Conveyance Structures for Canal Flows (Part - 1) - Civil Engineering (CE) PDF Download

Introduction 

A  canal  conveying  water  from  the  head works  has  to  run  for  large  distances and  has  to  maintain  the  water  levels  appropriately,  as  designed  along  its  length.  It  has to  run  through  terrains  which  generally  would  have  a  different  slope  small  than  the  canal .  The  surrounding  areas  would  invariably  have  its  own  drainage system ranging  from  small  streams  to  large  rivers . The  canal  has  to  carry  the  water  across  these  water bodies  as  well  as  across  artificial  obstacles  like  railway line  or roads .

The  main  structures  of  a canal system  for conveyance of canal flow  and  control  of  water levels  are  as  follows .

1. Pipe  conduits,  culverts  and  inverted  syphons  to  carry  flow  under railways  and  highways .
2. Aqueducts, syphon aqueducts, super-passage, canal siphon or level crossings across natural drainage courses or other depressions.
3. Transitions at changes in cross sections.

This  lesson  deals  with  the  concepts  of  planning, layout  and  design  of  canal  structures  for  flow  conveyance  across  artificial  and  natural obstacles.   

Structures for crossing canals across roads and railway lines 

These are structural elements to convey canal water under roads or railway lines. For small roads, carrying relatively less traffic, the pipe conduit is sufficient. A general view of the pipe conduit is shown in Figure 1 and its typical plan and cross section in Figure 2. For canals crossing under major highways and railway tracks, reinforced concrete culverts are more commonly adopted. These roads or railway crossings are usually having a straight profile along its length. The water level in the canal for this type of crossing is lower than the level of the obstruction it crosses, as may be noticed from Figure 2 and the flow through the pipe may be free or under mild pressure. 

FIGURE 1. Pipe conduits for canals crossing small roads

FIGURE 2. Plan and section of road crossing with pipe conduit

Pipe road crossings are relatively economical, easily designed and built, and have proven a reliable means of conveying water under a roadway.  Pipe  installations  are  normally  installed  by  cut  and  cover  method below minor roads but for important roads, where traffic cannot be interrupted, it may be accomplished by jacking the pipe through the roadway foundation. 

The inverted syphons are structures for canal water conveyance below roads, railway lines and other structures (Figure 3). The longitudinal profile is not exactly in a straight line and the central portion is seen to sag beneath the object to be crossed. The inverted syphon, therefore, is provided where the water level in the canal is about the same as the level of the obstruction (Figure 4).   

FIGURE 3. Inverted Syphon below roads showing rectangular section. Circular section also possible

FIGURE 4a. Canal Full Supply Level(FSL) and road level are nearly same

FIGURE 4b. An example of an inverted syphon of a small canal crossing a road

The  inverted syphon  is  a closed  conduit  designed  to run full  and  under  pressure . If  made of  pressure pipes , they  should  be  able  to  withstand  the  load  of  cover  and  wheel  from  outside  and  the  hydrostatic  head  from  inside . Transitions for changes in cross sections are nearly always used at inlet and outlet of a siphon to reduce head losses and prevent erosion in unlined canals caused by the velocity changes between the canal and the pipe. 

Structures for crossing canals across natural streams (cross drainage works)

These structural elements are required for conveying the canals across natural drainage. When a canal layout is planned, it is usually seen to cross a number of channels draining the area, varying from small and shallow depressions to large rivers. It is not generally possible to construct cross-drainage structures for each of the small streams. Some of the small drainage courses are, therefore, diverted into one big channel and allowed to cross the canal. However, for larger streams and river, where the cost of diversion becomes costlier than providing a separate cross-drainage work, individual structures to cross the canal across the stream is provided. 

There could be a variety of combinations of the relative position of the canal with respect the natural channel that is to be crossed. These conditions are shown in Figures 5 to 9. The notations used in the figures are as follows: 

(a) CBL: Canal Bed Level;  

(b) SBL: Stream Bed Level;  

(c) FSL: Canal Full Supply Level; and  

(d) HFL: Stream High Flood Level  

FIGURE 5. Relative position of canal (shown in cross section) with respect to a natural stream (shown in longitudinal section), when canal bed level is higher than stream high flood level

Figure 5 shows the relative position of canal (shown in cross-section) with respect to a natural stream (shown in longitudinal section), when canal bed level is higher than stream high flood level. 

FIGURE 6 . Relative position of a canal whose canal bed level is below the high flood of the stream.

Figure 6 shows the relative position of a canal whose bed level is below but full supply level is above the stream high flood level. 

FIGURE 7 . Canal with full supply level almost matching the high flood level of the natural stream

Figure 7 shows a canal with full supply level almost matching the high flood level of the natural stream. 

FIGURE 8 . Canal full supply level and bed levels below the levels of high Hood level and bed level of the stream, respectively.

Figure 8 shows a canal full supply level and bed levels below the levels of high flood level and bed level of stream, respectively. 

FIGURE 9 . Relation position of canal with respect to the natural stream where the canal full supply level is below the stream bed level

Figure 9 shows the relative position of canal with respect to the natural stream where the canal full supply level is below the stream bed level. 

In  general , the  solution  for  all  the  illustrated  conditions  possible  for  conveying  an  irrigation  canal  across  a  natural  channel  is  by  providing  a  water  conveying  structure  which  may: 

(a) Carry the canal over the natural stream;  

(b) Carry the canal beneath the natural stream; or  

(c) Carry the canal at the same level of the natural stream.