The Nile in 1904

by Sir William Willcocks

A good description of certain spills is given on page 112 of the Report, a good idea of scour in Plate XXIX., opposite page 110, while on page 181 Sir William Garstin makes the remark that the experience of American engineers has taught us that though in theory it may be possible to shorten or straighten a great river, in actual practice it is accompanied with almost insurmountable difficulties. If the new channel were dug and set working, in a few years it might be as crooked as the Albert Nile itself, unless it were protected with stone along its entire length.

In my project for deepening and widening both the Albert Nile and the Zeraf river to enable them to carry 600 cubic metres per second, I think I have given very solid reasons against abandoning the Albert Nile and sending the whole supply down the Zeraf river at a cost of £3,400,000.

I cannot but think that Sir William Garstin’s recent objections to the Albert Nile are founded on an oversight. He has, apparently, not kept the flood discharges of the Albert Nile at Gondokoro and above the Sobat mouth sufficiently apart from those of low supply. It is the Sobat flood, combined with the poor carrying capacity of the White Nile, which is the disturbing factor, and not any inherent viciousness in the Albert Nile itself. The Albert Nile has a good section, and, if it were trained in conjunction with the Zeraf river, would, I feel confident, discharge all the water required with a very moderate expenditure of money. This, I always understood, was Sir William’s own opinion. In such projects it is wise to remember Horace’s saying, “Naturam furcâ expellas tamenusque recurrit.”

38. Project for converting the basin irrigated lands of Upper Egypt into perennially irrigated lands.

—No consideration of this question would be complete without first examining into the changes which would be made in the regimen of the Nile flood by the contemplated conversion of basin irrigation into perennial irrigation. This question was examined very thoroughly by me in 1892 and 1893 and I give here my arguments for not anticipating any serious difficulties.

We have to consider the effect of the introduction of perennial irrigation on the regime of the Nile. The perfection of the perennial irrigation of the Delta north of Cairo will in no way affect the Nile in flood. The canals will continue to run as they do at present, and the question of conversion in Lower Egypt is therefore quite independent of the subject of flood protection. In Upper Egypt, however, we have 1,460,000 acres of basin irrigation; and as each acre receives in a low flood 80 cubic metres of water per day, in an ordinary flood 130 cubic metres per day, and in an extraordinary flood 170 cubic metres per day, while the demands of perennial irrigation are only 25 cubic metres per acre per day, it will readily be understood that we are dealing with a quantity of water which demands the greatest attention.

To foretell with exactitude the anticipated changes in the regime of the Nile, it is necessary to know first the daily gauges of the Nile at Assuân and Cairo for a period of at least twenty years, and the discharges corresponding to these gauges. The difference between these discharges represents the consumption of water. We have next to determine the amount of water which passes into the canals, the amount utilised in filling up the trough of the Nile and covering the berms, and the amounts evaporated and absorbed. We know that the last three items are constant while the canal discharges are variable and depend on the system of irrigation and, if our data are correct, we can tell with moderate certainty what changes in the level of the Nile will follow certain changes in the system of irrigation.

[Tables 65] and [67] of [Appendix L] contain the Assuân and Cairo gauges for typical years in a period of twenty years from 1873 to 1892, and the mean gauges of these twenty years. Finding it impossible to understand the Nile without first referring every gauge to some uniform standard, I have had to choose the line of reference. The mean high water level and the mean low water level are both available. In Egypt the mean high water level varies very considerably whether we take it in August and the early part of September when the basin canals are running full supply, or in the latter half of September when the canals are running only half supply, or in October when the basins are discharging back into the Nile. Early and quick rising floods have a different series of levels from slow and late floods; while again the recent works carried out in Upper Egypt by Col. Ross have so increased the discharging capacity of the canals that the flood gauges have been appreciably affected. All this points to the conclusion that the mean high flood is no satisfactory standard. The mean low flood on the other hand is much less liable to change and is very fairly constant from year to year. High floods are certainly followed by scouring out of the bed, and low floods by a silting up of the channel, but the changes are very moderate compared to those in high flood. I have chosen the mean low water level as the line of reference, and referred all gauges to it. From the mean of twenty years’ observations, this level at Assuân is R. L. 85 metres. By observations along the Nile generally, and by calculations at Cairo, I have fixed it at all important places north of Assuân. [Table 46] of [Appendix K] gives the Reduced Levels at different places, while it is also drawn on the longitudinal section of the Nile in [Plate XII]. It was on this system that the ancient Egyptian engineers worked the Nile. They however chose the mean high water level during the early part of the flood as their standard of reference and consequently made the so called cubits in the flood reaches of the Cairo gauge half cubits. This means a discharge of 1600 cubic metres per second and fairly represents the discharges of the basin canals in flood. When it is considered that the level of the Nile valley is raised by about 10 centimetres per 100 years it will be seen that the old Cairo gauge, which was a living record 1500 years ago, is to-day a meaningless anachronism. It has also to be compared with the Assuân gauge which was erected in Ismail Pasha’s time with an arbitrary zero some 90 centimetres below mean low water level, and which may be reading 17 cubits while Cairo may be recording 25 cubits. The Cairo gauges in winter and summer are no records of discharge as the afflux from the Barrage affects them. To find the discharge at Cairo during these months, I have added those of the Rosetta and Damietta branches and the Delta canals upstream of the Barrage. When the Nile falls below mean low water level, the gauges are recorded as minus quantities.

Discharge sites having been chosen for the Assuân, Assiout and Cairo gauges on the Nile, a continuous series of surface velocity observations, cross sections and slope measurements were made during 1892 and 1893 and the resulting discharges recorded. Curves of discharge have been drawn and referred to the gauges of twenty years and modified until finally a curve has been found which will suit any year whether it is a maximum or a minimum. In connection with this subject, it must be remembered that the Nile bed is raised by silt during low floods and scoured out during high floods and that consequently August and September discharges vary considerably at times from October and November discharges for the same gauge. In addition to this, it must also be borne in mind that the slope of water surface and that consequently the discharge of a flood during the rise is far greater than during the fall for the same gauge reading. Indeed the Nile often discharges more when it is 30 centimetres below its maximum and rising fast than when it has reached its maximum and begun to fall. It is owing to this fact that we often see the discrepancy of the Assuân gauge reaching its maximum a day before Halfa which is 350 kilometres higher up the river. The discharge depends on gauge and slope, and the gauge only records one element. Keeping these facts in my mind, I saw that it was of no use recording the gauges to two places of decimals and covering paper with useless figures, and consequently I have chosen the higher unit for a rising gauge and the lower for a falling gauge when I have been dealing with discharges.

Flood discharges have been taken of all the canals in Upper Egypt through 1892 and 1893 and have been recorded in >[Tables 48] and [49] of [Appendix K]. From these tables, [Table 47] has been compiled which gives rough approximate discharges of the canals corresponding to the Assuân gauges in the first half of the flood.