probability that sufficient space at the side for a by-wash will be
difficult to obtain, it would seem reasonable that in the calculation for
their section allowance should be made for the possible condition of the
whole length of the dam being converted into a weir, over which the
waters may flow without risk of injury to the dam, to a depth of, say, at
least twice that ever probable.

The topping of dams by floods is not uncommon, and if the extra strain
thus induced has not been allowed for, their destruction is nearly
certain, as instanced in more than one case in Algeria, where, although
the average rainfall is only 15 in. yearly, a depth of 6¼ in., or more
than one-third of the annual total, has been known to fall in twenty-four
hours.

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 31½ 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