Contents preface (VII) introduction 1—37

Table 10.1 Length of bed pitching (2)

Head over crest, H in metres	Total length of pitching in metres
Less than 0.30	3
0.30 to 0.45	3	+ 2D
0.45 to 0.60	4.5	+ 2D
0.60 to 0.75	6.0	+ 2D
0.75 to 0.90	9.0	+ 2D
0.90 to 1.05	13.5 + 2D
1.05 to 1.20	18.0 + 2D
1.20 to 1.50	22.5 + 2D

D = Drop in bed level.
Example 10.1 Design a 1.2 m Sarda fall for a channel carrying 25 m³/s of water at a depth of flow equal to 1.8 m. The bed width of the channel is 20 m.
Solution: Since the discharge exceeds 14 m³/s, the cross-section of the crest of Sarda fall is chosen as trapezoidal with sloping downstream (1 in 8) and upstream (1 in 3) faces.

CANAL REGULATION STRUCTURES

U/S wings
U/S FSL
	0.75		0.96
U/S BED	1.047
	2	3	1
		in
		in
			8
			1
1.2				2.22
			15.4

	D/S wings
Subsoil	D/S FSL
Bricks-on	HGL
-edge flooring	0.24
	6.00
4.76
	7.2

Warped wings (6.75) _Bed
1.5

Side pitching

pitching 1:10

11.4

365


^45° D/S BED

(All dimensions in metres)

Fig. 10.12 Longitudinal section of Sarda fall (Example 10.1) (not to scale)
Dimensions of crest: Using Eq. (10.18),
Width of crest, B = 0.55 h₁ + D

• 
  0.55 1.8 + 1.2
  

• 
  0.953 m
  

• 
  0.96 m (say)
  

Length of crest = Bed width of channel = 20.0 m

From Eq. (10.17),

L	QB	1/ 6	O	0.6
H = M	P
		M		0.45 2gL P
N					Q		O
L	25 ×	(0.96)^1/6	0.6
= M						P	= 0.753 m
N	0.45 ×		19.62 ×	20 Q

Height of crest above the upstream bed = h₁ – H

• 
  1.8 – 0.753
  

• 
  1.047 m
  

Height of crest above the downstream bed, d = h₁ + D – H

= 1.8 + 1.2 – 0.753 = 2.247 m

The base of the fall should be at least 0.5 m below the downstream bed level. Accordingly, Base width of fall = (1/3) (d + 0.5) + B + (1/8) (d + 0.5)

• 
  (11/24) (d + 0.5) + B
  

• 
  (11/24) (2.247 + 0.5) + 0.96
  

• 
  2.220 m
  

Cistern: Using Eq. (10.7),

Length of cistern,	Lc = 5	EHL
Assuming E ≅ H,	Lc = 5	0.753 × 1.2

366 IRRIGATION AND WATER RESOURCES ENGINEERING
= 4.753 m
= 4.76 m (say)
From Eq. (10.6),
Depth of cistern, x = (1/4) (EH_L)^2/3
= (1/4) (0.753 × 1.2)^2/3

= 0.234 m

= 0.24 m (say)
Upstream and downstream cutoffs:
Depth of the upstream cutoff = ^h₃¹ + 0.60

• 
  (1.8/3) + 0.60
  

• 
  1.2 m
  

Depth of the downstream cutoff = ^h₂¹ + 0.60

• 
  (1.8/2) + 0.60
  

• 
  1.5 m
  

Thickness of these cutoffs may be kept equal to 0.4 m.

Length of impervious floor: Assuming safe exit gradient to be equal to 1/5, one can write

GE =		1 F	H I
	π	G	J
		λ H	d K

Here, H = Head for no flow condition
= height of the crest above the downstream bed = 2.247 m d = the depth of the downstream cutoff = 1.5 m

∴

1

= GE

d

=

1

×

15.

