• Irrigation structures constructed for carrying the canal water safely over or under the drainage water are called cross drainage works. When a canal is taken off from the reservoir, it meets various natural drainage's before reaching the watershed line. In this range, cross drainage works are required to be constructed.

• The construction of cross drainage work is very costly. Hence, the alignment of canal shall be such as to have a minimum number of cross drainage works.

• Figure. 6.9 indicates how such works can be avoided or reduced by diverting the flow of the stream of changing the alignment of canal.

Necessity of C.D. Work.

1] Irrigation structures constructed for carrying the canal water safely over or under the drainage water are called cross drainage work.

2] When a canal is taken off from the reservoir, it meets various natural drainage's before reaching the watershed. In this range cross drainage works are required to the constructed.

Types of cross drainage works.

Following are the types of cross drainage works.

a] Aqueduct [Fig. 6.10 (a)]

b] Super passage [Fig. 6.10 (b)]

c] Level crossing [Fig. 6.10 (c)]

d] Inlet and outlet [Fig. 6.10 (d)]

• The crossing of canal and drain should be normal to each other. The banks of drainage should be high and stable at the point of crossing.

a] Aqueduct: The irrigation structure constructed for passing the canal water safely over the drainage water is an aqueduct. (S – 07, S – 02, W – 08, S – 10, W – 10, S – 12, W – 12)

1] Canal bed level is above HFL of Nala.

2] The discharge of drain is more in comparison to canal discharge or conditions.

3] The bed level of canal is sufficiently above the high flood level in the drain.

b] Construction: An aqueduct consists of a masonry or concrete trough of rectangular section supported on abutments of piers. The drain water flows below the trough through the abutments and piers. In this type, the canal is open for section.

Fig. 6.11 (a) shows the usual type of aqueduct provided. A trough is divided into compartments and roadway is provided. The width of trough is made less than the width of canal to reduce the cost of masonry structure.

Syphon aqueduct (S – 09, W – 12): [Fig. 6.11 (b)] when the high flood level of the drain is much higher above the canal the usual type of aqueduct cannot be provided. The drain is then siphoned as shown in Fig. 6.11. The bed of the drain expressed below the crossing to form internal siphon. The drain water flows under hydro-static pressure. Necessary slope given to the depressed drain to give self-cleaning velocity so as to avoid slitting.

Difference between aqueduct and siphon aqueduct.

Sr. No	Aqueduct	Siphon Aqueduct
1.	Drainage water flows freely under gravity.	The water runs under symphonic action.
2.	HFL of drain is sufficiently below the canal bed.	HFL of drain is higher than canal bed but lower than FSL of canal.
3.

Other Types of Aqueducts:

[1] Masonry Aqueduct: Suitable when width of drainage is more than 15 m.

[2] Irrigation Culvert: Suitable when the width of natural drain is from 2.4 m to 15 m.

[3] Irrigation Slab Drain: Suitable when the width of natural drain is less than 2.4 m.

4] Irrigation pipe Aqueduct: Suitable when irrigation channel is very small as compared to the drain water and width of drain is large. Refer to Fig. 6.13.

5] Super passage: It is provided when Nela bed level is above FSL of canal. When the drainage water at a point of crossing is taken over the canal, the structure is called super passage. It is just the reverse of aqueduct.

Super passages are constructed under the following conditions:

[1] The discharge of drain is small in comparison with the canal discharge.

[2] Sufficient clearance is available between the F.S.L. of canal and the drain bed.

In this type of structure, the canal is not open for inspection. If the slit is deposited in the barrels of the structure, it is difficult to clear it off.

Syphon super passage (Fig. 6.14) : In this case, the canal bed is depressed and a ramp is provided at the exit so that the deposition of slit is minimized.

It is constructed at the crossing of drain and a canal under the following conditions:

1] HFL of nala is between FSL of canal and bed level of canal.

2] The drain bed is at a higher level than F.S.L. of canal.

3] The clearance between the drain bed and F.S.L. of the canal is either insufficient or the drain bed is lower than the F.S.L. of canal, but higher than the bed of canal.

4] When the drainage is large in comparison to the canal, such a situation may occur in contour alignment of canal.

Comparison between Aqueduct an super passage.

At the end of main canal or to allow escape of the slity water into the waste channel and then to natural drain during rainy season escapes are provided. Irrigation structures constructed to escape extra water from the canal into some natural drain or nallah is called canal escape. They are located at 5 to 10 km c/c along the length of major canal near the natural drain.

Function: The canal escape is constructed for the purpose of wasting some of its water.

A canal escape is a structure constructed on an irrigation canal for the purpose of wasting some of its water.

Depending upon the purpose, there can be three types of escapes:

1] Canal scouring escape.

