great refinement in the design of the side ditches for the ordinary
case where the water is carried along the road for only a few hundred
feet. The ditches are made of ample capacity by using the commonly
accepted cross section for a road, which will be discussed in a later
paragraph. But where large areas must be drained by the road ditches,
it is desirable carefully to design the side ditches. The basis for
that design is too lengthy to be included herein, and reference should
be made to a standard treatise on the subject.

=Ordinary Design of Ditches.=--For grades of one per cent or less on
roads in the humid area, the bottom of the ditch should be at least
three and one-half feet lower than the traveled surface of the road,
except for very sandy soil. For grades greater than one per cent, this
depth may be decreased one foot, and for grades of four per cent and
upward, the depth may be still less. These general rules for depth are
susceptible of variation but are believed to be the minimum except in
arid or semi-arid climates. It is far better to be too liberal in
ditch allowance than to be too conservative. In arid or semi-arid
regions, the ditch design will be based on the necessity of providing
for flood flow and preventing damage through erosion. Ordinary
drainage requirements will be satisfactory with the ditch about one
foot deep.

If the topography is such that it is evident considerable storm water
will flow from the adjacent land to the road ditches, the design must
be modified to take this into account. Sometimes such water can be
diverted by ditches well back from the road, and thus prevented from
flowing into the side ditches along the roadway. It is especially
desirable to divert water, which would otherwise flow down the slope
of a cut, by means of a ditch on the hill-side above the upper edge of
the slope of the cut.

Ditches are not effective unless they afford a free flow throughout
their length and have an outlet to a drainage channel of ample
capacity. Therefore, ditch grades should be established by survey,
especially if the gradient is less than one per cent, and the
construction work should be checked to insure that the ditch is
actually constructed as planned. A few high places in the ditch will
greatly reduce the effectiveness, although these may appear at the
time of construction to be slight. Constricted places, such as might
be due to a small amount of loose earth left in the ditch, ar