A certain loss of water, in
times of excessive floods, must, in designing a dam, be ever expected,
and under favorable conditions may be estimated at 10 per cent. of the
total amount impounded.
As regards the choice of position for the dam of a reservoir, supposing
that it is intended to impound the water by throwing an obstruction
across a valley, it may be premised that to impound the largest quantity
of water with the minimum outlay, the most favorable conditions are
present where a more or less broad valley flanked by steep hills suddenly
narrows at its lower end, forming a gorge which can be obstructed by a
comparatively short dam. The accompanying condition is that the nature of
the soil, i.e., the character, strata, and lie of the rock, clay, etc.,
as the case may be, is favorable to assuring a good foundation. In Great
Britain, as a rule, dams for reservoirs have been constructed of
earthwork with a puddle core, deemed by the majority of English engineers
as more suitable for this purpose than masonry.
Earthwork, in some instances combined with masonry, was also a form usual
in the ancient works of the East, already referred to; but it would
appear from the experience of recent years that masonry dams are likely
to become as common as those of earthwork, especially in districts
favorable to the construction of the former, where the natural ground is
of a rocky character, and good stone easily obtained.
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