= 0.134

π

λ

5

2.247

H

∴

λ = 5.643

α =

b

= 10.24

d

1

∴

b = 10 × 1.5

∴Total floor length = 15.36 m ≅ 15.4 m (say)

Minimum floor length required on the downstream = 2h₁ + H_L + 2.4
= 2 × 1.8 + 1.2 + 2.4 = 7.2 m
So provide the downstream floor length equal to 7.2 m and the balance 8.2 m long impervious floor on the upstream side. Thickness of the concrete floor at various sections is decided as illustrated in Example 10.2.
Upstream protection:
Radius of curvature of the upstream wing walls = 5 to 6 times H

• 
  5 × 0.753 to 6 × 0.753
  

• 
  3.765 m to 4.518 m
  

• 
  4.0 m (say)
  

CANAL REGULATION STRUCTURES

367

Brick pitching on the upstream bed is provided at a slope of 1 : 10 for a distance equal to the upstream depth of flow. Drain holes of 20 cm diameter are provided at an interval of 4 m.

Downstream protection: The downstream wing walls are to be kept vertical for a distance of about 5 to 8 EH_L

= 5 0.753 × 1.2 to 8 0.753 × 1.2

• 
  4.75 m to 7.60 m
  

• 
  6.0 m (say)
  

Thereafter, the wing walls should be warped from the vertical to the side slopes of the channel. The top of the wing walls is given an average splay of about 1 : 2.5 to 1 : 4. The difference in the surface widths of flow in the rectangular and trapezoidal section is 1.8 × 1.5 × 2 = 5.4 m. For providing a splay of 1 : 2.5 in the warped wings, the length of the warped wings along the channel axis is equal to (5.4/2) × 2.5 = 6.75 m.

In accordance with Table 10.1, total length of bed pitching on the downstream side is equal to 9 + 2D

• 
  9 + 2.4
  

• 
  11.4 m
  

The bed pitching should be horizontal up to the end of the downstream wing walls and, thereafter, it should be laid at a slope of 1 in 10. A toe wall of thickness equal to 40 cm and depth equal to 1.0 m should be provided at the end of the bed pitching.

Side pitching is provided downstream of the wing walls up to the end of bed pitching. The side pitching may be suitably curtailed. Longitudinal section based on these computations has been shown in Fig. 10.12.
10.9. GLACIS FALL
In case of a glacis fall, the energy is dissipated through the hydraulic jump which forms at the toe of the glacis. Ideally, the profile of the glacis should be such that the maximum horizontal acceleration is imparted to the falling stream of water in a given length of the structure to ensure maximum dissipation of energy. It is obvious that in case of a free fall under gravity, there will be only vertical acceleration and no horizontal acceleration. Theoretically speaking, there will be some horizontal acceleration on a horizontal floor at the crest level because of the formation of an H₂ profile. This acceleration, however, would be relatively small. Hence, there should be a glacis profile, in between the horizontal and vertical, which would result in the maximum horizontal acceleration. Neglecting the acceleration on a horizontal floor at the crest level, Montague obtained the following profile which would yield maximum horizontal acceleration (1):

x = 2U y / g + y

(10.21)

where, U is the initial horizontal velocity of water at the crest, and x and y are, respectively, the horizontal and vertical distances from the crest to any point along the glacis. The parabolic glacis profile [Eq. (10.21)] is, however, difficult and costly to construct. As such, a straight glacis with a slope of 2(H) : 1 (V) is commonly used.

A straight glacis fall may be provided with a baffle platform at the toe of the glacis and a baffle wall at the end of the platform in order to hold the jump at the toe of the glacis. The baffle platform is followed by a cistern downstream of the baffle wall. Such a fall was first developed by Inglis (1) and is called an Inglis fall; this too is obsolete now.

368 IRRIGATION AND WATER RESOURCES ENGINEERING
The present practice is to use a fall (Fig. 10.13) with straight glacis of slope 2 (H) : 1(V) at the toe of which is provided a cistern followed by friction blocks and other suitable measures to hold the jump at the toe of the glacis. Such straight glacis falls may or may not be flumed and can be either meter or non-meter falls. For known specific energy E at the downstream and the energy loss H_L in the jump, one can determine the discharge intensity q using Blench curves (Fig. 9.17). The crest length L is obtained by dividing the total canal discharge Q by the discharge intensity q. While finalising the crest length, the following limits with regard to permissible fluming should be kept in mind (3):

Drop	Permissible minimum value of the ratio of
	crest length to the width of canal
Up to 1 m	0.66
Between 1 and 3 m	0.75
Above 3 m	0.85

