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. The next important point is to determine the
position of the dam, having regard to the suitability of the ground for
affording a good foundation and the impoundment of the requisite body of
water with the least outlay on embankment works.

It has been suggested that the floods of the valley of the Thames might
be controlled by a system of storage reservoirs, and notice was
especially drawn to this in consequence of the heavy floods of the winter
of 1875. From evidence given before the Royal Commission on Water Supply,
previous to that date it was stated that a rainfall of 1 in. over the
Thames basin above Kingston would give, omitting evaporation and
absorption, a volume of 53,375,000,000 gallons. To prevent floods, a
rainfall of at least 3 in. would have to be provided against, which would
mean the construction of reservoirs of a storage capacity of say
160,000,000,000 gallons. Mr. Bailey Denton, in his evidence before that
commission, estimated that reservoirs to store less than one tenth that