2] Surplus escape and

3] Tail escape

The sourcing escape is constructed for the purpose of scouring off excess slit from time to time. Escapes are also constructed to dispose off excess supplies of the parent channel. Excess supplies in the canal take place either during heavy rains or due to the closure of canal outlet by the farmers. In this case, the escapes save the d/s section of the canal from overflow of banks

Escapes are thus essential safety valves for the canals, and should be constructed at intervals. The canal leading the surplus water to a natural drain is known as escape channel. The capacity of escape channel should not be less than 50% of capacity of the parent channel at that point. A canal surplus escape may be weir type, with the crest of weir wall at F.S.L. of parent canal bed level [Fig. 6.17 (a)]. A tail escape [Fig. 6.17 (b)] is provided at the tail end of the canal, and is useful in maintaining the required F.S.L. at the tail end. The structure is weir with its crest level at the required F.S.L. of canal at its tail end.

Purpose or objective of canal escapes:

1. To remove surplus water from an irrigation channel into a natural drain.

2. To avoid damage to the channel by surplus water.

3. To provide the safety valve of canals.

4. It save the d/s section of the canal from overflow of banks.

CANAL OUTLETS.

INTRODUCTION.

An outlet may be denied as a device constructed in a canal to deliver designed discharge from a government channel to the field channel or water course (cultivator’s channel). It is a work which connects a government channel with the water course. Outlets are generally not provided on the main canal and branches, but are installed in such cases in the ditch distributaries constructed along the canals. In exceptional cases, when provided on main canal and branches these are called direct outlets. Since equitable distribution of the canal supplies is dependent on outlet, it must be such that it not only passes a known and constant quantity of water, but must also essentially be a measure of the discharge.

CLASSIFICATION OF OUTLETS.

The outlets are classified into three types, as under:

1. Modular Outlet: Also called modules or rigid modules. It is the outlet whose discharge is independent of the water leveI is in the distributary and the water course within reasonable working limits. The outlet discharge is thus constant within the working limits. The loss of head through the outlet is more. This type of outlet is either with moving parts or without moving parts. In the latter case, these are called rigid modules. Outlets with moving parts have disadvantages 1. Expensive being not simple to design and construct and 2. Liable to de arrangements due to increase in friction, rusting of the moving parts and any obstruction in the working of the parts caused by slit and weeds in the canal water.

Gibb’s Module: It is the most common type of modular outlet. It essentially consists of bell mouth inlet pipe curved upwards from which water is led into an eddy chamber, semicircular in plan, around which water flows giving rise to a free vortex flow (Fig. 14.2). a number of baffles are

provided in the eddy chamber with thin lower ends sloping against the direction of flow at the required height above the bottom to prevent increased discharge passing through the module. The number o baffles coming into action depends on the variation in head. With increase in the head causing flow, the water banks up at the outer circumference of the eddy chamber and impinges against the baffles imparting an upward, rotational direction of flow of the water, which spins round in the compartment between two successive baffles an finally drops on the incoming stream of water thereby dissipating excess energy and keeping the discharge constant. The disadvantage’s of the outlet are that it is costly and easily tempered with.

Discharge Formula

$Q = r_0 \sqrt{2g} (y_1 + h_o)^{3/2} [ \frac{m^2 – 1}{m^3} log_2 \ m + \frac{1}{m} log_e \ m - \frac{m^2 – 1}{2m^2} ]$

Where, Q = discharge (cumec), $r_o$ = radius of outer semi-circle of eddy chamber, $r_1$ = radius of inner semi-circle of eddy chamber, m = $r_o/r_1$, i.e. ratio of outer to inner radius, $y_1$ = water depth at inner circumference, and $h_o$ = head at outer circumference.

The formula holds good only for Gibb’s standard design in which m = 2, $h_o/D$ = 1/7, where D is the difference of level measured from the minimum water level in the distributary to the floor of eddy chamber.

2. Semi modular outlet: It is the outlet the discharge of which is independent of water level in the watercourse but dependent on the water levels in the distributary so long as the minimum working head required or its working is available. The outlet discharge thus increases with rise in the water level in the distributary properly designed semi modules can distribute more or less equitably various in the supply in distributary. Adjustable proportional module, open flume outlet and free fall pipe outlet belong to this class. In free fall outlet the exit end of pipe is placed higher than the water level in the water course (Fig. 14.6) such that the working head is the difference between water level in the distributary and centre of the pipe at exit end.

3. Non modular outlet: It is the outlet discharge is a function of the difference in levels between the water surfaces in the distributing channel and the water course. Variations in either effect the discharge. The outlet discharge thus increases with rise in water level in the dis-tributary or lowering of water level in the water course. Pipe outlet is a non-modular type of