^1:1 2:1

Fig. 10.13 Straight glacis fall
The level of the crest is H below the upstream total energy line. Here, H is the head over the crest up to the total energy line and is obtained by the discharge equation of a broad-crested weir,

Q = 1.71 LH^3/2

(10.22)

Here, Q is expressed in m³/s, and L and H are in metres. For meter falls, the width of the crest should at least be 2.5 H so that the crest behaves as a broad-crested weir, and the coefficient of discharge is fairly constant at different discharges.
The upstream face of the crest is kept inclined at a slope of 1 : 1. The downstream face of the crest, i.e., the glacis, is kept straight and inclined at a slope of 2 (H) : 1(V). The glacis is usually carried below the bed level of the downstream channel so that the toe of the glacis (or the level of the cistern) is 1.25 E₂ below the downstream total energy line. The depressed cistern increases the post-jump depth by about 25 per cent of the tail-water depth. This increased depth ensures formation of the jump at the toe of the glacis. The length of the cistern should not be less than 2 E₂. The cistern joins the downstream concrete floor through an upward slope of 5 (H) : 1 (V). The length of the downstream concrete floor (inclusive of the cistern) should be about five to six times the height of the jump. A concrete floor is also provided in the upstream canal immediately upstream of the crest for a length of about 2.0 m. In addition, a vertical cutoff is also provided at the downstream end of the concrete floor. The length and thickness of the concrete floor are checked against uplift force and exit gradient.
In case of flumed falls, suitable expansion has to be provided starting from the downstream end of the glacis. The expansion can be either a hyperbolic type or, simply, a 1 : 3

CANAL REGULATION STRUCTURES

369

straight expansion. The vertical side walls at the end of the cistern are so warped that at the end of the expansion, the slope of the side walls is equal to the side slopes of the downstream channel. It should be noted here that while deciding the width of the flumed portion, the presence of the downstream expansion was completely ignored. For the prevalent subcritical flow conditions, the tail-water depth at the upstream end of the expansion will be smaller than the tail-water depth in the downstream channel after the expansion. To ensure formation of the jump at the toe of the glacis, friction blocks and end cill (both of around 0.5 m height) are additionally provided on the concrete floor. These provisions along with the depressed cistern ensure formation of the jump at the toe of the glacis. These design specifications have stood the test of time and are, accordingly, used in practice.

Downstream of the concrete floor, brick pitching is provided on the bed up to the end of the expansion. Brick pitching is also provided on the upstream canal bed just upstream of the concrete floor. Toe walls are suitably provided to support the brick pitching.
For meter falls, the side walls immediately upstream of the crest are made curved with radius of curvature of five to six times the drop and subtending an angle of 60° at the centre. These curved wing walls are joined to the upstream canal banks. For non-meter falls, however, the side walls may only be splayed at an angle of 45° from the upstream edge of the crest and carried into the banks for about 1.0 m.
Example 10.2 Design a straight glacis fall for a drop of 2.25 m in the water surface level of an irrigation channel carrying water at the rate of 60 m³/s. The bed width and depth of flow in the channel are 30 m and 2.20 m, respectively.
Solution:
Area of flow cross-section = 30 × 2.20 + ₂¹ ( 2.20)² = 68.42 m²

60
∴ Mean velocity of flow in the channel = _68.42 = 0.877 m/s ∴ Velocity head = (0.877)²/(2 × 9.81) = 0.039 m

Post-jump specific energy in the downstream (lower level) channel, E₂ = 2.20 + 0.039 = 2.239 m

If the jump forms at the bed level of the downstream channel,
pre-jump specific energy = specific energy in the upstream channel + drop

• 
  2.239 + 2.25 = 4.489 m ∴ Energy loss in jump, H_L = 4.489 – 2.239 = 2.25 m
  

Using Blench’s curves (Fig. 9.17) for H_L = 2.25 m and E₂ = 2.239 m, q = 2.85 m³/s/m
Thus,
60

Length of crest = _2.85 = 21.05 m

This length is less than the permissible flumed width of channel which is 75% of 30 m, i.e., 22.5 m.

