Popular Science Monthly/Volume 62/April 1903/The Nile Dams and Reservoir
|THE NILE DAMS AND RESERVOIR.|
By SIR BENJAMIN BAKER, F.R.S.,
PAST PRESIDENT INSTITUTION OF CIVIL ENGINEERS.
THE Nile Reservoir at Aswân will contain over 1,000 million tons of water. This statement will probably convey little meaning to most people; and in truth the quantity may be made to appear either small or large at will by a judicious selection of illustrations. Thus the absolute insignificance to Egypt of 1,000 million tons of water in a reservoir, as compared with a reasonable rainfall, will be apparent at once when it is considered that the annual rainfall on the area included within the four-mile cab radius from Charing Cross is about 100 million tons, and that the rainfall on London and its suburbs within a thirteen-mile radius would, therefore, about suffice to fill the Nile Reservoir. On the other hand, we may, by choosing other illustrations, restore the Nile Reservoir to the dignity of its just position of one of the greatest engineering works of the clay. Thus the question of the water supply of London, and its prospective population of 111⁄4 millions, has been prominently before the public for some years; and many will remember what was termed the colossal project of our member, Sir Alexander Binnie, late of the London County Council, for constructing reservoirs in every reasonably available valley in Wales, to store up water for London, and to supply compensation water to the Welsh rivers affected thereby. Well, the united contents of the whole of those reservoirs would be less than half that of the great Nile Reservoir. Again, the Nile Reservoir would hold more than enough water for one year's full domestic supply to every city, town and village in the United Kingdom with its 42 million inhabitants. But possibly the best way of giving an idea of the magnitude of the work, and its utility to cultivation in a thirsty land, is by considering the volume of the water issuing from the reservoir during the three or four summer months, when scarcity of supply prevails in the river and the needs of the cultivators are greatest. At that time the flow from the reservoir will be equivalent to a river double the size of the Thames in mean annual flood condition. It will be recognized at once that a good many buckets would have to be set at work to bale out a river like that, and yet the scarcity of water in the Nile itself, and in the canals, during the months of April, May and June, is such that even dipping the water out of the channels in buckets has to be controlled by strict regulations. Thus, two years ago, when the Nile was below the average in summer discharge, it was decreed in Upper Egypt that the 'lifting machines,' which include the handoff, or bucket-and-pole system, and the sakieh, or oxen-driven chain of buckets, should be worked not more than from five to eleven consecutive days, and stop the following nine to thirteen days, between the middle of April and the middle of July; and the order in which the different districts were to receive a supply was carefully specified, so that, as far as possible, every crop should get watered once in about three weeks. When it is remembered that a single watering of an acre of land means, where shadoofs are concerned, raising by manual power about 400 tons of water to varying heights up to 25 feet, and that four or five waterings
are required to raise a summer crop, it will be seen what a vast amount of human labor is saved throughout the world by the providential circumstance that in ordinary cases water tumbles down from the clouds, and has not, as in Egypt, to be dragged up from channels and wells. Shadoof work, under average conditions, involves one man's labor for at least one hundred days for each acre of summer crop; so that even at 6d. per day for labor, the extra cost of cultivation due to the absence of rain would amount to 50s. per acre.
The great Nile Reservoir and Dam at Aswan, the Barrage at Asyut, and various supplementary works in the way of distributing canals and regulators, are designed with the object of mitigating the evils enumerated above, by supplying in summer a larger volume of water at a higher level in the canals, so that not only can more land be irrigated, but that labor in lifting water will be saved. When the International Commission, eight years ago, recommended the construction of a large reservoir somewhere in the Nile valley, I was desirous of knowing what would be the opinion of a real old-fashioned native landowner on the subject; and was introduced to one whose qualifications were considered to be of no mean order, as he was a descendant of the Prophet, very rich, and had been twice warned by the government that he would probably be hanged if any more bodies of servants he had quarrelled with were found floating in the Nile. He was a very stout old man, and, between paroxysms of bronchial coughing, he assured me that there could be nothing in the project of a Nile reservoir, or it would have been done at least 4,000 years ago. In contrast with this I may mention that, a few months ago, the most modern and enlightened of all the rulers of Egypt, the present Khedive, when visiting the dam, said he was proud that the great work was being carried out during his reign, and that the good services rendered by his British engineers was evidenced by the London County Council coming to his Public Works Staff for their chief engineer.
