Contents preface (VII) introduction 1—37

Waterway of the Stream:
Since the bed level of the flumed canal (i.e., 267.203) is below the HFL of the stream (i.e., 268.00), a siphon aqueduct will be suitable.

Lacey’s regime perimeter,

P = 4.75 Q

= 4.75 500 = 106.2 m

CROSS-DRAINAGE STRUCTURES

403

If the clear span of barrel is fixed as 8 m, the pier thickness is obtained as

0.55 barrel span in metres

= 0.55 8 = 1.56 m = 1.6 m (say)

The overall waterway for 12 spans of 8 m each and 1.6 m pier thickness would be 12 × 8 + 11 × 1.6 = 113.6 m

The clear waterway is 96 m.

For velocity of flow in the barrels equal to 2.5 m/s, the height of barrel should be

500	= 2.083 m
96 × 2.5

Providing a height of 2.0 m for barrels, the flow velocity in the barrels is

500

_{96 × 2.0} = 2.60 m/s

8 × 2

For these barrels, hydraulic radius, R = _{2 (8 + 2)} = 0.8 m

The loss of head through siphon barrels (16.8 m long) can be obtained from Eq. (11.2). Neglecting approach velocity,

h =

^F 1 + f

1

+ f

2

L

I V ^2

L

G

J

2g

H

RK

F

b I

where

f1 = 0.505,

and

f2

= aG 1 +

J

H

RK

For smooth cement plaster surface, a = 0.00316 and b = 0.03

∴

f2

F

+

0.03I

= 0.00316G 1

J = 0.00328

H

0.8 K

∴

h

= ^F 1 + 0.505 + 0.00328 ^16.8^I

(2.6)^2

= 0.54 m

L

G

J

2 × 9.81

H

0.8 K

∴

Upstream HFL = 268 + 0.54 = 268.54 m

Uplift pressure on the barrel roof (or flume trough):

Consider the downstream end of the flume and transition designed using Vittal and Chiranjeevi’s method.

For the chosen thickness of the trough slab as 0.6 m and the computed value of ∆z as 0.203 m, R.L. of the bottom of the trough (i.e., culvert roof) slab

• 
  267.0 + 0.203 – 0.6
  

• 
  266.603 m
  

					V ^2	(2.6)	2
Loss of head at the entry of the barrel = 0.505	2g = 0.505			= 0.174 m
2g
and velocity head in the barrel =	V ^2	=	(2.6)	2
2g	2g		= 0.345 m

404 IRRIGATION AND WATER RESOURCES ENGINEERING
Part of the hydraulic head available at the upstream end of the barrel, Fig. 11.11, is utilized in meeting the entry (to the barrel) loss and developing the velocity head in the barrel. Therefore, maximum uplift (at the upstream end) pressure on the trough slab (Fig. 11.11)

• 
  HFL – Entry loss – velocity head in the barrel
  

– RL of the bottom of the trough

• 
  268.54 – 0.174 – 0.345 – 266.603
  

• 
  1.418 m
  

Uplift pressure on the floor of the barrel : RL of barrel floor = 266.03 – 2.0

= 264.603 m
Let the floor thickness of the barrel be 1.5 m

∴	RL of the bottom of the barrel floor	= 264.603 – 1.5 = 263.103 m
∴	Static pressure on the barrel floor	= 265 – 263.103 = 1.897 m

The culvert floor should extend toward the upstream by a distance equal to the difference between HFL and the culvert floor level, i.e., 268 – 264.603 = 3.397 m ≅ 3.5 m (say)

Length of upstream contracting transition (of canal) = 16 m Half of the barrel span = 4 m

End of the culvert floor from the centre of the barrel = 3.5 + (16.8/2) = 11.9 m Total seepage head (at the upstream end of the culvert floor)

• 
  FSL of canal (in the flumed portion) – Bed level of the stream
  

• 
  (267.203 + 6.006) – 265.0 = 8.209 m
  

Therefore, residual seepage head at the centre of the barrel (calculated approximately)

• 
  _{8.209 ×} [31.9 − (16 + 4)]
  

(16 + 4 + 119.)

• 
  8.209 × 119. _{= 3.06 m}
  

319.
Therefore, total uplift at the bottom of the culvert floor = 1.897 + 3.06 = 4.957 m
As in case of other hydraulic structures, suitable wing connections and protection works on both the upstream and downstream sides of the canal and the stream at the site of the structure are to be provided.


EXERCISES

1. 
   What is the purpose of providing a cross-drainage structure ? In which reach of a canal, does the need of such structures arise and why ?
   

2. 
   Explain how the type of a cross-drainage structure may change with the shift in the site of the structure along the stream.
   

3. 
   Design a siphon aqueduct across a stream for the following data:
   

Canal

Full supply discharge = 56 m³/s

Bed width = 32 m

CROSS-DRAINAGE STRUCTURES	405
Depth of flow	= 2.0 m
Bed level	= 267.0 m
Stream
High flood discharge	= 425 m^3/s
High flood level	= 268.20 m
Bed level	= 265.50 m
General ground level	= 267.20 m

Make suitable assumptions, if required. Draw plan and sectional elevation of the designed siphon aqueduct.

11.4 The following are the data for a canal siphon:
Canal
Fully supply discharge	= 32 m^3/s
Full supply level	= 262.00 m
Bed level	= 260.00 m
Bed width	= 22.00 m
Side slope	= 1.5 (H) : 1 (V)
Stream
Design flood discharge	= 500 m^3/s
High flood level	= 263.50 m
Drainage bed level	= 261.00 m
Silt factor for stream bed	= 1.50
Other given data are as follows:
Permissible velocity through siphon barrels	= 2.5 m/s
Manning’s roughness coefficient for barrel friction	= 0.018
Barrel entrance loss coefficient	= 0.25
Thickness of barrel walls	= 0.30 m
Hinds’ contraction coefficient	= 0.25
Expansion coefficient	= 0.35

Assume any other data suitably. Work out the following items of design: (a) Barrel size and its vertical location,

(b) TEL, WSL, and BL at the six sections along the canal, and (c) Total uplift force on barrel roof.
REFERENCES
1. ...... IS Codes of Practice for Design of Cross-Drainage Works, IS: 7784 (Pt. 1)-1975 and IS: 7784 (Pt. 2/Sec 1-5)-1980-83.

2. 
   Yarnell, DL, Bridge Piers as Channel Obstructions, U.S.Dept. of Agriculture, Tech. Bull. No. 442, 1934.
   

2. 
   Bharat Singh, Fundamentals of Irrigation Engineering, Nem Chand & Bros, Roorkee, 1988.
   

2. 
   Vittal, N and VV Chiranjeevi, Open Channel Transition: Rational Method of Design, J. of Hyd. Division, Proc. ASCE, Vol. 109, Jan. 1983.
   

2. 
   Hinds, J, The Hydraulic Design of Flume and Siphon Transition, Trans. ASCE, Vol. 92, 1928.
   

2. 
   ...... Tech. Memorandum No. 9 of UPIRI, Roorkee, 1940.