Therefore, provide a crest of length = 22.5 m
so that q = 60/22.5 = 2.67 m³/s/m

370			IRRIGATION AND WATER RESOURCES ENGINEERING
From	Q = 1.71 LH^3/2
	H = (Q/1.71 L)^2/3
	F	60		I	2 /3
	= G			J	= 1.345 m
	H	171. × 22.5K

Hence, height of crest above the upstream channel bed
= 2.239 m – 1.345 m = 0.894 m For a meter fall, the width of crest should not be less than

2.5 H i.e., 2.5 × 1.345 = 3.363 m Therefore, provide the crest width as 3.70 m.

The level of cistern is kept 1.25 E₂ below the level of the downstream total energy line, i.e., 1.25 × 2.239 = 2.799 m. This means that the cistern is depressed below the downstream channel bed by an amount x = 2.799 – 2.239 = 0.56 m
(The value of E₂ corresponding to q = 2.67 m³/s/m and H_L = 2.25 m is 2.16 m).
Thus, 1.25 E₂ = 2.7 m. The lowering of the cistern level below the downstream bed would, therefore, be 2.7 – 2.16 = 0.54 m which is less than 0.56 m. Hence, the cistern is depressed by 0.56 m.
Length of cistern L_c = 2 E₂ = 2 × 2.239 = 4.478 ≅ 4.48 m
Length of the downstream concrete floor (inclusive of cistern length) should be sufficient to accommodate the jump within itself.
Post-jump depth = 2.20 m

The pre-jump depth for q = 2.67 m³/s/m and specific energy = E₂ + drop = 2.16 + 2.25

• 
  4.41 m is obtained from Montague’s curves (Fig. 9.27) as 0.30 m. ∴ Height of jump = 2.20 – 0.30 = 1.90 m
  

Hence,
Length of downstream floor = 5 × 1.90 = 9.50 m

On providing 2.4 m length of concrete floor upstream of the crest and the slopes of the upstream face of the crest and the glacis as 1 : 1 and 2 (H) : 1 (V), the total length of the impervious floor works out as
2.4 + 0.894 + 3.7 + 2 (0.894 + 2.25 + 0.56) + 9.50 = 23.902 m Let the depth of the downstream cutoff be 1.5 m

∴	α =	b		=	23.902	= 15.935
d		15.
	λ =	1	+	1 + α ^2	=	1 +	1 + (15.935)	2
				2				2	= 8.483
The exit gradient is calculated using the equation
					GE =	H	×	π	1
						d			λ

The value of H for the condition of no flow with water on the upstream up to the crest level is (2.25 + 0.894) m, i.e., 3.144 m. The corresponding value of H, when full supply discharge

CANAL REGULATION STRUCTURES

371

is flowing, works out to only 2.25 m. Therefore, the former condition is the most critical condition for the exit gradient. Accordingly,

G = (2.25 + 0.894)	×		1	= 0.229
E	15.		3.14	8.483

which is less than 0.25 and may be considered satisfactory. Using Eqs. (9.51) and (9.52),

1

−1F

λ − 2I

1

−1F

8.483 −

2I

φE =

cos

G

J

=

cos

G

J

= 0.223

π

λ

π

8.483

H

K

H

K

1

−1F

λ − 1I

1

−1F

8.483 − 1I

φD =

cos

G

J

=

cos

G

J

= 0.156

π

λ

π

8.483

H

K

H

K

Assuming a floor thickness of 0.5 m at the downstream end,
Correction for thickness (for φ^E) = ^{0.223 − 0.156} × 0.5 = 0.022 (negative) 15.
Corrected value of φ_E = 0.223 – 0.022 = 0.201
For no flow condition, the uplift pressure at the downstream end is equal to 0.201 × (2.25 + 0.894) = 0.632 m (w.r.t. the downstream bed). Ignoring the effect of the upstream cutoff (conservative approach), one can now draw the subsoil hydraulic gradient line by joining the uplift ordinate (0.632 m w.r.t. the downstream bed) at the downstream end of the impervious floor with the uplift ordinate (3.144 m w.r.t. the downstream bed, i.e., 0.894 m above the upstream end of the concrete floor) at the upstream end of the impervious floor by a straight line.
Similarly, for full supply flow condition the uplift pressure at the downstream end would be equal to 0.201 × (2.25) = 0.45 m above the downstream full supply level. The subsoil hydraulic gradient line for this case too has been shown in Fig. 10.14. Calculations for jump profile are given below:

