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
7. What is the reason in checking the ratio (i.e. design flow to full-bore
flow > 0.5) in circular pipe design? (HD4)
For checking of self-cleansing velocity for pipes, there is another criterion
to check design flow Q to full bore flow Qfull> 0.5. If this criterion is met, it
can be deduced that the design flow is always greater than self-cleansing
velocity.
The reason behind is that from the chart of circular pipes, when Q/ Qfull
>0.5, then the ratio of design velocity V to full bore velocity Vfull >1. After
confirming Vfull >1m/s, then it leads to V>1m/s. Hence, minimum velocity at
full bore flow should be checked.
8. What are the potential advantages in using best hydraulic section?
(HD5)
The best hydraulic section of an open channel is characterized by provision
of maximum discharge with a given cross sectional area. As such,
channels with circular shape is the best hydraulic sections while a
rectangular channel with channel width being equal to two times the height
of channel is the best hydraulic section among all rectangular sections. In
fact, the choice of best hydraulic section also possesses other advantages
than hydraulic performance. For instance, for a given discharge rate the
use of best hydraulic section could guarantee the least cross sectional area
of the channels. Substantial savings could be made from the reduction in
the amount of excavation and from the use of less channel linings.
9. What are the potential problems of high velocity in pipes? (HD4)
Flow velocity seldom causes abrasion problem for concrete pipes. Instead,
the particles carried by effluent in high velocity may create abrasion
problem of concrete. The abrasive effect is dependent on the size of
particles and velocity. In most circumstances, the problem of abrasion shall
be avoided for flow velocities less than 8m/s.
Very high velocity (i.e. more than 10m/s) could also induce significant
cavitation problem because air bubbles are formed from low water
pressure and they would collapse when entering a region of high water
pressure.
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