The Habra dam—­see Fig.  No. 13—­completed in 1871, was destroyed by a sudden flood of this kind in December, 1881.  This reservoir, with a storage capacity of 6,600,000,000 gallons, was intended for the irrigation of a cultivated bordering on the Mediterranean and the storage of floods.  The height of the dam was 116.7 ft. and was provided with a by-wash of 394 ft. in length, and outlets for irrigation formed by four cast iron pipes of 311/2 in. diameter through the dam.  It was composed of rubble set in hydraulic mortar, the latter composed of two parts of sand to one of hydraulic lime.

For getting rid of the large deposits of sand to which all reservoirs in that country are liable, two scouring outlets were provided of the same description as those in the old Moorish dams.  The profile was calculated from Delocre’s formula, and was correct in this respect, supposing the by-wash to have been sufficient.  But as it was otherwise, and the flood swept over the crest to the depth of about 3 ft., the enormous extra strain thus induced overthrew the dam and caused the destruction of several villages and the death of 209 persons.  It must be mentioned that when the reservoir was filling, the water percolated through the masonry, giving the face wall the appearance of a huge filter, which at the time was attributed to the porous nature of the sandstone used in construction, but which more probably was due to the washing of the green mortar out of the joints.

At the Hamiz dam, also in Algeria, the water was admitted in 1884, but it showed immediately signs of weakness, so that the water had to be run out and an immense retaining wall erected to strengthen the main dam.  Algeria seems to have been singularly unfortunate as regards the success of works of this description.  Water was admitted to the Cheurfas reservoir in January, 1885, and it at once began to make its way through permeable ground at one end of the dam.  The flushing sluice in the deepest part of the dam had become jammed, so that the pressure could not be relieved, and in February 30 ft. length of the dam was carried away, causing a flood in the river below.  At some distance down stream was the Sig reservoir.  The flood rushing down, topped this dam by 18 ft. and overthrew it also.

Allusion has been made to provision for scouring out sand and deposit, especially in the dams of Algeria and of Spain.  The amount of sand, etc., brought down by the floods is something enormous, and the question of the best means of getting rid of it has occupied much attention.  In the old Moorish reservoirs the flushing gallery, piercing the lower part of the dam, was closed by iron doors on the down stream face and blocked with timber at the upper end.  When required to be flushed out, laborers passed through the gallery and broke down the timber barrier, the silt forming a wall sufficiently thick to resist the pressure of the water for the time being, and allow of the retreat of the Forlorn Hope—­if the latter had luck—­before giving way.