ground. Where grades and other conditions permit canals should be made narrow and deep rather than wide and shallow, in order to lessen the surface exposed to evaporation.
Fig. 3. CANALS ON ROCK SLOPE WITH RETAINING WALLS.
Fig. 4. SEMI-CYLINDRICAL WOOD-STAVE FLUME.
Seepage should be guarded against. To this
end lining with stone laid in mortar or with
concrete or with cement mortar may be employed.
A great advantage in linings, if reasonably
smooth, is that they increase the carrying capacity
of the canals by lessening the friction, and
aid in maintaining it by lessening the
sedimentary
deposits and plant growths on their sides
and bottoms. Sometimes leakage may be
diminished by throwing powdered clay into the
water at the head of the canal. The sedimentary
matter naturally carried by the water will
often reduce the leakage in a few months or
years.
Fig. 5. WOOD IRRIGATION FLUME ON TRESTLE.
Flumes are most commonly built of wood,
with a rectangular cross-section, but in recent
years steel has been employed, particularly in
precipitous rocky locations, or where crossing
streams or deep ravines. The ordinary flumes
of boards or plank are subject to leakage. To
avoid this, and also to give a channel better
adapted to the flow of water, wooden staves are
now being employed, formed into a semicircular
or other shape designed to give a curved bottom.
The staves are held in place by steel rods
or bands, so arranged that they can be tightened
by turning nuts. The steel flumes are made of
thin plates riveted together. Flumes may rest
on mudsills, or timbers placed on the ground,
but, being generally designed to cross depressions,
they are more frequently supported on
trestles. The trestles, like the flumes, are
generally of wood, but they are sometimes of steel,
particularly where the flume proper is of that
material, or where the flume support must be
in spans, as at a stream crossing.
Fig. 6. STEEL IRRIGATION FLUME ON TRESTLE.
When, instead of valleys or streams, hills are
encountered, necessitating a long detour for
canals, tunnels are often employed. They do
not differ materially from other tunnels. If lining
is necessary, as in earth, or to reduce the
friction when in rock, it may be of brick,
concrete, or stone, and resembles that for masonry
aqueducts in tunnels. See Tunnels; Aqueducts.
Headworks of some kind are required for nearly all canals, flumes, or pipe-lines. In America they are generally of timber, but in much of the foreign work they are permanent structures of stone. The essential features are a bulkhead, gates, and wasteway. Where there is a dam at the head of the canal, the headworks may be at one end of it, or form a part of it.
