in, should the dam be drowned by flood during its erection, no harm
would accrue were it composed of Portland cement concrete, whereas should
it be of hydraulic mortar masonry, the wall would probably be destroyed
or, at all events, considerably injured by the mortar being washed out of
the joints. Portland cement, however, is only suitable for situations
where the foundation is absolutely firm, as, should there be the
slightest settlement, fissures would certainly be produced.

As regards foundations, the dam of the Puentes reservoir in Spain is
somewhat remarkable--see Fig. 12. Its height is 164 ft., and the profile
or cross section is of precisely the same character as that of the
Alicante dam, the latter being 135 ft. in height, 65 ft. wide at the
crest, and 65 ft. at the base, and erected about 300 years ago. At the
Puentes dam the flanks of the valley were reliable, but, as must be
frequently the case in such situations, the bed of the valley was
composed to a great depth of gravel, _debris_, and shaky strata. The
difficulty was overcome by throwing an arch, or arches, across the
valley, the abutments being formed by the solid rock on each side, and
building the dam upon this arching and filling in below the latter down
to a sufficient depth with walling.

Bearing in mind the sudden and great floods to which dams constructed in
such situations must be subjected, and, if the valley be very narrow, the
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 61/4 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