Canal Structures, Ingenuity at its Best
02 November 2015
If barrages and weirs are meant to raise water level in rivers and headworks to control flow, together making the irrigation system work, there are auxiliary structures on and along the canals that are examples of civil and hydraulic engineering at its ingenious best.
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| A distributary of Upper Swat Canal crosses a rivulet by aqueduct in the Yusufzai Plain |
As far as the layman is concerned, a canal is excavated and water let in by the headworks to make it flow. However, as the channel winds its way across the topography, it traverses variable conditions, the most common being a river or canal crossing. In such a situation, there are three possible variables: the canal and diagonal water body flow at the same level, the canal flows at a level higher than the other water body or the canal is lower than the one to be crossed.
In the first instance, where the two transverse streams flow at exactly the same level, canal engineers devised what can well be termed their most imaginative piece of work: the level crossing. This mechanism essentially maintains level because every canal is designed for a calculated maximum quantum of water. While less is fine, any more than the calculated flow can damage the canal from scouring the bed to causing breaches. A level crossing is most suitable where a hill torrent with uncontrolled flow crosses the canal.
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| The downside regulators of the level crossing to let water into the hill torrent after the canal crossing |
At the meeting point, as the transverse water bodies flow into each other, the downstream sides on both streams are equipped with gated regulators: though inflow from the torrent cannot be controlled and may multiply the flow of the canal, its downside regulator can let water out to keep the canal at the required level. This system is crucial in the event of spate in the hill torrent. In case of low inflow from the parent river into the canal, the hill torrent regulator can add to overall flow and keep the canal running to capacity.
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| The structure of the aqueduct is both practical and aesthetically pleasing |
The Upper Jhelum Canal, long celebrated for its singularity as a perennial canal that drew from the river without a barrage or weir, is also the only canal in Pakistan equipped with four level crossings, all of them harnessing the flow of hill torrents that can be unruly in spate.
Canal engineers of old recall when the normally somnolent lives of level crossing crews were jolted into action by floods surging down the hill torrents. Of particular note are the summer rains of 1960, 1967, 1973 and 1976. In fact, there were occasions when the road and rail bridges at Sarai Alamgir would be swept away but for the vigilance of the crews manning the four level crossings on Upper Jhelum Canal.
In the second case, where the canal flows at a higher level than the stream to be crossed, the aqueduct serves the purpose. There are about two dozen examples of aqueducts around the country, most being simple brickwork affairs. However, the structures on the Upper Swat Canal as it traverses the Yusufzai Plain are not just architectural pieces to convey the canal across, their builders also kept a sharp eye on aesthetics.
These structures are constructed with limestone, locally quarried and finely dressed to produce specific forms, especially in the prow-shaped piers. However, tiles, burnt to perfection, go into the construction of the heads of the arches and their keystones. The entire arrangement produces an eye-catching effect. To add to this attractive centrepiece is the blue strip of the hill stream below and the variegation of cultivated land in the middle ground set to pleasing advantage against a backdrop of distant hills. The aqueducts of the North West Frontier, renamed Khyber-Pakhtunkhwa, are aesthetically among the most appealing of canal structures anywhere in Pakistan.
In the third case of the canal passing at a lower level than the stream to be crossed, the siphon comes into play. A brick and concrete conduit dives under the transverse stream as if the canal of a sudden disappears into the ground. Except that it reappears on the far bank of the transverse watercourse.
Siphons, commonly used across the irrigation system of the country, are architecturally and photographically virtual non-entities. Other than a brick rampart marking the spot where the canal goes under and another on the far side where it emerges, there is no impressive architecture. Nor too is there any steel superstructure or ageing winches with geometric shapes to add to the beauty of brick and stonework. Siphons, if it may be said, are the poorest cousins in the wide array of canal fixtures.
And then there is the fall, the dumb waiter of canal engineering. This concrete construction is needed where the canal encounters a sudden drop in level. Since increased flow can enhance erosion, scouring the bed and banks of the channel, this construction, visible only when the canal is closed, breaks the fall as its concrete bed, extending several metres downstream, controls erosion.
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| A level crossing on Upper Jhelum Canal flowing at par with the hill torrent coming in from the right not in view. The inflow of the canal can be seen in the background while on the left is the outlet for excess water back into the bed of the hill torrent |
Though canals flowed in the subcontinent for centuries before the advent of the Raj, it was the scientific understanding of hydraulics that British engineers brought with them that resulted in establishing the greatest irrigation networks of the world. And so they roll on to this day, bringing untold prosperity to the country.
Previous: Begari Wah, Salam, Jekum Sahib Bahadur, Jamrao Canal, The Dragon’s Tail, Khanki Headworks, From Primeval Forest to Breadbasket, Lower Bari Doab Canal, Boundless Magnanimity, Upper Jhelum Canal, No Small Wonder, Upper Swat Canal, Defying Mountains, Sukkur Barrage, Fife Dream, Khirthar Canal, A touch of picturesque, Bambanwala-Ravi-Bedian Link Canal, Raiya Branch to the Rescue [BRB Canal], Sulemanki Headworks, Bloom the Desert
Labels: Book of Days 2015, Waters of Empire
posted by Salman Rashid @ 00:00,
2 Comments:
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At 3 November 2015 at 18:51,
Naeem Malik said...
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Salman,
Just a couple of weeks ago I had an argument with my friend about the canals built by he Raj. I said that they did a good job by constructing railways and canals. His counter argument was that the canals were bad because they caused a lot of flooding.
Please share your views and enlighten me.
Thank you,
Naeem.
PS: You write a lot, for me its difficult to read at this pace :) -
At 4 November 2015 at 10:32,
Salman Rashid said...
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Flooding by canals? Your friend is way off the mark. Next he'll tell you something evil about railways as well.
If your friend means water-logging, he still is wrong: we have to weigh one against the huge acreage that was brought under the plough and the food we got from this irrigation.
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