Flow in Sewers

The flow in sewers varies widely from hour to hour and also seasonally. The flow in sewer is said to be steady, if the rate of discharge at a point in a conduit remains constant with time, and if discharge varies with time, it is unsteady. If velocities and depth of flow are same from point to point along the conduit, the steady open channel is said to be uniform flow and non-uniform if either the velocity, depth or both are changing.

For the purpose of hydraulic design of sewers following assumptions are made:

(a)        The flow of wastewater in its sewers is steady and uniform. The unsteady and non-uniform waste water flow characteristics are accounted in the design by proper sizing of manholes.

(b)       The available head in waste water lines is utilized in overcoming surface resistance and, in small part, in attaining kinetic energy for the flow.

(c)        The designs of sewers are based on Peak Flow Discharge.