers which are partially full, the presence of sediment bed enhances higher frictional resistance and results in increasing the flow depths and subsequent decrease of velocity. The reduction of velocity will lead to further deposition of sediment owing to the decrease of sediment carrying capacity if the increase of capacity of sewers generated by the presence of sediment bed does not exceed the reduction in flow caused by the bed roughness.