27. THE ASWAN HIGH DAM

by Dr Hassan Zaky

The Aswan High Dam, called Sadd-el-Aali, is among the largest works of its kind ever undertaken. The need for the project arises from the fact that the flow of the River Nile, like that of many other rivers, is fickle and unsteady, being governed by intricate climatic conditions as yet uncontrolled by man. During a high flood the discharge at Aswan may be as much as 450,000 cubic feet per second, but in a low year it may not reach half that rate. The natural daily river supply normally varies from 300,000 cfs during floods to 15,000 cfs during periods of shortage.

In order to avoid the extreme fluctuations of drought and flood, and to obtain the maximum benefits from the river's potential resources, it was found necessary to devise measures for regulating its flow. It was decided that the only solution of the problem was the construction of a storage reservoir which could deal with the whole volume of the river and store all surplus water from one year to the next. For that a very large capacity is required; and further capacity is needed to allow for the accumulation of silt and to provide flood protection.

Hence came the idea of building a high dam on the Main Nile, at 4.5 miles upstream of the present Aswan Dam, for the development and utilisation of the water resources of the river to the best advantage.

The project calls for the construction of a rockfill dam some 365 feet in height from the river bed to the roadway that will cross the top. Flooding of Nubian land upstream of the dam will create the second largest man-made lake in the world: it will extend 315 miles upstream with a mean width of 5 miles, a maximum width of 13 miles and a maximum depth of 315 feet. The total quantity of water stored is estimated to be more than 104 million acre-feet, a little less than the capacity provided by the new Kariba Dam in Rhodesia, at present the largest man-made lake.

The guaranteed net annual draft from the Sadd-el-Aali reservoir is estimated to be 60 million acre-feet, or about 40 per cent in excess of the present irrigation requirements for both Egypt and the Sudan. This water, which will literally be saved from that now being wasted to the sea every year, will be used in the development of 1.7 million acres of agricultural land in Egypt and of about three times the present agricultural land in the Sudan.

The designing of the High Dam faced many difficulties, for it has to be constructed on a pervious foundation in the lake of the existing Aswan Dam. The whole extent of the main dam foundation will be covered with a layer of compacted dune sand about 82 feet deep. The rockfill above this layer will have a clay core made of Nile silt. The dam will be protected against seepage by two lines of defence, a horizontal impervious blanket in the upstream part and a vertical grout curtain descending in the pervious bed of the river to a depth of 690 feet until it strikes natural bed rock. This grout curtain will plug the voids in the layers of sedimentary materials underneath the dam.

At the early stages of the project it was proposed to pass the water from the reservoir to the downstream of the dam through seven diversion tunnels. However, it was found advantageous to substitute for the tunnels an open diversion canal. The diversion canal will be cut through the solid granite rock forming the east bank of the river. Where the canal crosses the line of the dam, six tunnels, with iron gates to control the flow, will be provided. It will be the largest canal in the world in terms of cross-sectional area. At the downstream end of the control tunnels, one of the biggest hydro-electric power plants in the world will be constructed. It is designed for an ultimate installation of twelve main generating units and at the ultimate stage the total annual energy generated will be 10,000 million kW with a production cost of 0.125 pence per unit of energy at Aswan and 0.5 pence when transmitted to Cairo. Such a supply of cheap energy will clearly be of immense value to Egypt's industrial development.

(from New Scientist, 9th March, 1961)