in height, and although now in ruins would formerly impound a
reservoir lake of from eight to ten miles long and three to four miles
broad. There is also the Kala-Weva tank, with a bund of twelve miles in
length, which would, if perfect, create a lake of forty miles in
circumference. Both of these ruined works are situated in Ceylon. The
third embankment of a similar character is that of the Cummum tank,
situated in the Madras Presidency, and which, though ranking among the
earliest works of Hindoo history, is still in such a condition as to
fulfill its original intention. The area of the reservoir is about
fifteen square miles, the dam about 102 ft. high, with a breadth at the
crest of 76 ft., and of the section shown in the diagram.
The by-wash is cut in the solid rock altogether clear of the dam; the
outlet culverts, however, are carried under the bank. We will now
consider generally the methods employed in determining the site,
dimensions, and methods of construction of reservoir dams adapted to the
varying circumstances and requirements of modern times, with a few
references to some of the more important works constructed or in
progress, which it will be endeavored to make as concise and burdened
with as few enumerations of dimensions as possible.
The amount of the supply of water required, and the purposes to which it
is to be applied, whether for household, manufacturing, or irrigation
uses, are among the first considerations affecting the choice of the site
of the reservoir, and is governed by the amount of rainfall available,
after deducting for evaporation and absorption, and the nature of the
surface soil and vegetation.
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