32 AGRICULTURAL EXPLORATIONS IN ALGERIA. 



is reported that seepage around and under the walls of the structure 

 still causes trouble, and some engineers question the permanent safety 

 of the work. 



The largest storage dam in Algeria is that across the Habra River, 

 7 miles south of the town of Perregaux. This structure, also, has been 

 the scene of a catastrophe, and a much more serious one than that 

 which occurred at St. Denis du Sig. The original dam, 1,506 feet long, 

 was built in two sections at an angle of 30 degrees, with the angle 

 pointing downstream. It was partly carried away on December 15, 

 1881, by excessive floods which overtopped the entire dam. This 

 disaster is generally attributed to the giving way of the soft founda- 

 tion material, and to water cutting around the east end through the 

 soft material. As a result of the break in the eastern end of the dam 

 400 persons were drowned and immense damage was done to property. 



The work of reconstruction was finished in 1886. The dam, as it 

 now exists, is essentialh' in three parts. The spillway on the west 

 end has a length of about 410 feet. The center of the dam crosses an 

 island which divides the stream into two channels. The portion of the 

 dam across the east channel is 13 feet higher than that over the west 

 channel. The reconstructed dam has a height of 131 feet, is 1,443 feet 

 long, 131 feet thick at the base, and 14.7 feet thick at the top. The 

 highest part of the dam, in the eastern section, consists of a wall 7.9 

 feet high and 4.9 feet thick, resting upon the top of the dam proper. 

 This was added to prevent overflow of the adjacent land by floods. 

 The event has shown the wisdom of this precaution, for in 1900 water 

 rose to within 2 feet of the top of the highest wall, and was 6 feet 

 higher than the crest of the spillwa^^ The total cost of the Habra 

 dam, from the inception of the enterprise, has been about $1,080,000. 



The reservoir formed b}' this structure has a capacity of 30,800 acre- 

 feet, and is intended to provide for the irrigation of about 100,000 

 acres, although so large an area has never been taken up under it. 

 The water from the reservoir is taken out at the base of the high or 

 eastern portion of the dam. A complicated apparatus has been devised 

 by which water passing through the sluice furnishes power to pump 

 water into a tank, which is situated upon a hill about 100 feet high at 

 the east end of the dam. The water thus elevated furnishes stored 

 power for the operation of the sluice gate. The gate is supposed to 

 be automatically raised and lowered as the water rises and falls in the 

 reservoir, but the mechanism has never proved altogether satisfactory. 



The Habra, with its tributaries, has a flow in summer of 18 second- 

 feet, but during unusual floods the discharge has been known to exceed 

 25,000 cubic feet per second. Although the drainage basin above the 

 Habra dam covers 3,859 square miles, the mean annual discharge of the 

 stream is estimated to amount to only about three and one-half times 

 the capacity of the reservoir. During the flood which occasioned the 



