For checking of self-cleansing velocity for pipes, there is another criterion to check design flow Q to full bore flow Q_full> 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/ Q_full>0.5,
then the ratio of design velocity V to full bore velocity V_full >1.

After confirming V_full>1m/s, then it leads to V>1m/s. Hence, minimum velocity at full bore flow should be checked.

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.

In the design of gravity drainage pipes, full bore flow capacity is normally adopted to check against the design runoff. However, one should note that the maximum flow rate does not occur under full bore conditions. The maximum discharge occurs when the water depth in circular pipes reaches 93.8% of the pipe diameter. Therefore, the use of full bore discharge is on the conservative side though the pipe’s maximum capacity is not utilized.

Similarly, the maximum velocity does not occur in full bore conditions and for circular pipes it occurs when the water depth is 81.3% of the pipe diameter. Hence, in checking for the maximum velocity of flow in pipes to avoid possible erosion by rapid flow, the use of full-bore velocity may not be on the conservative side.

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.

Darcy-Weisbach equation combined with the Moody Diagram is the accepted method to calculate energy losses resulting from fluid motion in pipes and other closed conduits. However, the Moody Diagram may not be suitable for usage in some conditions. For instance, the curve at transition region between laminar and fully turbulent rough pipe flow is applicable for pipes with interior roughness comparable to iron. Moreover, owing to the difficulty in determining pipe roughness, the accuracy of Moody Diagram is only about plus or minus 15%.

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.

Manning’s formula was proposed by Robert Manning (an Irish engineer) to calculate uniform flow in open channel. It is probably the most widely used uniform-flow formula around the world. Its extensive usage is due to the following reasons:

(i) The majority of open channel flows lies in rough turbulent region;
(ii) It is simple in form and the formula is well proven by much practical
experience.

Manning’s formula shall not be applied in situations where Reynolds number effect is predominant. For Chezy formula, it is less commonly used owing to insufficient information to find out equivalent roughness and it is not backed by sufficient experimental and field data. However, in smooth boundaries, Chezy formula shall be adopted instead of Manning’s formula.

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.

Manning’s Equation is commonly used for rough turbulent flow while Colebrook-White Equation is adopted for transition between rough and smooth turbulent flow.

For Manning’s Equation, it is simple to use and has proven to have acceptable degree of accuracy and is the most commonly used flow formula in the world. When using Colebrook-White Equation, it is observed that for very flat gradient (i.e. <1.5%) it tends to underestimate the flow because as gradient approaches zero, velocity also approaches zero. Hence, care should be taken when using Colebrook-White Equation for flat gradients.

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.