_Whenever it is possible to avoid it, no drain should have a decreasing
rate of fall as it approaches its outlet._
If the first hundred feet from the upper end of the drain has a fall of
three inches, the next hundred feet should not have less than three
inches, lest the diminished velocity cause silt, which required the speed
which that fall gives for its removal, to be deposited and to choke the
tile. This defect of grade is shown in Fig. 17. If the second hundred feet
has an inclination of _more_ than three inches, (Fig. 18,) the removal of
silt will be even better secured than if the fall continued at the
original rate. Some silt will enter newly made drains, in spite of our
utmost care, but the amount should be very slight, and if it is evenly
deposited throughout the whole length of the drain, (as it sometimes is
when the rate of fall is very low,) it will do no especial harm; but it
becomes dangerous when it is accumulated within a short distance, by a
decreasing fall, or by a single badly laid tile, or imperfect joint,
which, by arresting the flow, may cause as much mischief as a defective
grade.
Owing to the general conformation of the ground, it is sometimes
absolutely necessary to adopt such a grade as is shown in Fig. 19,--even to
the extent of bringing the drain down a rapid slope, and continuing it
with the least possible fall through level ground. When such changes must
be made, they should be effected by angles, and not by curves.
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