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Regime canals
Before the colonization of India by the British, a network of irrigation canals existed in the Punjab, besides many other areas of the country. This network had been designed and built during the Muslim rule.
However, information regarding the methodology of the canals’ design does not seem to exist or is simply not available to researchers. Unfortunately, the same is true of the design of buildings and other structures built by Muslim rulers.
Although they built beautiful monuments — Taj Mahal in Agra, Red Fort in Delhi and Badshahi Masjid in Lahore, to name just a few — they did not leave behind for posterity the details of the methods they employed.
In the early 1970s, I was interested in finding out if the engineers of Muslim India had the true conception of the water discharge of a canal (rate of flow of water per unit of time, say, per second). Therefore, I spent considerable time in trying to discover appropriate sources of such information but failed to find any.
From the physical evidence that they left behind — for example, the canals that remained in flow for a long time — it appears that Muslim engineers must have had a good understanding of the fundamental principles of hydraulics. However I failed to find any factual source material that could support such a thought.
Curiously, not a few but many canals built by Muslim engineers operated satisfactorily. By satisfactory operation we mean that the canals transported the quantity of water according to their designed capacities without suffering from the problems of sedimentation and erosion.
Sedimentation is the process of silt and sand depositing on the bed and banks of a canal. Deposition of sediments reduces the flow capacity of a canal and if this is excessive it might choke it completely. Excessive erosion, on the other hand, may breach the canal banks and let the water flow out over the adjoining land, flooding it.
It is imperative that a well-designed canal is free of such problems. Whatever sediments exist in the flowing water should keep moving with it. At the same time, the flow should not erode the canal-bounding surface. Such a canal is called a sediment-stable canal.
Among the canals that the British engineers inherited from the Muslim rulers, several were choked up and were out of operation due to a lack of maintenance. However, many flowed smoothly, transporting water for irrigation to lands where it was needed. The British engineers called the canals that operated satisfactorily the “regime canals”.
They tried to comprehend how these canals were designed by their predecessors, but didn’t find any methodology on the record. The next best thing that they tried to do was to model the flow characteristics of the regime canals into the new canals that they designed. For this purpose, they needed to develop empirical relationships between the flow parameters of the regime canals.
Therefore they collected the basic flow data from many regime canals. Among the data that they required were the discharge and geometrical parameters of the canals, including flow depth, width, and the bed slope. Flow velocity could be calculated if the discharge and flow area were known.
From these data, empirical formulae were developed and used for the design of new canals. For instance, they developed a formula connecting velocity and the flow depth of a regime canal. Later on, other formulae were developed, connecting each canal parameter with its design discharge.
A complete set of these formulae were in due time systematized and given by Gerald Lacey. These formulae were called Lacey’s formulae.
An intriguing fact that was noticed was that a velocity formula differed for the canals in one region from those located in another. It was argued, correctly, that the form and structure of such formulae were influenced by the size of the sand particles in which a canal flowed. Both the size of the sand particles and their quantity suspended in the flow and moving at the canal boundary (bed and banks) changed its water transporting capacity.
Information regarding the methodology of canal designs does not seem to exist. Unfortunately, the same is true of the design of buildings and other structures built by Muslim rulers
Lacey introduced the size of bed particles in his empirical formulae to account for their effect on flow. Sir Claude Inglis introduced both the size and sediment transport in his empirical formulae. These methods were used all over India reportedly with great success and a similar development took place in colonial Egypt, although the formulae that were used in the two countries were substantially different from each other.
These methods, however, were tentative because they were not derived from the laws of Newtonian mechanics. Also, in due time, a host of similar formulae were published and it became difficult to choose from them for use in a given situation. For practical reasons, many provincial departments specified which formulae were to be used for designing regime canals in their jurisdiction. Lacey’s formulae, for example, became the standard method of design in Punjab.
In Europe, interest in the movement of sand at the bed and in suspension of an open channel (a canal) was fundamentally focused on the principles of theoretical hydromechanics. Hans Einstein (son of the celebrated Albert Einstein) developed a bed load formula largely using theoretical methods. This is quite complicated for general use.
Later on, Hunter Rouse developed a theoretical formula for predicting the distribution of suspended sediments at various depths in an open channel flow. These approaches were very important, in their own right, for developing the mechanics of sediment transport but they could not be directly used for designing sediment-stable canals.
My interest in regime canals originated in Northern Nigeria, when I was given the charge of building a small irrigation scheme for irrigating some 300 acres of land at Tungan Tudu, which was located 35 miles northwest of Sokoto. I used an engineering book written by an Indian author for my guidance.
That was my first serious contact with the Lacey’s method. I was able to understand the technical shortcomings of the method, but was impressed nevertheless by its simplicity of use even though the problem at hand was rather complex. In a short time, I realized that Lacey’s method was not applicable for the soil conditions of the area where I was working.
Lacey’s method is applicable only in silty and non-cohesive soils, while the soils at Tungan Tudu were clayey. I did not use Lacey’s method but my interest in regime method was nonetheless sparked which led me to my research career.
I started with a concerted effort to find out if there was a common link between Einstein’s bed load formula and Lacey’s empirical equations. In my first research paper — “Rationalization of Lacey’s flow equations” — I showed some similarities between the two.
This paper attracted considerable attention from researchers working on sediment-stable canals. Both Lacey and Inglis, among several other distinguished researchers, discussed my paper. This was a tiny step towards building a bridge between theory and the empirical regime method. Work in this area is continuing.
After publishing this work, my interest drifted to some other problems in the field of sediment transport. However, when Gary Parker — now at University of Illinois — published an important paper, “Self-formed rivers with stable banks: part I” (Journal of Fluid Mechanics, 89(1), 1978), in which he developed an integral equation which defined the shape of stable rivers, I thought of developing a closed form solution of this equation.
I was able to obtain a simple linear solution which I published in a discussion on the “Critique of the regime theory for alluvial channels” by Michael Stevens (JHD, ASCE, 1987), together with an outline of the method of design of regime canals.
This method was quite complex and unsuitable for the design offices in Pakistan where simplicity is a supreme virtue. I was recognized for contributing the discussion on Michael Stevens’ paper and given the JC Stevens Award of American Society of Civil Engineers in 1990.
Although a lot of development has taken place in the field of sediment transport, there are still no theoretically sound and simple formulae available for designing sediment-stable canals. Theore-tical methods that can be used for design purposes are very complex.
Developing a simple, theoretically sound and reliable method for designing sediment-stable canals remains a challenge. But simplicity is also relative. In the computer age of the modern era, complex problems have been broken down by the iterative numerical methods of computation.
A structurally complex method, when appropriately programmed on a digital computer, becomes simple and routine. There is a compelling need to introduce more accurate methods of design in Pakistani engineering practice.
I doubt if an accurate method of design of the alluvial canals will have the simplicity of Lacey’s equations. So be it. We should look beyond Lacey.
We should try to develop new and more accurate methods and overcome their complexity with the use of computers. We need to introduce sophistication of the new knowledge in our engineering practice.
It is a fact that not many problems related to sediment transport — for example, design of stable canals, erosion of river channels, local scour around obstructions — are simple. While the existing knowledge in this field of enquiry is being enhanced, methods should be discovered to apply this knowledge in our engineering practice.
The writer is a US-based engineer who did his PhD from London University. Email: akramgill@yahoo.com
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