Page 222 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

5.24 THE WORK

FIGURE 5.16 Siphon and priming pump.

Siphons in which water moves slowly are likely to be stopped by air entering the top of the
outlet. This may be prevented by putting the outlet in a box or small pool so that the opening will
be under water. A slow current may also allow an air lock to form from the accumulation of air
or other gases escaping from the water in the pipe, or leaks from the outside.
Very small siphons may be started by mouth suction, and a medium size by inverting it so that the
ends are higher than the middle, filling it with water and holding the ends closed while placing it
in position. Or a tee connection in the top may be used to pour water in, keeping the ends plugged
until the pipe is full and the tee tightly plugged.
The most satisfactory way to start a large siphon is with a suction pump. A way to connect it is
shown in (B). A tee is placed on the outlet end, with the side opening reduced to fit the inlet hose
of a small diaphragm or centrifugal pump. Means are provided to prevent air from entering through
the lower opening of the tee, by means of a check valve, a screw plug, or a piece of plywood with
mud on it. With this stop in place, the pump is started and the air sucked out of the siphon so that
water from the pond is drawn through it into the pump. The stop on the main pipe then opens or
is removed, and the pump is shut off.
Channels. Channels may consist of natural watercourses; watercourses which have been enlarged,
straightened, or paved; or artificial ditches.
A streambed may be dredged to lower its level, to increase its depth or capacity, to keep it from
changing its course, or to change its course.
Level may be lowered to drain surface water from a swamp or pond, or to provide better under-
drainage for land in its vicinity.
Depth is usually increased to assist navigation, or to provide for more rapid runoff of floodwater.
Widening and straightening increase capacity, often at the expense of depth.
Streams normally wander in their courses, cutting away banks in some places and building them
in others. When valuable property or structures are threatened by these changes, the channel may
be artificially shaped to direct the force of the water away from them. This may involve turning
the water back to its original direction, or forcing it to flow in a new one.