Cross Drainage Works

Cross Drainage Works Points : cross drainage works, types of cross drainage works, cd work carrying canal over the drainage, cd work carrying drainage over the canal, cd work admitting the drainage water into the canal, canal escape 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 reservoir to 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, when 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. Types of Cross — Drainage Works: Depending upon the relative levels and discharges, cross-drainage works may be of the following types: (i) CD Work carrying canal over the drainage In this type of C.D .work the canal is carried over the natural drain. The advantage of such arrangement is that the canal, running perennially, is above the ground and is open to inspection. Also, the damage done by flood is rare. However, sometimes during heavy floods the foundation can be scoured, or the waterway of the drain may be chocked with trees etc. This is the usual type of work constructed when the drain is very big in comparison to the section of the canal. The structures that fall under this type are:
1. Aqueduct
2. Syphon aqueduct.

and (c) show the aqueduct and syphon aqueduct respectively. As is clear from fig (b), the H.F.L of the drain is much below the bottom of the canal through in the case pi aqueduct so that drainage water flows freely under gravity. However, in the case of a syphon aqueduct, the H.F.L of the drain 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.

(ii) CD Work carrying drainage over the canal: In this type of CD work, drainage is carried over the canal. The advantage of this type is that the CD works them selves 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
1. Super-passage
2. Canal siphon

Fig (b) shows a super-passage. A super-passage is similar to an aqueduct except that in this case the drain is over the canal. The F.S.L. of the canal is lower than the underside of the trough carrying drainage water. Thus, the canal water funs under gravity. Fig. (c) shows a canal syphon, or simply syphon. In this, case, the levels are such that the F.S.L. of the canal is much above the bed level of the drainage trough, so that the canal runs under 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 minimized

(iii) CD Work admitting the drainage water Into the canal: In this type of work, the canal water and the drainage water are permitted to interming with each other. The only advantage of this type of work is its low initial cost. Such type of works have the following disadvantages:

(i) Regulation of such a work is difficult and requires additional staff.
(ii) The canal has to be designed to carry the increased flood discharge of the drain.
(iii) The faulty regulation of gates may damage the canal.
(iv) There is additional expenditure of silt clearance.

Following are the structures under this type CD work
1. Level crossing
2. Inlet and outlets.

Fig (a) shows the schematic plan of a level crossing which is constructed in the circumstance when the beds 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 the opposite bank. The work consists of (i) construction of a crest, with its top at the F.S.L. of the canal, at the u/s junction with the canal, (ii) provision of the head regulator across the drainage at its d/s junction with the canal, and (iii) 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 of the canal 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 through it to the downstream of the drainage. The accurate supplies in the canal are maintained by a cross-regulator. Level crossings are suitable for canals of all sizes. They are, however, specially suited to crossings of very Large drains when the cost of other cross- drainage works would be very high.

A canal inlet [Fig. (b)] is constructed when cross-drainage flow is small, and its water may be absorbed into the canal with out causing appreciable rise. Ho ever, 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 inlets and outlet should be the same. There may be one outlet for two or three inlets,

Canal Escape 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 scouring escape is constructed for the purpose of scouring off excess silt 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 that case the escapes save the d/s section of the canal from over flow of bank.

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 paint. A canal surplus escape may be well type, with the crest of weir wall at F.S.L. of parent canal bed level (Fig (a)]. A tail escape 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 type with its crest level at the required F.S.L of canal at its tail end.