USE OF CONCRETE PIPE IN IRRIGATION. 27 
mistake to depend on valves that may be closed entirely to relieve 
pressures, as such valves may all be closed at one time. Safety 
devices will be discussed under another heading. 
SETTLING BASINS AND SCREENS. 
Settling basins (fig. 6) should be installed in a pipe line where 
vegetable or earthy material may clog the pipe. Settling basins 
are often necessary where water is carried from a hilly territory in 
an open ditch, as such conditions are favorable for picking up debris, 
and where the ditch terminates at the beginning of a pipe line. It 
is usual in this case to install the basin at the junction of the ditch 
and pipe line as shown in figure 7. The dimensions of the settling 
basin depend upon the quantity of solid material transported in the 
ditch or pipe line. It must be large enough in area to check the 
velocity of the water sufficiently to allow the solid particles to settle, 
and deep enough to collect sand and other trash for a considerable 
period, otherwise it will require cleaning at too frequent intervals. 
Ordinary sand is transported in a pipe or ditch at a velocity of 
about one-half to two-thirds foot per second and fine gravel at an 
average velocity of about 1 foot per second. If the grades are such 
that velocities suddenly become too low to carry solid particles, 
settling basins should be installed or the pipe is liable to become 
clogged. Small settling basins, such as are installed at the intake 
of a field lateral with a main pipe or ditch, are usually cleaned out 
by hand. Large basins in main ditches should be located so that the 
sand can be flushed out. If a ditch runs parallel to a river bed or 
natural drain it is usual to install basins so that the waste will flush 
into the river or drain. Basins are also provided at points where the 
pipe crosses a ravine or other waterway. In this case solid materials 
can be flushed out readily. In case of flat grades, however, it is 
necessary to clean out the collected material by hand. 
Screens are often installed with settling basins for small lateral 
ditches that carry a large quantity of floating trash, in order to 
prevent orchard valves from becoming clogged. Some are placed 
in the basin at an angle of about 30 degrees to the vertical. In other 
cases multiple screens (fig. 6) are inserted in grooves. The one 
nearest the intake pipe may be of chicken wire having about 1J 
inch clear openings and each successive screen having a finer mesh of 
wire down to one-half or one-eighth of an inch. The first screen 
collects the larger trash such as leaves and small floating sticks, while 
the other screens catch the finer particles. The screens are made on 
frames that slide in grooves in the sides of the concrete box and can 
be readily removed and cleaned. 
If trash is carried in large quantities it is often necessary to devise 
some method of greatly increasing the screened area. This may be 