The old system of irrigation, which the descendant of the Prophet looked back upon with regret, was little more than a high Nile flooding of different areas of land or basins surrounded by embankments. Less than a hundred years ago, perennial irrigation was first attempted to be introduced, by cutting deep canals to convey the water to the lands when the Nile was at its low summer level. When the Nile rose, these canals had to be blocked by temporary earthen dams, or the current would have wrought destruction. As a result, they silted up, and had to be cleared of many millions of tons of mud each year by enforced labor, much misery and extortion resulting therefrom. About half a century ago, the first serious attempt to improve matters was made by the construction of the celebrated barrage at the apex of the Delta. This work consists, in effect, of two brick arched viaducts crossing the Rosetta and Damietta branches of the Nile, having together 132 arches of 16 feet 4 inches span, which were entirely closed by iron sluices during the summer months, thus heading up the water some 15 feet, and 'throwing it at a high level into the main irrigation canals below Cairo. The latter are six in number, the largest being the central canal at the apex of the Delta, which, even in the exceptionally dry time of June, 1900, was carrying a volume of water one-fourth greater than the Thames in mean flood, whilst the two canals right and left of the two branches of the river carried together one half more than the Thames, and the Ismailieh Canal, running down to the Suez Canal, though starved in supply, was still a river twice the size of the Thames at the same time of the year. At flood times the discharges of all the canals are, of course, enormously increased. It will be recognized at once, therefore, that, as in the summer months the whole flow of the Nile is arrested and thrown into the aforesaid canals, the old barrage will always remain the most important work connected with the irrigation of Egypt. It was constructed under great difficulties by French engineers, subject to the passing whims of their Oriental chiefs. About fifteen years elapsed between the commencement of the work and the closing of all the sluices, and another twenty years before the structure was sufficiently strengthened by British engineers to fulfil the duties for which it was originally designed. All the difficulties arose from the nature of the foundations, as the timber sheet piling wholly failed to prevent the substructure from being undermined by the head of water carrying away the fine sand and silt upon which the barrage was built. At Asyût, cast-iron sheet piling was used, as will hereafter be described. It is impossible to say what the cost of the old barrage has been from first to last, but probably nearly ten times that of the recently-completed Asyût Barrage. Forced labor was largely employed in its construction, and at one time 12,000 soldiers, 3,000 marines, 2,000 laborers, and 1,000 masons were at work at the old barrage.
In connection with the Nile Reservoir, subsidiary weirs have been constructed below the old barrage to reduce the stress on that structure. The system adopted was a novel one, reflecting great credit on Major Brown, Inspector-General of Irrigation in Lower Egypt. His aim was to dispense almost entirely with plant and skilled labor; and so, without attempting to dry the bed of the river, he made solid masonry blocks under water, by grouting rubble dropped by natives into a movable timber caisson. Both branches of the Nile were thus dammed in three seasons, at a cost, including navigation locks, of about half a million sterling. Many other subsidiary works have been and will be constructed, including regulators, such as that on the Bahr Yusuf Canal.
By far the most important of the works constructed to enable the water stored up in the great reservoir to be utilized to the greatest advantage is the Barrage across the Nile at Asyût, about 250 miles above Cairo, which was commenced by Sir John Aird and Co. in the winter of 1898, and completed this spring. As already stated, in general principle this work resembles the old barrage at the apex of the Delta; but in details of construction there is no similarity, nor in material, as the old work is of brick and the new one of stone.
The total length of the structure is 2,750 feet, or rather more than half a mile, and it includes 111 arched openings of 16 feet 1 inches span, capable of being closed by steel sluice gates 16 feet in height. The object of the work is to improve the present perennial irrigation of lands in Middle Egypt and the Fayoum, and to bring an additional area of about 300,000 acres under such irrigation, by throwing more water at a higher level into the great Ibrahimiyah Canal, whose intake is immediately above the Barrage (Fig. 2).
The piers and arches are founded upon a platform of masonry 87 feet wide and 10 feet thick, protected up and down by a continuous and impermeable line of cast-iron grooved and tongued sheet piling, with cemented joints. This piling extends into the sand bed of the river to a depth of 23 feet below the upper surface of the floor, and thus cuts off the water and prevents the undermining action which caused so much trouble and expense in the case of the old barrage. The height of the roadway above the floor is 41 feet, and the length of the piers up and down stream 51 feet. The river bed is protected against erosion for a width of 67 feet up stream by stone pitching, with clay
puddle underneath to check infiltration, and down stream for a similar width by stone pitching, with an inverted filter-bed underneath, so that any springs which may arise from the head of water above the sluices shall not carry sand with them from underneath the pitching.