Pre-jump Froude number,	F1 =	2.67 / 0.32	= 4.7
			9.81 × 0.32
Therefore, from Eq. (9.20),		= (5.08 × 4.7 – 7.82) × 0.32 = 5.14 m
X

Values of distance from the trough of the jump, x , and the corresponding height of the water surface profile above the trough of the jump have been computed using Fig. 9.3.

x(m)	1.0	2.0	3.0	4.0	5.0	6.0	7.0	8.0	9.0	10.0
x/			0.195	0.39	0.58	0.78	0.97	1.17	1.36	1.56	1.75	1.95
X
y(m)	0.395	0.677	0.917	1.13	1.34	1.48	1.62	1.75	1.80	1.83

The jump profile has been plotted in Fig. 10.14.

The thickness of floor at different locations can now be computed using Eq. (9.41) on the basis of the larger of the two uplift pressures and assuming a value of G = 2.30.
Radius of curvature of upstream wing walls = 5 × 25 = 11.25 m
Provide brick pitching for a length of about 5 m upstream of the concrete floor. Providing a splay of 1 in 3 for the straight downstream expansion, the length of the downstream expansion will be 11.25 m. Provide brick pitching from the downstream end of the concrete floor to the downstream end of the expansion.
The longitudinal section of the glacis fall based on these computations is shown in Fig. 10.14.

372

0.039
2.20	1.345	3.70
	0.894	1.00
	0.40

All dimensions are in metres

IRRIGATION AND WATER RESOURCES ENGINEERING

U/S TEL

Subsoil H.G.L. (Full supply) 0.039

Subsoil H.
G.L. (No flow)
	0.40	0.40	2.2
	0.632
0.56	0.90		0.70	1.5
4.48
	1.60
	9.40
23.902

Fig. 10.14 Longitudinal section of glacis fall (Example 10.2) (not to scale)
10.10. DISTRIBUTARY HEAD REGULATOR
The distributary head regulator is constructed at the upstream end (i.e., the head) of a channel where it takes off from the main canal or a branch canal or a major distributary. The distributary head regulator should be distinguished from the canal head regulator which is provided at the canal headworks where a canal takes its supplies from a river source. The distributary head regulator serves to (i) divert and regulate the supplies into the distributary from the parent channel, (ii) control silt entering the distributary from the parent channel, and (iii) measure the discharge entering the distributary.
For the purpose of regulating the supplies entering the offtaking channel from the parent channel, abutments on either side of the regulator crest are provided. Piers are placed along the regulator crest at regular intervals. These abutments and piers have grooves (at the crest section) for the purpose of placing planks or gates. The supplies into the offtaking channel are controlled by means of these planks or gates. The planks are used for small channels in which case manual handling is possible. The span of hand-operated gates is also limited to 6 to 8 m. Mechanically-operated gates can, however, be as wide as 20 m.
An offtaking channel tends to draw excessive quantity of sediment due to the combined effects of the following:
(i) Because of their smaller velocities, lower layers of water are more easily diverted into the offtaking channels in comparison to the upper layers of water.
(ii) Sediment concentration is generally much higher near the bed.
(iii) Sediment concentration near the banks is usually higher because of the tendency of the bottom water to move towards the banks due to difference in central and near-bank velocities of flow.
As such, if suitable steps are not taken to check the entry of excessive sediment into the offtaking channel, the offtaking channel will soon be silted up and would require repeated sediment removal.
Sediment entry into the offtaking channel can be controlled by causing the sediment to concentrate in the lower layers of water (i.e., near the bed of the parent channel upstream of the offtaking point) and then letting only the upper layers of water enter the offtaking channel.
Concentration of sediment in lower layers can be increased by providing smooth bed in the

CANAL REGULATION STRUCTURES

373

parent channel upstream of the offtaking point. The smooth channel bed reduces turbulence which keeps sediment particles in suspension. In addition, steps which accelerate the flow velocity near the banks would also be useful. It should also be noted that the alignment of the offtaking channel also affects the sediment withdrawal by the offtaking channel. Hence, the alignment of the offtaking distributary channel with respect to the parent channel needs careful consideration. The angle of offtake may be kept between 60° and 80° to prevent excessive sediment withdrawal by the offtaking channel. For all important works, the alignment of offtaking channels should be fixed on the basis of model studies.

