A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
10. What are the effects of sewer sediment on hydraulic performance?
(DS1)
The presence of sediment in sewers has adverse effects on the hydraulic
performance of sewers [13]. 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.
11. What are the functions of wetwells? (P1)
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.
12. Should sewer manholes be designed as watertight? (S3)
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
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