It is easy enough to construct dams and barrages on paper, but wherever water is concerned the real difficulty and interest is in the practical execution of the works, for water never sleeps, but clay and night is stealthily seeking to defeat your plans. On the Nile the conditions are very special, and in some respects advantageous. There is only one flood in the year, and within small limits the time of its occurrence can be foretold, and arrangements made accordingly. It would have been impossible to have carried out the Nile works on the system adopted had the river been subject to frequent floods. The working season for below-water work on the Nile lies practically between November and July, for nothing would be gained by starting the temporary enclosing embankments, or sudds, when the river was at a higher level than it is in November; nor would it be possible at any reasonable cost to prevent the sudds from being swept away by the flood in July. At Asyfit the mode of procedure was to enclose the site of the proposed season's work by temporary dams or sudds of sandbags and earthwork, then to pump out and keep the water down by powerful centrifugal pumps, crowd on the men, excavate, drive the cast-iron sheet piling, build the masonry platform and piers, lay the aprons of puddle and pitching, and get the work some height above low Nile level before the end of June, so that the temporary dams should not require reconstruction after being swept away by the flood. The busiest months were May and June, when in the year 1900 the average daily number of men was 13,000. It is also then the hottest; the shade temperature rising to 118 degrees. To keep the water down, seventeen 12-inch centrifugal pumps, throwing enough water for the supply of a city of two million inhabitants, had to be kept going, and in a single season as many as one and a half million sandbags were used in these temporary dams. The bed of the river being of extremely mobile sand, the constant working of the pumps occasionally drew away sand from under the adjoining completed portions of the foundations, necessitating the drilling of many holes through the 10-foot thick masonry platform, and grouting under pressure with liquid cement. About 1.000 springs also burst up through the sand, each one of which required special treatment. A new regulator had to be constructed for the Ibrahimiyah Canal, with nine arches and sluices, to control the high floods and prevent damage to the Canal and the works connected therewith.
Asvut, as already observed, is about 250 miles above Cairo. The great dam at Aswân is 600 miles above the same point. Between Asyfit and Aswan the remains of many temples exist, of far greater interest and importance than those at Philæ. The latter ruins, however, have attracted more attention in recent days, because, being situated immediately above the Dam, the filling of the reservoir will partially flood Philæ Island during the tourist season.
It would be idle to speculate as to who first thought of constructing a reservoir in the Xile valley, or who first arrived at the conclusion that the site of the present dam above Aswân was the best one. Mr. Willcocks, one of the ablest engineers of the Public Works Department of Egypt, who was instructed by Sir William Garstin to survey various suggested sites for a dam between Cairo and Wady Haifa, unhesitatingly decided that the Aswan site was the best, and the majority of the International Committee, who visited the sites in 1894, came to the same conclusion. This conclusion had, however, been anticipated by Sir Samuel Baker more than forty years ago, from mere inspection of the site without surveys. In suggesting a series of dams across the Nile to form reservoirs from Khartoum downwards, he wrote: "The great work might be commenced by a single dam above the first cataract at Aswân, at a spot where the river is walled in by granite hills. By raising the level of the Nile 60 feet, obstructions would be buried in the depths of the river, and sluice-gates and canals would conduct the shipping up and down stream." This single dam, proposed by Sir Samuel Baker forty years ago, is in effect the one which is now on the point of completion. Mr. Willcocks' original design consisted practically of a group of independent dams, curved on plan, and the arrangement of sluices and dimensions of the dam differed considerably from those of the executed work. There is no doubt that the single dam, 11⁄4 miles in length, constitutes a more imposing monumental work than a series of detached dams, and that it also offered greater facilities to a contractor for the organization of his work and rapid construction; and, further, the straight dam is better able to resist temperature stresses from extreme heat without cracking. Two dams across the Nile, the old barrage and the Asyût Barrage, have already been described; and it will be hardly necessary to say, therefore, that the Aswân Dam is not a solid wall, but is pierced with sluice openings of sufficient area for the flood discharge of the river, which may amount to 15,000 tons of water per second. There are 180 such openings, mostly 23 feet high by 6 feet 6 inches wide; and where subject to heavy pressure, when being moved, they are of the well-known Stoney roller pattern.