For the purpose of regulating the discharge in the distributary, it is essential to measure the discharge for which one can use gauge-discharge relationship of the distributary. However, this relationship is likely to change with the change in the channel regime. Hence, it is advantageous to use head regulator as a metering structure too.
10.11. CROSS REGULATOR
A cross regulator is a structure constructed across a canal to regulate the water level in the canal upstream of itself and the discharge passing downstream of it for one or more of the following purposes (4):
(i) To feed offtaking canals located upstream of the cross regulator. (ii) To help water escape from canals in conjunction with escapes.

(iii) To control water surface slopes in conjunction with falls for bringing the canal to regime slope and section.

(iv) To control discharge at an outfall of a canal into another canal or lake.
A cross regulator is generally provided downstream of an offtaking channel so that the water level upstream of the regulator can be raised, whenever necessary, to enable the offtaking channel draw its required supply even if the main channel is carrying low supply. The need of a cross regulator is essential for all irrigation systems which supply water to distributaries and field channels by rotation and, therefore, require to provide full supplies to the distributaries even if the parent channel is carrying low supplies.
Cross regulators may be combined with bridges and falls for economic and other special considerations.
10.12. DESIGN CRITERIA FOR DISTRIBUTARY HEAD REGULATOR AND
CROSS REGULATOR
The effective waterway of a head regulator should not be less than 60 per cent of the width of the offtaking canal. It should be fixed such that the mean velocity of flow at full supply condition does not exceed 2.5 m/s. The overall waterway should at least be 70 per cent of the normal channel width (at mid-depth) of the offtaking channel at the downstream of the head regulator. For cross regulators, waterway should be decided so that the resulting afflux does not exceed 0.15 m.
The crest level of a head regulator should be such that the full supply discharge of the offtaking channel can be passed even when the parent channel is running with low supplies of the order of two-thirds of the fully supply discharge of the parent channel. It should be possible to maintain full supply level in the parent channel downstream of the offtake by means of a cross regulator. The water level at the location of offtake should be computed using back water computations. The level of the crest of the head regulator is obtained by subtracting the amount of head required from the computed water level at the offtake. In any case, the crest level

374 IRRIGATION AND WATER RESOURCES ENGINEERING
should not be lower than the bed level of the offtaking channel. Usually, the crest level of the head regulator is 0.3 to 0.6 m higher than the crest level of the cross regulator. The amount of head over the crest of the head regulator, H is computed from the equation (3),

Q = CBeH^3/2

(10.23)

where, Q is the full supply discharge of the offtaking channel, and B_e is the effective width of waterway and is given as

Be = Bt – 2 (NKp + Ka) H

(10.24)

Here, B_t is the overall width of the waterway (i.e., the length of the crest), N the number of piers, and K_p and K_a are contraction coefficients for piers and abutments, respectively. Values of K_p range from 0.005 to 0.02 while those of K_a range from 0.1 to 0.2.
C is a suitable discharge coefficient whose value can be taken as 1.84 for sharp-crested weirs (the crest width being less than 2/3 H) and 1.705 for broad-crested weirs (the crest width being greater than 2.5 H) for free flow conditions (3). If the flow is submerged, the values of C will have to be suitably modified depending upon the submergence ratio.
The crest of the cross regulator should be at least 0.15 m above the bed of the canal but should not be higher than 0.4 times the normal depth of the upstream canal (4). The crest width should be greater than 2/3 H and should be sufficient to accommodate the gate cill. The upstream and downstream slopes of the crest can be at slopes of 2 (H) : 1 (V). Impervious floor and cutoffs will be designed from considerations of hydraulic jump, uplift pressures, safe exit gradient, and scour depth. Similarly ‘flexible’ protection works at the upstream and downstream ends of the impervious floor will be provided in the form of block protection, inverted filters, and launching apron.

