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Why sewers are designed to run partially full?
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• The sewers are necessarily designed for the partially full flow condition to maintain an open channel flow conditions and also to be provide the space above the surface of the sewage for proper ventilation.
• The sewers must be designed for a depth of flow 'd' which should be at least half of the diameter 'D' i.e. 0.5 D and it is also should not be more than 0.8 D at the peak flow condition.
• Need to maintain the minimum and maximum velocities in the sewers.
• The sewage has a considerable proportion of organic and inorganic matter so if the pipe to cause abstraction to the flow of sewage. so it necessary to maintain the minimum velocity of the flow to control such depositions in the sewers.
• These velocities are called as self cleansing velocity. It must be maintained at least once in a day, during the peak flow of the sewage at all the sections of the sewerage system.
• On the other hands the velocity should not be too high, to cause erosion of the pipe material of the sewers to reduce the life of the sewers.
• This velocity is called as non scouring velocity. Generally it is kept limited up to $3.0 m/s$.
• The important point to note about the velocity of flow in partially flow sewers:-

1) The velocity of the flow is equal to the full flow velocity when the flow is half full.

2) The velocity of the flow is more than the full flow velocity when the flow is more than the half full.

3) The velocity of the flow is declining when the flow is less than half full.

4) The velocity of flow is to be maintained of the self cleaning velocity, if the sewer is running less than half full.

5) The velocity of flow is maximum when the depth of flow is 0.82 D.

6) The carrying capacity of the sewer is maximum when the depth of the flow is equal to 0.9 D.