Although the preliminary studies of Mr. Willcocks and the other government engineers occupied some four years, there was neither time nor money to sink shafts in the bed of the river, to ascertain the real character of what was called in the engineer's report 'an extensive outcrop of syenite and quartz diorite clean across the valley of the Nile,' giving 'sound rock everywhere at a very convenient level.' Unfortunately, the rock proved to be unsound in many places to a considerable depth, with schistous micaceous masses of a very friable nature, which necessitated carrying down the foundations of the dam sometimes more than 40 feet deeper than was originally anticipated or provided for in the contract. As the thickness of the dam is nearly 100 feet at the base, this misapprehension as to the character of the rock involved a very large increase in the contract quantity and cost of the granite masonry of the dam. The total length of the dam is about l1⁄4 miles; the maximum height from foundation, about 130 feet; the difference of level of water above and below, 67 feet; and the total weight of masonry over one million tons. Navigation is provided for by a 'ladder' of four locks, each 260 feet long by 32 feet wide.
As remarked in the case of Asyût, the difficulties in dam construction are not in design, but in the carrying out of the works. It would not be too much to say that any practical man standing on the verge of one of the cataract channels, hearing and seeing the apparently irresistible torrents of foaming water thundering down, would regard the putting in of foundations to a depth of 40 feet below the bed of the cataract in the short season available each year as an appalling undertaking. When the rotten rock in the bed was first discovered, I told Lord Cromer frankly that I could not say what the extra cost or time involved by this and other unforeseen conditions would be, and that all I could say was that, however bad the conditions, the job could be done. He replied that he must be satisfied with this assurance, and
say that the dam had to be completed whatever the time and cost. With a strong man at the head of affairs, both engineers and contractors—who often are suffering more anxiety than they care to show—are encouraged, and works, however difficult, have a habit of getting completed, and sometimes, as in the present case, in less than the original contract time.
The contract was let to Sir John Aird and Co., with Messrs. Ransomes and Rapier as subcontractors for the steelwork, in February, 1898, and they at once commenced to take possession of the site of the works, and of as much of the adjoining desert as they desired in order to construct railways, build dwellings, offices, machine shops, stores and hospitals, and provide sanitary arrangements, water supply, and the multitudinous things incidental to the transformation of a remote desert tract into a busy manufacturing town. Two months after signing the contract the permanent works were commenced, and before the end of the year thousands of native laborers and hundreds of Italian granite masons were hard at work. On February 12, 1899, the foundation stone of the dam was laid by H.R.H. the Duke of Connaught. Many plans were considered by the engineers and contractors for putting in the foundations of the dam across the roaring cataract channels, and it was finally decided to form temporary rubble dams across three of the channels below the site of the great dam, so as to break the force of the torrent and get a pond of comparatively still water up stream to work in. Stones of from one ton to twelve tons in
weight were tipped into the cataract, and this was persevered with until finally a rubble mound appeared above the surface of the water. The first channel was successfully closed on May 17, 1899, the depth being about 30 feet and the velocity of current nearly fifteen miles an hour. In the case of another channel, the closing had to be helped by tipping railway wagons themselves, loaded with heavy stones, and bound together with wire ropes, making a mass of about 50 tons, to resist displacement by the torrent.
These rubble dams were well tested when the high Nile ran over them; and on work being resumed in November, after the fall of the river, water-tight sandbag dams or sudds were made around the site of the Dam foundation in the still waters above the rubble dams, and pumps were fixed to lay dry the bed of the river. This was the most exciting time in the whole stage of the operations, for no one could predict whether it would be possible to dry the bed, or whether the water would not pour through the fissured rock in altogether overwhelming volumes. Twenty-four 12-inch centrifugal pumps were provided to deal if necessary with one small channel; but happily the sandbags and gravel and sand embankments staunched the fissures in the rock and interstices between the great boulders covering the bottom of this channel, and a couple of 12-inch pumps sufficed. The open rubble dam itself, strange to say, checked the flow sufficiently to cause a difference of nearly 10 feet in the level of the water above and below;
but when the up-stream sandbag dam was constructed the difference was 20 feet, so that the down-stream sandbag dam was a very small one compared with the other.