10.13. CONTROL OF SEDIMENT ENTRY INTO AN OFFTAKING CHANNEL
When water is withdrawn by an offtaking channel from the parent canal carrying sediment-laden water, it is essential that the offtaking channel also withdraw sediment in proportion to its water discharge. For achieving proportionate distribution of sediment between the offtaking channel and the parent canal, measures such as silt vanes, groyne walls, and skimming platform are constructed.
10.13.1. Silt Vanes
Silt vanes (also known as King’s vanes) are thin, vertical, and curved walls made of plain or reinforced concrete. The recommended dimensions of silt vanes are shown in Table 10.2 and Fig. 10.15. The dimensions are intended only as rough guides and can be used for skew offtakes also. The height of the vanes may be about one-fourth to one-third of the depth of flow in the parent canal (5). The thickness of the vanes should be as small as possible. Faces of vanes should be smooth. The spacing between the vanes may be kept about 1.5 times the vane height.
Table 10.2 Dimensions (in metres) of silt vanes (5)

W	0.60	1.2	1.8	2.4	3.0	3.6	4.6	6.0	7.6	9.0	10.6	12.0
X	1.2	1.5	2.1	2.4	3.0	3.6	4.6	5.4	6.0	7.0	7.8	8.5
Y	0.6	1.2	1.5	1.8	2.4	2.7	3.0	4.0	5.2	6.0	6.6	7.6
Z	1.2	1.2	1.5	1.8	2.4	2.7	3.0	3.6	4.2	5.2	5.8	6.6
R	9.0	9.0	10.0	12.0	18.0	21.0	24.0	30.0	35.0	44.0	50.0	57.0

Note: See Fig. 10.15 for meaning of symbols used in this table.

CANAL REGULATION STRUCTURES

375

Parent
Canal R

X

Z

Y
W
Head of offtaking canal

Flow

Fig. 10.15 Silt vanes (5)
Reverse vanes (Fig. 10.16) may have to be provided in cases in which the width of the parent canal is small and the sediment deflected towards the edge of the parent canal is likely to be deposited there due to low velocities.
Top of bank Side pitching




Reverse

vanes

45°

Z

Top of bank		Side pitching

Fig. 10.16 Reverse vanes (5)
10.13.2. Groyne Wall (Curved Wing)
A curved vertical wall (also known as Gibb’s groyne wall) extending from the downstream abutment of the offtaking channel into the parent channel (Fig. 10.17) causes the offtaking channel to draw its share of sediment load. The wall should extend at least up to 3/4 of the width of the offtake. It may, however, preferably extend up to the upstream abutment of the offtaking canal. The nose of the wall should be pointed and vertical, and thickness of the wall should increase gradually. The top of the wall is kept at least 30 cm above the full supply level

376 IRRIGATION AND WATER RESOURCES ENGINEERING
of the parent channel (6). If the offtaking channel has a slope milder than that of the parent channel, the curved wall may not be useful to the desired extent, and has to be employed in conjunction with sediment vanes (6).

Parent channel

30° to 45°

Fig. 10.17 Curved wing wall (6)
10.13.3. Skimming Platform
A skimming platform consists of an RC slab placed horizontally in the parent channel in front of the offtake (Fig. 10.18). It works on the principle of sediment excluder (Sec. 13.11) and is suitable only where the parent channel is deep (say 2 m or more) and the offtake is comparatively small (7).

Parent channel

30°

60°

1.5 to 3 m

Offtake
Fig. 10.18 Skimming platform (7)
10.14. CANAL ESCAPES
A canal escape is a structure to dispose of surplus or excess water from a canal. A canal escape essentially serves as a safety valve for the canal system. It provides protection of the canal against possible damage due to excess supplies which may be on account of either a mistake in releasing water at headworks, or a heavy rainfall due to which there may be sudden reduction

CANAL REGULATION STRUCTURES

377

in demand, making the cultivators close their outlets. The excess supply makes the canal banks vulnerable to breaches or dangerous leaks and, hence, provision for disposing of excess supply in the form of canal escapes at suitable intervals along the canal is desirable. Besides, emptying the canal for repairs and maintenance and removing a part of sediment deposited in the canal can also be accomplished with the help of the canal escapes. The escapes are usually of the following types (8):

(i) Weir or surface escape: These are weirs or flush escapes constructed either in ma-sonry or concrete with or without crest shutters which are capable of disposing of surplus water from the canal.
(ii) Sluice escapes: Sluices are also used as surplus escapes. These sluices can empty the canal quickly for repair and maintenance and, in some cases, act as scouring sluices to facilitate removal of sediment.
Location of escape depends on the availability of suitable drains, depressions or rivers with their bed level at or below the canal bed level for disposing of surplus water through the escapes, directly or through an escape channel.

