along
a side hill, the ditch should be dug so as to have walls of solid,
untouched ground, and not made banks. The top of the solid bank on the
lower side should be fully three feet wide. In such cases the top soil
is first removed for the width of the ditch and bank ; the material
excavated to form the ditch is used to raise the lower bank, and in
time consolidates to firm ground, thus increasing the capacity of the
ditch.
The
digging of ditches is usually let by contract at a given sum per rod,
and heavy cuts per cubic yard. It is customary to excavate large
ditches with a slope of 6o° for the upper and 650 for the
lower bank. These slopes, of course, the engineer will vary in
accordance with the ground encountered. In practice they are changed
eventually by erosion and denudation; but experience seems to warrant
the above-mentioned slopes as the best to be adopted in laying out such
works.
In
large mining ditches constructed with high grades and running large
amounts of water, the erosion and consequent enlargement of the ditch
(when kept in order) is noticeable ; moreover, the banks gradually
become solidified, and thereby the loss by leakage and absorption is
decreased. It is roughly estimated that the capacity of a
well-constructed ditch which is properly kept up is increased about 10
per cent, in eight years.
Ditches
poorly built in the beginning subsequently require large and constant
expenditures, and lose considerable amounts of water. The annual cost
of running and maintaining large ditches, including all repairs and
taxes, is estimated to be $400 per mile.
Examples of Ditches.—Among
the principal ditches in the State are the North Bloomfield, the
Milton, the Eureka Lake, the San Juan, the South Yuba Canal, the
Excelsior or China ditch, the Bouyer, the Union, the El Dorado, the
Spring Valley and Cherokee, the Hendricks and the La Grange.
North Bloomfield.—The North Bloomfield main
