Engineering Wonders of the World
Volume I

THE NILE DAMS AND THE ASSOUAN RESERVOIR. 387 Fig.’ 3.—DIAGRAM TO SHOW PRINCIPLE OF BASIN AND PERENNIAL IRRIGATION. bank of the Nile. In the background we have the desert hills, forming the limit of cultivable , . .. land, and between these and the river the fertile valley, composed of a deposit brought down by the river in past ages. The land nearest the desert lies lower than that bordering upon the Nile. In a phrase, “ the high land is always on the river bank and the low land near the desert; ” and it is indeed seldom_ eight or nine times in a century—that the land marked A in the plan is totally sub- merged. Two methods of irrigation are followed— basin irrigation and perennial irrigation. Under natural conditions, with the country devoid of canals or embankments, as the Nile rose, the flood water would spread over all the land lying at a low level and submerge it for about two months. At the end of that period, as the flood subsided, the water would drain off and leave behind a rich deposit of alluvial mud. This land, known as Rai land, is sown while still moist, and without further watering the winter harvest is reaped therefrom—in Central Egypt—about the middle of March. That simple process when brought under con- trol illustrates the principle of basin irrigation. An embankment, B, constructed on each side of the river, limits the extent of land naturally flooded to the part marked D, and prevents the flood water passing to the low- lying land until required. This land, L, is divided into a series of basins by raised em- bankments 12 feet high, and through them runs the canal, C, with its intake and main regulator at E, and other distributing regu- lators at K, V, etc. In mid-August, when the Nile is in flood, the canal C is opened, and the waters are allowed to enter the various basins (numbered 1, 2, 3, 4, 5, 6, 7) until all are flooded to a depth of about five feet. At the end of thirty days, when the deposit has settled, and the river has subsided, the water is allowed to escape into the Nile by making a communi- cation between the lowest basin and the river at Z, and permitting the contents of the other basins to escape through the same channel by sluices or breaches in the dividing banks at P, Q, R. Towards the end of November the land is sown, as described above, and the winter crop, consisting of wheat, beans, clover, etc., gets sufficient moisture from the mud. If the canal C were merely a flood or inundation canal (of