Page 331 - Moving the Earth_ The Workbook of Excavation

LANDSCAPING AND AGRICULTURAL GRADING

LANDSCAPING AND AGRICULTURAL GRADING 7.25

Wells. Water may be obtained from wells, occasionally by natural flow but more often by
pumping. Such water, if used in the immediate vicinity, involves minimal piping and distribution
difficulties. The most serious problem is the likelihood of using too much water, so that wells must
be deepened or new ones drilled to keep in touch with the falling water table. In many localities
such overpumping will eventually reach salty or alkaline water unfit for agriculture.
River water may be pumped up to the land to be irrigated, but more often flows onto the land
from a higher point in the river. A ditch or canal is cut from the stream, and run horizontally or
with a very slight gradient along the side of the valley. The land between the canal and the river
is irrigated by gravity flow from the canal into ditches, and from them onto the fields.
Most rivers have a wide variation in level, so that such a canal might be subject to being left
dry at times and flooded at others. A more constant level can be obtained by damming the stream
below the canal entrance to keep the water high enough to enter it, and by providing gates and a
protective dike to prevent flooding. A dam and control gates above the inlet will permit regulation
of the river level and inflow.
In dry climates, streams may flow only during the winter and spring or in occasional floods.
Water from such a source may be useful for crops which will mature in the wet season or as a sup-
plement to water obtained from wells, but the cost of handling it may not be justified.

Reservoirs. Such streams may be utilized by building one or more impounding reservoirs upstream.
These will permit storing of the heavy flow of some months to be released in drier periods. Large
reservoirs of this type may be ideal sites for hydroelectric plants, as the irrigation water can be
used to turn turbines on its way through the dam.
Most of the large irrigation systems of the United States depend on storage reservoirs.
Lakes may be used as sources of water. It may be pumped up onto adjoining land or conducted
through siphons or canals to lower land. Occasionally, a lake may be tapped by means of a tunnel
and used for stream regulation in the same way as an artificial reservoir.

Sediment. Direct river flow into an irrigation canal carries varying quantities of sediment with the
water. The flat gradients and slow motion of the water in the system will cause extensive silting,
which will ultimately reduce water capacity, may cause continuous trouble in operation of gates and
other devices, and will clog pipes. The design should take this quality of the water into account. Very
dirty water requires oversized waterways and steepened gradients, particularly in pipes; cleanout
traps or flushing devices for pipes; and access to all ditches for cleaning by machinery or by hand.
Upstream reservoirs catch most, or sometimes all, of the nonsoluble sediment. Any residue
may be still further reduced by settling basins.
Dirty water has certain advantages. In most localities it is good for the land, carrying topsoil
and minerals which help to replace natural losses. It also tends to seal leaks in canals and ditches
so that wastage by seepage is reduced.
Its disadvantage is the greatly increased cleaning and maintenance work required, which is so
important a problem that most irrigation engineers will secure clean water whenever possible.
Canals. The main canal may be an unlined ditch or a ditch lined with special impervious soils
or a concrete or other artificial lining. See. Fig. 7.18. If it is carried well below natural grade at
any point, it is customary to use a lined tunnel or concrete pipe at such a section.
The unlined ditch is the most economical to construct, and may be used where soils are imper-
vious, where the water carries enough sediment to seal leaks, or where the supply of water is so
large that seepage losses are less important than the expense of preventing them.
A ditch may be lined, either in sections or as a whole, by placing a layer of clay or impervious
silt on the bottom and sides. This may be placed in a dry ditch, or fine clay may be added to the
irrigation water until it has coated the canal.
A ditch must have bank slopes which will not cave or slump into the water. Vegetation may be
used to hold dirt banks and will permit steeper slopes, but its consumption of water is often exces-
sive. Banks should be protected from erosion by surface water.
Concrete lining is expensive construction, and if properly done is most satisfactory. Such leaks
as occur will generally be localized and comparatively easy to find and repair.