A cross- drainage work is a structure carrying the discharge of a natural stream across a canal intercepting the stream. When a canal is to be taken to the watershed, it crosses a number of natural streams in the distance between the reservoirto the watershed. Once the canal is taken to the watershed, no cross-drainage works are normally necessary except when the canal leaves the watershed for some distance forming a loop. However, the canal is aligned as a contour canal, a number of cross-drainage works are necessary.

A cross-drainage work is generally a very costly item, and should be avoided as far as possible by (i) diverting one stream into another, or (ii) changing the alignment of the canal so that it crosses below the junction of two streams.

8.12.1 Types of Cross-drainage Works:

Depending upon the relative levels and discharges, cross-drainage works may be of the following types:

1. C.D Works carrying canal over the drainage:

In this type of C.D work, the canal is carried over the natural drain. The advantages of such arrangement is that the canal, running perennially, is above the ground level and is open to inspection. Also, the damage done by the flood is rare. However, sometimes during heavy floods the foundation can be scoured, or the water way of the drain may be choked with trees etc. This is the usual type of work constructed when the drain is very big in comparison to the section of canal. The structure that fall under these types are:

(a) Aqueduct

(b) Syphon Aqueduct.

Figure134b and c shows the aqueduct and syphon aqueduct respectively. As is clear from Fig 134b, the HFL of the drain is much below the bottom of the canal trough in the case of aqueduct so that the drainage water flows freely under gravity. However, in case of syphon aqueduct, the HFL is much higher above the canal bed, and the water runs under syphonic action through the aqueduct barrels. In this case, the water surface level of the flood is depressed when it passes under the canal trough. The bed of the drainage is also lowered.

(a) Plan of crossing

(b) Aqueduct

(c) Syphon Aqueduct

1. C.D. works carrying drainage over the canal:

In this type of C.D. works, drainage is carried over the canal. The advantage of this is that the C.D works themselves are less liable to damage than the earth work of the canal. The major disadvantage of this work is that the perennial canal is not open to inspection. Also, if the silt is deposited in the barrels of the work, it is difficult to clear it out.

The structures that fall under this type are:

(a) Super-passage

(b) Canal Syphon.

Figure 135b shows a super passage. A super passage is similar to an aqueduct except that in this case the drain is over the canal. The FSL of the canal is lower than the underside of the trough carrying drainage water. Thus, the canal water runs under gravity. Fig 135c shows a canal syphon, or simply syphon. In this case, the levels are such that the FSL of the canal is much above the bed level of the drainage trough, so that the canal runs under the syphonic action under the trough. The canal bed is lowered and a ramp is provided at the exit so that the trouble of silting is minimised.

(a) Crossing

(b) Super passage

(c) Syphon

1. C.D. works admitting the drainage into the canal.

In this type of work, the canal water and the drainage water are permitted to intermingle with each other. The only advantage of this type of work is its low initial cost. Such type of workshas the following disadvantages:

(a) Regulation of such work is difficult and requires additional staff.

(b) The canal has to be designed to carry the increased flood discharge of the drain.

(c) The faulty regulation of the gates may damage the canal.

(d) There is additional expenditure of silt clearance.

Following are the structures under this type of C.D. works:

(i) Level crossing (ii) Inlet and outlet.

Figure 136a shows the schematic plan of level crossing which is constructed in the circumstance when the bed of the canal and drainage are practically at the same level. In this type of work, the drainage water is passed into the canal and then taken out at opposite bank. The work consists of (i) construction of crest, with its top at the FSL of the canal, at the u/s junction with the canal, and (ii) a cross regulator across the canal at its d/s junction with the drainage. When the drainage does not carry any water, its regulator is closed while the cross regulator is kept fully open so that the canal flows without any interruption. During the floods, however, the drainage regulator is opened so that the flood discharge, after spilling over the crest and mixing with the canal water, passes it to the d/s of the drainage. The accurate supplies in the canal are maintained by a cross regulator. Level crossing are suitable for canals of all sizes. They are, however, especially suited to crossings of very large drains when the cost of other cross-drainage would be very high.

A canal inlet is constructed when cross-drainage flow is small, and its water may be absorbed into the canal without causing appreciable rise. However, if the canal is small, an outlet may be constructed to pass out the additional discharge which has entered the canal. It is not necessary that the number of inlet and outlet should be the same.