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 1^ 

 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 



