Conveyance Structures for Canal Flows (Part - 2) - Civil Engineering (CE) PDF Download

Structures to carry canal water over a natural stream 

Conveying a canal over a natural watercourse may be accomplished in two ways: 

(a) Normal  canal  section  is  reduced  to  a rectangular  section  and  carried  across  the  natural  stream  in  the  form  of  a  bridge  resting  on  piers  and  foundations (Figure         10) .  This  type  of  structure  is  called  a  trough  type  aqueduct .

(b) Normal  canal  section is  continued  across  the  natural  stream  but  the  stream  section  is  flumed   to  pass  through  ‘barrels’ or rectangular passages (Figure 11). This           type is called a barrel type aqueduct. 

FIGURE 10. Trough type aqueduct

FIGURE 11. Barrel type aqueduct

Typical  sections  and  plans  of  a trough  type  and  a  barrel  type  aqueducts  are  shown  in  Figures  12 and 13  respectively. 

FIGURE 13a. A typical plan of a barrel type aqueduct

FIGURE 13b. Cross sections of the barrel type aqueduct shown in Fig. 13a.

For the aqueducts, it may be observed from Figures 12 and 13 that the HFL of the natural stream is lower than the bottom of the trough (or the roof of the barrel). In this case, the flow is not under pressure, that is, it has a free surface exposed to atmospheric pressure. 

In case the HFL of the natural stream goes above the trough bottom level (TBL) or the barrel roof level (BRL), then the flow in the natural watercourse would be pressured and the sections are modified to form which is known as syphon aqueducts (Figures 14 and 15).

FIGURE 14. Section through a syphon aqueduct showing condition of pressured flow in natural drain

FIGURE 15. Plan of a syphon aqueduct if flow in natural drain is pressured

It may be observed that the trough type aqueduct or syphon aqueduct would be suitable for the canal crossing a larger stream or river, whereas the barrel type is suitable if the natural stream is rather small. The relative economics of the two types has to be established on case to case basis.

Further, the following points maybe noted for the two types of aqueducts or siphon aqueducts: 

Trough type: The  canal  is  flumed  to  not  less  than  75  percent  of  the  bed  width  keeping  in  view  the  permissible  head loss  in  the  canal .Transitions  3:1  on  the upstream  and  5:1  on  the  downstream  side  are  provided  to  join  the flumed section  to  the  normal  canal  section . For  the  trough-type syphon aqueduct  the designer must  consider  the  upward  thrust  also  that  might  act during  high  floods  in  the  natural  stream  when  the  stream water  flows  under  pressure  below the trough base and  for  worst  condition , the  canal  may  be  assumed  to  be  dry  at  that  time. The  dead  weight  of  the  trough  may  be  made  more  than  that  of  the  upward  thrust  or it  may  be  suitably  anchored   to  the  piers  in  order  to  may  be  counteract  the  uplift  condition  mentioned . 

Barrel type: The barrel   may be made up of RCC, which could be single or multi-cell, circular or rectangular in cross section. Many of the earlier structures were made of masonry walls and arch roofing. Precast RCC pipes may be economical for small discharges. For barrel-type syphon aqueducts, the barrel is horizontal in the central portion but slopes upwards on the upstream and downstream side at about an inclination of 3H : 1V and 4H : 1V respectively. A self-cleaning velocity of 6m/s and 3m/s is considered while designing RCC and masonry barrels respectively.