EGYPT. 



231 



THE GREAT DAM AT ASSOUAN, ON THE NILE. 



up-stream face is pitched at a less abrupt angle 

 in order to secure greater stability. The height 

 of crest or roadway above the zero mark of the 

 long-established water-gage is 25 meters, and the 

 high-water level as at present designed is 22 me- 

 ters above the same zero. When full, the reser- 

 voir will hold 1,165,000,000 cubic meters of water, 

 and its set-back will reach 143 miles above the 

 dam. The total cost of construction as given by 

 the best English authorities was 2,400,000, and 

 the amount of work accomplished is divided as 

 follows : Excavation, 775,000 cubic meters ; mason- 

 ry, 496,000 cubic meters; brickwork, 7,000 cubic 

 meters; ashlar, 41,400 cubic meters; sluice-gates, 

 2,240 square meters; lock-gates, 530 square me- 

 ters. At these figures, the initial rate of water- 

 storage is about 1 cent for each cubic foot, but 

 as this must diminish in a direct but irregular 

 ratio from year to year, it will eventually disap- 

 pear altogether when balanced against the accru- 

 ing benefits. 



Technically, the dam is described as " insub- 

 mergible " that is to say, it is not intended that 

 the Avater shall at any time flow over its lip or 

 crest. The reason for this is, that the Nile car- 

 ries in suspension such enormous quantities of 

 silt that if the flow is entirely checked at any 

 point the suspended matter settles to the bottom. 

 In the case of a reservoir, no matter how large, 

 it is evident that in a few years it would be 

 filled up, and its purpose for irrigation would be 

 defeated. This dam, therefore, is provided with 

 an elaborate system of sluice-gates, 65 of them 

 being but 3 meters above the zero mark ; then 

 come 75 at 8 meters, 18 at 12 meters, and 22 at 

 16 meters above zero. These different groupings 

 of the sluice-ways can be seen in part in the ac- 

 companying illustration, and some idea can be 

 formed of the way in which the discharge of 

 the river can be regulated through these great 

 openings, each of which is fitted with its own 

 hand-operated machinery for the effective open- 

 ing and closing of its gates. 



During the period of high water that is begin- 

 ning about the middle of July, when the per- 

 centage of solid matter in suspension is at its 

 highest, the river will be permitted to flow 

 through the sluices with as little hindrance as 

 possible, bearing its rich alluvial tribute to be 

 deposited in the lower valley. When all the 

 sluices are open there will be little perceptible 

 difference in the appearance of the river channel 

 a short distance above and below the dam from 

 what has always existed. As the flood subsides, 

 usually some time in November, the water be- 

 comes comparatively clear, and the gates will 

 then be closed. Under average conditions the 

 reservoir should be at its full level in January, 

 and this can be maintained until May, when the 

 need of reserve water begins to be felt in the 

 agricultural regions. From this time until the 

 next flood is due in July the gates will be open, 

 regulating the quantity of water in the lower 

 Nile according to the special climatic conditions 

 of the season. 



At the western end of the great dam is a navi- 

 gating channel with locks, each 75 meters long 

 and 9 meters wide, affording ample accommo- 

 dation for any river craft likely to be in use 

 upon the Nile. The lock-gates were designed 

 by F. B. M. Stoney, who is also the designer and 

 patentee of the sluice-gates used in the dam. 

 The heavier lock-gates are worked by hydraulic 

 power. 



The present high-water line of the reservoir 

 is as stated above; but the structure was inten- 

 tionally designed to sustain a pressure result- 

 ing from a higher level, 28 meters above the zero 

 mark being contemplated. This will nearly or 

 quite double the capacity of the reservoir, and 

 as the expense of raising the dam will be com- 

 paratively small (250,000), it may be effected 

 before many years. 



Curious and interesting problems are involved 

 in this initial step toward harnessing the great 

 river of the Pharaohs. Similar dams or weirs 



