half
of the actual depth. As a general proposition, this is the most
economical form of profile for water-channels with given side slopes.
The amount of excavation is the least in that channel where the wetted
perimeter for a given area is the smallest. In practice the forms
commonly adopted for ditches and flumes are trapezoidal and
rectangular.
With rectangular profiles the resistance due to friction is the smallest when the width is twice the height.
Of trapezoidal profiles, the half of a regular hexagon is generally used in canals and ditches.
Circular and square profiles are employed only in stone, wood, and iron constructions.
Narrow and Deep vs. Broad and Shallow Ditches.—In
a mountainous country narrow and deep ditches with steep grades will
generally be found preferable to large conduits with gentler slopes.
The first cost of excavation is much less, as is also the cost of
repairs rendered necessary by snows and severe storms, the narrower
aqueduct being more easily protected. The experience of the
ditch-builders in this State has been uniformly favorable to these
steep grades, but little trouble being caused by the washing of the
banks due to high "velocities. In the valleys with ashy soil such
grades, of course, would not be practicable.
Ditches
in California with carrying capacities as large as 80 cubic feet per
second have been built, and are now in successful operation, with
grades of sixteen to twenty feet per mile.
Excavating the Ditch.:—Before
the work of excavating is commenced the line is cleared of trees and
underbrush for a sufficient width to render work afterwards easv and
to prevent subsequent damage to the ditch. All trees which are liable
to fall and injure the work should be removed before construction
begins. On a flume line the brush for at least ten feet on each side is
burned as a precaution against fire. So far as possible, and especially
