Engineering Wonders of the World
Volume I

r 388 ENGINEERING WONDERS OF THE WORLD. which there are many in Egypt), it would run dry as soon as the river had fallen below the canal bed at E, and no more Perennial crOpS COuld be raised on these Irrigation. .r Rai lands unless other means of irrigation were provided. If, however, the canal bed, C, be cut deeper than the lowest summer level of the river at E, another method of irrigation, the perennial, is possible. Let us assume the canal to be constructed for per- ennial irrigation. Since the gradient of the canal bed is much more gradual than that of the Nile Valley—1 in 20,000 as against 1 in 12,000—if we follow the course of the canal from its inflow at E (where the land is higher than the water level of the canal) in a direc- tion parallel to the river, there will come a point, say at V, where the water level of the canal and the land level will be the same. From that point onward the farther we pro- ceed the lower will be the level of the land relatively to the water level in the canal. Eventually the point M will be attained, whereat th© water level of the canal will command even the land, marked lying above the flood level of the Nile, follows that below the point V, at X, M, etc., wheresoever it may be required for irrigation, the water will flow on to the land by gravitation. In summer, the basins, 1, 2, 3, 4, 5, 6, 7—some of these, it may be said, have an area of 40,000 acres—which had been flooded and drained off in the winter, are divided up by distributing canals and low banks into diminutive basins, and are flooded to a depth of two or three inches according to the requirements of the crops. It should be remembered, in connection with the perennial system of irrigation, that, where gravitation is relied on solely, the area of cul- tivable land each year is determined by the general level of the Nile in summer. The lower the Nile at E the farther away from the inflow will be the point V, where the water level of the canal C and the land level are c A, It Y, the same. It should also be noted that when the average level of the low Nile falls below the normal, the volume of water in the canal is decreased, and the area of land capable of being reached is diminished. We have seen that in ordinary circum- stances the water level of the canal C is the same at point V as the level of the adjacent fields. If, however, a barrage were built across the river, down-stream of the intake E, so that during high Nile the flood water would be allowed to pass freely, and during the summer the river could be dammed back, the point at which the level of the canal water and the level of the adjacent fields were the same would then be brought proportionately nearer the intake—say, to the point K. The effect of this would be to increase the volume of water in the canal C, to enlarge greatly the area of irrigable land, and to reduce the effects of a “ bad ” Nile. Such, in fact, is the purpose served by the Assiout Barrage. Comparing our illustration (Fig. 3) with this important work and the great canal it serves, W may be said to represent the Assiout Bar- rage, and canal C the Ibrahimiyeh Canal. In the foregoing description of perennial irrigation we have assumed that both means —flooded basins for winter crops, and gravita- tion irrigation (Sefi water) for summer crops— were used. The crops so cultivated are wheat, beans, barley, maize, durra, etc. But cotton cannot be cultivated under the same condi- tions, for the seed of this most important crop is sown in March, and the . . Cotton, harvest is not ripe for picking until September, at the middle of what is known as the flood season. So the land used for cotton cannot be irrigated on the basin system. During the flood season it is watered in a manner similar to that employed through- out the summer, by a system of miniature waterways running off the main canal. Where, as often happens, the canal or river is not sufficiently high to allow water to pass by