Contents preface (VII) introduction 1—37

Fig. 9.28 Montague’s curves

EXERCISES

1. 
   How does seepage endanger safety of a structure on permeable foundation ?
   

2. 
   Differentiate between Bligh’s creep theory and Khosla’s method for the analysis of seepage be-low hydraulic structures.
   

350 IRRIGATION AND WATER RESOURCES ENGINEERING

3. 
   Sketch the hydraulic gradient line for the weir profile, shown in Fig. 9.29, considering the case of no flow at pond level. Slope correction for the slope (2 : 1) is 6.5 per cent. Also compute the value of the exit gradient.
   

Pond level

102.50
101.00

1. 
   1
   

	2
99.00	98.00
0.5m

97.00

0.5m

1. 
   94.00
   

92.00
Fig. 9.29 (Exercise 9.3)

4. 
   Using Khosla’s method, check the safety of the weir profile, shown in Fig. 9.30, against piping and uplift (at point A). Safe exit gradient may be assumed as 1 in 5.
   

134.00

129.00
Pond level

130.75

1:4

1	: 5

128.40
A 127.00

126.40

122	122	126.00
	16 m	35 m

120
Fig. 9.30 (Exercise 9.4)

5. 
   For the diversion structure, shown in Fig. 9.31, it was observed that wide cracks have developed immediately, downstream of the intermediate sheet pile, allowing the entire seepage water to escape through the cracks. Check the safety of the cracked structure, against uplift at B and also piping. The safe exit gradient is 0.20, correction for slope A is 4.5 per cent, and for slope B is 1.0
   

per cent.

SURFACE AND SUBSURFACE FLOW CONSIDERATIONS FOR DESIGN OF CANAL STRUCTURES

351

Pond level

259.60

258.50

:1 1
256.00

257.00

1	:
	3

255.00		Slope	‘B’		Slope ‘A’				Crack
								B	255.00
													254.50
253.25								254.00			8.5 m
	6 m					2 m
											250.00	250.00

Fig. 9.31 (Exercise 9.5)

REFERENCES

1. 
   Silvester, R, Hydraulic Jump in all Shapes of Horizontal Channels, J. of Hydraulics Division, Proc. ASCE, Vol. 90, No, HY1, Jan. 1965, pp. 23-55.
   

2. 
   Rajaratnam, N, Hydraulic Jumps, Advances in Hydroscience, Vol. 4, Academic Press, New York, 1967, pp. 197-280.
   

3. 
   Rajaratnam, N and K Subramanaya, Profile of the Hydraulic Jump, J. of Hydraulics Division, Proc. ASCE, May 1968.
   

4. 
   Crump, ES, A Note on an Approximate Method of Determining the Position of a Standing Wave, Lahore, 1930.
   

5. 
   Swamee, PK, Sequent Depths in Prismatic Open Channels, CBIP, J. of Irrigation and Power, Jan. 1970, pp. 45-61.
   

6. 
   Kindsvater, CE, The Hydraulic Jump in Sloping Channels, Transactions of ASCE, Vol. 109, 1944, pp. 1107-1154.
   

7. 
   Bradley, JN and AJ Peterka, The Hydraulic Design of Stilling Basins: Hydraulic Jumps on a Horizontal Apron (Basin I), J. of Hydraulics Division, Proc. ASCE, Vol, 83, No. HY 5, 1957, pp. 1-24.
   

8. 
   Rajaratnam, N, The Forced Hydraulic Jump, Water Power, Vol. 16, 1964, (pp. 14-19 and 61-65).
   

9. 
   Ranga Raju, KG, MK Mittal, MS Varma, and V Ganesan, Analysis of Flow over Baffle Blocks and End Sills, J. of Hydraulic Research, Vol. 18, No. 3, 1980.
   

10. 
    Basco, DR and JR Adams, Drag Forces on Baffle Blocks in Hydraulic Jumps, J. of Hydraulics Division, Proc. ASCE, Dec. 1971.
    

11. 
    Darcy, H, Recherches Hydrauliques, Paris, 1856.
    

12. 
    Khosla, AN, NK Bose, and EM Taylor, Design of Weirs on Permeable Foundations, CBIP Publica-tion No. 12, 1954.
    

13. 
    Bligh, WC, Practical Design of Irrigation Works, 1910.
    

14. 
    Lane, EW, Security of Masonry Dams on Earth Foundation from Under-seepage or Piping, Tech-nical Memorandum No. 303 of the United States Bureau of Reclamation. (This has also ap-peared as paper No. 1919 in Transactions of American Society of Civil Engineers in 1935).
    

15. 
    Terzaghi, C, Effect of Minor Geological Details on the Safety of Dams, Technical Publication of
    

the Institute of Mining and Metallurgical Engineers, No 215, 1929.

10
CANAL REGULATION STRUCTURES
10.1. GENERAL
Canal regulation structures are hydraulic structures which are constructed to regulate the discharge, flow velocity, or supply level in an irrigation channel. These structures are necessary for efficient working as well as for the safety of an irrigation channel. Canal regulation structures can be classified as follows:
(i) Canal fall: The canal fall (or, simply, the ‘fall’ or ‘drop’) regulates the supply level in a canal by negotiating the change in its bed elevation necessitated by the difference in ground slope and canal slope.
(ii) Distributary head regulator: This controls the supply to an offtaking channel from the parent channel.
(iii) Cross regulator: This structure controls the water level of a channel and the dis-charge downstream of another hydraulic structure.
(iv) Canal escape: Canal escape disposes of extra supplies when the safety of a canal is endangered due to heavy rains or closure of outlets by farmers.
10.2. CANAL FALL
A canal fall is a hydraulic structure constructed across a canal to lower its water level. This is achieved by negotiating the change in bed elevation of the canal necessitated by the difference in ground slope and canal slope. The necessity of a fall arises because the available ground slope usually exceeds the designed bed slope of a canal. Thus, an irrigation channel which is in cutting in its head reach soon meets a condition when it has to be entirely in filling. An irrigation channel in embankment has the disadvantages of: (i) higher construction and maintenance cost, (ii) higher seepage and percolation losses, (iii) adjacent area being flooded due to any possible breach in the embankment, and (iv) difficulties in irrigation operations. Hence, an irrigation channel should not be located on high embankments. Falls are, therefore, introduced at appropriate places to lower the supply level of an irrigation channel. The canal water immediately downstream of the fall structure possesses excessive kinetic energy which, if not dissipated, may scour the bed and banks of the canal downstream of the fall. This would also endanger the safety of the fall structure. Therefore, a canal fall is always provided with measures to dissipate surplus energy which, obviously, is the consequence of constructing the fall.
The location of a fall is primarily influenced by the topography of the area and the desirability of combining a fall with other masonry structures such as bridges, regulators, and so on. In case of main canals, economy in the cost of excavation is to be considered. Besides, the relative economy of providing a large number of smaller falls (achieving balanced earth work and ease in construction) compared to that of a smaller number of larger falls (resulting in

352

CANAL REGULATION STRUCTURES

353

reduced construction cost and increased power production) is also worked out. In case of channels which irrigate the command area directly, a fall should be provided before the bed of the channel comes into filling. The full supply level of a channel can be kept below the ground level for a distance of up to about 500 metres downstream of the fall as the command area in this reach can be irrigated by the channels offtaking from upstream of the fall.