The masonry of the dam is of local granite, set in British Portland cement mortar. The interior is of rubble, set by hand, with about 40 per cent, of the bulk in cement mortar, four sand to one of cement. All the face-work is of coursed rock-faced ashlar, except the sluice linings, which are finely dressed. This was steam-crane and Italian masons' work. There was a great pressure at times to get a section completed before the inevitable rise of the Nile, and as much as 3,600 tons of masonry were executed per day, chiefly at one point in the dam. A triple line of railway, and numerous trucks and locomotives, were provided to convey the materials from quarries and stores to every part of the work. The maximum number of men employed was 11,000, of whom 1,000 were European masons and other skilled men (Figs. 3, 4 and 5).
Mr. Wilfred Stokes, chief engineer and managing director of Messrs. Ransomes and Rapier, was responsible for the detailed designing and manufacture of the sluices and lock-gates; 140 of the sluices are 23 feet high by 6 feet 6 inches wide, and 40 of them half that height; 130 of the sluices are on the 'Stoney' principle, with rollers, and the remainder move on sliding surfaces. The larger of the Stoney sluices weigh 14 tons, and are capable of being moved by hand under a head of water producing a pressure of 450 tons against the sluice.
There are five lock-gates, 32 feet wide, and varying in height up to 60 feet. They are of an entirely different type to ordinary folding lock-gates, being hung from the top on rollers, and moving like a sliding coach-house door. This arrangement was adopted for safety, as 1,000 million tons of water are stored up above the lock-gates, and each of the two upper gates is made strong enough to hold up the water, assuming the four other gates were destroyed (Fig. 6). When the river is rising, the sluices will all be open, and the red water will pass freely through, without depositing the fertilizing silt. After the flood, when the water has become clear, and the discharge of the Nile has fallen to about 2,000 tons per second, the gates without rollers will be closed, and then some of those with; so that between December and March the reservoir will be gradually filled. The reopening of the sluices will take place between May and July, according to the state of the Nile and the requirements of the crops.
Between December and May, when the reservoir is full, the island of Philæ will in places be slightly flooded. As the temples are founded partly on loose silt and sand, the saturation of the hitherto dry soil would cause settlement, and no doubt injury to the ruins. To obviate this risk, all the important parts, including the well-known Kiosk, or 'Pharaoh's bed' have been either carried on steel girders or underpinned down to rock, or, failing that, to the present saturation level. It need hardly be said that, having regard to the shattered condition of the columns and entablatures, the friability of the stone, and the running sand foundation, the process of underpinning was an exceptionally difficult and anxious task. There were few men to whom I would have entrusted the task, but amongst those was Mat Talbot—one of the well-known Talbots who have done such splendid service as non-commissioned officers in the army of workmen employed by contractors during the past forty years; and well has he justified his reputation at home where his last job was the most difficult part of the Central London Railway and the commendation of Dr. Ball, who had charge of the works at Philæ.
It would be invidious to single out for special acknowledgment the services of members of a staff, where all have enthusiastically done their best for the accomplishment of the great work projected and patiently persisted in against all opposition, by Lord Cromer and his trusty lieutenant, Sir William Garstin, Under Secretary of State for Public Works. The successive Director-Generals of the Reservoirs were Mr. Willcocks, Mr. Wilson, and Mr. Webb; the chief engineers at Aswân, Mr. Fitzmaurice and Mr. May, and at Asyût, Mr. Stephens. The almost unprecedented labor and anxiety of arranging all the practical contractors' details of supply of labor, materials, and execution of the work fell upon the shoulders of Mr. Blue, except as regards Asyût, where Mr. McClure relieved him of a part of his responsibility.
As regards the initial stages of the project, I may say that when the Egyptian Government informed me that they wanted the works carried out for a lump sum, and no payment to be made to the contractor until the works were completed, I felt it would be idle to invite tenders until some arrangement had been made as to finance. As in other cases of doubt and difficulty, therefore, I went to my friend, Sir Ernest Cassel, and the difficulties vanished. The way was then clear for getting offers for the work. Sir John Aird and Co. were the successful competitors, and they have completed a largely increased quantity of work in less than the contract time, to the entire satisfaction of the Egyptian Government and of every one with whom they have been associated. The same recognition is due to Messrs. Ransomes and Rapier, and their able engineer and manager, Mr. Wilfred Stokes, who was unexpectedly called upon to complete all the complicated machinery of the sluices and gates in one year under the contract time, and did it.
- Address given before the Royal Institution of Great Britain.