Sewer manholes should preferably be designed to be watertight owing to the following reasons:

(i) Water-tightness is important in sewer system. Otherwise, leakage of sewage from manholes would seriously contaminate the environment.

(ii) In the event of high water table, water would infiltrate into sewer manholes. Therefore, a higher volume of sewage would be delivered to sewage treatment plant for treatment and this essentially increases the cost for treating this additional volume of water.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Wetwells are designed to store temporarily water/sewage before it is pumped out. They are usually provided for sewage and stormwater pumping stations and they serve the following functions:

(i) They assist in attenuating the fluctuations of flow owing to the diurnal variation of sewage discharge.

(ii) The wetwells serve as sump pits where the suction pipes are inserted and the fluid level in the sumps can be employed for the control of opening and closure of pumps.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

For solids in large sewers, forces on solids position them at different flow heights depending on their specific gravity.

There are generally two modes of movement of solids in small sewers, namely, floating and sliding dam. The floating mechanism operates when the size of solid is small when compared with the diameter of sewer. Solids move with the wave of sewage. On the other hand, the sliding mechanism functions when the size of solid is large when compared with the diameter of sewer. The sewage waves build up behind the solids which act as a barrier at the base of sewer. When the waves store sufficient energy to overcome the friction between solids and sewer invert, the solid would move along the sewer.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

When excessive pipe deformation occurs, it may impair the joint performance and affect the strain in pipes. Based on the information by PIPA, there is a 5% reduction of flow capacity when the pipe is deflected by 15%. Hence, pipe deflection has impact on flow capacity but its effect is not significant.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.