EXERCISES

1. 
   Why does the need of a canal fall arise ? Describe different types of fall.
   

2. 
   What are the functions of a distributary head regulator and a cross regulator ?
   

3. 
   A Sarda fall is to be designed for a drop of 1.5 m in a channel 20 m wide and carrying 20 m³/s of water discharge at a depth of 1.5 m. Determine: (i) the crest dimensions, (ii) the minimum length of floor to be provided on downstream, and (iii) the length and depth of the cistern.
   

4. 
   Design 1.25 m Sarda fall on a channel carrying 22 m³/s of water with bed width and water depth of 16.0 m and 1.75 m respectively. The channel cross-section is trapezoidal with side slopes as 1.5 (H) : 1 (V). Draw the plan and longitudinal section of the fall.
   

5. 
   Design a 2.5 m straight glacis fall on a channel carrying 35 m³/s and having a bed width of 25 m and full supply depth of 1.8 m. The slope of the glacis is to be kept 2.5 (H) : 1 (V). Safe exit gradient may be taken as 1/6.
   

REFERENCES
1. ...... Irrigation Canal Falls, CBIP Publication No. 10, New Delhi, 1935.

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

2. 
   Varshney, RS, SC Gupta and RL Gupta, Theory and Design of Irrigation Structures, Nem Chand & Bros., Roorkee, 1982.
   

4. ...... IS: 7114-1973, IS Code for Criteria for Hydraulic Design of Cross Regulators for Canals.

5. ...... IS:6522-1972, IS Code for Criteria for Design of Silt Vanes for Sediment Control in Offtaking Canals.
6. ...... IS:7871-1975, IS Code for Criteria for Hydraulic Design of Groyne Walls (Curved Wing) for Sediment Distribution at Offtake Points in a Canal.
7. ...... IS:7880-1975, IS Code for Criteria for Hydraulic Design of Skimming Platform for Sediment Control in Offtaking Canal.
8. ...... IS:6936-1973, IS Guide for Location, Selection and Hydraulic Design of Canal Escapes.


11
CROSS-DRAINAGE STRUCTURES

11.1. NEED OF CROSS-DRAINAGE STRUCTURES
Aligning a canal on the watershed of an area is necessary so that water from the canal can flow by gravity to fields on both sides of the canal. However, a canal taking off from a river at A (Fig. 11.1) has to necessarily cross some streams or drainages (such as at a, b, c, and d in the figure) before it can mount the watershed of the area at B. In order to carry a canal across the streams, major cross-drainage structures have to be constructed. Once the canal is on the watershed at B, usually no cross-drainage structure is required except in situations when the canal has to leave a looping watershed (such as DEF in Fig. 11.1) for a short distance between D and F, and may cross tributaries (as at e and f ). Cross-drainage structures are constructed to negotiate an aligned channel over, below, or at the same level of a stream (1).

	Tributaries
O	A
	a
c	b
d
River	River
B

D e

E

f
F
Tributaries

OBDEF Watershed
ABDF Canal alignment
Fig. 11.1 Canal alignment between offtake and watershed
11.2. TYPES OF CROSS-DRAINAGE STRUCTURE
The cross-drainage structures can be classified under three broad categories depending on whether the structure is built to negotiate a carrier channel over, below, or at the same level as the stream channel.

378

CROSS-DRAINAGE STRUCTURES

379

11.2.1. Structures for a Carrier Channel Over a Natural Stream
The structures falling under this category are aqueducts and siphon aqueducts. Maintenance of such structures is relatively easy as these are above ground and can be easily inspected. When the full supply level (FSL) of a canal is much higher than the high flood level (HFL) of a stream which, in turn, is lower than the bottom of the canal trough, the canal is carried over the stream by means of a bridge-like structure, which is called an aqueduct. The stream water passes through the space below the canal such that the HFL is lower than the underside of the canal trough, Fig. 11.2

Topofdowel	Service road
	Slope 1.5:1
Z	Flow
ofbank	Canal	Canal
	bed	wind
		wall
Top	Slope 1.5:1
	Right bank

Stream

bed

Transition	Stream	flow
1 in 2
	Side wall